NEET Biology

Structural Organization In Animals

Introduction To Animal Tissues

A group of cells similar in structure, function, and origin is called tissue. In a tissue, cells may be dissimilar in structure and function, but they are always similar in origin.

• The term “animal tissue” was coined by Bichat.
• Bichat is known as the “father of histology.”
• The term “histology” was coined by Mayer (1819).
• The study of tissues is called histology, Along with cytology (study of cells), histology was placed under the term “microscopic anatomy.”
• Marcello Malpighi is considered to be the founder of microscopic anatomy.

Based On Their Location And Function, Animal Tissues Are Classified Into Four Types:

Epithelial Tissue

Cells of the epithelium are set very close to each other, separated by thin films of extracellular material. Neighboring cells are held together by cell junctions. The epithelial tissue rests on a non-cellular basement membrane, which separates it from the underlying connecting tissue.

The Basement Membrane Is A Non-Cellular Membrane Made Up Of Two Layers: The upper thin layer is called the basal lamina, made up of glycoproteins and mucopolysaccharides and secreted by epithelial cells and the lower thick fibrous layer is called the reticular lamina made of reticular fibers and collagen fibers, which are a part of underlying connective tissue.

• Blood vessels are absent in the epithelial tissue. Materials are exchanged between epithelial cells and ves¬sels of the connective tissues by diffusion across the basement membrane.
• The epithelial tissue is classified into simple and compound epithelium.

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Specialized Junctions Between Epithelial Cells: To provide mechanical support to tissues, the plasma membrane of the adjacent epithelial cells is modified to form structures called as intercellular junctions.

Fight Junctions (Zonula Occludens): They help to prevent substances from leaking across the tissue. Plasma membranes in the apical parts become tightly packed together or are even fused.

Interdigitations: These are interesting, finger-like processes of the cell membranes of the adjacent cells.

Intercellular Bridges: These are minute projections that arise from adjacent cell membranes. They make contact with one another.

Gap Junctions: they facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules, and sometimes big molecules also.

Intermediate Junctions (Zonula Adherens): These usually occur just below tight junctions. The intercellular space at these places contains a clear, low electron-density fluid. There is a dense plaque-like structure on the cytoplasmic side of each plasma membrane from which fine microfilaments of actin (protein) extend into the cytoplasm. There are no intercellular filaments between the adjacent cell membranes. There is an adhesive material at this point. They probably serve anchoring functions.

Desmosomes (Macula Adherens): They perform cementing to keep the neighboring cells together. These are like zonula adherens but are thicker and stronger and have disc-like junctions. They have intercellular proteins. The plaque-like structures (protein plates) are much thicker. The microfilaments that extend from microfilaments are called tonofibrils. Desmosomes serve anchoring functions.

Hemidesmosomes (single-sided desmosomes) are similar to desmosomes, but the thickening of the cell membrane is seen only on one side. Hemidesmosomes join epithelial cells to the basal lamina (outer layer of basement membrane).

Simple Epithelium

• Simple epithelium is formed of a single layer of cells.
• The adjacent cells are held together by means of a desmosome, resting on the basement membrane.
• Simple epithelium occurs mainly on secretory and absorptive surfaces.
• It helps in nutrition, excretion, and secretion, but not in protecting the underlying tissue.

1. Squamous Epithelium
   • It consists of a layer of thin, flat, scale-like cells with prominent nuclei.
   • The cells have irregular boundaries that fit closely into those of neighboring cells.
   • It forms the inner lining of lung alveoli and blood vessels (endothelium).
   • It is also known as pavement or tessellated epithelium.
2. Cuboidal Epithelium
   • It has cells that are polygonal in outline, but ap-pear cuboidal in vertical section.
   • It lines small salivary and pancreatic ducts and thyroid vesicles.
   • The cells participate in secretion, excretion, and absorption.
   • The cells of cubical epithelium in absorptive surfaces often bear microvilli on their free ends. This gives a brush-like appearance to their free border. They are, therefore, called brush-bordered cubical epithelial cells, for example, in the proximal tubules of kidneys.
   • Microvilli greatly increase the area of the free surface of the cell and thereby enhance absorption.
3. Columnar Epithelium
   • It is characterized by the presence of tall cells that are shaped like polygonal columns.
   • The nucleus is usually located at the base of the cell.
   • Columnar epithelium covers the inner surface of the intestine, stomach, and gall bladder. It also occurs in gastric and intestinal glands.
   • Its function is secretion or absorption.
   • The intestinal mucosa is lined by brush-bordered columnar epithelium which is highly absorptive.
4. Ciliated Epithelium
   • The ciliated epithelium consists of columnar or cubical cells bearing cilia on their free surfaces.
   • The function of the cilia is to move particles, free cells, or mucus in a specific direction over the epithelial surface.
   • Ciliated columnar epithelium lines the inner surfaces of some hollow organs such as fallopian tubes, bronchioles, and small bronchi.
   • The ciliated columnar epithelium lining the ventricles of the brain and spinal canal is called as ependyma.
   • Cilia Is Of Two Types: Kinocilium is motile cilium with 9 + 2 organization whereas stereocilium is non-motile and ciliated columnar epithelium and does not contain basal granule. 9 + 2 organization is also absent. Stereocilia are found in some parts of the male reproductive tracts such as the epididymis and vas deferens.
5. Pseudostratified Epithelium
   • Pseudostratified epithelium covers the inner linings of the trachea and large bronchi.
   • Although made up of a single layer of columnar cells, it appears two-layered, because some cells are shorter than the others and have their nuclei at different levels.
   • The shorter cells lack cilia and secrete mucus which traps particles on the epithelial surface, whereas the longer cells are ciliated.
   • Ciliary movements propel the mucus and the particles toward the larynx.
   • Pseudostratified non-ciliated columnar epithelium tissue is found in the urethra of the male and parotid salivary glands.
   • Squamous cells show some keratinization.

Epithelial Tissue Points To Remember

Special Types Of Epithelium

1. Neuro Sensory Epithelium: In between pillar-shaped supporting cells, modified sensory cells are present. On the free end, sensory hair is present. The base of these cells is attached to the sensory nerve. For example,
   • Gustatory Epithelium: Covers the taste bud of the tongue and receives taste sensation.
   • Olfactory Epithelium: The Schneiderman membrane receives smell sensation.
   • In Stato-acoustic Organ: Lining of the internal ear.
   • In The Retina Of The Eye: Receives optic sensation.
2. Myoepithelium: Around mammary and sweat glands
3. Pigmented Epithelium (Cuboidal): In the retina of eye

Compound Epithelium

• It consists of more than one layer of cells.
• Only the cells of the deepest layer rest on the basement membrane.
• Being multilayered, compound epithelia have little role in secretion or absorption, but they provide protection to underlying tissues against mechanical, chemical, thermal, and osmotic stresses.
• Compound epithelia may be stratified or transitional.

1. Stratified Epithelium
   • The stratified epithelium has many layers of epithelial cells.
   • The deepest layer is formed by cuboidal cells.
   • But the morphology of the superficial layers varies in the different kinds of stratified epithelia.
   • In stratified cuboidal epithelium, the superficial cells are cuboidal.
   • It lines the inner surfaces of larger salivary and pancreatic duct
   • Stratified non-keratinized squamous epithelium covers moist surfaces such as those of the buccal cavity, pharynx, and esophagus.
   • It has several superficial layers of living squamous cells and deeper layers of interlinked polygonal cells,
   • Stratified keratinized squamous epithelium covers the dry surface of skin. It has many superficial layers of horny, scale-like remains of dead squamous cells, and several deeper layers of living polygonal cells.
   • Heavy deposits of the insoluble protein keratin in the dead superficial cells make the epithelium impervious to water and highly resistant to mechanical abrasions.
   • In contrast, non-keratinized stratified epithelium cannot prevent water loss and can afford only moderate protection against abrasions.
2. Transitional Epithelium
   • Transitional epithelium is much thinner and more stretchable than stratified epithelium.
   • It has a single layer of cuboidal cells at the base, two to three middle layers of large polygonal or pear-shaped cells, and a superficial layer of large, broad, rectangular, or oval cells.
   • It lines the inner surface of the urinary bladder and ureters.
   • It allows considerable expansion of these organs to accommodate urine because stretching considerably flattens and broadens the cells of superficial and middle layers.
3. Glandular Epithelia
   • The cells of glandular epithelia are generally columnar or cuboidal.
   • The Glandular Epithelium Can Be Classified Into Two Types: unicellular, consisting of isolated glandular cells (for example, goblet cell of the alimentary canal), and multicellular (for example, salivary glands), consisting of a cluster of cells.
   • A gland with a single unbranched duct is called a simple gland.
   • The secretory part of the gland consists of epithelial cells arranged in the form of tubes (tubules) sacs (acini, alveoli), or a combination of both.
   • The duct is also made up of epithelial cells.
   • A gland with a branched system of ducts is called a compound gland.
   • In these glands, the secretory tubule or acinus may be coiled or branched and open into the single duct of the gland.
   • Compound glands are present in the pancreas and sub-mandibular salivary glands.

Types Of Simple Glands

1. Simple Tubular: Simple tubular glands are present in the intestine (for example, crypts of Lieberkuhn).
   • Simple Alveolar: The terminal part forms the alveolus, for example, mucous glands in the skin of frogs, and poison glands in toads.
   • Simple Coiled Tubular: The terminal part is coiled, for example, sweat glands.
   • Branched Tubular: Gastric glands in the stomach.
   • Branched Alveolar: Example, sebaceous gland.
2. Types Of Compound Glands
   • Compound Tubular Gland: Example, mammary glands of prototherians.
   • Compound Saccular Or Alveolar Gland: Example, salivary glands, (sub-maxillary and sub-lingual).
   • Compound Tubulo Alveolar Or Tubulo Saccular: They are tubular as well as alveolar and are found in mammary glands, pancreas, parotid salivary glands, Cowper’s glands, and Bartholin’s glands.
3. Exocrine And Endocrine Glands: Exocrine glands have a secretory portion that contains the cells for secretion of milk, digestive enzymes, mucus, saliva, ear wax, oil, and ducts, which transport their secretions to the respective sites of action, for example, salivary gland, tear gland, gastric gland, and intestinal glands. Endocrine glands are glands of “internal secretion” whose secretions are usually secreted directly into the blood. Examples are follicle-stimulating hormone from the anterior pituitary or thyroxin from the thyroid. When a gland performs both exocrine and endocrine functions, it is called a mixed gland or heterocrine gland (for example, pancreas, testes, ovaries).
4. Classification Of Glands Based On The Mode Of Secretion
   1. Holocrine Glands: In holocrine glands (for example, the sebaceous gland), the product of secretion is shed with the whole cell leading to its destruction.
   2. Merocrine Glands: When the secretory granules leave the cell by exocytosis (simple diffusion) with no loss of other cellular material, the glands are called merocrine glands (for example, pancreas, salivary glands, intestinal glands, and sweat glands)
   3. Apocrine Glands: In apocrine glands (for example, mammary glands and axillary sweat glands), only the apical portion of the cytoplasm is discharged along with the secretory product.

Connective Tissue

All connective tissues in the body are developed from mesoderm. O. Hartwig called them mesenchyme because they originated from embryonic mesoderm. Only connective tissue constitutes 30% of the total body weight (muscle: 50%, epithelium: 10%, nervous: 10%). On the basis of the matrix type, connective tissue is of three types:

1. Connective Tissue Proper: Soft and fibrous matrix.
2. Connective Tissue Skeleton: Dense and mineralized matrix. Due to the deposition of minerals, it becomes hard.
3. Connective Tissue Vascular: Liquid and fiber-free matrix

Connective Tissue Proper: Connective Tissue Proper is composed of three components:

1. Different types of cells,
2. Fibers, and
3. Matrix.

Cells Of Connective Tissue Proper

• Fibroblast Cells
  • The largest cell of connective tissue proper.
  • Maximum in number.
  • The cell body and nucleus both are oval-shaped.
  • A branched cytoplasmic process arises from these cells so they appear irregular in shape.
  • Rich in rough ER because their main or primary function is to produce fibers. Fibers are composed of protein.
  • They are the chief matrix-producing cells.
  • Old fibroblast cells (fibrocytes) are inactive cells.
  • These are also considered undifferentiated cells of connective tissue because they can be modified into osteoblast and chondrioblast cells to produce bone and cartilage.
  • Fibroblast Cells Function: To produce fibers and to secrete matrix.
• Plasma Cell (Cart Wheel Cell)
  • Less in number.
  • Amoeboid in shape.
  • Chromatin material is arranged like spokes in a wheel, hence is also called cartwheel cells.
  • According to research, these cells are formed by the division of lymphocytes. So they are also called clones of lymphocytes.
  • Plasma Cell (Cart Wheel Cell) Function: Produce, secrete, and transport antibodies.
• Mast Cells/Mastocytes
  • Numerous, amoeboid, and small in size.
  • Structurally and functionally similar to basophils.
  • Two- to three-lobed S-shaped nucleus.
  • The cytoplasm contains basophilic granules which can be stayed with the basic dye methylene blue.
  • Mast Cells/Mastocytes Function: These are important cells of connective tissue proper as they perform certain important functions:
  • Histamine is a protein, a vasodilator, which increases the permeability of blood capillaries. It takes part in allergy and inflammatory reactions.
  • Serotonin, also called 5-hydroxytryptamine, is a protein, a vasoconstrictor, and decreases blood circulation, but increases blood pressure. At the site of cut or injury, serotonin causes a decrease in blood loss.
  • Heparin, a mucopolysaccharide, is a natural anti-coagulant that prevents the clotting of blood in blood vessels by preventing the conversion of prothrombin into thrombin.
• Adipose Cells/Fat Cells: These arc oval-shaped cells that store fat. Fat is collected in the form of fat globules formed by the fusion of small oil droplets. On the basis of the number of fat globules, adipocytes are of two types:
  1. Monolocular Adipocytes/White Fat Tissue Cell
     • In these cells, a single large and central fat globule is present.
     • The nucleus and cytoplasm is peripheral and the cytoplasm is less in amount.
     • Due to the compression of the fat globule, the nucleus becomes flattened in shape.
     • These adipocytes form white fat.
  2. Multilocular adipocytes/brown fat tissue cell
     • In these cells, two to three fat globules are distributed in the cytoplasm around the nucleus.
     • The cytoplasm is more in quantity.
     • The nucleus is rounded and found in the center
     • These adipocytes form brown fat.
• Mesenchymal Cells
  • Less in number. Small-sized with cytoplasmic process having an irregular shape.
  • Oval-shaped nucleus
  • These are undifferentiated cells of connective tissue because they can transform into any cell of connective tissue proper (totipotent in nature).
  • Mesenchymal Cells Function: To form other cells of connective tissue.
• Macrophages/Histiocyte/Plasmatocytes
  • These are the second largest in size and number.
  • These are amoeboid in shape with bean or kidney-shaped nuclei.
  • Cytoplasm quantity is more agranular, but due to the presence of more number of lysosomes, it appears granular.
  • They are phagocytic in nature and destroy bacteria and viruses by phagocytosis. They arise by the fusion of monocytes.
  • Also called as the scavenger cells of connective tissue, because they destroy dead or damaged cells to clean connective tissue.
  • They are named differently in different organs:
    • Lung: Dust cells
    • Liver: Kupffer cells
    • Blood: Monocytes
    • Brain: Microglia cells
    • Thymus gland: Hessels granules
    • Spleen: Reticular cells
• Lymphocytes
  • Less in number and small in size, having an amoeboid shape.
  • A large nucleus is present and cytoplasm is present as a peripheral layer. Cytoplasm quantity is less.
  • They produce, transport, and secrete antibodies.
  • They divide to form the plasma cells of connective tissue properly.

Fibers

• Collagen Fibers (White Fibers)
  • They are shining white fibers composed of collagen protein (tropocollagen).
  • It is present in maximum quantity in vertebrates (only collagen fibers constitute one-third part of connective tissue fibers in human beings).
  • They are wavy and tough fibers and are always arranged in bundles called fascia.
  • On boiling, they convert into gelatin.
  • They can be digested by pepsin enzyme.
• Elastic Fibers (Yellow Fibers)
  • Precursor in color and composed of elastin protein.
  • They are branched fibers but are always arranged singly. Branches of these form a network.
  • In these fibers, maximum elasticity is present.
  • They are highly resistant to chemicals.
  • When boiled they do not dissolve.
  • They can be digested by trypsin enzyme.
• Reticular Fibers
  • Precursor of collagen fibers, delicate with no elasticity.
  • Also known as argyrophilic fibers, since they can be stained with silver salts.
  • They are composed of reticulin protein; highly branched fibers that always form dense networks.
  • These are mainly distributed in lymphoid organs such as the spleen or lymph nodes.

Differences Between Collagen, Elastic, And Reticular Fibers

Matrix: Matrix is composed of mucopolysaccharide which is present in the form of hyaluronic acid.

Types of Connective Tissue Proper

Loose Connective Tissue: It consists of cells scattered within an amorphous mass of proteins that form a ground substance. The gelatinous material is strengthened by a loose scattering of protein fibers such as collagen, and elastin, which makes tissue elastic, and reticulin, which supports the tissue by forming a meshwork.

• Adipose Tissue
  • Adipose tissue is a connective tissue rich beneath the skin, around kidneys in mesentery and bone marrow.
  • Besides fibroblasts, macrophages, collagen fibers, and elastic fibers, the adipose tissue also contains large, spherical, or oval cells called fat cells or adipocytes.
  • The cytoplasm and organelles in adipocytes are pressed by fat globules into a narrow annular layer just beneath the plasma membrane.
  • The adipose tissue synthesizes, stores, and metabolizes fat.
  • There are two kinds of fatty tissues. In the white adipose tissue, there is a single large fat droplet in the cells surrounded by a small amount of the cytoplasm.
  • The brown adipose tissue cell, on the other hand, has many small droplets of fat, suspended in a considerably larger amount of cytoplasm. Whereas brown fat cells contain many mitochondria, white fat cells have comparatively few.
  • The color in the brown fat is due to the high concentration of iron-containing cytochrome pigments.
  • Brown fat is particularly found in newborn babies and hibernating mammals.
  • It accounts for 5-6% of the body weight of the newborn rabbit and also of man.
  • Brown fat has a larger capacity for generating heat.
  • It is because of brown fat that new-boni mammals generally do not shiver in spite of lower temperatures outside.
  • Brown fat cannot be a substitute for food. Adipose tissue may be examined from the fat bodies of frogs or from the skin of rabbits.
  • Adipose Tissue Functions:
    • Prevents heat loss by forming a heat-insulating layer beneath the skin.
    • Forms shock-absorbing cushions around kidneys and eyeballs.
    • Acts as a food reserve.
• Areolar Tissue: Areolar tissue occurs beneath the epithelia of many hollow visceral organs, skin, and in the walls of arteries and veins. It contains different types of cells:
  1. Fibroblasts: They are the principal cells of this tissue. They are irregularly shaped flat cells with long proto¬plasmic processes. Fibroblasts synthesize two kinds of protein collagen and elastin. Fibroblast secretes the major amount of matrix.
  2. Macrophages/Histiocytes/Plasmatocytes: They are phagocytic in nature.
  3. Mast Cells/Mastocytes: They are irregular or ovoid cells and contain basophilic granules which are made of:
     • Histamine: An inflammatory substance produced during allergic reactions.
     • Heparin: Natural anti-coagulant.
     • Serotonin: Vasoconstrictor.
  4. Plasma cells/cartwheel cells synthesize antibodies. The areolar tissue joins different tissues and forms the packing between them and helps to keep the organs in place and in normal shape.

Dense Connective Tissue: Fibers and fibroblasts are compactly packed in dense connective tissues. The orientation of fibers might show a regular or irregular pattern and is called dense regular or dense irregular tissues, respectively.

• In the dense regular connective tissues, the collagen fibers are present in rows between many parallel bundles of fibers, for example, tendons and ligaments.
• Dense irregular connective tissue has fibroblasts and many fibers (mostly collagen) that are oriented in different directions. This tissue is present in the skin, perimysium, perineurium, and around bones as periosteum.
• White Fibrous Tissue: It carries only a few fibroblasts scattered amidst the dense network of thick collagen fiber bundles. It has great tensile strength. The presence of white fibrous tissue at the joints between skull bones makes them immovable.
• Tendon: It is a dense, strong, and fibrous connective tissue with thick parallel bundles of collagen fibers. A few flat, elongated tendon cells lie in single rows between the fiber bundles. The tendon forms the strong inextensible attachment of a skeletal muscle to a bone. Colloidal protein gelatin is obtained by boiling collagen.
• Ligament: Ligaments connect the bones at the joints and hold them in position. A sprain is caused by excessive pulling of ligaments. They are made of bundles of elastic fibers and a few collagen fibers. Many-year-old mummies still have their arteries intact due to well-preserved elastic fibers.

Difference Between Tendon And Ligament

Reticular Tissue: It consists of star-shaped reticular cells whose protoplasmic processes form a network, These cells are phagocytic in function. Matrix and some other types of cells are are also found in the spaces of the network. Reticular tissue is present in the spleen, lymph nodes, bone marrow, etc.

Supportive Connective Tissue

Cartilage: Cartilage is a solid but semi-rigid and flexible connective tis¬sue. Chondrocytes are large, blunt, angular cartilage cells. They occur in clusters of two or three cells in small spaces (lacunae) scattered in the matrix.

1. Hyaline Cartilage: In hyaline cartilage, the matrix is apparently fiberless and glass-like (hyaline), but trans-lucent. It occurs in the larynx, nasal septum, tracheal rings, and costal cartilage. It gives those structures a definite but pliable form. White fibrocartilage carries thick dense bundles of collagen fibers between rows of chondrocytes in lacunae. It occurs in joints between vertebrae. Its collagen fibers make such joints strong but less clastic and only slightly movable.
   • Nucleus Pulposus: In the center of the intervertebral disc, a soft area is present called nucleus pulposus which is supposed to be a remnant of notochord.
2. Elastic Cartilage: It contains a dense network of elastic fibers between scattered chondrocytes. It forms the Eustachian tube, epiglottis, and pinna of the ear. The elastic fibers make those organs considerably elastic and pliable.
3. Calcified Cartilage: Initially, it is like hyaline cartilage but later on it gets hardened like bone due to the dep¬osition of calcium salts, for example, supra scapula of a frog’s pectoral girdle, pubis of the pelvic girdle of the frog.

Bone: It is a solid, rigid connective tissue. The matrix of the bone has the deposition of apatite salts of calcium and phosphorus. For example, hydroxyapatite salts and fluorapatite salts.

• 60-70% of bone is made up of inorganic matter and 30-40% is made up of organic matter.
• If the bone is put in dil. HCI will become decalcified, soft, and flexible. Nothing will happen to the bone if we put the bone in KOH.
• Osteoblasts are bone-forming cells that secrete ossein protein.
• Osteocytes are bin cells, they are metabolic inactive cells present in the lacuna.
• Bone is a solid, rigid, and strong connective tissue. Its matrix is heavily deposited with apatite salts of calcium and phosphorus. Flat irregular spaces called lacunae occur in the solid matrix. Each lacuna lodges a nathone cell or osteocyte. A bone cell has irregular¬shaped and long cytoplasmic processes. These processes extend into minute canals (canaliculi) radiating from each lacuna.
• Compact bone forms the dense outer layers of all bones. It is composed of many parallel, longitudinal column-like structures called Haversian systems, cemented to each other. Haversian canals are connected to each other by Volkmann’s canals. In each Haversian system, several concentric layers (lamellae) of bony matrix encircle a longitudinal central canal (Haversian canal. This canal carries blood vessels and nerves. Lacunae-containing osteocytes occur in a layer between two lamellae.

Spongy Bone: The ends of long bones are composed of an open lattice of bone called spongy bone. The spaces within contain marrow’, where most blood cells are formed. It carries no concentric organization like the Haversian system. It consists of a network of many fine irregular bony plates or trabeculae. Each trabecula consists of many irregularly arranged lamellae with lacunae between them. It has red bone marrow. Spongy bone is also called as cancellous bone and is found in epiphysis, i.e., the ends of long bones.

Differences Between Bone And Cartilage

Differences Between A Dried Bone And Decalcified Bone

Connective Tissue Points To Remember

Types Of Bones

• Cartilage Bones/Endochondrial/Replacing Bones: They are formed by the replacement of cartilage by the bone, for example, hu-merus, femur, vertebrae, ribs, and girdle bones, except clavicle. Chondroblasts are cartilage-eater cells.
• Membrane/investing bone/dermah Examples, skull bones, clavicle, etc. The bones are formed in the dermis of the skin and are invested over the already present cartilage.
• Sesamoid Bones: They are formed by the ossification of the tendons, for example, the patella.
• Visceral Bones: They are those bones that get detached from the skeleton and come to lie in visceral organs, for example,
  • os Cordis: Present in the interventricular septum of the heart of deer.
  • os Falciparum: Palm of mole.
  • os Penis: Penis of rats and carnivores.
  • os Palbebrae: In the eyelids of a crocodile.
  • os Rostralis: Snout of pig.
• Bone China: Porcelain was first made in China during the Tang dynasty. English found a new way of making porcelain with bone ash. Bone china is a form of porcelain made from burned animal bones. Bone ash is mixed with kaolin and white clay. The bone ash increases the porcelain’s translucence.
• Word Roots And Origin Of Periosteum: Greek peri means “around” and osteon means “bone.”

Fluid Connective Tissue (Blood)

• Blood is a fluid connective tissue.
• Its cells are quite distinct from other connective tissue cells both in structure and function.
• The extracellular material in the blood is a fluid devoid of fibers.
• Fluids outside the cells are generally called extracellular fluids (ECF).
• Blood is heavier than water.
• The extracellular material in the blood is a straw-colored, slightly alkaline (pH = 7.4), aqueous fluid called plasma.
• Constituents, having characteristic forms, float in the plasma. They are collectively called the formed elements of blood. They include the blood cells and blood platelets.
• Blood cells are of two types: erythrocytes and leukocytes.
• Blood circulates within blood vessels in higher animals.

Plasma

• Plasma contains three major classes of plasma proteins, viz., serum albumin, serum globulins, and fibrin¬ogen.
• Plasma proteins serve as a source of proteins for tissue cells.
• Tissue cells may utilize plasma proteins to form their cellular proteins.
• Additionally, albumin and globulins retain water in blood plasma by their osmotic effects.
• A fall in plasma proteins leads to the movement of excessive volumes of water from blood to tissues. That is why hands and feet get swollen with accumulated fluid  (edema) in persons suffering from dietary deficiency of proteins.
• Albumins and globulins also transport many substances such as thyroxine and Fe³⁺ in combination with them.
• One class of globulins, called immunoglobulins, acts as antibodies.
• Plasma proteins also maintain the blood pH by neutralizing strong acids and bases. Thus, they act as acid-base buffers.
• It is a slightly alkaline, non-living intercellular substance that constitutes about 60% part of the blood.
• It is a pale yellow but transparent and clear fluid.

Composition of Plasma: Plasma forms 55-60% by the blood volume.

1. Water: Water alone forms about 90% to 92% of the plasma. Solids form about 8% of the plasma.
2. Mineral Salts: These are chlorides, bicarbonates, sulfates, and phosphates of sodium, potassium, calcium, iron, and magnesium. All salts constitute about 0.9% of plasma. Buffer of the blood is sodium bicarbonate.
3. Nutrients: These include glucose, fatty acids, phospholipids, cholesterol, fats, amino acids, nucleosides, etc. Mineral salts have been mentioned above.
4. Plasma Proteins: They constitute about 7-8% of plasma. These mainly include albumin 4.4%, globulin 1.5-2%, prothrombin, and fibrinogen both 0.3%.
5. Defence Proteins: Immunoglobulins which act as antibodies and some other substances, such as lysozyme and properdin (a large protein) are always found in the plasma. They destroy bacteria, viruses, and toxic substances that may enter into the blood from outside.
6. Excretory Substances: These include ammonia, urea, uric acid, creatinine, etc.
7. Dissolved Gases: The water of blood plasma contains oxygen, carbon dioxide, and nitrogen in dissolved form.
8. Anticoagulant: Blood plasma contains a conjugated polysaccharide, heparin, which prevents the coagulation of blood inside blood vessels.
9. Hormones: These are secreted and released in blood by endocrine glands.
10. Vitamins And Enzymes: Different kinds of vitamins and enzymes are present in the blood plasma.

Functions Of Blood Plasma: These can be summarized as under:

1. Transport,
2. Retention of fluid in the blood,
3. Maintenance of blood ph,
4. Body immunity,
5. Prevention of blood loss,
6. Conducting heat to skin for dissipation, and
7. Uniform distribution of heat all over the body.

Blood Glucose

• Glucose is mainly absorbed in the small intestine.
• Glucose is also absorbed in the stomach.
• After absorption, glucose reaches the blood.
• Excess of glucose is converted into glycogen by insulin hormone in the liver and muscles.
• Whenever it is required, glycogen is changed back into glucose by glucagon hormone.
• Usually blood glucose level is about 80-100 mg per 100 mL of blood, 12 hours after a normal meal. But its concentration rises soon after a carbohydrate-rich diet.
• If the blood glucose level exceeds 180 mg per 100 mL, it starts appearing in urine. This condition is called glucosuria.
• Fasting glucose is 70-110 mg/dL. Glucose PP is 110 mg/dL (PP: post-prandial or after breakfast).
• If it is higher, it causes diabetes mellitus (hyperglycemia).
• If it is less, it causes hypoglycemia (less amount of glucose in the blood).

Blood Cholesterol

• Usually, cholesterol is considered a harmful substance. But it is quite useful in limited amount.
• Cholesterol is used in the synthesis of biomembranes, vitamin D, bile salts, and steroid hormones.
• Its normal amount is 80-180 mg in 100 mL of blood plasma.
• Cholesterol comes in the blood plasma either by intestinal absorption of fats by the synthesis from the liver or by both.
• Saturated fats such as ghee and butter increase cholesterol level in the blood.
• Increased blood cholesterol may lead to its deposition in the internal wall of the blood vessels such as arteries and veins which cause high blood pressure and heart problems.

Functions Of Plasma Proteins

1. Prevention Of Blood Loss: Fibrinogen and prothrombin play a role in blood clotting.
2. Retention Of Fluid In The Blood: Albumin helps in osmotic balance.
3. Body Immunity: Certain globulins called immuno-globulins (glycoproteins) act as antibodies in blood and tissue fluid. Antibodies belong to a class of proteins called as immunoglobulins.
4. Maintenance of pH: Plasma proteins serve as acid-base buffers. It means they maintain pH of the blood by neutralizing acids and bases.
5. Transport Of Certain Materials: Thyroxine (hormone) is bound to albumin or specific globulin for transport in the plasma.
6. Distribution Of Heat: Plasma proteins help in the uniform distribution of heat all over the body.
7. Enzymes: Some proteins acting as enzymes also occur in the plasma.

Blood Cells

1. Erythrocytes

• Erythrocytes (red blood corpuscles or RBCs) are the most numerous of the formed elements of blood.
• Their most important characteristic feature is the presence of hemoglobin (Hb), the red oxygen-carrying pigment.
• The total number of erythrocytes per microliter (1μL = 1 mm³ =10^-6 L) of blood is known as the total count of RBC.
• It averages 5 million and 4.5 million in adult men and adult women, respectively.
• The total count would be low in anemia and after profuse bleeding.
• On the contrary, the abnormal rise in the total count of RBC is called polycythemia.
• Anemia is caused due to the deficiency of folic acid, vitamin B₁₂, and hemoglobin.
• The size and shape of erythrocytes vary in different classes of animals.
• In fishes, amphibians, reptiles, and birds, erythrocytes are usually nucleated, oval, and biconvex. But in mammals, they are non-nucleated, biconcave, and circular.
• Only camel and llama possess oval red blood corpuscles.
• Human erythrocytes measure 7-8 μm (1 μm = 10^-6 m) in diameter and 2μm thickness near the rim.
• Old and damaged erythrocytes are phagocytosed and destroyed by macrophages.
• The pigment part (porphyrin) of hemoglobin is then catabolized to the yellow pigment bilirubin which is excreted in the bile.
• The pale yellow color of plasma is largely due to bilirubin.
• If a sample of blood is rendered non-coagulable by adding potassium or sodium oxalate and then centrifuged at a high speed in a graduated centrifuge tube (hematocrit tube), the centrifugal force rapidly sediments the erythrocytes to the bottom of the tube. They become packed into a solid, red. bottom layer while plasma forms a clear, fluid upper layer. On the upper surface of the erythrocyte layer, leukocytes form a thin, buff-colored layer.
• From the graduations on the tube, the relative volume of erythrocytes may be read as a percentage of the total blood volume. This is called the hematocrit value or packed cell volume.
• It normally forms 45% of the blood volume.

RBCs of mammals are circular, biconcave, and non-nucleated, except those of the family Camelidae (camel, which has non-nucleated and oval RBCs). The largest RBCs are found in amphibia. The smallest RBCs are found in mammals.

• In mammals, the smallest RBCs are found in “musk deer,” Tragiilns jammies (1.5 pm).
• In mammals, the largest RBCs are found in elephants. (9.4 pm).
• The graveyard of RBC is the spleen.
• Life span of RBCs in men is 120 days, in frogs 100 days, and in rabbits 80 days.
• The radioactive chromium method (Cr⁵¹) is used for the estimation of the life span of RBC.

Count of RBCs:

In embryo = 8.5 million/mm³

In man = 5-5.5 million/mm³

In woman = 4.5 million/mm³

Daily destruction of RBCs = 1%

ESR (Erythrocyte Sedimentation Rate): It is measured by Wintrobe’s method. It is the rate of the settling down of RBCs. It is also estimated by Westergen’s method.

• ESR is very useful in diagnosing various diseases including tuberculosis.
• ESR in men is 0-5 mm/h and in women 0-7 mm/lt, according to Westergen’s method.

Hemocytometer: It is an instrument for counting the number of both WBCs and RBCs.

Rouleaux: In resting and slow-flowing blood, RBCs aggregate to form rouleaux (RBCs are piled on top of each other). Fibrinogen favors rouleaux formation.

Bone Marrow: It is the main site for the formation of RBC. The volume of bone marrow at the time of birth is 70 mL. In adults, the volume of bone marrow is 4000 mL.

Structure Of RBC Of Man: Biconcave, nonnucleated, and bounded by Donnan’s membrane (plasma membrane of RBC). Hemoglobin is filled in RBC which is a respiratory pigment.

Normal Range Of Hemoglobin

• Infants: 16.5 ± 3.0 g/dL (dL = deciliter)
• Children 3 Months: 11.0 ± 1.5 g/dL
• Childen 10-12 Months: 13.0 ± 1.5 g/dL
• Men: 15.5 ± 2.5 g/dL
• Women: 14.0 ± 2.5 g/dL

Structure Of Hemoglobin: Each molecule of hemoglobin contains four molecules of heme and one molecule of globin. These are attached by coordinate bonds. Heme is a protoporphyrin compound and has four pyrrole groups joined together to form a ring structure. In Hb, Fe is present in (Fe⁺⁺) ferrous form. Heme is 5% and Globin is 95%. Globin is made of four polypeptide chains.

2. Leukocytes

• Leukocytes (white blood corpuscles or WBC) are devoid of hemoglobin and are consequently colorless
• Leukocytes are nucleated blood cells.
• They are of two major classes: granulocytes (with cytoplasmic granules) and agranulocytes (without granules).
• Granulocytes are of three types, viz., neutrophils, eosinophils, and basophils, each with lobed nuclei.
• Agranulocytes are of two types, viz., lymphocytes and monocytes.
• Neutrophils and monocytes protect the body against microbes by phagocytosing them.
• Lymphocytes secrete antibodies in the blood to destroy microbes and their toxins.
• The number of leukocytes per microliter (1 μL = 1 mm³ = 10^-6 L) of blood is called the total count of WBC. It is 6000-8000/mm³ of blood normally.
• It may rise abnormally in acute infections (for example, pneumonia), inflammations (for example, appendicitis), and malignancies (for example, leukemia).
• In some conditions such as folic acid deficiency, the total count falls abnormally (leukopenia).
• The total count of WBCs is also of diagnostic value in many diseases.
• Monocytes have kidney-shaped nuclei.
• The process by which monocytes and neutrophils squeeze out through thin capillary walls is diapedesis.

1. Neutrophils: They are maximum in number, stain equally with both basic and acidic dyes, and have many lobed nuclei, granules are in abundance in the cytoplasm, and help in phagocytosis.
2. Eosinophils: They have a bilobed nucleus. They get stained with acidic stains. Their number increases during allergic reactions (eosinophilia).
3. Basophils: They get stained with basic dyes.  Their nucleus is S-shaped. Coarse granules are few in the cytoplasm. Basophils release heparin and histamine in the blood and have a function similar to the mast cells.
4. Lymphocytes: They have large and rounded nuclei. The cytoplasm forms a thin peripheral film. They have their stem cells in the bone marrow and are differentiated in the bone marrow or in the thymus. Lymphocytes are of two types: B- lymphocytes and T lymphocytes. B-lymphocytes produce antibodies against antigens and they mature in the bone marrow.
5. Monocytes: They are the largest leukocytes (12-15 pm). The nucleus is kidney-shaped. They are produced from bone marrow monoblast cells. They help in phagocytosis.

Differences Between Different Types Of Leukocytes

3. Blood Platelets

• Also called thrombocytes, blood platelets are non-nucleated, round, or oval, biconvex disc-like bodies.
• They are 2-3 pm in diameter and their number normally varies from 0.15 to 0.35 million/mm3 or 150,000-350,000 platelets/mm3.
• They bud off from the cytoplasm of very large megakaryocytes of the bone marrow.
• Their normal life span is about a week.
• When a blood vessel is injured, platelets get clumped at the injured spot and release certain chemicals called platelet factors. These promote blood coagulation.
• Thrombocytopenia is a decrease in the platelet count and purpura is a group of bleeding diseases due to thrombocytopenia.

Blood Coagulation

• When blood oozes out of a cut, it sets into gel within a few minutes. This is called coagulation.
• Coagulation is brought about by the hydrolysis of soluble fibrinogen of plasma to insoluble fibrin. This is catalyzed by an enzyme called thrombin.
• Fibrin precipitates as a network of fibers. This network traps many blood cells, particularly RBCs, to form a red solid mass called the blood clot.
• The clot seals the wound in the vessel to stop the bleeding.
• The straw-colored fluid left after clotting of blood is called serum.
• Serum cannot be coagulated as it lacks fibrinogen.
• Thrombin occurs in normal blood as an inactive globulin called prothrombin.
• It must be activated to thrombin before blood coagulation can occur.
• In the case of injury to a blood vessel, coagulation-promoting substances called thromboplastins are released into the blood from clumped platelets and damaged tissues.
• Thromboplastins help in the formation of the enzyme prothrombinase. This enzyme hydrolyzes prothrombin to thrombin to initiate coagulation.
• Ca²⁺ ions are essential for both activation and action of thrombin.
• Blood normally contains an anticoagulant, heparin, which prevents the activation of prothrombin. Heparin is released from “mast cell” granules.
• Blood also contains antithrombin, which inhibits any thrombin formed accidentally.
• Blood drawn from a blood vessel can be kept coagulated by adding a pinch of oxalate (sodium or potassium oxalate) to it.
• Oxalate precipitates Ca²⁺ and consequently prevents coagulation.
• Chilling of blood also delays coagulation because lesser temperature depresses the action of courage-location-promoting enzymes.

Blood Cells Points To Remember

ABO Blood Clotting Factor

• Karl Landsteiner reported for the first time ABO blood groups in human beings.
• A, B, and O blood groups were discovered by Landsteiner (1900), while the AS blood group was found by de Castello and Steini (1902).
• Agglutinogens (antigens) are present on the surface of red blood corpuscles and agglutinins (antibodies) are found in the blood plasma. Both antigens and antibodies are proteins.
• When two different types of blood are mixed, the red blood corpuscles form a clump.
• The clumping of red blood corpuscles is called agglutination

Clotting Factors

• Thirteen factors help in blood clotting.
• These factors are mainly produced in the liver.
• Vitamin K is required in the synthesis of these clotting factors.
• These factors are represented in Roman numbers.
  • 1: Fibrinogen
  • 2: Prothrombin
  • 3: Thromboplastin
  • 4: Ca⁺² (cofactor in each step of blood clotting)
  • 5: Proaccelerin
  • 6: Accelerin (rejected)
  • 7: Proconvertein
  • 8: AHG (anti-hemophilic globin; absent in hemophilia-A)
  • 9: Christmas factor
  • 10: Stuart factor
  • 11: PTA (plasma thromboplastin antecedent)
  • 12: Hagman factor
  • 13: FSF factors (fibrin-stabilizing factor) (Laki Lorand factor)
• Other natural anticoagulants
  • Hirudin: Found in leech.
  • Anophelin: Found in female Anopheles
  • Lampredin: Found in Petcromyzon (Lamprey)
  • Cumerin: Obtained from plants
  • Warfarin: Obtained from plants
• To collect blood in a bottle in a blood bank, artificial anticoagulants such as sodium citrate, and sodium oxalate. EDTA (ethylenediaminetetraacetic acid), etc., are used. These chemicals act as calcium-binding units and remove Ca⁺² ions from the blood.

Blood Group

• Agglutination is due to the interaction of antigens and antibodies.
• There are two kinds of antigens that are named A and B.
• There are also two kinds of antibodies which are called a and b.
• Antigen A and antibody a are incompatible (antagonistic) cause self-clumping and cannot exist together. The same is the case with antigen B and antibody b. Thus, A and b can exist together and B and a can exist together.
• Corpuscle factors A and B can occur together if their antagonistic plasma factors a and b are not present.
• Plasma factors a and b can occur together if their antagonistic corpuscle factors A and B are absent.

Blood Groups With Corresponding Antigens And Antibodies

Rh Factor

• Another antigen, Rh antigen, similar to one present in Rhesus monkeys (hence Rh), is also observed on the RBC surface of the majority (nearly 80%) of humans.
• In India, the percent ratio of Rh is 97% Rh positive and 3% Rh negative. In the world, it is 80% Rh positive and 20% Rh negative.
• Individuals in which Rh antigen is present are called Rh positive (Rh⁺) and those in whom this antigen is absent are called Rh negative (Rh^–).
• An Rh^– person, if exposed to Rh⁺ blood, will form specific antibodies against the Rh antigens. Therefore, Rh group should also be matched before transfusions.
• A special case of Rh incompatibility (mismatching) has been observed between the Rh^– blood of a pregnant mother and with Rh⁺ blood of the fetus.
• Rh antigens of the fetus do you get exposed to the Rh-blood of the mother in the first pregnancy as the two types of blood are well separated by the placenta. However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the Rh⁺ blood from the fetus.
• In such cases, the mother starts preparing antibodies against Rh antigen in her blood. In the case of her subsequent pregnancies, the Rh antibodies from the mother (Rh^–) can leak into the blood of the fetus (Rh⁺) and destroy the fetal RBCs. This could be fatal to the fetus or could cause severe anemia and jaundice to the baby. This condition is called erythroblastosis foetalis. This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Muscular Tissue

Muscles cause movements of limbs and internal organs and also locomotion of the organism. Cells of muscle tissue can shorten forcefully and again return to the relaxed state. This specialized property is called contractility.

• It is based on the organized arrangement of some protein filaments in the cytoplasm of a muscle cell.
• The cell shortens or relaxes according to the relative positions of different intracellular filaments.
• Whenever adequately stimulated, muscle cells respond by contracting. This property of the muscle tissue is responsible for various movements in an animal.
• Muscle cells are usually called muscle fibers because they are thin and elongated.
• In higher animals, some muscles remain associated with the skeleton, but many others form walls of visceral organs, blood vessels, and heart.
• Muscle tissue may be classified into striated, non-striated, and cardiac muscles, according to their structure, location, and functions.

Striated/Skeletal/Voluntary Muscles: Such muscles are attached to bones by tendons. A voluntary muscle is composed of long bundles of striated muscle fib¬ers. Each fiber is a long, unbranched, cylindrical cell. It shows transverse striations in the form of regular alternate dark (A) and light (I) bands.

• At the center of the band is a fine, dense Z band or Z-line (Krause’s membrane). The plasma membrane covering the fiber is called sarcolemma. The cytoplasm inside the fiber is called sarcoplasm.
• The sarcoplasm contains many long, thin, unbranched, cross-striated cylindrical structures called myofibrils. They are arranged along the long axis of the fiber. Dark A bands of neighboring myofibrils are located side by side, and so also are their light I bands. This gives a cross-striated appearance to the entire muscle fiber also.
• A-band has both actin and myosin filaments. The portion of A band, where actin filaments are absent is called H zone. Z line or Krauze’s membrane is a dark membrane that bisects I band or isotropic band.
• Muscle is rich in proteins. Most of these proteins occur as two types of filaments arranged longitudinally in myofibrils. The thick filaments are made up of the protein myosin. Myosin filaments are located inside A bands.
• Thin filaments are more numerous. They are composed of the protein actin. From a fine, dense, dark Z band at the center of each I band, actin filaments extend through I band and encroach between myosin filaments up to a considerable distance into A band.

Each segment of the myofibril from one Z band to the next, functions as a contractile unit and is called a sarcomere. Various parts of a sarcomere have a specific arrangement of actin and myosin filaments as given below.

I Band: Has only actin filaments

A Band: Has both actin and myosin filaments

H Band: Has only myosin filaments

Z Line: A membrane to which actin filaments are attached on both sides.

Differences Between Single-Unit And Multi-Unit Smooth Muscles

Non-Striated or Smooth Muscles: Non-striated fibers do not show cross-striations; instead, they look smooth. Smooth muscles cannot be moved voluntarily. Hence, they are also called involuntary muscles. Functionally, smooth muscles are of two types: single-unit and multi-unit smooth muscles.

• Single-unit smooth muscles are composed of muscle fibers closely joined together. All its fibers contract together as a single unit. They may contract automatically and rhythmically. Such smooth muscles occur on the walls of hollow visceral organs such as the urinary bladder and gastrointestinal tract.
• Multi-unit smooth muscles are composed of more independent muscle fibers, not so closely joined together. Individual fibers of such smooth muscles contract as separate units. These occur at hair roots and in the walls of large blood vessels, for example, erector pili muscles.
• Smooth muscle fibers are elongated spindle-shaped cells. They are packed parallel to each other in branching bundles. Each fiber contains a single, spindle-shaped nucleus at its thick central part. The smooth muscle fiber is generally shorter than a striated muscle fiber. Mitochondria and other organelles are less extensive and protein filaments are not regularly arranged to give rise to striations.

Cardiac Muscle: Cardiac muscle occurs in the heart. It possesses considerable automatic rhythmicity and generates its own wave of excitation. The excitation can also pass directly from one fiber to another in the cardiac muscles. It is not under voluntary control. It shows cross-striations, but striations are much fainter than those of striated muscle.

Between the cardiac muscle fibers, intercalated discs are present. They are specialized regions of the cell membrane of two adjacent fibers. The intercalated discs function as boosters of the contraction wave and permit the wave of contraction to be transmitted from one cardiac fiber to another.

Cardiac muscle cells are short cylindrical cells joined end to end to form rows. They possess abundant cytoplasm with myofibrils (sarcoplasm) and numerous mitochondria and glycogen granules. This is because they need a large amount of energy. Faint but regular, alternately dark and light bands give rise to cross-striations in the cardiac muscle fibers and indicate regular and alternate arrangements of thin and thick filaments in the fiber.

Sarcomeres are also present. Cardiac muscle cells frequently branch to form junctions with neighboring cells. Where two cardiac muscle cells meet end to end, a dense zig-zag junction is formed between them. It is called an intercalated disc. The longest refractory period is present in cardiac muscles.

Differences Between Striated, Non-Striated, And Cardiac Muscles

Nerve Tissue

Ordinary connective tissue is absent inside the central nervous system. The neurons are held together by supportive cells called neuroglia cells. A nerve tissue is made up of neurons and neuroglia cells. A neuron has a large cell body with two or more, thin protoplasmic processes extending from it.

• One of the processes called the axon is long and conducts nerve impulses away from the cell body. Axon ends in a number of small branches on muscle fibers, gland cells, or other neurons. The remaining one or more processes conduct nerve impulses towards the cell body and are called dendrites or dendrons.
• The axon terminals may form intercommunicating junctions, called synapses, with dendrite terminals, cell bodies, or even axons of other neurons. Nerve impulses pass between neurons through the synapse with the help of chemicals, such as acetylcholine, which are termed neurotransmitters.
• The cell body of a neuron is called the soma. It has various shapes.
• Both soma and the processes are covered by a plasma membrane.
• The soma contains abundant granular cytoplasm and a large nucleus.
• To serve the high-energy needs for impulse conduction, neurons have many mitochondria.

• Light microscopy shows many small conical, angular, or rhomboidal and highly basophilic structures in the cytoplasm of soma and dendrites, called Nissl bodies which are absent in the axon and the axon hillock. Nissl bodies are made up ofribosomes, ER, and m-RNA.
• The processes which arise from neurons are called as neuritis. These are of two types: dendrites and axons.
• Dendrites conduct the nerve impulse toward the nerve cell body and are called as afferent processes.
• Axon is a single, usually long process. The part of cyton from where the axon arises is called as axon hillock. The cell membrane of the axon is called axolemma and its cytoplasm is known as axoplasm. The axon divides to form an axon ending, each with a synaptic knob. The synaptic knobs contain mitochondria and se¬cretory vesicles. The vesicles contain neurotransmitters such as nor-adrenaline, adrenaline, acetylcholine, etc.

Synapse

• Nerve signals travel from neuron to neuron all over the body. These associations are called synapses.
• Synapse is a junction between the axon endings of one nerve fiber and the dendrite of the other.
• At a synapse, the membrane of the axon and dendrites are not in physical contact with each other but there is a narrow intercellular gap, 10-20 nm across, separating the axon tip and target cell. This gap is a synaptic cleft. The neurotransmitter is always released from axon endings and not by dendrites, so there is only one-way transmission of nerve impulses.

Types Of Neurons: Based on the number of nerve processes, neurons are of four types

1. Unipolar Neurons: These have only axons but no dendrons and are found only in early embryos.
2. Bipolar Neurons: These have two processes, one axon, and another dendron, and are found in the olfactory epithelium and retina of eye.
3. Multipolar Neurons: These have many processes arising from the cell body; out of them, one (longer) acts as an axon and the remaining as dendrites. Multipolar neurons are most common and are found in the brain and spinal cord.
4. Pseudo-Unipolar Neurons: They are actually bipolar but appear like unipolar. A single process arises first which divides to form dendrite and axon. This is found in the dorsal root ganglion of the spinal cord.

Non-Polar Neurons: Each neuron bears several branched processes which are not differentiated into axons or dendrites. On the basis of function, neurons are of three types:

1. Sensory (Receptor Or Afferent) Neurons: They connect sense organs with the central nervous system (brain and spinal cord).
2. Motor (Effector Or Efferent) Neurons: They connect the central nervous system to the effectors (muscles and glands). They carry motor impulses from the central nervous system to the effectors.
3. Interneurons (Connector, Relaying, Or Adjustor Neurons): They are present in the central nervous system and occur between the sensory and motor neurons for distant transmission of impulses. They are neither sensory nor motor.

The extended axon or dendrite of a neuron is called a nerve fiber. It is generally elongated axon. There are two basic types of nerve fibers:

1. Myelinated/Medullated Nerve Fibers: These are with the myelin sheath. Myelin sheath is formed by the spiral wrapping of the Schwann cell membrane around the axon. Outside the myelin sheath, a neurilemma is present. Myelin sheath is absent at certain points called nodes of Ranvier. In myelinated nerve fibers, the impulse jumps from one node of Ranvier to the other, which is called saltatory conduction of the impulse. The node of Ranvier is without myelin but with neurilemma. Myelinated nerve fibers are found in cranial and spinal nerves.
2. Non-Inyelinated/Non-Medullated Nerve Fibers: These are not covered with myelin sheath and, hence are called non-myelinated or non-medullated nerve fibers. They do not possess nodes of Ranvier but have neurilemma. Myelinated nerve fibers are generally thicker than non-myelinated ones. These fibers are enclosed by Schwann cells that do not form a myelin sheath around these axons and are commonly found in autonomous and somatic neural system.

Nerve: A nerve is a collection of nerve fibers surrounded by connective tissue membranes.

• The membrane of the nerve fiber is a neurilemma; outside this, each nerve fiber is surrounded by a layer of connective tissue called the endoneurium.
• A nerve consists of several bundles of nerve fibers called fasciculi.
• Each fasciculus is surrounded by a layer of connective tissue called the perineurium.
• A dense layer of connective tissue that surrounds the entire nerve made of a number of fasciculi is called epineurium.
• A Nerve Can Be Of The Following Types:
  • Sensory Nerve: It is made up of only sensory nerve fibers surrounded by connective tissue membrane. It carries the impulse from the receptor to the CNS.
  • Motor Nerve: It is made up of motor nerve fibers, which carry the impulse from CNS to the effector organs, i.e., muscles or glands to bring about their movement.
  • Mixed Nerve: It has both sensory and motor nerve fibers. All the spinal nerves in our body are mixed.

Neuroglia Cells/Glial Cells: Neuroglia cells are undifferentiated cells with no Nissl granules. They are of the following types:

1. Astrocytes/Macrocytes: They are large in size with a number of protoplasmic processes. They form a maximum number of glial cells. They help in the repair of nerve tissue and form the blood-brain barrier.
2. Oligodendrocytes: They arc with few protoplasmic processes and form myelin sheath in CNS. There is no neurilemma inside the central nervous system. In the absence of Schwann cells, myelin is formed by the spiral wrapping of nerve fibers by the processes of oligodendrocytes. They are a type of neuroglia cells.
3. Microglial Cells: They are mesodermal in origin. They are smallest in size with few feathery processes and help in phagocytosis.

Nerve Tissue Points To Remember

Cells, tissues, organs, and organ systems split up the work in a way that ensures the survival of the body as a whole and exhibits division of labor. A tissue is defined as a group of cells along with intercellular substances performing one or more functions in the body.

• Epithelial tissues are sheet-like tissues lining the body’s surface and its cavities, ducts, and tubes.
• Epithelia have one free surface facing a body fluid or the outside environment.
• Their cells are structurally and functionally connected at junctions.
• Epithelial tissue is classified into different categories on the basis of the shape and function of cell.
• Diverse types of connective tissues bind together, support, strengthen, protect, and insulate other tissues in the body.
• Soft connective tissues consist of protein fibers as well as a variety of cells arranged in a ground substance.
• Cartilage, bone, blood, and adipose tissues are specialized connective tissues.
• Cartilage and bone are both structural materials.
• Blood is a fluid tissue with transport functions.
• Adipose tissue is a reservoir of stored energy.
• Muscle tissue, which can contract (shorten) in response to stimulation, helps in the movement of the body and specific body parts.
• Skeletal muscle is the muscle tissue attached to bones
• Smooth muscle is a component of internal organs.
• Cardiac muscle makes up the contractile walls of the heart.
• Connective tissue covers all three types of tissues.
• Nervous tissue exerts the greatest control over the response of the body.
• Neurons are the basic units of nervous tissue.

Earthworm (Pheretima Posthuma)

Phylum: Annelida

Class: Oligochaeta

Genus: Pheretima

Species: Posthuma

There are several types of earthworms. The most common genus of earthworms is Pheretima in India and Lumhricus in Europe. Pheretima has 500 species, 13 of which are found in India.

Habitat: Earthworm is a reddish brown terrestrial animal that inhabits the upper layer of moist soil where it lives inside burrows during the daytime.

• Earthworm inhabits those soils that have abundant organic matter.
• An acre of good moist soil can have up to 50,000 animals. Burrow is made by boring and swallowing the soil.
• The burrows are vertical or oblique.
• They are 30-45 cm deep during moist season but may go as deep as 2 m in summer.
• The burrows are lined by debris or mucus secreted by the animals and are wider at the base.
• During winter, the animal drags organic debris into its burrow and plugs the mouth of the burrow.
• This keeps the burrow warm.
• Even the mouth of the burrow is hidden from view by leaves and small stones.
• The area of the burrow can be recognized by fecal pellets called worm castings.

Habit: Earthworm is nocturnal because it is sensitive to higher light intensities.

• It partly creeps out of burrows during the night for search of food.
• It is only during the rainy season that the earthworm comes out of the burrow even during the daytime.
• After heavy rainfall, they can be seen crawling on the ground in large numbers.
• If the burrow is left, the animal does not re-enter the same.
• It digs a new burrow by pushing the body through the soft soil as well as by eating its way through the soil.
• The worm keeps its skin moist through mucus, coelomic oozing, and from the moisture of the soil. The animal respires through the skin.
• The body of an earthworm is long and cylindrical has about 100-120 segments (metameres). The first segment is called as buccal segment or peristomium which bears a very small terminal opening the mouth.

A small projection is also present which hangs over the crescent-shaped mouth and is called prostomium. It serves as a wedge to force open cracks in the soil into which earthworms may crawl. It is sensory in nature.

• The skin of earthworms is brown due to the presence of porphyrin pigment which protects the earthworm from UV radiation.
• In all the body segments, except the first, last, and clitellum, there is a ring of S-shaped setae, embedded in the epidermal pit at the middle of each segment (perichaetine).
• Setae are chitinous structures and are not dissolved in KOH.
• In the intersegmental grooves of 5/6, 6/7, 7/8, and 8/9 segments, four pairs of spermathecal pores are present which are the opening of spermathecae.
• A thick band of glandular tissue clitellum (cingulum) surrounds segments 14-16, forming a thick girdle. Its glands secrete mucus and albumin and also form the cocoon.
• On the ventral surface of the 18th segment, a pair of male genital apertures is present and on the ventral surface of the 14th segment, a median female genital aperture is present.
• On the ventral side of each of the 17th and 19th segments, circular raised pairs of genital papillae are present which help in reproduction.
• The dorsal surface of body is marked by a dark median mid-dorsal line (representing dorsal blood vessels) along the longitudinal axis of the body. The ventral surface is distinguished by the presence of genital openings (pores).

Internal Morphology: The body wall of earthworms is thin, soft, and slimy. From the surface inward, it consists of the cuticle, epidermis, muscular layers, and coelomic epithelium.

Cuticle: It is a thin and clastic non-cellular protective membrane. It is formed of collagen fibers secreted by the underlying epidermis.

Epidermis: This is a single layer of epithelium of tall, columnar cells which arc distinguished into four types as follows:

1. Supporting Cells: Unspecialized epithelial cells that form the major part of the epidermis.
2. Glandullar Cells: These are of two types more numerous and club-shaped mucussecreting goblet cells and fewer, narrower albumen cells. The mucus, se¬creted by these cells, keeps the body wall moist and slimy. It is also used to lubricate and smoothen the walls of burrows.
3. Basal Cells: These are shorter, conical cells wedged in between narrower basal parts of other cells.
4. Sensory Cells: These are narrow, columnar cells occurring, here and there, in small groups. Each sensory cell has small sensory hairs at its free end.

Muscular Layer: Beneath the epidermis is the musculature of the body wall. It consists of a thin outer layer of circular and about twice thicker, inner layer of longitudinal muscle fibers. The circular muscle layer is a continuous sheet around the body, but the longitudinal layer is broken into several longitudinal strips or bands, separated from each other by thin connective tissue partitions. These bands appear elliptical or club-shaped in the transverse section. Numerous granules of porphyrin pigment are found scattered in the circular muscle layer.

Coelomic Epithelium: Next to the longitudinal muscle layer is a thin, membrane-like, mesodermal epithelium of flattened, squamous cells. It is the outer envelope of coelomic cavity and hence called the parietal or somatic layer of coelomic epithelium or peritonium.

Coelom Or Body Cavity: Earthworm has schizocoel type of body cavity. It lies between the body wall and the alimentary canal.

• A layer of peritoneum lines both the surfaces, the outer parietal in contact with the body wall and the inner visceral in contact with the alimentary canal.
• The coelom of the first four segments is continuous or undivided.
• The coelom is divided by septa from the fourth and fifth segments onward.
• The septa lying between segments 5/6, 6/7, 7/8, 8/9 9/10, and 10/11 are thick and muscular.
• One of the two septa, either between the eighth and ninth or between the ninth and 10th segments are absent.
• The first six septa are cone-like and run obliquely backward from the body wall to the gut wall.
• The first nine septa, i.e., up to septum 13/14 are without perforations.
• The remaining septa beginning from the septum 14/15 are perforated by numerous apertures.
• Coelomic fluid is milky white and alkaline. It has a fluid matrix of watery plasma containing proteins, salts, and numerous minute nucleated corpuscles. Corpuscles are of the following four types.
• Phagocytes: They move like Amoeba and engulf harmful germs.
• Leucocytes: These are smaller and fewer. Their function is not fully understood.
• Mucocytes: These are elongated, vase-like corpuscles, one end forms an expanded fan-like structure and the other narrow end contains a nucleus. The function of these cells is not known.
• Eleocytes: They are formed by mitosis of the yellow cells of the visceral peritoneum. They contain glycogen and fat and distribute their food to various tissues.
• The coelomic fluid serves as a hydrostatic skeleton to assist the musculature of the body wall in bringing about the locomotion of the body. It oozes out upon the body surface through dorsal pores, keeping the body wall moist to facilitate respiration and to destroy bacteria and other harmful microorganisms.

Locomotion: The earthworm does not have specialized locomotory organs.

• The locomotion is brought about by the circular and longitudinal muscles of the body wall, aided by the chitinous curved setae embedded in the skin.
• Due to the contraction of the circular muscles of the anterior end, the latter becomes thin, elongated, and extends forward.
• At the same time, the setae of the anterior end hold the ground firmly and prevent the animal from slipping backward.
• Now the circular muscles of the anterior end relax and the longitudinal muscles contract.
  It causes the shortening and thickening of the anterior segments, and thus, the posterior part of the body is pulled ahead.
• The process is repeated and the worm is able to move forward with speed. Earthworms move at the rate of about 15 cm per minute.

Alimentary Canal: The alimentary canal is a straight tube and runs between the first and last segment of the body.

• In the first to third segments, the buccal cavity is present; the pharynx is present in the fourth segment.
• From the fifth to seventh segment, the oesophagus is present.
• Gizzard is present in the eighth segment.
• The ninth to 1 4th segment is a tubular stomach. Calciferous glands are present in the stomach, which produce CaCO₃, to neutralize the humic acid.
• The intestine starts from the 15th segment onward and continues till the last segment.
• A pair of short and conical intestinal caeca projects from the intestine on the 26th segment. They secrete amylolytic enzyme which digests starch. Other enzymes are lipase, cellulase, invertase, etc.
• Surrounding the pharynx there are pharyngeal or salivary glands, made up of masses of chromophil cells, they produce mucin for the lubrication of the food, and also a proteolytic enzyme that can digest some proteins.
• Associated with the intestine arc chloragogen cells which are supposed to be excretory in function.
• The intestine is divided into the pre-typhlosolar region: This runs from the 15th to the 26th segment, it has a small villi.
• Typhlosolar Region: It starts from 27th segment and extends up to 23-25 segments in front of theanus. Typhlosole is a large villus as an internal median fold of the dorsal wall of intestine. This enhances the effective area of absorption after digestion. Post-type solar region: Also known as rectum, it is present in the last 23-25 segments and opens to the outside through a terminal anus.
• Lymph Glands: These are white fluffy bodies that are found arranged on either side of the dorsal vessel from the 26th segment and extend to the successive segments. These glands are believed to produce phagocytes of the coelomic fluid.

Circulatory System

• Earthworms is the first to evolve a closed circulatory system in the evolution of animals.
• The respiratory pigment is hemoglobin which remains dissolved in the plasma, RBC are absent.
• Blood glands are present in the fourth, fifth, and sixth segments. They produce blood cells and hemoglobin.
• There are two main blood vessels: the dorsal vessel is the largest vessel and blood flows forward from the posterior to anterior end. The dorsal vessels have a contractile wall; valves are present. Before the 13th segment, it becomes a distributing vessel and behind the 13th segment, it becomes a collecting vessel. The ventral vessel is the main distributing vessel, blood flows backward from the anterior to a posterior end; valves are absent.
• Latero-oesophageal vessels are paired vessels that extend from the first to 13th segment.
• The supra-oesophageal vessel is unpaired and extends between 9th and 13th segments.

Hearts: There are four pairs of hearts with valves.

1. Two pairs of lateral hearts, one pair in the seventh segment and one pair in the ninth segment.
2. Two pairs of lateral oesophageal hearts in the 12th and 13th segments.
3. There are two pairs of lateral loops in which valves are absent. One pair is in the tenth segment and one pair is in the 11th segment. Blood flows in an upward direction in them.

Earthworm Points To Remember

Earthworm mainly removes nitrogenous waste in the form of urea in soil. But when plenty of water is available, earthworms become ammonotelic. So, earthworm is both ureotelic and ammonotelic.

Excretory System: Excretory organs occur as segmentally arranged coiled tubules called nephridia. They deliver the wastes through a pore to the surface of the body wall or into the digestive tube. Three main types of nephridia

1. Pharyngeal Nephridia: These are situated in the segments 4, 5, and 6. They open in the anterior part of the alimentary canal, i.e., the buccal cavity and pharynx. They are without nephrostome and are enteronephrictype.
2. Integumentary Nephridia: These are scattered in the body wall. They are the smallest, V-shaped, without nephrostome, and are exophoric type. In clitellar segments, they form forests of nephridia.
3. Septal Nephridia: These are the largest, attached to both faces of each intersegmental septum behind the 15th segment. Septal nephridia are the only nephridia with nephrostome or funnel. The terminal duct opens into the septal excretory canal. These canals, in turn, open into two supraintestinal excretory canals.

Septal nephridia are enteronephric. Hence, final excretory products are poured into the intestine. Enteronephric condition is an adaptation for the conservation of water or osmoregulation. The excretory products of earthworms are urea (about 50%), ammonia (about 40%), and traces of creatinine. Earthworms are mainly ureotelic.

Nervous System: The nervous system of earthworms consists of central, peripheral, and sympathetic nervous systems.

Central Nervous System

• The central nervous system consists of a brain ring/cricopharyngeal ring and a ganglionated double ventral nerve cord.
• The circumpharyngeal ring occurs in the third and fourth segments. It has a brain consisting of two suprapharyngeal ganglia, two circumpharyngeal connectives, and a pair of subpharyngeal ganglia.
• The ventral nerve cord arises from subpharyngeal ganglia, which is double, solid, and bears paired ganglia in each segment.

Peripheral Nervous System

• The peripheral nervous system comprises nerves that extend from CNS to supply various parts.
• The nerves are mixed in nature.
• Two pairs of nerves arise from the brain and innervate the prostomium and buccal cavity.
• Nerves from cricopharyngeal connectives supply segments 1 and 2.
• Subpharyngeal ganglia send nerves to the second, third, and fourth segments.
• Each segmental ganglion (actually paired) sends out three pairs of segmental peripheral nerves, one pair from the anterior part and two pairs from posterior part.
• They supply various structures in each segment. All the segmental nerves are mixed in nature, i.e., containing both sensory (afferent) and motor (efferent) nerve fibers.

Sympathetic Nervous System: The sympathetic nervous system consists of various nerve plexes present in the wall of the alimentary canal.

Sense Organs

• A group of specialized cells are found in the skin and lining of the buccal cavity.
• Photoreceptors are located in the prostomium and dorsal epidermis. They perceive light intensity with the help of phagosomes (optic organelle).
• Thigmoreceptors are located in the ventral and lateral epidermis.
• Olfactoreceptors are located in the lining of the buccal cavity.
• Gustatory receptors are located in the lining of the buccal cavity.

Respiratory System

• Earthworm has no special respiratory organs.
• Gaseous exchange takes place simply through the skin, which is thin and highly vascular.
• Effective gaseous exchange takes place only when the skin is moist.
• The skin is kept moist due to damp earth, secretion of the mucus by the epidermal gland cell, and oozing of coelomic fluid through the dorsal pores.

Reproduction

• Earthworms is hermaphrodites.
• There are two pairs of testes present in the 10th and 11th segments. They are surrounded by two testes sacs lying ventrally, one in the 10th and the other in 11th segment.
• There are two pairs of seminal vesicles, one pair in the 11th and the other pair in 12th segment.
• Maturation of sperms occurs in seminal vesicles.
• The testis sac of 10th segment communicates with the seminal vesicles of 11th segment and the testis sac of 11th segment communicates with the seminal vesicles of 12th segment. From each testis sac, the vas deferens carries the sperms up to the 18th segment where they join the prostatic duct from the prostate gland.
• Four pairs of spermathecae are present, one pair in each of the sixth, seventh, eighth, and ninth segments. They receive and store the spermatozoa of another earthworm during copulation.
  One pair of ovaries lies in the 13th segment, which opens through a median aperture on the 14th segment.
• Accessory glands are present on the ventral surface of the 17th and 19th segments, which open through genital papillae. These are a part of the male reproductive system.
• Testes mature earlier (protandrous).
• Development is direct and there is no larval form.

Cockroach (Periplaneta Americana)

Phylum: Arthropoda

Class: Insecta

Genus: Periplaneta

Species: Americana

Morphology: The body is divided into three distinct regions—head, thorax, and abdomen.

• Their size ranges from 1/4 inch to 3 inches (0.6-7.6 cm).
• The head is hypognathus (facing downward) and is formed by the fusion of six segments.
• Anteriorly, the head bears mouth which is provided with appendages collectively called mouth parts which are used in chewing, cutting, and swallowing.
• The mouth parts consist of a pair of mandibles and maxillae, labium forming the lower lip, and a labrum forming upper lip. Within the cavity enclosed by mouthparts, there is a median flexible lobe called hypopharynx which acts like a tongue.
• Thorax consists of three segments—prothorax, mesothorax, and metathorax.
• A pair ofwings arise from mesothorax which are thick and leathery and are called elytra or tegmina. A pair of membranous wings used in flying arise from metathorax. In houseflies and mosquitoes, the metathoracic wings are reduced to halteres for balancing.

Habit And Habitat: Cockroaches are worldwide and found in such places where darkness, warmth, dampness, and plenty of organic debris is available.

• Cockroaches are nocturnal and omnivorous.
• Three species of cockroaches are commonly found in India: Periplanata americana, Blatta orientalis, and Blatta germanica.
• Periplanata americana is the largest and most common species. It is commonly called American cockroach, Bombay canary, or ship cockroach. In both sexes, wings are present which are larger than the body.
• Blatta orientalis is a black or dark brown mediumsized species. The female has rudimentary wings which are not helpful in flight. The male has wings that are short and are not present up to the end of the body.
• Blatta Germania is the smallest of the domestic species of cockroaches. It is pale yellow-brown in color.

External Features (P. Americana)

• The body is narrow, elongated, bilaterally symmetrical, and flattened, measuring about 3-4.5 cm (34-53 mm) in length and 1.5-2 cm in breadth.
• The color is reddish brown with a pale yellow area around the edge of the pronotum and two dark patches over it.

Exoskeleton

• The entire body of the cockroach is covered with a thick, hard, chitinous cuticle, which is secreted by the epidermis, forming the exoskeleton.
• The exoskeleton of each segment consists of four plate-like pieces called sclerites. The dorsal sclerite is called tergum or tergite, the ventral sclerite is called sternum or sternite, and two lateral sclerites are called pleura or pleurites.
• The sclerites of each segment are joined with each other and with those of the adjacent segments by means of soft and flexible articular or arthrodial membranes. This gives the sclerites some freedom of movement upon each other at their edges.
• The stiff, immovable bristles or spines covering the body and its appendages are in fact the outgrowths of the cuticle while the movable hair-like setae occurring at some places are secreted by special trichogen cells of the hypodermis lying below the cuticle.

Segmentation

• Embryologically, the body of a cockroach is formed of 20 segments: six in the head, three in the thorax, and 11 in the abdomen.
• Due to complete or incomplete fusion between some segments during development, the number of distinct segments is reduced in the adult.
• All segments of the head are fused. The thorax consists of three segments. Only 10 segments are retained in the abdomen of the adult. Of these, only the first seven are distinct in females and the first nine in males. The remaining hinders abdominal segments from becoming small and modified into external genitalia that are hidden under the last distinct segment.
• When wings are removed, the three regions—head, thorax, and abdomen—become distinctly visible. A small, soft, and mobile neck or cervical connects the head with the thorax.

Head

• It is small and triangular, and its narrow end is bent downward in a hypognathous position, i.e., at an angle of 90° with the long axis of the body.
• On each lateral side, it bears a large and blackish compound eye.
• At the base of each antenna, on the inner side, a small rounded and light-colored area called fenestra or ocellar spot representing the simple eye is present. The endoskeleton of head is called the tentorium.
• All sclerites of the head are fused, forming a strong head capsule exhibiting only faint lines of fusion.
• Cephalic Appendages: The head appendages include a pair of antennae, mandibles, first maxillae, and second maxillae (fused as single labium or lower lip), one labrum or upper lip, and one hypopharynx.
• Antennae: These are a pair of long, thread-like appendages, extending forward from an antennal socket located dorsally upon the head capsule near the eye. These are mobile and act as tactile, thermal, and olfactory receptor organs. Each is formed of several small segments called podomeres. The first basal podomere, called scape, is the largest. The second, called pedicel, is narrow and elongated. The remaining long, slender, and many-jointed parts of each antenna is called a flagellum.

Mouth Parts: The remaining cephalic appendages are small and located around the mouth. Hence, together these appendages comprise the mouth parts of the cockroach. These help in “biting and chewing” its food.

Labrum (Upper Lip): It is the broad, flattened termi¬nal sclerite of the dorsal side of the head capsule, movably articulated to the clypeus that acts as the upper lip. It has an epipharynx (chemoreceptors) on its inner side.

Mandibles: Thick, hard, and triangular appendages beneath the labrum, one on each lateral side of the mouth, which bear pointed, teeth-like processes called denticles.

First maxillae: Located on each side of the mouth next to mandibles. These serve to hold food particles in between the mandibles for cutting and chewing. They also bear olfactory receptors.

Labium (Lower Lip): The second maxillae are fused together forming a single large structure that covers the mouth from the ventral side, hence called the “lower lip” or labium. It bears tactile and gustatory sensory setae.

Hypopharynx: It is a small, cylindrical mouthpart, sandwiched between the first maxillae and covered by labrum and labium on the dorsal and ventral sides, respectively. It bears several sensory setae on its free end and the opening of a common salivary duct upon its basal part.

Thorax

• The thorax comprises three segments—prothorax, mesothorax, and metathorax.
• The three thoracic segments are covered by relatively thicker and larger tergites called nota.
• The notum of prothorax called pronotum is very large and covers the neck also. Each of the mesonotum and metanotum bears a pair of wings.

Thoracic Appendages: Each thoracic segment bears a pair of walking legs. Each walking leg consists of segments

1. Coxa.
2. A triangular short rod-like trochanter, articulated with coxa and femur
3. A long, spiny femur.
4. A spring-like tibia which represents the longest segment.
5. A long tarsus is divided into five tarsomeres or podomeres. The last tarsomere is called pretarsus forming the claws and bearing an adhesive arolium or pulvillus. Similar but smaller adhesive pads called plantulae are located at each joint of the tarsus.

Abdomen: It is the largest and the broadest; relatively more flattened and softer part behind the thorax. There are 10 tergites. In both males and females, the eighth and ninth tergites are mostly covered by the seventh. The 10th tergum is somewhat bowl-shaped and posteriorly bifurcated into two lobes.

• Ventrally, the abdomen has nine stemites in males and seven in females.
• In females the last stemming (seventh) is larger and boat-shaped and together with indistinct eighth and ninth stemites, it forms a chamber-like structure called gynatrium the posterior part of this chamber is called the oothecal chamber. Behind this chamber, the seventh stemite bifurcates into two prominent oval plates called apical lobes. Female gonopores is located between them.
• In males, the ninth stemite bears a pair of spine-like anal styles.
• In both male and female cockroaches, several small chitinous appendages are located around the gonopore. These help in reproduction and hence are called gonapophyses.
• At several places, certain processes of the exoskeleton extend into the body and form endoskeletal elements that provide attachment to muscles and are hence called apodemes.

Abdominal Appendages

• Abdominal segments lack locomotory appendages. There are certain small structures associated with gonopore, which are different in male and female cockroaches.
• The 10th tergum posteriorly bears a pair of many jointed anal cerci. They bear minute sensory hair sensitive to sound and other vibrations.
• The ninth sternum of males bears, in addition, a pair of small and spine-like, unjointed anal styles.

Digestive System Of Cockroach

The alimentary canal is long and somewhat coiled, di-visible into three main parts, namely, foregut, midgut, and hindgut.

Foregut (stomodeum) is lined by a cuticle and is differentiated into five parts: buccal chamber, pharynx, esophagus, crop, and gizzard. The crop is used for the storage of food.

• The gizzard is muscular and internally provided with six cuticular teeth that crush the food. A stomodeal valve is present between the gizzard and mesenteron.
• The midgut (mesenteron or ventriculus) is short, tubular, and lined with glandular endoderm.
• At the anterior end of the mesenteron, there are six to eight blind glandular hepatic or gastric cecae which secrete digestive enzymes.
• Internally mesenteron is not lined by cuticle but it is covered by a very thin and transparent peritrophic membrane formed of chitin and proteins.
• The peritrophic membrane is secreted by the gizzard which serves to protect the wall of the midgut from abrasion due to the friction of food particles.
• The peritrophic membrane is permeable to digested food and enzymes in the mesenteron.
• The hindgut (proctodeum) is broader than the midgut and comprises the ileum, colon, and rectum.
• The wall of the rectum is provided with six rectal papillae. They help in the absorption of water and salts.
• The cockroach is omnivorous. It feeds on all sorts of organic debris.
• The digestive enzymes of saliva are mainly zymase and amylase.
• Most of the nutrients of food are digested in the crop.
• The absorption of digested food takes place in the mesenteron.

Respiratory System of Cockroach

• The blood of cockroaches is not responsible for the transportation of gases. It serves as a stationary medium for the exchange of gases.
• A complicated system of numerous, shiny, transparent, and branched air tubes or tracheae are found for gaseous exchange in the hemocoel cavity. There are six longitudinal tracheal tubes—two dorsal, two ventral, and two lateral, which are interconnected by transverse commissures. Chitinous rings prevent the collapse of the trachea.
• Atmospheric air enters into and escapes out from this system through 10 pairs of slit-like apertures called stigmata or spiracles located on the lateral sides of the body. Two pairs of these are thoracic and eight pairs are abdominal. The openings of spiracles are regulated by the sphincters.

• Thoracic spiracle, arc somewhat larger. One pair of these is between prothorax and mesothorax and the other between mesothorax and metathorax, upon re¬spective pleurites.
• The first pair of abdominal spiracles is dorsolateral upon the tergite of the first abdominal segment, but the remaining seven pairs are present upon the pleurites of the second to eighth segments.
• Each spiracle is surrounded by a ring-like sclerite called peritreme.

Mechanism Of Respiration

• Several tergo-stemal muscles extend vertically between the tergites and stemites of all abdominal segments.
• Harmonious contractions and relaxations of these at regular intervals cause rhythmic expansion and compression of the abdomen leading to inspiration (with relaxation) and expiration (with contraction) of air.
• At rest, the oxygen requirement is less, and tracheolar ends get filled with tissue fluid.
• The movement of O₂ is along the pressure gradient as the tracheolar ends are losing oxygen to the cells for performing cellular respiration.
• O₂ requirement increases during activity.
• Tracheolar fluid is withdrawn out of Tracheoles.
• Alternate expansion and contraction of the abdominal cavity occurs involving tergo-stemal muscles and abdom¬inal muscles.
• A high level of CO₂ in the abdominal cavity makes tergostemal muscles and abdominal muscles to contract pushing out the air from the tracheal system to the outside through spiracles.
• With relaxation, the abdomen expands, i.e., tracheal trunks and tracheae expand, and as a result, air rushes into tracheae and tracheoles via spiracles, which results in inspiration.

Circulatory System of Cockroach

• Blood vascular system is open and lacunar type. Body cavity contains blood (hemolymph) which bathes viscera in it, therefore known as hemocoel.
• The blood vascular system consists of a tubular heart, a blood vessel called the anterior aorta, and a system of ill-defined blood spaces or sinuses.

Heart: It is a long elongated tube situated in the mid-dorsal line of the thorax and the abdomen immediately beneath the terga.

• The heart consists of 13 chambers.
• The last two posterior chambers are very small.
• The chambers are separated from one another by deep constrictions.
• The opening of each chamber into another is guarded by valves that allow blood from behind to forward.

Blood Sinuses: The large body cavity or hemocoel is divided by two membranous horizontal partitions, into three wide and flattened sinuses—the dorsal pericardial sinus containing the heart, the middle perivisceral sinus containing the gut, and the ventral perineural sinus or sternal sinus containing the nerve cord.

• The partition between the pericardial and perivisceral sinuses is called the dorsal diaphragm and between the perivisceral and perineural sinuses is called ventral diaphragm.
• The sinuses intercommunicate by pores in the respective diaphragms.
• A pair of fan-like, triangular alary muscles in the floor of the pericardial sinus in each segment reinforces the dorsal diaphragm by their broad bases and also con¬nects it by their pointed tips with the tergite of the segment.

Circulation Of Hemolymph

• The pumping force that propels the hemolymph is provided by the pulsations of the heart. The respiratory movements of the abdomen and contraction of alary muscles increase this force.
• From the pericardial sinus, the hemolymph enters into heart through the ostia. When the heart is filled, it contracts from behind forwards. This is its systole phase. Soon the heart becomes relaxed in its diastole phase. Then the next systole follows after a short time interval called diastasis. Thus, the heart pulsates about 50 times/ min.
• During systole, the valve-like ostia closes, preventing the backflow of hemolymph into the pericardial sinus. Therefore, some of its hemolymph is pumped into seg-mental vessels while most of it is poured into the head sinus through the terminally opening anterior aorta.
• From the head sinus, the hemolymph flows backward into the thorax and abdomen. While flowing backward from the head sinus, the hemolymph remains in the ventral part due to the presence of esophagus in the dorsal part. So, it fills into the perineural sinus.
• From the perineural sinus, the hemolymph now flows into the perivisceral sinus through the pores of the ventral diaphragm in the abdominal region.
• Then from the perivisceral sinus, it flows into the pericardial sinus through the pores of the dorsal diaphragm. Then, during the heart’s diastole, it fills in the heart through the ostia.

Excretion In Cockroach: Cockroach is uricotelic. In cockroaches, the following structures help in excretion:

1. Malpighian tubules
2. Fat bodies
3. Nephrocytes
4. Cuticle
5. Uricose glands in some species

1. Malpighian Tubules: Malpighian tubules are attached at the junction of the midgut and hindgut. Excretory products, dissolved in hemolymph and absorbed by Malpighian tubes, are discharged into the hindgut.
2. Fat Bodies: Some fat bodies are also present in hemocoel which have mycetocytes, urate cells, oenocytes, and trophocytes.
3. Nephrocytes: Nephrocytes are present in the lateral wall of the heart and help in the excretion and storage of nitrogenous waste.
4. Uricose Glands: In some species, in males, uricase glands are present on the periphery of mushroom glands. These glands synthesize uric acid. Malpighian tubules are analogous to mammalian kidneys, and bodies are analogous to vertebrate liver.
5. Nervous System of Cockroach: It consists of a series of fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side.
6. Central Nervous System: The central nervous system consists of the brain or supraesophageal ganglion. The brain gives of a pair of short, stout cords, the circumocsophageal connectives, which encircle the esophagus and pass downward and backward over the subesophageal ganglion situated below esophagus.

From the suboesophageal ganglion, a double ventral nerve cord passes backward into the thorax, which bears three ganglia in the thorax and six in the abdomen.

Peripheral Nervous System: The peripheral nervous system consists of nerves, which are given off from the ganglia so as to innervate all the parts of the body.

Sympathetic or Somatogastrlc Visceral Nervous System: The stomatogastric nervous system consists of a frontal ganglion which is situated on the dorsal side of the esophagus in the head. From this ganglion, a median unpaired recurrent nerve reaches the visceral ganglion situated on the crop.

Various nerve branches are given off from the visceral ganglion. The frontal ganglion is joined with the central nervous system by nerves that connect it to circumoesophageal commissures.

Sense Organs of Cockroach: Receptor cells are present on the general body surface.

• Proprioreccptors: They are for hearing or receiving sound vibrations. Auditory receptors are present on the antennae and anal cerci.
• Thigmoreceptors: They are the receptors for touch and arc present on antennae, maxillary palps, and legs.
• Olfactory Receptors: They receive various smells and are present on antennae and palps.
• Gustatory Receptors: They are for the sense of taste and are present on maxillae and labial palps.

Eyes: Cockroach has compound eyes. Each compound eye is formed of about 2000 hexagonal ommatidia. Each ommatidium has a biconvex lens or cornea. Below the lens, there are collagen cells that secrete the lens.

• Below the collagen cells is a transparent crystalline cone surrounded by four vitrellae or cone cell.
• The vitrellae secrete the crystalline cone. All this forms the focusing or dioptrical region.
• Below the cone, there is a refractive body, the rhabdome, surrounded by seven retinular cells.
• Each ommatidium is isolated from the other by an iris pigment sheath and retinal pigment sheath.
• The image formed is an apposition or mosaic vision, composed of as many separate but adjacent images as there are ommatidia.
• In mosaic vision, images are sharp but separate and the eye can use only in bright light.
• In cockroach vision is mosaic and an apposition image is formed (although cockroach is nocturnal).
• If pigmented iris sheath is removed from the compound eye of insects, only a superposition image will be formed.

Reproductive System of Cockroach (Male)

• In cockroaches, the sexes are separate, so it is dioecious.
• The testes of cockroaches are located in the abdominal segments 4, 5, and 6.
• The mushroom gland consists of two types of tubules:
• The long slender tubules, utriculi majors or peripheral tubules, and
• Short tubules, utriculi breviores, make up the major part of the gland. It is present in the sixth to seventh abdominal segments which function as an accessory reproductive gland.
• Small seminal vesicles are also found associated with mushroom glands.
• All sperms of a seminal vesicle are glued together into a large bundle called a spermatophore.
• Spermatophore has a three-layered wall: the inner layer is secreted by utriculi majores; the middle layer is secreted by the ejaculatory duct; and the outer layer is secreted by the phallic gland or conglobate gland.
• There are three asymmetrical chitinous structures called male gonapophyses or phallomeres. Right phallomere, left phallomere (largest), and ventral phallomere (smallest).

Reproductive System of Cockroach (Female)

• Female organs consist of ovaries, oviducts, vagina, genital chamber, spermathecae, colleterial glands, and female gonapophysis (ovipositor processes).
• The ovaries of cockroaches are located in the abdominal segments 2 to 6. Each ovary consists of eight ovarioles.
• Two oviducts from each side open into a common oviduct or vagina which opens into the genital chamber by the female genital pore. A pair of spermathecae (left larger pyriform sac) are present near the female genital pore.

• A pair of colleterial glands also open in the genital chamber.
• The genital pouch or gynatrium is divisible into a genital chamber in front and an oothecal chamber (vestibulum) behind.
• Female genitalia consist of three pairs of chitinous processes hanging from the roof of the oothecal chamber into its cavity.
• The ootheca of cockroaches contains 14 to 16 fertilized eggs. It is formed of a protein secreted by colleterial glands. On average, females produce 9-10 oothecae.
• The nymphs of cockroaches emerge from the ootheca. A nymph resembles an adult in general structure but lacks wings and mature reproductive organs. The next to last nymphal stage has wing pads but only adult cockroaches have wings.
• The instar is a stage in the development of insects (larval instar, nymphal instar). The period between two successive molts in insects is termed stadium.
• In Periplaneta americana, the nymph grows by molting about 13 times to reach the adult form, and in Blatta orientalis, it molts about six times.

Frog (Rana Tigrina)

Phylum: Chordata

Class: Amphibia

Order: Anura

Genus: Rana

Species: tigrina

The most common frog found in India is the Indian bullfrog. It is the largest frog and is named as bullfrog because of its large size and loud call.

• Indian bullfrog is found in freshwater marshes, ditches, ponds, and shallow lakes.
• They undergo aestivation (summer sleep) in summer and hibernation (winter sleep) in winter.
• They are carnivorous (feeding upon other animals, insects, etc.), poikilothermic, i.e., the body temperature changes with the environment.
• They develop protective coloration to camouflage, i.e., to hide in surroundings.
• Frogs belong to the phylum Chordata, subphylum Vertebrata or Craniata, superclass
• Gnathostomata, class Amphibia, and Genus Rana.
• The most common species is known as Rana tigrina (Indian bullfrog).
• The scientific name of the common toad is Bufo melanostictus. Frogs exhibit sexual dimorphism.
• Male and female are distinguishable externally only during breeding season when the males develop nuptial pad in the first digit of the forelimbs.
• Vocal sacs are well developed in males so they produce louder sounds as compared to females which are devoid of vocal sacs.
• The vocal sacs help to produce mating calls.
• The total number of bones in a frog is 153.

External Morphology: Skin is made up of the epidermis and dermis. Mucous glands are present in the dermis and their ducts open at the surface.

• Blood capillaries and pigment cells (chromatophores) are present in the dermis.
• Skin is without scales or any other cover or exoskeleton.
• Body is divisible into head and trunk, neck is absent. The trunk is provided with a pair of fore and hind- limbs. The hind limbs are much larger and muscular than the forelimbs. Forelimbs end in four digits and the hind limbs end in five digits. The digital formula of forelimbs is 02233. The digital formula of hind limbs is 22343.
• Shank or crus is associated with hind limbs.

Sexual Dimorphism

• The male and female frogs exhibit certain differences in their external features, which become more pronounced during breeding season.
• Generally, male frogs are larger than females.
• During the breeding season, however, the females become bloated with large ovaries and numerous ova and appear considerably larger.
• Only the males possess a pair of ventrolateral, wrinkled, pouch-like vocal sacs located a little behind the mouth.
• These sacs become especially large and distensible in the breeding season.
• By inflating these repeatedly with air from the lungs, the males produce a loud croaking sound meant to call the females for copulation (amplexus).
• The sound is actually produced by a pair of vocal cords in the larynx; the sacs only increase its pitch, such as resonators.
• The females produce a low-pitched sound by their vocal cords alone.
• The forelimbs in both male and female frogs bear small particular pads dorsally at the joints of digits, but the males also possess a special nuptial, copulatory, or amplcxusary pad on the ventral side of the first finger of each forelimb.
• Normally, these pads appear merely as rough patches but during breeding season, these become thick and sticky.
• In amplexus, the male strongly grips a female under her armpits by means of these pads.

Internal Morphology: Digestive System

• Since the frogs are carnivorous, their alimentary canal is short in length.
• Tadpole larva is herbivorous, so the alimentary canal is very long and coiled in the form of offspring.
• The mouth is present as a terminal, wide opening.
• It opens into the bucco-pharyngeal cavity, which contains numerous maxillary teeth arranged along the margin of the upper jaw. Vomerine teeth are present under the roofof the buccopharyngeal cavity.
• The lower jaw is toothless.
• Opening of the Eustachian tube, vocal sacs (only in males), gullet, and glottis can be seen clearly in the buccopharyngeal cavity.
• The muscular tongue is bilobed at the tip and free from behind. It is used for capturing the prey.
• The gullet opens into a narrow and short tube-like esophagus, which continues in a large and distended stomach.
• It contains a thick muscular layer, which helps in verting food into chyme.

• It secretes gastric juice containing HCl and proteolytic enzymes. The stomach is followed by a coiled small intestine.
• The intestinal wall has numerous finger-like folds called villi and microvilli, projecting into its lumen to enhance the surface area for absorption of the digested food.
• The first part of the small intestine lying parallel to the stomach is called the duodenum. The intestine continues into a wider rectum, opening into the cloaca.
• The urinary bladder opens into the cloacal chamber through the ureter.
• The gastric and intestinal glands occur in the walls of the stomach and intestine, respectively.
• The other important digestive glands associated with the alimentary canal are the liver and pancreas.
• The liver secretes bile which is temporarily stored in the gall bladder before being released into the duodenum.
• Bile helps in the digestion of food by changing its pH from acidic to alkaline and by emulsifying the fats.
• The liver does not secrete any digestive enzymes. The pancreas is an irregular, elongated gland, situated in a thin mesentery, and lies parallel to the stomach.
• It produces pancreatic juice containing digestive enzymes such as trypsin, amylopsin, etc.

Respiratory System

• Three Types Of Respiration Are Observed: cutaneous, buccopharyngeal, and pulmonary.
• Cutaneous respiration on land is through the body’s surface. During hibernation and aestivation, frog respires only through this method.
• Buccopharyngeal respiration occurs through the lining of the buccal cavity. It occurs only when the frog is out of water. The mucus membrane of the buccal cavity is moist which dissolves oxygen whose diffusion occurs into the blood capillaries.
• Pulmonary Respiration: Lungs in frogs are not efficient respiratory organs because only mixed air enters into them and they mainly function as hydrostatic organs.
• Lungs are a pair of thin-walled, translucent sacs with an inner surface divided into alveoli by septa. Pulmonary respiration has a maximum frequency of 20/min.
• It occurs when more energy is required. The mouth and gullet are kept closed during pulmonary respiration.
• Respiratory movements in pulmonary respiration are because of the buccopharyngeal cavity which acts as a force pump. These movements are carried out by a set of paired muscles—stemohyal and pterohyal muscles.
• Stemohyal muscles are attached to hyoid and coracoid processes and clavicles of the pectoral girdle and, on contraction, depress the buccal floor enlarging the buccopharyngeal cavity.
• Pterohyals are attached in between the hyoid and prootics of the skull and, on contraction, lift the floor of the buccal cavity.
• With the depression of the buccal floor, air enters the buccal cavity through the nares.
• External nerves are then closed by pushing the tuberculum prelingual and the movable pre maxillae.
• It is followed by the raising of the buccal floor by peroneal muscles which reduce the volume and the air is pushed into the lungs where the exchange of gases takes place.
• The buccal floor is again lowered enlarging its volume which draws air into the buccal cavity.
• External nares are opened followed by raising the buccal floor, pushing the air out through external nares.
• The sound-producing organ of a frog is the laryngotracheal chamber. It is supported by one cricoid, two arytenoids, and two pre-arytenoid cartilages. It has a pair of muscle strands (vocal cords) that actually produce sound. The male frog has vocal sacs which act as resonating chambers.

Circulatory System

• The circulatory system in frogs is closed type.
• The heart lies enclosed by a thin, transparent, two-layered sac, pericardium.
• Frog s heart is a three-chambered structure made up of two upper auricles and a single lower ventricle.
• The two additional chambers connected to the heart of the frog are sinus venosus and truncus arteriosus.
• Frogs also possess two well-developed portal systems: the renal portal system and the hepatic portal system. Frog also has two pairs of lymph hearts.

Nervous System

• The nervous system is organized into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves), and an autonomic nervous system (sympathetic and parasympathetic chains of ganglia).
• There are 10 pairs of cranial nerves.
• The brain is enclosed in a bony structure or brain box (cranium) which has two occipital condyles for attachment with the first vertebra (atlas).
• The brain is divided into forebrain, midbrain, and hindbrain.
• The forebrain includes olfactory lobes, paired cerebral hemispheres, and unpaired diencephalon. The midbrain is characterized by a pair of optic lobes.
• The hindbrain consists of the cerebellum and medulla oblongata.
• Medulla oblongata passes out through the foramen magnum and continues into the spinal cord which is contained in the vertebral column.
• Jacobson’s organ, also called the vomeronasal organ, opens into the nasal chamber and acts as an additional olfactory organ.

Sensory Organs

• Eye
  • The eye is guarded by an immovable upper eyelid, movable low er eyelid, and transparent nictitating membrane.
  • The outer sclerotic ring is cartilaginous and the cornea is the part exposed. In the middle, the highly vascular, pigmented layer is choroid.
  • Iris is yellow-pigmented and perforated by a central aperture, the pupil.
  • The retina is the innermost coat of the eyeball and consists of an inner pigmented layer and an outer receptor layer.
  • Rods and cones are light-sensitive structures found in the retina.
  • Rods have rhodopsin or visual purple meant for night vision while cones have iodopsin responsible for color vision in daylight.
  • The anterior chamber present in the front of the lens is filled with aqueous humor while the chamber behind the lens is a posterior chamber, filled with vitreous humour.
  • The eyeball is moved in the eye orbit by a set of six muscles—two are oblique and four are recti.
  • Besides, there are retractor bulbs and levator bulbs muscles for intrusion or protrusion of the eyeball into the eye orbit, respectively.
• Ears
  • The ear of frog has only a middle and internal ear.
  • The tympanic membrane is present at the body surface.
  • The middle ear has a single bone called columella auris.
  • Its outer end is attached to the eardrum while the inner end is attached to the stapedial plate.
  • The pressure of air in the middle ear is controlled by Eustachian tubes.
  • The membranous labyrinth or internal car consists of a utriculus, sacculus, and semicircular canals. Endolynrph fills the membranous labyrinth.
• Excretory System
  • The main organ of excretion is a pair of kidneys.
  • These compact, dark red, and bean-like structures are situated a little posteriorly in the body cavity on both sides of the vertebral column.
  • The frog excretes urea and thus is a ureotelic animal.
  • Urea is carried by blood into the kidney where it is separated and excreted.
  • Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons.
  • The ureter emerges from the kidney as urinogenital ducts in males.
  • A common ureter opens into the cloaca.
  • A thin-walled urinary bladder is present ventral to rectum which also opens in the cloaca.

Reproductive System: The male reproductive organs consist of a pair of yellowish ovoid testes, which are found to adhere to the upper part of the kidneys by a double fold of peritoneum called mesorchium.

• Vasa efferentia are 10-12 in number and, after arising from the testes, run through the mesorchium and enter the kidneys of their side.
• In kidneys, these open into Bidder’s canal which finally communicates with the urinogenital duct. This duct emerges from the kidneys and finally opens into the cloaca.
• The cloaca is a small, median chamber that is used to pass fecal matter, urine, and sperm to the exterior.
• A pair of ovaries, situated near the kidneys, comprises the female reproductive organs. However, these have no functional connection with the kidneys.
• A pair of oviducts opens into the cloaca, separately
• The release of ovum in females is termed as spawning.
• A mature female can lay 2500-3000 ova at a time.
• Fertilization is external and takes place in water.
• Development involves a larval stage called tadpole.
• Tadpole undergoes metamorphosis to form an adult.

 

Structural Organization In Animals Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: Cardiac muscles have striations and fiber is nucleated and involuntary.

Reason: Intercalated discs form the three-dimensional network of cardiac muscle fiber.

Answer: 3. If Assertion is true, but Reason is false.

Question 2. Assertion: Multipolar neurons have several efferent processes.

Reason: Axons are the afferent processes of a neuron.

Answer: 4. If both Assertion and Reason are false.

Question 3. Assertion: Blood circulation is absent in epithelium tissue.

Reason: Blood vessels are unable to pierce the basement membrane.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: Reticular fibrous connective tissue is called as embryonic tissue.

Reason: Reticular fibrous connective tissue is mainly found in the embryonic stage.

Answer: 4. If both Assertion and Reason are false.

Question 5. Assertion: Epithelia are highly regenerative.

Reason: When epithelia get damaged, they regenerate more rapidly than other tissues.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

Question 6. Assertion: Platelets play an important role in blood clotting.

Reason: In the blood oozing from an injury, the platelets disintegrate and release thromboplastin that initiates clotting.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

Question 7. Assertion: Brown fat produces more energy.

Reason: Brown fat is composed of molecular adiposity

Answer: 3. If Assertion is true, but Reason is false.

Question 8. Assertion: Simple cuboidal epithelium is called as germinal epithelium.

Reason: The cuboidal cells of gonads form gametes.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

Question 9. Assertion: Heparin is an anticoagulant found in mammals.

Reason: Heparin prevents the conversion of prothrombin to thrombin.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

Question 10. Assertion: Epithelium cells get their nutrients from adjacent cells.

Reason: In epithelium tissues, large intercellular spaces are present.

Answer: 4. If both Assertion and Reason are false.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is tine, but Reason is false.
4. If both Assertion and Reason are false.

Question 11. Assertion: Earthworm is brown or claycolored.

Reason: Because of the presence of pigment porphyrin.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 12. Assertion: Chloragogen cells are considered analogous to the liver of vertebrates.

Reason: Because it is concerned with the storage of reserve food, deamination of proteins, formation of urea, etc.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 13. Assertion: Earthworms is saprozoic.

Reason: Because it feeds on small insects.

Answer: 3. If Assertion is tine, but Reason is false.

Question 14. Assertion: Earthworm is hermaphrodite.

Reason: Because in earthworms both sexes are separate.

Answer: 3. If Assertion is tine, but Reason is false.

Question 15. Assertion: Earthworms are the enemy of fanner.

Reason: Because they destroy the crop in the field.

Answer: 4. If both Assertion and Reason are false.

Question 15. Assertion: In the body of earthworms, porphyrin pigment is found.

Reason: Because it protects earthworms from chemicals.

Answer: 3. If Assertion is tine, but Reason is false.

Question 16. Assertion: In earthworms, development is direct.

Reason: Because in development larval stage is not found.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 17. Assertion: In the anus of earthworms, depressor muscles are found.

Reason: These muscles help in the elimination of excretion from the rectum.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 18. Assertion: In cockroaches, inspiration is an active process.

Reason: It is due to the contraction of tergosternai muscle.

Answer: 4. If both Assertion and Reason are false.

Question 19. Assertion: In frogs, sinus venosus is present.

Reason: In mammals and birds, the remnant of sinus venosus has taken part in the formation of SA node.

Answer: 2. If both Assertion and Reason are true, but the Rea¬son is not the correct explanation of the Assertion.

Question 20. Assertion: Septal nephridia take part in osmoregulation.

Reason: They are entcronephric.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 21. Assertion: In Periplcineta, only superposition or overlapping images are formed.

Reason: Retinal pigment sheath remains contracted throughout life.

Answer: 4. If both Assertion and Reason are false.

Question 22. Assertion: The pharyngeal gland of earthworms includes chromophil cells, which secrete sativa.

Reason: The salivary amylase of earthworms is essential to digest carbohydrates.

Answer: 3. If Assertion is tine, but Reason is false.

Question 23. Assertion: The head of the cockroach is hypognathus.

Reason: The proximal part of the lower lip of a cockroach is called the pastmentum.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 24. Assertion: The heart of a cockroach is neurogenic.

Reason: The heartbeat rate in cockroaches is 49 per minute.

Answer: 2. If both Assertion and Reason are true, but the Rea¬son is not the correct explanation of the Assertion.

 

Breathing And Exchange Of Gases Respiration

Respiration may be defined as the process in which oxygen is taken inside the body from the environment for the oxidation of food to release energy and the carbon dioxide so produced during oxidation is expelled out of the body.

The process of oxidation of food is complex. It involves a series of complex biochemical reactions and the released energy is stored in the form of ATP molecules. However, for simplicity sake, the multistep reaction can be briefly expressed as follows:

Types Of Respiration: Based on the mode of gas exchange between the body cells and the surrounding air, there are mainly two types of respiration: direct and indirect respiration.

1. Direct Respiration

• There is a direct exchange of gases between the carbon dioxide of body cells and the oxygen of water, and there is no blood for the transport of gases.
• The exchange of gases occurs on the principle of diffusion.
• It is found in unicellular organisms such as aerobic bacteria and protists (for example., Amoeba) and metazoans such as sponges, coelenterates (for example., Hydra), flatworms, roundworms, and insects.

2. Indirect respiration

• There is no direct contact between the body cells and the surrounding air or water.
• The source of oxygen is called the respiratory medium.
• Indirect respiration is found in larger and more complex forms of animals.
• These organisms have some specialized organs such as gills (most crustaceans, mollusks, insect larvae, and all fishes and amphibians), lungs (snails, amphibians, and all reptiles, birds, and mammals).
• In this mode of respiration, the transportation of oxygen and carbon dioxide between the respiratory organs and the body cells is brought about by the blood of the circulatory system.

All physical and chemical reactions in which atmospheric air oxidizes food in the body cells resulting in the production of energy and liberation of CO₂ are included in respiration. Based on the mode of nutrient oxidation, respiration is of the following types:

Read and Learn More NEET Biology Notes

1. Anaerobic Respiration

• It occurs when nutrients are oxidized without using O₂ (also called fermentation).
• In yeast, glucose forms ethyl alcohol and CO₂. In bacteria and muscles, glucose is converted into lactic acid.
• Endoparasites also respire anaerobically.
• Anaerobic respiration is a low energy-yielding process.
• In mature RBCs also, anaerobic respiration occurs due to the loss of mitochondria along with other cell organelles.

2. Aerobic respiration

• Cells utilize O₂ for oxidizing nutrients.
• O₂ is used either from atmospheric air or from water. It involves:

1. External respiration (breathing or ventilation): It is the exchange of oxygen present in the surrounding gaseous or liquid medium and carbon dioxide in the blood through a liquid medium by diffusion across the body surface or respiratory surface. It is purely a physical process that depends upon the principle of diffusion and no energy is produced.
2. Internal respiration: It is the exchange of oxygen present in blood and carbon dioxide in the body cells through tissue fluid at the cellular level. Internal respiration involves energy production and is a physicochemical process.
3. Cellular respiration: It is the utilization of O₂ by cells for energy product; and result-dependent release of CO₂.

Respiratory Organs In Animals

The process of exchange of O^–, from the atmosphere with CO₂ produced by the cells is called breathing, commonly known as respiration. Shows various examples of their specific respiratory organs.

Organisms With Their Specific Respiratory Organs:

Human Respiratory System

Shows the human respiratory’ pathway including the organs responsible for carrying out the gas exchange in the human body.

Following are the organs involved in carrying out the human respiration:

1. Nasal cavity

• The nasal cavity is the first part of the respirator)’ system.
• It opens to the exterior through the nostrils.
• The nasal cavity is divided into nasal chambers by nasal septum. Each nasal chamber can be differentiated into three regions:
• Vestibular region: The first part starting from the nostril has oil glands and hair to prevent the entry of large dust particles.
• Respiratory part: Highly vascular and involved in the conditioning of air as the air gets warmed and moist. Arising from the tire wall of each nasal chamber, there are three shallow’ bony ridges called nasal conchae. These are individually named as superior, middle, and inferior. Superior lies within the olfactory part; middle and inferior lie in the respiratory part. The conchae are covered with mucus membranes and greatly increase the surface of the nasal chambers.

Olfactory’part: It is the upper part, lined by olfactory epithelium for smell.

2. Pharynx

• From the nasal cavity, the air enters the pharynx.
• It serves as a common passage for both air and food.
• The opening into the windpipe or trachea is a narrow slit the glottis.
• The glottis is protected against the entrance of food by a triangular flap of tissue, the epiglottis.

3. Human Larynx

• Until puberty, there is little difference in the size of the larynx (voice box) in males and females.
• The larynx opens into the tire oropharynx by a slit-like aperture, the glottis.
• The larynx is composed of irregularly shaped cartilages attached to each other by ligaments and membranes.

The main cartilages are:

• Thyroid cartilage: It is the most prominent, C-shaped, dorsally incomplete cartilage called Adam’s apple as it is apple-shaped and more prominent in males.
• Cricoid cartilage: It lies below the thyroid cartilage, and its shape is like a signet ring.
• Arytenoid cartilages: These are two roughly pyramid-shaped cartilages forming the posterior wall of the larynx. Corniculate cartilages: These are two conical nodules of elastic fibro-cartilage which lie at the apices of arytenoid cartilages.
• Cuneiform: These are two small, elongated, club-shaped nodules of elastic fibro-cartilage that lie above and anterior to corniculate cartilages. These connect epiglottis to arytenoid cartilage.

4. Trachea

• The trachea is a tube about 12 cm long with C-shaped (dorsally incomplete) rings of hyaline cartilage in its walls.
• These rings of cartilage make the wall non-collapsible.
• It is internally lined by pseudo-stratified ciliated columnar epithelium.
• Cilia beat to push out the mucus.

5. Bronchi

• The walls of the bronchi are likewise supported by cartilaginous rings.
• Each bronchus divides and redivides further into smaller bronchioles.

Epiglottis: It is a single leaf-shaped cartilage that projects into the pharynx.

• Thus, a total of nine cartilages are present, three paired (3, 4,5) and three unpaired (1,2, 4).
• The thyrohyoid membrane is a broad, flat membrane attached to the hyoid bone above and to the thyroid cartilage below.
• Inside the larynx are present two pairs of vocal cords, one pair of false vocal cords which have little to do with sound production, and the second inner pair of true vocal cords.
• When the air is forced through the larynx, it causes vibration of the true vocal cords, and sound is produced.
• The pitch of sound is determined by the tension on vocal cords. The greater the tension, the higher the pitch.

• Epithelium gradually changes from pseudo-stratified ciliated columnar epithelium in the bronchi to ciliated simple cuboidal epithelium in the terminal bronchiole.
• Incomplete rings of cartilage are gradually replaced by plates of cartilage that finally disappear in a distal bronchiole.
• The terminal bronchiole is further subdivided into respiratory bronchioles. Respiratory bronchioles open into alveolar ducts → atria → alveolar sacs → alveoli. From respiratory bronchioles onward, the wall is very thin and is made up of ol’ simple squamous epithelium.

6. Lungs

• The lungs occupy the greater part of the thoracic cavity.
• Surrounding each lung is a double-walled sac within the walls of which lies the pleural cavity.
• The right lung is divided into three lobes and the left into two.
• Inside the lung, each bronchus divides into numerous bronchioles, each of which terminates into an elongated saccule, the alveolar duct, which bears on its surface air sacs or alveoli.
• The number of alveoli in the human lungs has been estimated to be approximately 300 million.
• The lungs are covered by a thin double layer of simple squamous epithelium called the pleura.
• The outer or parietal pleuron remains attached to the wall of the thoracic cavity.
• The space between the two pleural membranes contains pleural fluid for reducing friction and makes the movement of the lung easy.
• Inflammation of the pleura causes a disease called pleurisy.
• Lungs are pink at birth.
• They become dark grey and mottled in adults due to the deposition of carbonaceous materials.
• Darkening increases in smokers and persons exposed to pollutants.
• The right lung is shorter by about 2.5 cm due to the raised position of the diaphragm on the right side to accommodate the liver.
• The left lung is longer. It is, however, narrower than the right lung because it contains a cardiac notch for accommodating an asymmetrically placed heart. The left lung is divisible by an oblique fissure into two lobes, left superior and left inferior.
• The right lung has two fissures, horizontal and oblique. They divide the right lung into three lobes: right superior, right middle, and right inferior. The lobes are divided internally into segments and segments into lobules.
• There are eight segments in the left lung and 10 segments in the right lung. On average, an adult’s right lung weighs 625 g, while the left lung weighs 565 g.

Mechanism Of Breathing

Respiratory movements in man are carried out with the help of intercostal muscles and diaphragm. There are two phases of each breathing movement, inspiration and expiration.

1. Inspiration (inhalation): It involves the intake of fresh air in the alveoli of the lungs. It includes an active process and consumes mechanical energy. There are two types of inspiratory muscles: phrenic and external intercostal muscles.

• Phrenic Muscles: These muscles extend from the diaphragm to the ribs and vertebral column.
• External Intervestal Muscles: These are 11 pairs of muscles present between 12 pairs of ribs dor- ventrally and laterally. When phrenic muscles contract, the diaphragm is flattened whereas when external intercostal muscles contract, the ribs are pulled forward, upward, and outward. The thoracic cavity increases in all directions which results in the increase in volume and decrease in pressure. So, air moves into the lungs.

2. Expiration: It involves the expelling of air of high pco₂ out of the body. During rest, the expiration is a passive process and simply involves the relaxation of inspiratory muscles (phrenic and external intercostal muscles). These decrease the volume of the thoracic cavity. But during forceful expiration, two expiratory muscles also help in expiration: the abdominal and internal intercostal muscles.

• Abdominal muscles: These extend front ribs to abdominal organs. When these muscles contract, the abdominal visceral organs are pulled upward, toward the diaphragm. So the diaphragm becomes more convex and the thoracic cavity decreases anteroposteriorly.
• Internal intercostal muscles: Like external intercostal muscles, internal intercostal muscles are also in 11 pairs present between the ribs. When these contract, ribs arc pulled backward, downward. and inward. So thoracic cavity decreases dorsoventrally and laterally. Due to the above changes, a larger amount of air is passed out.

Pulmonary Volumes

• Tidal volume (TV): It is the volume of air inspired or expired involuntarily in each normal breath. It is about 500 mL of air in an average young adult man.
• Inspiratory reserve volume (1RY): It is the maximum volume of air that a person can inhale over and above tidal volume by the deepest possible voluntary inspiration. It is about 25003000 mL.
• Expiratory reserve volume (ERV): It is the volume of air that can expire over and above the tidal volume with maximum effort. It is about 1000-1100 mL.
• Residual volume (RY): It is the volume of air left in the lungs even after maximum forceful expiration. It is about 1100— 1200

Pulmonary Capacities: Pulmonary capacities are the combination of two or more pulmonary volumes. These include

Inspiratory capacity (IC): This is equivalent to TV 4 IRV. It is about 3000-3500 rnL. Functional Residual Capacity (FRC): It is equivalent to ERV 4 RV. It is about 2500 mL. Vital capacity (VC): It is equivalent to IRV + TV + ERV. It varies from 3500 mL to 4500 mL depending on the age, sex, and height of a person.

Total lung capacity (TLC): It is equivalent to TV + IRV 4 RV + ERV. It is about 5800 mL. With the exception of FRC, RV, and TLC, all other lung values and lung capacities can be measured with the help of a simple spirometer. All pulmonary volumes and capacities are about 20-25% less in women than in men, more in athletic people than asthmatics.

Physiological Shunt And Shunted Blood: Whenever the ventilation/perfusion ratio is below normal, there is inadequate ventilation to provide the O₂ required to fully oxygenate the blood flowing through the alveolar capillaries.

Therefore, a certain fraction of venous blood passing through the pulmonary capillaries does not become oxygenated. This fraction is called shunted blood. A small degree of shunt is nonthermal and may be described as a physiological shunt. Normally, this is about 2% of the cardiac output.

Physiologic Dead Space: When ventilation of some of the alveoli is great but alveolar blood flow (perfusion) is low, there is far more O₂ available in alveoli than can be transported away from the alveoli by the flowing blood.

Hence, physiological dead space is the air that is inhaled by the body in breathing but does not take part in gaseous exchange. About one-third of every resting breath is dead space. In adults, it is usually in the range of 150 mL. Hence, shunt and dead space are due to deviations in the ventilation/perfusion ratio.

Gaseous Exchange

• Alveoli are the primary sites of exchange of gases.
• The exchange of gases also occurs between blood and tissues.
• O₂ and CO₂ arc are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient.
• The solubility of gases as well as the thickness of membranes involved in diffusion are also among the important factors that can affect the rate of diffusion.

• The partition between the two includes alveolar epithelium, alveolar epithelial basement membrane, thin interstitial space, capillary basement membrane, and capillary endothelial membrane. This whole part is called the respiratory membrane and cumulatively forms a membrane of 0.5 μm thickness.
• The limit of exchange between alveoli and pulmonary blood is called diffusing capacity. It is defined as the volume of gas that diffuses through the respiratory membrane in one minute for a particle pressure difference of 1 mm Hg.
• Solubility of the gases in lipids also affects diffusing capacity.
• The diffusing capacity of CO₂ is 20 times more than that of oxygen which is about double than that of nitrogen.
• Surfactant: A surface active agent lecithin secreted by type II alveolar epithelial cells reduces surface tension between the alveolar fluid and air. It prevents the collapsing of lung alveoli.
• The exchange of gases between the alveoli and blood in the lungs, and the blood and tissues is the result of differences in the partial pressure of respiratory gases, that is, oxygen and carbon dioxide.

External Respiration or Gaseous Exchange at Alveoli Level: The partial pressure of O₂ in atmospheric air is 159 mm of Hg and that of CO₂ is 0.3 mm of Hg.

• The partial pressure of oxygen po₂ in alveolar air is 104 mm Hg and it is only 40 mm Hg in the arterial capillaries of lungs. Therefore, oxygen from the alveolar air rapidly diffuses into the blood capillaries due to its higher po_2.
• In the expired air, it changes to 116 mm of Hg.
• Similarly, pco₂ in the blood reaching the alveolar capillaries is 45 mm Hg whereas pco₂ in the alveolar air is 40 mm Hg. Therefore, CO₂ rapidly leaves blood capillaries and reaches alveoli.
• pco₂ level in the expired air is 32 mm Hg.

Partial pressure (mm Hg) and percentage of respiratory gases during inspiration and expiration:

Internal Respiration or Gaseous Exchange at Tissues Level: The gaseous exchange between blood and body tissues is also due to differential partial pressures. The po₂ and po₂ of the arterial blood reaching the tissues are 95 and 40 mm Hg, respectively. The po₂ and pco₂ °f tissues are 20 and 52 mm Hg, respectively. Therefore, oxygen quickly leaves the blood and enters the cells whereas CO₂ produced in the tissues leaves the cells and enters the blood.

Factors That Decrease Oxygenation

Various factors responsible for the decrease in the combination of oxygen with blood are as follows:

1. Low blood volume
2. Anemia
3. Low Hb
4. Poor blood flow
5. Pulmonary diseases

Transport Of Gases In Blood

The exchange of oxygen and carbon dioxide takes place in between the lungs and blood. The greater part of oxygen diffuses into the blood, and at the same time, carbon dioxide diffuses out. Below are explained two types of gas transport systems: oxygen transport and carbon dioxide transport.

Oxygen Transport

• Each deciliter of blood carries 19.8 mL of O₂ ol which 5 mL diffuses into tissues.
• 3% of O₂ transported is dissolved in plasma and 97% is carried by RBCs. Four Fe²⁺ ions of each hemoglobin can bind with four molecules of O₂ and it is carried as oxyhemoglobin.

Hb₄+4O₂ → HB₄O₈

• Oxyhemoglobin dissociates near tissues due to an increase in acidity and a decrease in pH. It can also be caused due to high temperatures.
• In a normal person, the hemoglobin level is about 15 per 100 mL.
• The capacity of 1 g of hemoglobin to combine with O₂ is 1.34 mL. Therefore, arterial blood carries about 20 mL of O₂/100 mL of blood.
• Under normal conditions, the O₂ level falls to about 14.4 mL/100 mL in the venules.
• It indicates that under normal conditions, approximately 5 mL of oxygen is transported by blood.
• Under strenuous conditions or during exercise, the O₂ level falls to about 4.4 mL/100 mL, i.e., approximately 15 mL of O₂ is transported by Hb during exercise.
• Bohr’s effect: The relationship between po₂ and the percent saturation of hemoglobin when represented on a graph is termed an oxygen-hemoglobin dissociation curve and is sigmoid in shape. A rise in pco₂ or fad in pH decreases the oxygen affinity of hemoglobin, raising the p₅₀ value. This is called Bohr’s effect (p₅₀ value is the value of po₂ at which hemoglobin is 50% saturated with oxygen to form oxyhemoglobin).
• Conversely, a fall in pco₂ and a rise in pH increase the oxygen affinity of hemoglobin and shift the curve to the left. Fetal hemoglobin has a higher affinity for 02 because it binds BPG less strongly. Therefore, the oxygen-hemoglobin dissociation curve for fetal hemoglobin will appear on the left side.

Myoglobin present in the muscles also has more affinity for O₂. But since it has only one he’ group, the curve obtained will be hyperbolic, not sigmoid.

Points To Remember

Carbon monoxide poisoning: If a person sleeps in a closed room with a lamp burning, the absence of a sufficient amount of oxygen causes an incomplete combustion of carbon and produces carbon monoxide in the room. As the person inhales carbon monoxide, it diffuses from the alveolar air to the blood and binds to hemoglobin forming carboxyhemoglobin. The latter is a relatively stable compound and cannot bind with oxygen.

So, the amount of hemoglobin available for oxygen transport is reduced. The resulting deficiency of oxygen causes headaches, dizziness, nausea, and even death. Carbon monoxide combines with hemoglobin at the same point on the hemoglobin molecule as oxygen. It binds with hemoglobin 250 times faster than oxygen.

SARS (severe acute respiratory syndrome)

• The first patient of SARS was reported on February 26, 2003, in China.
• The causative agent is human coronavirus.
• It is a new member of the influenza virus family which is considered a mutant form of the influenza virus.

Carbon Dioxide Transport

CO₂ is transported in three ways:

1. In dissolved state: About 7% of CO₂ is transported after getting dissolved in plasma. The pco> in arterial blood is 40 mm of Hg and in venous blood, it is 45 mm of Hg. About 0.3 mL of extra CO: is carried per 100 mL of blood in this form.

2. As bicarbonate: Nearly 70% of CO₂ is transported from tissues to lungs in this form. CO₂ diffuses from tissues into the blood and enters RBCs. It combines with water to form H₂CO₃ which dissociates into H+ and HCO₃T Being catalyzed by carbonic anhydrase, it is a fast step.

H⁺ ions combine with hemoglobin replacing its association with K⁺ and forming hemoglobinic acid.

Due to this, the level of HCO₃^– increases in RBCs, which start coming out of it along the concentration gradient. To maintain ionic balance, Cl^– moves in from plasma into RBCs. In the plasma, HCO₃^– combines with Na⁺ or K⁺ to form NaHCO₃ or KHCO₃.

3. As carbamino-Hb: About 20-25% of CO₂ is transported in this mode. CO₂ combines with the NH₂ group of Hb and forms carbamino-Hb. This combination of CO₂ and Hb is a reversible reaction.

Release of CO₂ in the alveoli of the lung: In the pulmonary capillaries, CO₂ starts diffusing out into the alveoli.

• Carbamino-Hb spits into CO₂ and Hb. As Hb of RBC takes up O₂, it releases H+ in RBC.
• H+ starts combining with the available HCO₃^– in RBC to form H₂CO₃ which splits into H₂O and CO₂, and CO₂ starts diffusing out (reverse of reactions).
• As a result, HCO₃^– from plasma starts moving in along the concentration gradient, and for ionic balance, Cl-starts move out. This way CO₂ is released into the lungs.

Hamburger’s phenomenon: HCO₃ ions diffuse out into plasma and Cl^– ions enter into the RBCs at the level of tissues (internal respiration). This is known as the chloride shift or Hamburgers phenomenon. At the level of external respiration or alveoli, Cl^– moves out as HCO₃^– moves in. This is called the reverse of the chloride shift.

Haldane’S Effect

• It is related to the transport of CO₂ in the blood. It is based on the simple fact that oxyhemoglobin behaves as a strong acid and releases an excess of H+ ions which bind with bicarbonate (HCO₃^–) ions to form H₂CO₃ which dissociates into H₂O and CO₂.
• Secondly, due to increased acidity, CO₂ loses the power to combine with hemoglobin and form carbamino-hemoglobin.
• The effect of oxyhemoglobin formation or dissociation on CO^–, transport is called Haldane’s effect.

Regulation Of Breathing

1. Respiratory Center

• As the breathing muscles are voluntary, the breathing pattern can be altered voluntarily (cerebral cortex activity).
• The normal respiratory rhythm (12-14 min) is controlled by the nervous system (mainly the medulla).
• Three groups of respiratory centers are involved—dorsal respiratory group, ventral respiratory group, and pneumotaxic center.
• The signals from the dorsal respiratory group (located in the dorsal portion of the medulla) are transmitted to the diaphragm and maintain normal breathing patterns.
• The ventral respiratory group (located anterolaterally to the dorsal inspiratory group) is involved only in enhanced respiratory requirements. It regulates both inspiration as well as expiration.
• The pneumatic center is located in the pons and controls the switch-off point of inspiration.
• When sending a weak signal, inspiration lasts for about 5 seconds causing complete filling of the lungs.
• When sending a strong signal, the rate of breathing increases, i.e., the duration of inspiration and expiration decreases and complete filling of lungs is not possible.

2. Chemical control

• Central chemoreceptors are present in the medulla oblongata (CNS) whereas the peripheral chemoreceptors are located in the walls of systemic arteries (aortic body with aortic arch and carotid body with carotid artery).
• They relay impulses to the respiratory center over the cranial nerves of PNS.
• Increased pco₂, increased H⁺ concentration, and decreased po₂ input from the central and peripheral chemoreceptors cause the inspiratory area to become highly active, and the rate and depth of breathing increase.
• Severe hypoxia depresses the activity of central chemoreceptors and the respiratory center.
• The respiration rate decreases or breathing ceases altogether, and po₂ falls lower and lower (asphyxia).

Disorders Of The Respiratory System

Asthma: It is difficulty in breathing, causing wheezing due to inflammation of the bronchi and bronchioles.

Emphysema: It is a chronic disorder in which alveolar walls are damaged due to which the respiratory surface is decreased. One of the major causes of this is cigarette smoking.

Occupational respiratory disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.

Bronchitis: It is the inflammation of the bronchi which is characterized by hypertrophy and hyperplasia of scro-mucous glands and goblet cells lining the bronchi. The symptom is regular coughing with thick greenish-yellow sputum that indicates the underlying infection, resulting in the excessive secretion of mucus. It may also be caused by cigarette smoking and exposure to air pollutants like carbon monoxide.

Bronchial asthma: This is characterized by spasms of the smooth muscles present in the bronchiole walls. It is generally caused due to hypersensitivity of the bronchiole to foreign substances present in the air passing through it. The symptoms of the disease may be coughing or difficulty in breathing mainly during expiration. The mucus membranes on the wall of the air passage start secreting excess amounts of mucus, which may clog the bronchi, as well as bronchioles.

Pneumonia: It is the infection of the lungs by Streptococcus pneumoniae and leads to the accumulation of mucus and lymph containing dead WBCs in alveoli, impairing gaseous exchange. Uptake of oxygen is adversely affected, and as a result, the oxygen level of the blood falls. Sometimes, it may be caused by other bacteria or fungi, protozoans, viruses, and mycoplasma.

Occupational lung disease: It is caused because of exposure to potentially harmful substances such as gas, fumes, or dust present in the environment where a person works. Silicosis and asbestosis are common examples, which occur due to chronic exposure of silica and asbestos dust in the mining industry. It is characterized by the fibrosis (proliferation of fibrous connective tissue) of the upper part of the lung causing inflammation.

Prevention and cure: Almost all occupational lung diseases express symptoms after chronic exposure, i.e., 10-15 years, or even more. Not only this, diseases such as silicosis and asbestosis are incurable. Hence, the person likely to be exposed to such irritants should adopt all possible preventive measures. These measures include:

• Minimizing the exposure of harmful dust at the workplace.
• Workers should be well-informed about the harm of the exposure to such dust.
• Use of protective gear and clothing by workers at the workplace.
• Regular health check-ups.
• Holiday from duty at short intervals for workers in such areas.
• The patient may be provided with symptomatic treatment such as bronchodilators and antibiotics in order to remove the underlying secondary infection.

Tuberculosis: A bacterial disease caused by Mycobacterium tuberculosis. This decreases the elasticity of the lungs making them fibrous.

Whooping cough or pertussis: An infectious disease caused by the bacterium Bordetella pertussis.

Sleep apnoea syndrome (SAS): Persons with snoring habits suffer from sleep apnoea syndrome because their respiratory tract closes on inhalation.

Cough: A reflex in lower respiratory passages followed by forceful expulsion of air to remove an irritant.

Sneeze: A reflex similar to cough but applies to nasal passageways instead of lower respiratory passages.

Hypercapnia: Excess of CO₂ in the body.

Cyanosis: This means blueness of skin because of the excessive amount of deoxygenated Hb.

Periodic breathing: A breathing characterized by slowly waxing and waning respiration occurring over and over again about every 40-60 s. The acute effects of voluntary hyperventilation demonstrate the interaction of the chemical respiratory regulating mechanisms.

• When a normal individual hyperventilates for 2-3 min, then stops and permits respiration to continue without exerting any voluntary control over it, there is a period of apnea. This is followed by a few shallow breaths and then by another period of apnea, followed again by a few breaths (periodic breathing).
• The cycles may last for some time before normal breathing is resumed. The apnea apparently is due to CO₂ lack because it does not occur following hyperventilation with gas mixtures containing 5% CO₂. During an apnea, the alveolar Po₂ falls and PCo₂ rises.
• Breathing resumes because of hypoxic stimulation of the carotid and aortic chemoreceptors before the CO₂ level has returned to normal.
• A few breaths eliminate the hypoxic stimulus and breathing stops until the alveolar Po, falls again. Gradually, however, Pco₂ returns to normal, and normal breathing resumes. Periodic breathing occurs in various disease states and is often called Cheyne-Stokes respiration.

Atelectasis: It means the collapse of the alveoli. When a bronchus or bronchiole is obstructed, the gas in the alveoli beyond the obstruction is absorbed and the lung segment collapses. The collapse of the alveoli is called atelectasis.

• The atelectatic area may range in size from a small patch to a whole lung. Some blood is diverted from the collapsed area to better-ventilated portions of the lung and this reduces the magnitude of the decline in arterial Po₂. When a large part of the lung is collapsed, there is an appreciable decrease in lung volume.
• The intrapleural pressure, therefore, becomes more negative and pulls the mediastinum, which in humans is a fairly flexible structure to the affected side. Another cause of atelectasis is the absence or inactivation of surfactant, the surface tension depressing agent normally found in the thin fluid lining the alveoli.
• This abnormality is a major cause of failure of the lungs to expand normally at birth. The collapse of the lung may also be due to the presence in the pleural space of air (pneumothorax), tissue fluids (hydrothorax, chylothorax), or blood (hemothorax).

Breathing And Exchange Of Gases Points To Remember

Terms associated with breathing

• Eupnea: Nonnal breathing
• Hypopnea: Slow breathing
• Hyperpnea: Rapid breathing
• Apnea: No breathing
• Dyspnea: Painful breathing
• Orthopnea: Difficult breathing except in an upright position
• Tachypnea: Rapid shallow breathing
• Chronic obstructive pulmonary disease (COPD): Examples, are emphysema, and chronic bronchitis.

Artificial respiration

• Artificial respiration may be required whenever breathing is depressed or absent, for example. during drowning, cardiac arrest, electric shock, carbon monoxide poisoning, or a head injury.
• A variety of methods of artificial respiration can be used.
• The commonly used methods are mouth-to-mouth respiration, mouth-to-nose ventilation, resuscitator, tank respirator, etc.
• Mouth-to-mouth respiration: In this, the patient is placed on his back on a firm surface. The head is kept at a slightly lower level than his stomach.
• The mouth and throat are cleared off from the mucus, food, etc. The operator closes the patient’s nostrils to prevent the leakage of air, takes a deep breath, and blows into the patient’s mouth until the chest is seen to rise. The operator then takes his mouth quickly away to allow passive exhalation. The process is repeated about 10-15 times a minute in adults.
• A normal person can usually survive as much as 20 mm Hg of continuous positive pressure in the lungs, but exposure to more than 30 mm Hg for a few minutes may cause death.

 

Breathing And Exchange Of Gases Assertion Reasoning Questions

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: The pneumatic center controls the rate of respiration.

Reason: Primarily, it controls the switch-off point of inspiration.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2. Assertion: Asthmatic patients use bronchodilator drugs as well as inhalers for symptomatic relief.

Reason: Asthma is characterized by the spasm of smooth muscles in the wall of bronchioles due to allergen.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3. Assertion: A major part of carbon dioxide is transported in the form of sodium bicarbonate.

Reason: 0.3 ml of carbon dioxide is transported per 100 mL of blood in dissolved shite in the plasma of blood.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 4. Assertion: In cockroaches, inspiration is a passive process.

Reason: The expansion of the abdominal cavity allows the space for expansion of the tracheal trunk. As a result, air enters through spiracles.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 5. Assertion: The diffusion of carbon dioxide is 20 times faster than oxygen.

Reason: It is due to the difference in partial pressure as well as the solubility of diffusing gases.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 6. Assertion: Oxidation of nutrients releases bond energy.

Reason: Oxidation of nutrients is done by using molecular oxygen.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 7. Assertion: Aerobic respiration involves the exchange of respiratory gases twice.

Reason: Exchange occurs from lung to heart and then heart to lung.

Answer: 3. If Assertion is true, but Reason is false.

Question 8. Assertion: Respiratory gas exchange occurs through osmosis.

Reason: Respiratory gas goes from the lower partial pressure region to the region of higher partial pressure.

Answer: 4. If both Assertion and Reason are false.

Question 9. Assertion: The first step of gas exchange occurs through the body surface in some animals.

Reason: The body surface or membrane of amphibia is thick in nature.

Answer: 3. If Assertion is true, but Reason is false.

Question 10. Assertion: Abdominal muscle is related to respiration in animals.

Reason: Relaxation of abdominal muscles draws in air.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Body Fluids And Circulation Introduction

A circulatory system consists of a heart (pumping organ), arteries and arterioles, venules (bringing blood back to the heart), and capillaries (connecting arterioles and venules). Circulation is of two types:

1. Single circulation: When blood flows through the heart only once during its course of circulation, for example., invertebrates and fish.
2. Double circulation: When blood flows through the heart twice during its course of circulation, for example, amphibians, reptiles, birds, and mammals.

In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin, and the right atrium gets deoxygenated blood from other body parts. However, they get mixed up in a single ventricle which pumps out mixed blood (incomplete double circulation).

In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria, respectively, passes onto the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., the two separate circulatory pathways are present in these organisms; hence, these have complete double circulation.

Depending upon the medium of transportation, the circulatory system is divided into a water vascular system and a blood vascular system.

Water vascular system: Water is the medium of transportation, for example, in sponges (water canal system), in Hydra (gastrovas- cular system), and in starfish (ambulacral system).

Blood vascular system:  Blood is the medium of transportation. It is of two types:

1. Open circulatory system: Blood flows through vessels that open into tissue spaces or membrane-lined sinuses, for example, arthropods, non-cephalopod mollusks, and tunicates.
2.  Closed circulatory system: Blood flows through the heart, vessels, and finely branched capillaries, without coming in direct contact with body tissues or body cavities. It was discovered by William Harvey. A comparison between open and closed circulatory systems is mentioned.

Read and Learn More NEET Biology Notes

Comparison Between Open And Closed Circulatory Systems:

Types Of Heart

1. On the basis of type of blood:
   • Venous heart: Receives deoxygenated blood only. For example, fish (as the heart receives deoxygenated blood from all over the body except gills).
   • Arteriovenous heart: Receives deoxygenated blood from the body and oxygenated blood from the lungs/gills. For example, amphibians, reptiles, birds, mammals, etc.
2. On the basis of the origin of impulse:
   • Myogenic heart: The impulse for a heartbeat originates in heart muscles (pacemakers), for example. chordates and mollusks (octopus).
   • Neurogenic heart: The impulse for a heartbeat is brought about by nerves, for example, in most invertebrates.
3. Based on structure, the hearts are of the following types:
4. Tubular hearts: For example, insects (cockroach: 13-chambered heart)
5. Pulsating vessels: Annelids, holothurians, and Amphi- oxus.
6. Chambered hearts: Hearts of vertebrates and mollusks.
7. Ampullar accessory hearts: Branchial hearts of cephalopods (for example, octopus), insects, heart bulbils of Amphioxus, and lymph hearts of frogs.

Structure Of The Human Heart

The human heart is situated in the pericardial cavity, in mediastinal space. Its covering is called pericardium which is double-walled—outer parietal pericardium and inner visceral pericardium.

• In between these two is the present pericardial cavity, filled with pericardial fluid.
• The wall of the heart is made up of three layers. The outermost is the epicardium, the middle myocardium (consisting of cardiac muscles), and the innermost lining is the endocardium.
• The cardiac muscles are syncytial in functional terms because of intercalated discs. These are function points between two muscle spindles with an electrical resistance of 1/400th of any membrane.
• The human heart is four-chambered.
• Sinus venous and conus arteriosus are not present in the human heart.
• The auricle or atrium is divided by an interatrial septum into right and left auricles.
• On this septum, a depression fossa ovalis is present which is a remnant of embryonic foramen ovale, an aperture present between right and left auricles.
• Three large veins pour blood into the right auricle by separate pores.
• Eustachian valve guards the opening of post caval.
• A coronary or Thebesian valve is present at the opening of the coronary sinus.
• Two pulmonary veins bring oxygenated blood into the left auricle.

• The internal structure of the human heart
• The opening of the pulmonary vein is without any valve as its opening is oblique, which prevents the backflow of blood.
• The ventricle is also divided by an interventricular septum.
• Auricles open into ventricles by separate atrioventricular apertures.
• On the right side is present a tricuspid valve and on the left side is present a bicuspid or mitral valve.
• The cuspid valves are connected below to the walls of ventricles by chordae tendinae which terminate on papillary muscles.
• The right ventricle receives the deoxygenated blood from the right atrium and pumps it into the pulmonary trunk arising from it.
• The pulmonary trunk bifurcates to form right and left pulmonary arteries which supply deoxygenated blood to the lungs of the respective side.
• The left ventricle receives oxygenated blood from the left atrium and sends it into the ascending aorta which puts blood into the coronary arteries and the systemic circulation of the body.
• The origin of the pulmonary trunk and ascending aorta is guarded by a set of three semilunar valves in each.
• This internal structure of the human heart is to prevent the backflow of blood.
• The inner surface of the ventricle has a number of irregular muscular ridges called trabeculae camae or colum- nae camae.

• Large protrusions—papillary muscles—are also present. These are inserted at a ventricular wall at one end and continued at the other end with collagenous cords called chordae tendinae.
• These prevent the pushing of flaps into the atrium during ventricular contraction.
• In the right ventricle, a muscular band connects the interventricular septum with the parietal wall of the ventricle. It is called the moderator band which is a component of larger septomarginal trabecula.
• The left ventricle has the thickest muscles because it pumps blood to the whole body.

Working Of The Human Heart

The heart shows alternate contraction and dilation of its chambers. Contraction is known as systole, while dilation is called diastole.

• The heart is said to be in the state of joint diastole when all its four chambers are in a relaxed state. In this state, the atria receive blood. Atrial systole forces the blood from atria into the ventricles; the left ventricle receives oxygenated blood whereas the right ventricle receives deoxygenated blood.
• As the ventricular systole starts, the cuspid valves close, and the blood from the ventricles is forced into the great arteries.
• Oxygenated blood from the left ventricle enters the aortic arch and is carried to all parts of the body except the lungs.
• Deoxygenated blood from the right ventricle reaches the pulmonary arch and is carried to the lungs.
• As the ventricles begin to relax, the semilunar valves close and prevent the return of blood from the two arches.
• The action potential of heart muscles differs from that of skeletal muscles.
• One heartbeat completes in 0.8 s (atrial systole = 0.1 s, atrial diastole = 0.7 s, ventricular systole = 0.3 s, and ventricular diastole = 0.5 s). Thus, there are 72 heartbeats per minute on average when a person is performing normal work.

Cardiac Cycle

During a heartbeat, there is contraction and relaxation of auricles and ventricles in a specific sequence. The contraction phase is known as systole, while the relaxation phase is known as diastole. A series of events that occur during a heartbeat is known as the cardiac cycle.

• During the joint diastole phase, the blood flows into the right auricle from the superior vena cava and inferior vena cava.
• The blood also flows from the auricles to their respective ventricles through the atrioventricular valves.
• There is no flow of blood from the ventricles to the aorta and its main arteries as the semilunar valves remain closed in this phase.
• At the end of joint diastole, the auricles contract or they enter into the systolic phase. In this phase, it now forces most of its blood into the ventricles which is still in the diastolic phase.
• During auricular systole, the blood cannot pass back into the superior and inferior vena cava because they are compressed by the atrial contraction. Thus, the atrium acts as a main vessel to collect and pump venous blood into the ventricles. Thus, at the end of atrial systole, the atria get empty.
• After the atrial systole is over, the atrial muscles relax and the blood enters into the atrial diastolic phase.
• During atrial diastole, it again gets filled up with venous blood coming from the superior and inferior vena cava.
• Along with the atrial diastole, the ventricular systole starts. This results in an increased pressure of blood in the ventricle and it rises more than the pressure of blood in the atria. Soon the atrioventricular valves are closed, and thus, the backflow of blood is prevented. This closure of AV valves at the beginning of ventricular systole produces a sound “lub” and is known as the first heart sound. Initially, when the ventricle starts contracting, the pressure of blood within it is lower than the pressure of blood within the aorta and so the semilunar valves do not open. Therefore, the ventricles now contract as closed chambers. As the ventricular systole progresses, the pressure of blood within the ventricles increases more than that of the aorta. As a result, the semilunar valves now open, and blood flows (with a speed) into the aorta and its branches.
• The period between the closure of the AV valve and the opening of the semilunar valve is called isovolumetric systole/contraction.
• The backflow of blood in the atria is prevented as the AV valves remain closed.
• Now at the end of ventricular systole, ventricular diastole starts.
• As the atria are still in diastole, all four chambers are now in diastole. This is known as joint diastole.
• In the ventricular diastolic phase, the pressure of blood in the ventricles falls below the pressure of blood in the aorta. So the semilunar valves get closed to prevent the backflow of blood from the aorta to the ventricles. This closure of semilunar valves at the beginning of ventricular diastole produces a sound “dupp” and is known as the second heart sound.
• After the closure of the semilunar valves, the ventricles become closed chambers again. Also, as the ventricular pressure is more than the atrial pressure, the AV valves remain closed. However, as the ventricular diastole continues, the pressure of blood in the ventricles falls below the pressure of blood in the atria. At this point, the AV valves open and blood starts flowing again from the relaxed atria to the relaxed ventricles.
• The duration between the closure of semilunar valves and the opening of AV valves is called isovolumetric diastole/relaxation.
• When the joint diastole is over, the atrial systole starts and the blood is pumped into the ventricles.

The ventricular filling of the blood can be divided into three phases:

• First rapid filling: With the beginning of ventricular diastole, intraventricular pressure declines, and AV valves open. Due to this, blood already stored in the atria rushes into the ventricles rapidly. This also creates a sound, the third sound of heart.
• Diastasis/slow filling: After the first rapid rushing of blood, the blood keeps on entering the ventricles, though at a slow rate (diastasis).
• Second rapid filling: This occurs with the atrial systole which again causes rapid squeezing of blood into the ventricles. This creates the foolish sound of the heart.

Points To Remember

Cardiac output rises during exercise. During severe exercise, it may rise to even 20 L/min, about four to fivefold the normal resting value of about 5 L/min. The rise in the cardiac output helps the body during exercise by enhancing manifold the supply of nutrients and oxygen to the contracting muscles.

• HDLs (high-density lipoproteins): High levels of HDL in the blood may help to reduce our risk of coronary heart disease. HDLs are good lipoproteins. They contain 40-45% proteins, 5-10% triglycerides, 30% phospholipids, and 20% cholesterol. They remove excess cholesterol from body cells and the blood and transport it to the liver for elimination. Because HDLs prevent the accumulation of cholesterol in the blood, a high HDL level is associated with a decreased risk of coronary artery disease.
• LDLs (low-density lipoproteins): LDLs contain 25% protein, 5% triglycerides, 20% phospholipids, and 50% cholesterol. When present in excessive numbers, LDLs deposit cholesterol in and around smooth muscle fibers in arteries, forming “fatty plaques” that increase the risk of coronary artery disease. For this reason, the cholesterol in LDL is known as “bad” cholesterol.

Heartbeat

Physiological Properties of Cardiac (Heart) Muscle

• Excitability and contractility.
• It obeys the all-or-none principle.
• It has a longer refractory period, therefore it never develops fatigue.
• It does not show summation or tetanus.
• It shows conductivity and rhythmicity.

Origin Of Heartbeat

• The mammalian heart is a myogenic heart, i.e., the heartbeat originates from the muscles. However, it is regulated by nerves.
• In the right atrium, near the region where the superior vena cava opens, a specialized group of muscle fibers called sinus-auricular node (SA-node) is present from where the heartbeat originates.
• It is also called a pacemaker and is richly supplied with blood capillaries. A wave of contraction (systole) originates from it and spreads over to the whole heart.

Conduction Of Heartbeat

• At the junction of the right atrium and right ventricle, a tissue called auriculo-ventricular node (AV-node) is present that picks up the wave of contraction propagated by the SA-node.
  This is also known as the bundle of His.
• Its branches spread over the ventricles forming the Purkinje system.
• The wave of contraction spreads over the ventricle through the AV node and its Purkinje system.

Regulation Of Heartbeat

• The normal activities of the heart are regulated intrinsically, i.e., auto-regulated by specialized muscles (nodal tissue), hence the heart is called myogenic.
• A special neural center in the medulla oblongata can moderate cardiac function through the autonomic nervous system (ANS).
• Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heartbeat, the strength of ventricular contraction, and thereby the cardiac output.
• On the other hand, parasympathetic neural signals (another component of ANS) decrease the rate of heartbeat, speed of conduction of action potential, and thereby the cardiac output. This happens because these nerves release chemicals (hormones) when stimulated. Adrenal medullary hormones can also increase the cardiac output.
• High levels of potassium and sodium ions decrease heart rate and strength of contraction.
• An excess of calcium ions increases heart rate.
• Increased body temperature during fever increases heart rate.
• Strong emotions such as fear, anger, and anxiety increase heart rate, resulting in increased blood pressure.
• Mental states such as depression and grief decrease heart rate.
• The heartbeat is somewhat faster in females.
• The heartbeat is fastest at birth, moderately fast in youth, average in adulthood, and above average in old age.

Heart Sounds: Heart sounds are caused due to the sudden closure of the heart valves. There are mainly two sounds:

First sound: Occurs at the onset of ventricular systole and is caused due to the sudden closure of AV valves and the ejection of blood from the ventricles. It is dull and pronounced as L-U-B.

Second sound: Occurs at the onset of ventricular diastole and is caused by the sudden closure of the semilunar va ves o e aorta and pulmonary artery- It is short and sharp like the word
D-U-B.

The sequence of both these sounds is like first sound → second sound → pause → first sound → second sound → pause and so on. If damage occurs, as in rheumatic fever, blood may leak out through the valves, and a characteristic sound murmur is produced.

Pulse Rate: The blood is pumped from the ventricles of the heart into the aorta to be distributed to all the body parts. This happens during the ventricular systole and is repeated every 0.8 seconds.

• The blood from the aorta then goes to other arteries of the body. This causes a rhythmic contraction of the aorta and its main arteries and is felt as regular jerks or pulse in them.
• It can be felt in the regions where arteries are present superficially such as the wrist, neck, and temples.
• The pulse rate is, therefore, the same as that of the heartbeat rate.

Blood Vessels And Course Of Blood CircuLation

Arterioles: Arterioles are small arteries that deliver blood to capillaries. Arterioles also have smooth muscles on their walls.

Contraction and relaxation of these muscles alter the diameter of arterioles and thereby, respectively, reduce and increase the blood flow through them.

Capillaries: Capillaries were discovered by Malpighi in 1661.

• Capillaries are the smallest blood vessels in the body.
• A capillary has no muscular wall.
• Its wall is made of a single layer of flat endothelial cells and is consequently permeable to water and small solutes, but not to proteins and other macromolecules.
• The diameter of the capillary lumen is from 7.5 pm to 75 pm. Only about 5-7% of the total blood volume is contained in the capillaries.

Venules: Venules are small vessels that continue from capillaries and merge to form veins. They drain blood from capillaries into veins.

Veins: Veins have less elastic tissues and smooth muscles than arteries.

• One major difference between an artery and a vein is that a vein has a thin muscular wall.
• Veins contain valves to prevent the backflow of blood.
• Valves are necessary in veins but not in arteries because the pressure in veins is too low to push the blood.
• Weak valves can lead to varicose veins or hemorrhoids.
• All veins carry deoxygenated blood except pulmonary veins.
• Pulmonary veins carry oxygenated blood from the lungs back to the heart.
• Blood vessels that carry blood from the lungs to the heart are called pulmonary veins.
• The wall of veins is collapsible (non-collapsible in arteries).
• The lumen of veins is wider and narrower than the arteries.

• Vasa vasorum: A network of small blood vessels that supply blood to large blood vessels.

The course of Blood Circulation: There are two types of blood circulation pathways in the human body:

Pulmonary circulation: The circulation starts with the pumping of deoxygenated blood by the right ventricle which is carried to the lungs where it is oxygenated and returned to the left atrium.

Systemic circulation: In systemic circulation, the oxygenated blood is pumped by the left ventricle to the aorta. The oxygenated blood is carried to all the body tissues and the deoxygenated blood from there is collected by the veins and returned to the right atrium.

Main Arteries and Veins in the Human Body

Our human body has a network of different types of arteries and veins running across all organs, providing nourishment to them through oxygenated blood, and bringing back the deoxygenated blood to the heart. Table 18.2 describes all sorts of arteries and veins present in the human body.

Main Arteries And Veins Present In the Human Body:

Coronary Circulation: Two coronary arteries—right and left—branch from the ascending aorta. The left coronary artery passes inferiorly to the left auricle and divides into the anterior
interventricular and circumflex branches.

• The anterior interventricular branch or left anterior descending (LAD) artery is in the anterior interventricular sulcus and supplies oxygenated blood to the walls of both ventricles and the interventricular septum.
• The circumflex branch lies in the coronary sulcus and distributes oxygenated blood to the walls of the left ventricle and left atrium.
• The right coronary artery supplies small branches (atrial branches) to the right atrium.
• It continues inferiorly to the right auricle and divides into the posterior interventricular and marginal branches.
• The posterior interventricular branch follows the posterior interventricular sulcus and supplies the walls of the two ventricles and the interventricular septum with oxygenated blood.
• The marginal branch in the coronary sulcus transports oxygenated blood to the myocardium of the right ventricle.
• Most parts of the body receive branches from more than one artery, and where two or more arteries supply the same region, they usually connect.
  The connections called anastomoses provide alternate routes for blood to reach a particular organ or tissue.
• The myocardium contains many anastomoses, connecting branches of one coronary artery or extending between branches of different coronary arteries.
• In a resting person, heart muscles can remain alive if they receive as little as 10-15% of their normal blood supply, but the person may have little ability to engage in activities.

Coronary Veins: As blood passes through the coronary circulation, it delivers oxygen and nutrients and collects carbon dioxide and wastes.

• It then drains into a large vascular sinus on the posterior surface of the heart called the coronary sinus, which empties into the right atrium.
• A vascular sinus is a venous space with a thin wall that has no smooth muscle to alter its diameter.
• The principal tributaries carrying blood into the coronary sinus are the great cardiac vein, which drains the anterior aspect of the heart, and the middle cardiac vein, which drains the posterior aspect of the heart.

Blood Pressure

If a person is persistent above 140 mm Hg systolic and above 90 mm Hg diastolic blood pressure, he/she is said to have high blood pressure or hypertension.

• There are a number of factors responsible for hypertension.
• Over-eating and obesity are the most important factors for hypertension.
• Physical and emotional stresses such as fear, worry, anxiety, sorrow, etc., also cause hypertension.
• People living in big cities usually suffer from hypertension. Smoking is also a cause.
• Physical and mental rest are essential for a patient suffering from hypertension.
• Sometimes, cholesterol starts depositing on the walls of blood vessels when there are high levels of cholesterol in the blood. This causes the arteries to lose their elasticity and they become stiff. This is known as arteriosclerosis or hardening of arteries and as a result, the blood pressure rises. This is one of the main causes of heart attack.
• Frank-Starling law: Two great physiologists Frank and Starling said that the greater the heart muscle is stretched during the filling phase, the greater will be the quantity of blood pumped into the aorta.

Electrocardiogram (ECG): Electric changes in the cardiac chambers follow a specific sequence. These changes can be recorded with the help of an apparatus—electrocardiograph.

• The record is called ECG.
• It is represented as PQRST, where P stands for depolarization of atria, QRS depolarization of ventricles, and T repolarization of ventricles.
• Defects in cardiac function or structure are recorded in the ECG.
• For the purpose of recording, metal electrodes or leads are attached to each arm and leg with the help of straps after cleaning and putting a special jelly, which improves electrical conduction.

• An additional electrode is placed on the chest with the help of a rubber suction cup.
• Then the electrocardiograph is switched on.
• The electrical current of the heart is detected and amplified by the machine and is transmitted to
  the recording pen that draws a wavy line, called the deflection waves (electrocardiogram).
• A normal electrocardiogram is composed of a P wave, a QRS complex, and a T wave.
• The QRS complex has three separate Q, R, and S waves.
• The P wave is a small upward wave that indicates the depolarization of the atria or the spread of impulse from the sinus node throughout the atria.
• The second wave, i.e., the QRS complex, begins after a fraction of a second of the P wave.
• It begins as a small downward deflection (Q) and continues as a large upright (R) and triangular wave, ending as a downward wave (S) at the base. This is the expression of the ventricular depolarization.
• The potential generated by the recovery of the ventricle from the depolarization state is called the repolarization wave.
• In electrocardiography, the PQ interval (also called PR interval) is the time taken by the impulse to travel through the atria, AV node, and the rest of the conducting tissues.
• During rheumatic fever and in arteriosclerotic heart disease (i.e., the formation of plaques and calcification), the PQ interval lengthens. This is due to the inflammation of the atria and atrioventricular node.
• The normal PR interval lasts for 0.16s.
• The enlarged Q and R waves are an indication of myocardial infarction.
• The ST interval is the representation of time between the end of the spread of impulse through ventricles and its repolarization.
• The ST segment is elevated in acute myocardial infarction and depressed in a condition when the heart muscles receive insufficient oxygen.
• The ventricular repolarization is represented as a T wave.
• When the heart muscles receive insufficient oxygen, then the T wave is flattened.

Pacemaker

During the pumping action of the heart, the atria and ventricles contract rhythmically. The impulse of this wave of contraction begins every time from the SA node (sinus node) present in the right atrium. Thus, it can be said that the SA node controls the heartbeat, and hence, it is the natural pacemaker of the heart.

• The pacemaker is the rhythmic center, which establishes a pace of activity.
• Sometimes, a component of the impulse conduction system is disrupted, causing irregularity in the heart rhythm, such as failure to receive the atrial impulse by the ventricle or completely independent contraction of the atria and ventricles. Such types of patients are provided with an artificial electronic device, which regularly sends a small amount of electrical charge to maintain the rhythmicity of the heart. This device is known as an artificial pacemaker, which is implanted subcutaneously in the upper thoracic region having a connection with the heart.
• In patients having the symptoms of ventricular escape (Stokes, Adams syndrome), in which the atrial impulse suddenly fails to be transmitted to the ventricle, which may last for a few seconds to a few hours even, the artificial pacemaker is connected to the right ventricle for controlling its rhythm.
• The artificial pacemaker consists of a pulse-generator containing a cell (solid state lithium cell) to produce electrical impulse, the lead in the form of a wire, which transmits the impulse, and an electrode, which is connected to the portion of the heart where the impulse is to be transmitted.

Portal Systems

Hepatic portal system: Inferior mesenteric, superior mesenteric, duodenal, and lienogastric veins join to form a hepatic portal vein. It pours blood from the digestive system into the liver. This blood is collected by hepatic veins and poured into the canal to be returned to the heart.

• Renal portal system: In fishes and amphibians, the renal portal system is also found which is reduced in reptiles and birds and is absent in mammals.
• Hypophyseal portal system: A hypophyseal portal vein collects blood from the hypothalamus and enters the anterior lobe of the pituitary.

Lymphatic System

The lymphatic system comprises lymph, lymphatic capillaries, lymphatic vessels, lymphatic ducts, and lymphatic nodes.

• Lymphatic capillaries: They lie close to the blood capillaries but end blindly. They have extremely thin walls. They are composed of a single layer of endothelial cells.
• Lymphatic vessels: The lymphatic capillaries unite to form larger lymphatic vessels. They are composed of an outer coat of fibrous tissue, a middle coat of muscular tissue, and an inner lining of endothelial cells. The lymphatic vessels have numerous valves. The lymph vessels of intestinal regions absorb the digested fats. They are milky in appearance and are called lacteals.
• Thoracic duct: The lymphatic vessel of the left side begins at the cisterna chyli, present at the level of (anterior to) the first and second lumbar vertebrae. It discharges its lymph into the left subclavian vein.
• Right lymphatic duct: The lymphatic vessels of the right side of the thorax, head, and neck unite to form the right lymphatic duct. It discharges its lymph into the right subclavian vein.
• Lymphatic nodes: The lymphatic vessels bear lymph nodes at intervals and are abundant in the neck, armpit, and groin. The lymph is filtered through lymph nodes which contain phagocytic white blood corpuscles and macrophages which eat harmful microorganisms and foreign particles from the lymph. Lymph nodes also add lymphocytes and antibodies.
• Lymph movement: The lymph flows slowly and moves from lymphatic vessels to lymphatic ducts to the venous system. Blocking of lymph flow causes edema.
• Lymphoid organs: The organs which secrete lymph are called lymphoid organs. Besides the lymph nodes, tonsils, thymus gland, Peyer’s patches, liver, and spleen are the other lymphoid organs that secrete lymph.

Functions of Lymph: Lymph acts as a “middle man” that transports various proteins, hormones, etc., to the body cells brings carbon dioxide and other metabolic wastes from the body cells, and finally pours the same into the venous system.

• Lymph nodes produce lymphocytes. Lymph takes lymphocytes and antibodies from the lymph nodes to the blood.
• It absorbs and transports fat and fat-soluble vitamins from the intestine. Lymph capillaries present in the intestinal villi are called lacteals which are associated with the absorption and transportation of fat and fat-soluble vitamins.
• It brings plasma protein macromolecules synthesized in the liver cells and hormones produced, in the endocrine glands to the blood. These molecules cannot pass into the narrow blood capillaries but can diffuse into the lymphatic capillaries.
• The lymph maintains the blood volume. As soon as the blood volume reduces in the blood vascular system, the lymph rushes from the lymphatic system to the blood vascular system.

Diseases Of Heart

High Blood Pressure (Hypertension): Hypertension is the term for blood pressure that is higher than normal (120/80).

• In this measurement, 120 mm Hg (millimeters of mercury pressure) is the systolic or pumping pressure and 80 mm Hg is the diastolic or resting pressure.
• If the repeated checks of blood pressure of an individual are about 140/90 (140 over 90) or higher, it shows hypertension.
• High blood pressure leads to heart diseases and also affects vital organs such as the brain and kidneys.

Coronary Artery Disease: Coronary artery disease (CAD), often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle,

• It is caused by the deposits of calcium, fat, cholesterol, and fibrous tissues, which make the lumen of arteries narrower.

Angina: Angina is also called angina pectoris.

• A symptom of acute chest pain appears when enough oxygen does not reach the heart muscles.
• Angina can occur in men and women of any age, but it is more common among the middle-aged and elderly.
• It occurs due to conditions that affect the blood flow.

Heart Failure: Heart failure means the state of the heart when it is not pumping blood effectively enough to meet the needs of the body.

• It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease,
• Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).

Body Fluids And Circulation Points To Remember

1. The lowest level of glucose is in the hepatic vein.
2. The highest levels of amino acids are present in the hepatic vein.
3. The highest level of urea is in the hepatic vein and the lowest in the renal vein.
4. The largest vein in the human body is the inferior vena cava.
5. The largest artery in the human body is the aorta.
6. The smallest blood vessel in the body is the blood capillary.
7. A giraffe’s blood pressure may be the highest of all animals because it has to pump blood to the head through a long neck.
8. One species of Antarctic fish is the only fish known to have white blood. It has no red pigment in its blood.
9. Frog has two pairs of lymph hearts to pump the lymph back into veins.
10. Thrombopenia: Decrease in blood platelet count.
11. Erythropoietin: Hormone secreted by the juxta-glo- merular cells of the kidneys.
12. Circulatory shunts in the fetus: The fetus bypasses the pulmonary system through two shunts—foramen ovale (opening in the interatrial septum) and ductus arteriosus (the connection between dorsal aorta and pulmonary arch). Shunts are sealed after birth.
13. Lungfish have three-chambered hearts—two auricles and one ventricle.
14. Crocodiles, alligators, and gavialis have four-chambered hearts—two auricles and two ventricles.
15. The heart of a fish is called a venous heart because it receives and pumps deoxygenated blood.
16. In the human heart, auricles are called atria (singular atrium).
17. Nereis and Amphioxus do not have hearts. The heart of the prawn contains oxygenated blood.
18. There is single blood circulation in fish hearts. Hearts of amphibians, reptiles, birds, and mammals have double blood circulation. Foramen of Panizzae is
    present in between two systemic arches (they arise from the heart) of the heart of lizards and crocodiles.
19. An average human heart is about 12 cm. The average weight of a male human heart is 300 g, and that of a . female heart is 250 g. The opening of the coronary sinus into the right atrium is guarded by the Thebesian valve. The opening of the inferior vena cava into the fight atrium has an Eustachian valve. Fossa
    this is a depression on the interatrial septum.
20. Coronary angiography: When the contrast medium dye is injected into coronary arteries (arteries of the heart) and pictures are taken, it is known as coronary angiography.
    ess calcium ions cause an increased heartbeat.
21. RBCs fail to mature if there is a deficiency of vitamin Bj2 and folic acid.
22. Papillary muscles are found in the heart of mammals.
23. Keber’s organs or pericardial glands discharge excretory products into the pericardial cavity in the freshwater mussel.
24. “Blue baby” is the name given to an abnormal human baby who has a hole in the auricular or ventricular septum and hence the oxygenated and deoxygenated blood gets mixed in the heart.
25. An insect larva has red blood. The larva of the genus Chi-ronomusis is called a “blood worm.”
26. The red color of his larva is due to hemoglobin, which has the power to attract and store oxygen and give it off to the tissues as they require it. Such larvae are able to live in burrows constructed by them in the mud.
27. Vasa vasorum are small blood vessels that supply blood to the larger blood vessels.
28. A blue whale has the largest heart.
29. Cardiomegaly is heart enlargement.
30. Angiology: The study of blood vascular and lymphatic systems.
31. Venoms of bees and cobra contain Jecithinase which when injected into the bloodstream by sting or bite breaks down lecithins and produces iysolechhins which in turn cause rupturing of the RBC cell membrane (cell lysis).
32. Marey’s law: Heart rate is inversely related to systemic blood pressure.

 

Body Fluids And Circulation Assertion-Reasoning Questions

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: The cardiac impulse that originates from the SA node in the mammalian heart cannot spread directly from the atria to the ventricles.

Reasoning: In the mammalian heart, there is no continuity between cardiac muscle fibers of atria and those of ventricles except AV bundles.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2. Assertion: The first phase of ventricular filling is rapid and causes the third sound of the heart.

Reasoning: It is because of auricular systole.

Answer: 3. If Assertion is true, but Reason is false.

Question 3. Assertion: “Dub” is a long and sharp sound.

Reasoning: It is caused by the closing of AV valves.

Answer: 4. If both Assertion and Reason are false.

Question 4. Assertion: The portal system consists of veins that start from capillaries and end into capillaries.

Reasoning: All vertebrates have a hepatic portal system.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 5. Assertion: Arterioles possess smooth muscles on their walls.

Reasoning: These smooth muscles help in regulating blood volume flowing through a tissue or organ.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 6. Assertion: The open circulatory system is more efficient than the closed circulatory system.

Reasoning: The blood flows far more rapidly in the open circulatory system than in the closed one.

Answer: 4. If both Assertion and Reason are false.

Question 7. Assertion: The heart of a fish contains only deoxygenated blood.

Reasoning: Oxygenated blood does not return back to the heart in fishes.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 8. Assertion: The cardiac impulse is said to be myogenic.

Reasoning: The rate of formation and conduction of cardiac impulses can be changed by the action of nerves.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 9. Assertion: The left ventricle of the heart has a thinner wall than that of the right ventricle.

Reasoning: The left ventricle needs to pump blood to nearby lungs only,

Answer: 4. If both Assertion and Reason are false.

Question 10. Assertion: The AV bundle is essential for the conduction of cardiac impulses.

Reasoning: There is no continuity between the cardiac muscle fibers of the auricles and those of the ventricles.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 11. Assertion: The AV node is also called the pacemaker of the heart.

Reasoning: It is because of the fact that the AV node determines the rate of heartbeat.

Answer: 4. If both Assertion and Reason are false.

Question 12. Assertion: There is no mixing of oxygenated and deoxygenated blood in the human heart.

Reasoning: Valves are present in the heart, which allow the movement of blood in one direction only.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 13. Assertion: Hypotension is observed in arteriosclerotic patients.

Reasoning: In the condition of arteriosclerosis, arteries gain their elasticity and get stiffened.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 14. Assertion: ECG is of immense diagnostic value in cardiac diseases.

Reasoning: Defects in cardiac functions can be reflected in changes in the pattern of electrical
potential recorded in the ECG.

Answer: 4. If both Assertion and Reason are false.

Question 15. Assertion: An artificial pacemaker can replace the SA node of the heart.

Reasoning: This is because an artificial pacemaker is capable of stimulating the heart electrically to maintain its beats.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Molecular Basis Of Inheritance

DNA

• DNA (deoxyribose nucleic acid) is a long polymer of deoxyribonucleotides.
• The length of DNA is usually defined as the number of nucleotides (or a pair of nucleotide referred to as base pairs or bp) present in it.
• This is the characteristic of an organism.
• A few examples are sited in table.

Chemical Structure of DNA Polynucleotide

• The basic unit of DNA is a nucleotide which has three components: a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group.
• There are two types of nitrogenous bases: Purines (adenine and guanine) and pyrimidines (cytosine and thymine).
• Cytosine is common for both DNA and RNA (ribose nucleic acid), and thymine is present in DNA only.
• Uracil is present in RNA at the place of thymine.
• A nitrogenous base is linked to the pentose sugar through an N-glycosidic linkage to form a nucleoside such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine, and deoxythymidine.
• When a phosphate group is linked to 5′-OH of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending on the type of sugar present) is formed.
• Two nucleotides are linked through 3′-5′ phosphodiester linkages to form a dinucleotide.
• More nucleotides can be joined in such a manner to form a polynucleotide chain.
• The polymer, thus, formed has a free phosphate moiety at the 5′-end of sugar, which is referred to as the 5′-end of polynucleotide chain.

Read and Learn More NEET Biology Notes

• Similarly, at the other end of the polymer, the sugar has a free 3′-OH group which is referred to as the 3′-end of polynucleotide chain.
• Backbone in a polynucleotide chain is formed due to sugar and phosphates (phosphodiester bond). Nitrogenous bases linked to the sugar moiety project from the backbone.
• In RNA, every nucleotide residue has an additional -OH group present at the 2′-position in the ribose.
• Also, in RNA, uracil is found at the place of thymine (S-methyluracil, another chemical name for thymine). In 1953, James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed double helix model for the structure of DNA.
• One of the hallmarks of their proposition was “base pairing between the two strands of polynucleotide chain.” However, this proposition was also based on the observation of Erwin Chargaff (that for a double-stranded DNA, the ratios between adenine and thymine and that between guanine and cytosine are constant and equal.
• Chargaff (1950) made observations on the bases and other contents of DNA. These observations or generalizations are known as Chargaff’s rules.
  • Purine and pyrimidine base pairs are present in equal amounts, i.e., adenine (A) + guanine (G) = thymine (T) + cytosine (C).
  • The molar amount of purine (adenine) is always equal to the molar amount of pyrimidine (thy- mine). Similarly, guanine is equalled by cytosine.
  • Deoxyribose (sugar) and phosphate occur in equimolar proportions.
  • The ratio (A+T)/(G+C) is constant for a species. It is called base ratio. It is 1.52 for humans and 0.93 for E. coli.
• Base pairing is a very unique property of polynucleotide chains.
• Both strands of DNA are said to be complementary to each other. Therefore, if the sequence of bases in one strand is known, then the sequence in the other strand can be predicted.
• Thus, if one DNA strand has A, the other will have T; and if one has G, the other will have C.
• Therefore, if the base sequence of one strand is CAT TAG GAC, the base sequence of the other strand will be GTA ATC CTG.
• Hence, the two polynucleotide strands are complementary to one another.
• Also, if each strand from a DNA or parental DNA acts as a template for the synthesis of a new strand, the two double-stranded DNA or daughter DNA produced will be identical to the parental DNA molecule.

Salient Features of DNA Double Helix

• DNA double helix is made of two polynucleotide chains, where the backbone is constituted by sugar- phosphate and the bases project inside.
• The two chains have anti-parallel polarity. It means, if one chain has polarity 5’P→ 3’OH, the other has 3’OH→ 5’P.
• The bases in the two strands are paired through hydrogen bonds (H-bonds), forming base pairs. Ad- enine forms two hydrogen bonds with thymine from the opposite strand and vice versa. Guanine is bonded with cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.
• The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability to the helical structure.
• The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently, the distance between the base pairs in a helix is approximately equal to 0.34 nm. It is because of specific base pairing with a purine lying opposite to pyrimidine which makes two chains 2 nm thick.

Packaging Of DNA Helix

• The average distance between two adjacent base pairs is 0.34 nm (0.34 x 10m or 3.4 A).
• The length of DNA for a human diploid cell is 6.6 x 10° bp x 0.34 x 10 m/bp = 2.2 m.
• This length is far greater than the dimension of a typical nucleus (approximately 10 m).
• The number of base pairs in E. coli is 4.6 × 10°. Total length is 1.36 mm.
• The long-sized DNA is accommodated in a small area (about 1 um in E. coli) only through packing or com- paction.
• DNA is acidic due to the presence of a large number of phosphate groups.
• Compaction occurs by folding and attachment of DNA with basic proteins-polyamine in prokaryotes and histone in eukaryotes.

DNA Packaging in Prokaryotes

• DNA is found in cytoplasm in supercoiled state.
• The coils are maintained by non-histone basic proteins such as polyamines.
• RNA may also be involved. This compact structure of DNA is called nucleoid or genophore.

DNA Packaging in Eukaryotes

• DNA packaging in eukaryotes is carried out with the help of lysine- and arginine-rich basic proteins called histones.
• The unit of compaction is called nucleosome.

• There are five types of histone proteins: H1, H2A, H2B, H3, and H4.
• Four of these occur in pairs to produce histone octamer (two copies of each-H2A, H2B, H3, and H1) called nu-body or core of nucleosome.
• Their positively charged ends are directed outside.
• They attract negatively charged strands of DNA.
• About 200 bp of DNA are wrapped over nu-body to complete about 13 turns.
• This forms a nucleosome of size 110 x 60 Å (11 x 6 nm).
• The DNA present between two adjacent nucleosomes is called linker DNA.
• It is attached to H, histone protein.
• The length of linker DNA varies from species to species.
• Nucleosome chain gives a “beads-on-string” appearance under electron microscope ).

• The nucleosomes further coil to form a solenoid.
• It has diameter of 30 nm as found in chromatin.
• The beads-on-string structure in chromatin is packaged to form chromatin fibers that are further coiled and condensed at the metaphase stage of cell division to form chromosomes.
• Packaging at higher level requires additional set of proteins (acidic). These are collectively referred to as non-histone chromosomal (NHC) proteins.
• NHC proteins are of three types:
  • Scaffold or structural NHC protein
  • Functional NHC protein, e.g., DNA polymerase and RNA polymerase
  • Regulatory NHC protein, e.g., HMG (high mobil- ity group) proteins that control gene expression)
• In a typical nucleus, some regions of chromatin are loosely packed (and stain light) and are referred to as euchromatin.
• The chromatin that is more densely packed and stains dark is referred to as heterochromatin. Specifically, euchromatin is said to be transcriptionally active while heterochromatin is said to be transcriptionally inactive.

Chemical Composition of Chromosome

A chromosome consists of the following chemical compositions:

• DNA: 40%
• RNA: 1.2%
• Histone protein: 50%
• Acidic proteins: 8.5%
• Lipid: Traces
• Ca, Mg2, Fe12: Traces

Search For Genetic Material

The following experiments prove that DNA is the genetic material.

Evidence from Bacterial Transformation

• The transformation experiments conducted by Frederick Griffith in 1928 are of great importance in establishing the nature of genetic material.
• He used two strains of bacterium Diplococcus or Streptococcus pneumoniae or Pneumococcus, i.e., S-3 and R-2.

• • Smooth (S) or capsulated type: These have a mucous coat and produce shiny colonies. These bacteria are virulent and cause pneumonia.
  • Rough (R) or non-capsulated type: Mucous coat is absent and these produce rough colonies. These bacteria are non-virulent and do not cause pneumonia.
    The experiment can be described in the following four steps:
  • Smooth-type bacteria were injected into mice. The mice died as a result of pneumonia caused by bacteria.
    S strain Injected into mice → Mice died ii.
  • Rough-type bacteria were injected into mice. The mice lived and pneumonia did not occur.
    R strain Injected into mice →→ Mice lived
  • Smooth-type bacteria, which normally cause disease, were heat-killed and then injected into mice. The mice lived and pneumonia was not caused.
    S strain (heat-killed) → Injected into mice → Mice lived
  • Rough-type bacteria (living) and smooth- type heat-killed bacteria (both known not to cause disease) were injected together into mice. The mice died due to pneumonia and virulent smooth-type living bacteria could also be recovered from their dead bodies.
    S strain (heat-killed) + R strain (living) →Injected into mice → Mice died
• From the fourth step of the experiment, he concluded that some rough-type bacteria (non-virulent) were transformed into smooth-type bacteria (virulent).
• This occurred perhaps due to the absorption of some transforming substance by rough-type bacteria from heat-killed smooth-type bacteria.
• This transforming substance from smooth-type bacteria caused the synthesis of capsule which resulted in the production of pneumonia and the death of mice.
• Therefore, transforming principle appears to control genetic characters (e.g., capsule, as in this case). However, the biochemical nature of genetic material was not defined from his experiments.

Biochemical Characterization of Transforming Principle

• Later, Avery, Macleod, and McCarty (1944) repeated the experiment in vitro to identify the biochemical nature of transforming substance. They proved that this substance is DNA.

• Pneumococcus bacteria cause disease when capsule is present. Capsule production is under genetic control.
• In the experiment, rough-type bacteria (non-capsulated and non-virulent) were grown in a culture medium to which DNA extract from smooth-type bacteria (capsu- lated and virulent) was added.
• Later, the culture showed the presence of smooth-type bacteria also in addition to rough type.
• This is possible only if the DNA of smooth-type bacteria was absorbed by the rough-type bacteria which developed capsule and became virulent.
• This process of transfer of characters of one bacterium to another by taking up DNA from solution is called transformation.
• When DNA extract was treated with DNase (an enzyme that destroys DNA), transformation did not occur.
• Transformation occurred when proteases and RNases were used. This clearly shows that DNA is the genetic material.

Evidence from Experiments with Bacteriophage

• T2 bacteriophage is a virus that infects bacterium E. coli and multiplies inside it.
• T2 phage is made up of DNA and protein coat.
• Thus, it is the most suitable material to determine whether DNA or protein contains information for the production of new virus (phage) particles.
• Hershey and Chase (1952) demonstrated that only the DNA of the phage enters the bacterial cell and, therefore, contains necessary genetic information for the assembly of new phage particle.

• The functions of DNA and proteins could be found out by labeling them with radioactive tracers.
• DNA contains phosphorus but not sulfur.
• Therefore, phage DNA was labeled with p32 by grow- ing bacteria infected with phages in culture medium containing 32p.
• Similarly, the protein of phage contains sulfur but no phosphorus.
• Thus, the phage protein coat was labeled with S35 by growing bacteria infected with phages in another culture medium containing 35S.
• After the formation of labeled phages, three steps were followed:
  • Infection: Both types of labeled phages were al- lowed to infect normally cultured bacteria in sep- arate experiments.
  • Blending: These bacterial cells were agitated in a blender to break the contact between virus and bacteria.
  • Centrifugation: The virus particles were separated from the bacteria by spinning them in a centrifuge.
• After centrifugation, the bacterial cells showed the presence of radioactive DNA labeled with p32 while radioactive protein labeled with $35 appeared on the outside of bacteria cells (i.e., in the medium).
• Labeled DNA was also found in the next generation of phage.
• This clearly showed that only DNA enters the bacterial host and not the protein.
• DNA, therefore, is the infective part of virus and also carries all genetic information.
• This provided the unequivocal proof that DNA is the genetic material.

Properties of Genetic Material

• Following are the properties and functions which should be fulfilled by a substance if it is to qualify as genetic material.
  • It should be chemically and structurally stable.
  • It should be able to transmit faithfully to the next generation, as Mendelian characters.
  • It should also be capable of undergoing mutations.
  • It should be able to generate its own kind (replication).
• This can be concluded after examining the above written qualities. DNA is more stable and is preferred as genetic material due to the following reasons:
  • Free 2’OH of RNA makes it more labile and easily degradable. Therefore, DNA in comparison is more stable.
  • The presence of thymine at the place of uracil also confers additional stability to DNA.
  • RNA being unstable mutates at a faster rate.

RNA World

• RNA was the first genetic material.
• There are evidences to suggest that essential life processes such as metabolism, translation, and splicing evolved around RNA.
• RNA used to act as a genetic material as well as a catalyst.
• There are some important biochemical reactions in systems that are catalyzed by RNA catalysts and not by proteinaceous enzymes (e.g., splicing).
• RNA being a catalyst was reactive and, hence, unsta- ble.
• Therefore, DNA has evolved from RNA with chemical modifications that make it more stable.
• DNA being double stranded and having complementary strand further resists changes by evolving a process of repair.
• RNA is an adapter, structural molecule, and in some cases catalytic.
• Thus, RNA is a better material for the transmission of information.

Replication Of DNA

• The Watson-Crick model of DNA immediately suggested that the two strands of DNA would separate.
• Each separated or parent strand serves as a template (model or guide) for the formation of a new but complementary strand.
• Thus, the new or daughter DNA molecules formed would be made of one old or parental strand and an- other newly formed complementary strand.
• This method of formation of new daughter DNA molecules is called the semi-conservative method of replication.
• The following experiment suggests that DNA replication is semi-conservative.
• Messelson and Stahl (1958) conducted experiment using heavy nitrogen (15N) to determine whether the concept of semi-conservative replication is correct.
• They used cesium chloride (CSCI) gradient centrifugation technique for this purpose.
• A dense solution of CsCl, on centrifugation, forms density-gradient bands of a solution of lower density at the top that increases gradually towards the bottom with highest density.
• If the DNAs of different densities are mixed with CsCl solution, these would separate from one another and would form a definite density band in the gradient along with CsCl solution.
• Meselson and Stahl created DNA molecules of different densities by using normal nitrogen, 14N, and its heavy isotope, ‘N.
• For this purpose, E. coli was grown in 15NH,Cl-containing culture medium for many generations, to make bacterial DNA completely heavy.
• This non-radioactive or heavy DNA (incorporating 15N) had more density than the DNA with normal nitrogen (14N).
• Bacteria were then transferred to the culture medium containing only normal nitrogen (NH,CI). The change in density was observed by taking DNA samples periodically.

• If DNA replicates semi-conservatively, then each heavy (N) DNA strand should separate and each separated strand should acquire a light (N) partner after one round of replication.
• This should be a hybrid DNA made of two strands, i.e., 14N-15N.
• Meselson and Stahl observed that such DNA was actually half-dense indicating the presence of hybrid DNA molecules.
• After the second round of replication, there would be four DNA molecules.
• Of these, two molecules would be hybrid (14N-15N) showing half density as earlier and the remaining two molecules would be made of light strands (14N-14N). Thus, after the second generation, the same half-dense band (4N-15N) was seen but the density of light bands (14N-14N) increased.
• Meselson and Stahl’s work as such provided the confirmation of the Watson-Crick model of DNA and its semi-conservative replication.
• Taylor proved the semi-conservative mode of chromosome replication in eukaryotes using tritiated thymidine in the root of Vicia faba (faba beans).
• Cairns proved the semi-conservative mode of replication in E. coli by using tritiated thymidine (H3-tdR) in the autoradiography experiment.
• He proposed the 6-model for replication in circular DNA.

Mechanism of DNA Replication

DNA replication involves the following four major steps:

• Initiation of DNA replication
• Unwinding of helix
• Formation of primer strand
• Elongation of new strand

Initiation of Replication

• The replication of DNA always begins at a definite site called the origin of replication.
• Prokaryotes have single origin of replication.
• It is called ori-c in E.coli.
• On the other hand, eukaryotes have several thousand origins of replication.

Unwinding of Helix

• DNA replication requires that the double helical parental molecule is unwound so that its internal bases are available to the replication enzymes.
• Unwinding is brought about by enzyme helicase which is ATP-dependent.
• The unwinding of DNA molecule into two strands results in the formation of Y-shaped structure, called replication fork.

• These exposed single strands are stabilized by a protein known as single-strand binding (SSB) protein.
• Due to unwinding, a supercoiling develops on the end of DNA opposite to the replicating fork.
• This tension is released by enzyme topoisomerase.

Formation of Primer Strand

• A new strand is to be synthesized opposite to the parental strands. DNA polymerase 3 is the true replicase in E. coli. It is incapable of initiating DNA synthesis, i.e., it is unable to deposit the first nucleotide in a daughter (new) strand without the primer.)
• Another enzyme, known as primase, synthesizes a short primer strand of RNA.
• The primer strand then serves as a stepping stone (to start error-less replication).
• Once the initiation of DNA synthesis is completed, this primer RNA strand is then removed enzymatically.

Elongation of New Strand

• Once the primer strand is formed, DNA replication occurs in 5′-3′ direction, i.e., during the synthesis of a new strand, deoxyribonucleoside triphosphates (dATP, dGTP, dTTP, dCTP) are added only to the free 3’OH end.
• Thus, the nucleotide at the 3′ end is always the most recently added nucleotide to the chain.
• As DNA replication proceeds on the two parental strands, the synthesis of daughter or new strand occurs continuously along the parent 3’5′ strand.
• It is now known as the leading daughter strand.
• The synthesis of another daughter strand along the other parental strand, however, takes place in the form of short pieces.
• This is called the lagging daughter strand. These short pieces of DNA are known as the Okazaki fragments. These segments are about 1,000-2,000 nucleotides long in prokaryotes.
• Hence, DNA replication is semi-discontinuous. The discontinuous pieces of lagging strand are joined together by the enzyme DNA ligase (after the removal of primer) to form continuous daughter strand.
• Thus, two DNA molecules are now formed from one molecule.
• Each of these daughter DNA molecules is made of two strands, of which one is old (parental) and the other one is new or complementary strand.
• DNA polymerase is the most important enzyme of DNA replication.
• DNA polymerases are of three types in prokaryotes: DNA polymerase I, II, and III.

• DNA polymerases 2 and 3 have only 3′ 5′ exonuclease activity.
• Kornberg (1956) succeeded in demonstrating the in vitro synthesis of DNA molecule using a single strand of DNA as a template.
• He extracted and purified an enzyme from E. coli which was capable of linking free DNA nucleotides, in the presence of ATP as an energy source, to form complementary strand.
• He called it DNA polymerase. DNA polymerase I is called the Kornberg enzyme.
• In eukaryotes, DNA polymerases are of five types: DNA polymerase a, ẞ, 7, 8, and ɛ.
• In eukaryotes, the replication of DNA takes place at the S-phase of the cell cycle. The replication of DNA and the cell division cycle should be highly coordinated. A failure in cell division after DNA replication” results in polyploidy (a chromosomal anomaly).

Structure Of RNA

RNA is present in all living cells. It is laevo rotatory and is responsible for learning and memory. It is found in the cytoplasm as well as the nucleus.

Types of RNA

• In bacteria, there are three major types of RNAs: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).
• All three RNAs are needed to synthesize a protein in a cell.
• The mRNA provides the template, the tRNA brings aminoacids and reads the genetic code, and the rRNAs play structural and catalytic role during translation.
• RNA is generally involved in protein synthesis but in majority of plant viruses, it serves as a genetic material. Therefore, there are two major types of RNAs: genetic RNA and non-genetic RNA.

Genetic RNA

Fraenkel-Conrat showed that RNA present in TMV (tobacco mosaic virus) is a genetic material. Since then, it is established that RNA acts as a genetic material in most plant viruses.

Non-Genetic RNA

Non-genetic RNA is commonly present in cells where DNA is the genetic material. It is synthesized on DNA template. It is of the following three major types:

• mRNA
  • It was reported by Jacob and Monad.
  • It carries genetic information present in DNA and, so, is called working copy.
  • It constitutes about 3-5% of the total RNA pre- sent in the cell.
  • The molecular weight varies from 25,000 to 1,00,000.
  • It is about 300 nucleotide long at minimum.
  • Structure of prokaryotic mRNA: It is polycistronic in nature, i.e., several cistrons (functional part of DNA) form a single mRNA. Thus, each mRNA has a message to produce several polypeptides. Average life is 2 min.
    At the 5′-end near initiation codon, a sequence of fixed bases is found. It is called the Shine Dal- garno (SD) sequence (5’AGGAGGU3′). It helps in the correct binding of ribosomal subunit (30 S) on it.

• • Structure of eukaryotic mRNA: It is monocistronic in nature and has a message to produce on polypeptide only. It is metabolically stable and its life varies from a few hours to days.
  • UTR (untranslated region): These are sequences of RNA before the start or initiation codon and after the stop or termination codon. These are not translated and are transcribed as part of the same transcript as the coding region. Such UTRS pro- vide stability to mRNA and also increase translational efficiency.
• rRNA
  • rRNA was reported by Kuntz. It is the most stable type of RNA and is a constituent of ribosomes. It forms about 80% of the total cellular RNA.
  • In eukaryotes, four types of rRNAs are found: 28 S, 18 S, 5.85 S, and 5 S, whereas in prokaryotes, three types of rRNAs are found: 23 S, 16 S, and 5 S. These are synthesized by genes present on the DNA of several chromosomes found within a region known as nucleolar organizer.
• tRNA
  • The existence of tRNA was postulated by Crick. It is also known as soluble RNA (SRNA). tRNAs are the smallest molecules that carry amino acids to the site of protein synthesis. These constitute about 15% of the total cellular RNA.
  • tRNA acts as an intermediate molecule between the triplet code of mRNA and the amino acid se- quence of polypeptide chain. All tRNAs have almost the same basic structure. There are over 60 types of tRNAs.
  • Structure of tRNA: The cloverleaf model of tRNA is a two-dimensional model suggested by Holley et.al. A tRNA molecule appears like a cloverleaf, being folded with three or more double helical regions (stem), having loops also. The three-dimensional structure of tRNA was proposed to be inverted L-shaped by Kim and Klug.
  • All tRNA molecules commonly have a guanine residue at their 5′ terminal end. At their 3′ end, un- paired CCA sequence is present. Amino acid gets attached at this end only. The number of nucleo- tides varies from 77 (tRNA alanine) to 207 (tRNA1y- rosine).
• There are three loops in tRNA.
  • Amino acyl synthetase binding loop, also called DHU loop.
  • Ribosomal binding loop with seven unpaired bases. It is also called TVC loop.
  • Anti-codon loop with seven unpaired bases. Out of the seven bases in the anti-codon loop, three bases act as anti-codon for a particular triplet codon present on mRNA.

Gene Expression

• DNA, being the genetic material, carries all the informations necessary to program the functions of a cell by controlling the synthesis of enzymes or proteins.
• Beadle and Tatum put forward a theory-one gene one enzyme-in support of the earlier hypothesis that enzymes are proteinaceous in nature and each is produced by a single gene.
• They conducted experiments on the nutritional strains of pink mold, Neurospora crassa.
• This fungus grows on simple nutrient medium and has the ability to synthesize all its cellular components. Such an organism is called prototroph.
• An organism that is unable to synthesize a particular cellular metabolite such as an amino acid or a coenzyme is called auxotroph.
• Beadle and Tatum produced arginine (an amino acid) auxotrophs (mutants of Neurospora unable to synthesize arginine) by giving X-rays treatment to the cells. Arginine synthesis passes through the following path.

• They found that any step of this metabolic chain could be blocked by a mutation in a specific enzyme catalyzing the reaction, each enzyme representing a different gene product.
• Thus, Beadle, and Tatum reached a conclusion that each gene functions to produce a single enzyme.
• Some proteins, e.g., hemoglobin and other quaternary proteins, are made up of two or more than two poly- peptide chains. After this, the “one gene one enzyme” theory was modified into “one gene one polypeptide” hypothesis by Yanofsky. Later, Jacobson and Balti- more proposed “one mRNA one polypeptide” hypothesis.
• Gene and protein: A gene expresses itself by protein synthesis.
• When a particular gene is expressed (i.e., controls a function or a reaction), its information is copied into another nucleic acid, i.e., mRNA, which in turn directs the synthesis of specific proteins.
• These concepts form the central dogma of molecular biology. This has been shown by F. Crick in.

• An exception to this one-way flow of information was reported in 1970.
• H. Temin and D. Baltimore independently discovered reverse transcription in some viruses.
• These viruses can code an enzyme, reverse transcriptase, which can code DNA on RNA template.
• This discovery was important in understanding cancer and, hence, these two scientists were awarded Nobel Prize.
• Commoner suggests circular flow of information.

Mechanism Of Protein Synthesis

The process of protein synthesis consists of two major steps:

• Transcription or synthesis of mRNA on DNA
• Translation or synthesis of proteins along mRNA

Transcription

• The transfer of genetic information from DNA to mRNA is known as transcription. The segment of DNA that takes part in transcription is called transcription unit. It has three components.
  • A promoter
  • The structural gene
  • A terminator
• Promoter sequences are present upstream (5′ end) of the structural genes of a transcription unit.
• The binding sites for RNA polymerase lie within the promoter sequence.
• Certain short sequences within the promoter sites are conserved. In prokaryotes, at 10 bp upstream from the start point lies a conserved sequence described as -10 sequence TATAAT or the “Pribnow box” and -35 sequence TTGACA or the “recognition sequence.”

• Structural gene is a part of that DNA strand which has 3′ 5′ polarity, as transcription occurs in 5′ → 3′ direction. This strand of DNA that directs the synthesis of mRNA is called the template strand. The complementary strand is called non-template or coding strand. It is identical in base sequence to RNA transcribed from the gene, only with U in place of T.
• Terminator is present at the 3′ end of coding strand and defines the end of the process of transcription.

Mechanism of Transcription

• RNA polymerase binds to the promoter region of DNA and the process of transcription begins.

• RNA polymerase moves along the DNA helix and unwinds it.
• One of the two strands of DNA serves as a template for RNA synthesis.
• This results in the formation of complementary RNA strand.
• It is formed at a rate of about 40 to 50 bp/s.
• RNA synthesis comes to a stop when RNA polymerase reaches the terminator sequence.
• The transcription enzyme, i.e., RNA polymerase, is only of one type in prokaryotes and can transcribe all types of RNAs.
• RNA polymerase is a holoenzyme that is represented as (αBB’w)σ.
• The molecular weight of holoenzyme is 4,50,000.
• The enzyme without σ subunit is referred to as core enzyme.
• Though the core enzyme is capable of transcribing DNA into RNA, transcription starts non-specifically at any base on DNA.
• It is σ subunit which confers specificity. Rho factor (p) is required for the termination of transcription.
• Sigma factor (o): Binds to the promoter site of DNA and initiates transcription.
• Core complex: It continues the transcription.
• Rho factor (p): It terminates the transcription; its molecular weight is 55,000.

Transcription in Eukaryotes

• In eukaryotes, the promoter site is recognized by the presence of specific nucleotide sequence called TATA box or the Hogness box (7 base pair long-TATATAT or TATAAAT) located 20 bp upstream to the start point.
• Another sequence is the CAAT box present between -70 bp and 80 bp.
• In eukaryotes, the RNA polymerases are of three types: RNA polymerase I, RNA polymerase II, and RNA polymerase III.
• The functions of different RNA polymerases in eukaryotes are as follows:
  • RNA polymerase 1-5.8 S, 18 S, and 28 S rRNA synthesis.
  • RNA polymerase 2-hnRNA (heterogeneous nuclear RNA) and mRNA synthesis.
  • RNA polymerase 3-tRNA, scRNA (small cytoplasmic RNA), 5S rRNA, and snRNA (small nuclear RNA) synthesis.
• The gene in eukaryotes, however, is made of several pieces of base sequence coding for amino acids called exonic DNA, separated by stretches of non-coding sequences, commonly called intronic DNA.
• Thus, the information on the eukaryotic gene for assembling a protein is not continuous but split.
• This discovery is the result of works of Richard J. Roberts and Philip Sharp.
• They shared 1993 Nobel Prize for Physiology and Medicine for the discovery of “split genes” in higher organisms.
• Most eukaryotic genes contain very long base sequences, all of which do not necessarily form mature mRNA.
• The coding DNA sequences of the gene are called exons and the intervening non-coding DNA sequences are called introns.
• All introns have GU at the 5′ end and AG at the 3′ end. Depending on the size of gene, the number and length of exons may vary from a few to more than 50 nucleotides. Exons alternate with stretches of DNA that contain no genetic information introns.
• The nascent RNA synthesized by RNA polymerase II is called hnRNA.
• It contains both unwanted base sequences (transcribed from introns) alternated with useful base sequences (transcribed from exons).
• This primary transcript is converted into functional mRNA after post-transcriptional processing which involves three steps:
  • Modification of 5′ end by capping: Capping at the 5′ end occurs rapidly after the start of transcription.

• • • The guanosine methylated at the seventh position is added at 5′ with the help of enzyme guanyl transferase.
    • Cap is essential for the formation of mRNA-ribosome complex.
    • Translation is not possible if cap is lacking, be- cause cap is identified by 18S rRNA of ribosome unit.
  • Polyadenylation at 3′ end (tailing): Poly (A) is added to the 3′ end of newly formed hnRNA with the help of enzyme poly A polymerase. It adds about 200-300 adenylate residues.
• Splicing of hnRNA (tailoring): In eukaryotes, the coding sequences of RNA (exons) are interrupted by non-coding sequences (introns).
  • snRNA and protein complex called small nuclear ribonucleoprotein (or snRNPs or snurps) play important role in this process.
  • Here, the introns are removed and the exons come in one plane.
  • This process is called splicing through which a mature mRNA is produced.
• Normally, mRNA carries the codons of a single complete protein molecule (monocistronic mRNA) in cukaryotes, but in prokaryotes, it carries codons from several adjacent DNA cistrons and becomes much longer in size (polycistronic mRNA).

Genetic Code

• The term genetic code was coined by Gamow.
• DNA (or RNA) carries all the genetic information.
• It is expressed in the form of proteins.
• Proteins are made up of 20 different types of essential amino acids.
• The information about the number and sequence of these amino acids forming proteins is present in DNA and is passed on to mRNA during transcription.
• Genetic code is an mRNA sequence containing coded information for one amino acid. It consists of three nu- cleotides (triplet).
• Thus, for 20 amino acids, 64 (4 x 4 x 4 or 43 = 64) minimum possible permutations are available.
• This important discovery was the result of experiments by Marshall W. Nirenberg and J. Heinrich Matthaei and later by H.G. Khorana. Nirenberg and Khorana shared the 1968 Nobel Prize with R.W. Holley who gave the details of tRNA structure. Nirenberg and Mathaei used a synthetic poly(U) RNA and deciphered the code by translating this as polyphenylalanine. Hargobind Kho- rana, using synthetic DNA, prepared polynucleotide with known repeating sequence, CUCUCUCUCUCU; it produced only two amino acids-leucine (CUC) and serine (UCU).

Properties of Genetic Code

• Triplet code: Three adjacent nitrogen bases constitute a codon which specifies the placement of one amino acid in a polypeptide.
• Start signal: Polypeptide synthesis is signaled by two initiation codons-AUG or, rarely, GUG. The first or initiating amino acid is methionine. The initiating codon on mRNA for methionine is AUG. Initiating methionine occurs in formylated state in prokaryotes. At other positions, methionine is non-formylated. Both these methionines are carried by different tRNAs. Rarely, GUG also serves as initiation codon. It normally codes for valine but if present at the initiating position, it would code for methionine. So, GUG is an ambiguous codon.
• Stop signal: Polypeptide chain termination is signaled by three termination codons-UAA (ochre), UAG (amber), and UGA (opal). They do not specify any amino acid and were, hence, called nonsense codons. Whenever present in mRNA, these bring about termination of polypeptide chain and, thus, act as stop signals. Codons UAA, UAG, UGA, AUG, and GUG are also called punctuation codons.
• Commaless: The genetic code is continuous and does not possess pauses (meaningless base) after each tri- plet. If a nucleotide is deleted or added, the whole genetic code will read differently. Thus, a polypeptide having 50 amino acids shall be specified by a linear sequence of 150 nucleotides. If a nucleotide is added or deleted in the middle of this sequence, the amino acids before this will be the same but subsequent amino acids will be quite different.
• Universal code: The genetic code is applicable universally, i.e., a codon specifies the same amino acid from a virus to a human being or a tree.
• Non-overlapping code. Each codon is independent and one codon does not overlap with the next one.
• Degeneracy of code: Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. Only tryptophan and methionine are specified by single codons. All other amino acids are specified by 2-6 codons. The latter are called degenerate codons.

Wobble Hypothesis

• A change in nitrogen base at the third position of a codon does not normally cause any change in the expression of the codon because the codon is mostly read by the first two nitrogen bases (wobble hypothesis of Crick).
• The mutation that does not cause any change in the expression of the gene is called silent mutation.
• The position of the third nitrogen base in a codon which does not influence the reading of the codon is termed as wobble position.
• It pairs with the first position in anticodon. It means that the specificity of codon is determined by the first two bases.
• Wobbling helps one tRNA to read more than one codon and, thus, provides economy in the number of tRNA molecules at the time of translation.

Mutations and Genetic Code

• In a hypothetical mRNA, for example, the codons would normally be translated as follows.

• The insertion of single base G between the third and fourth bases produces a completely different protein from the earlier one.

 

• Similarly, the deletion of single base C at the fourth place produces a new chain of amino acids and, hence, a different protein.

 

• The insertion or deletion of one or two nitrogenous bases changes the reading frame from the point of insertion or deletion.
• Such mutations are called frame shift mutations.
• However, the insertion or deletion of three or its mul- tiple bases leads to the insertion or deletion of one or multiple codons. Hence, one or multiple amino acids and reading frames remain unaltered from that point onwards.
• This forms the genetic basis of proof that codon is a triplet and that it is read in a contiguous manner.

Translation

• Translation is the mechanism by which triplet base sequences of mRNA molecules are converted into a specific sequence of amino acids in a polypeptide chain. It occurs on ribosomes.
• The major steps are as follows:
  • Activation of amino acids: In the presence of enzyme aminoacyl-tRNA synthetase (E), specific amino acids (AA) bind with ATP.
  • Charging of tRNA: The AA, -AMP-E, com- plex formed in the first step reacts with a specific tRNA. Thus, amino acid is transferred to tRNA. As a result, the enzyme and AMP are liberated.
  • Formation of polypeptide chain: It is completed in three steps.
    • Chain initiation: It requires three initiation factors in prokaryotes-IF3, IF2, and IF.
      Nine initiation factors are required in eukaryotes. These are elF2, eIF3, eIF1, eIF4A, eIF4B eIF4c, eIF4D, eIFs, and eIF6.
      • Binding of mRNA with smaller subunit of ribosomes (30S/40S): IF, is involved in prokaryotes while eIF2 is involved in eukaryotes. It involves interaction be- tween the Shine Delgarno sequence and the 3′ end of 16S rRNA in prokaryotes.
      • Binding of 30S/40S-mRNA complex with tRNA: Non-formylated methionine is attached with tRNA in eukaryotes and formylated methionine in prokaryotes. IF2 is involved in prokaryotes while eIF3 is involved in eukaryotes.
      • Attachment of larger subunit of ribosomes: Initiation factors in eukaryotes are eIF, and eIF4. Initiation factor in prokaryotes is IF.
    • Chain elongation
      • Ribosomes have two sites for binding amino acyl tRNA: (1) amino acyl or A site (acceptor site) and (2) peptidyl site or P site (donor site).
      • A second charged tRNA molecule along with its appropriate amino acid approaches the ribosome at the A site close to the P site.
      • Its anticodon binds to the complementary codon of mRNA chain.
      • A peptide bond is formed between the COOH group of the first amino acid (methionine) and the NH2 group of the second amino acid.
      • The formation of peptide bond requires energy and is catalyzed by enzyme peptidyl transferase. The initiating formyl methionine or methionine tRNA can bind only with the P site.
      • All other newly coming aminoacyl tRNA bind to the A site.
      • The elongation factors required for prokaryotes are EF-Tu and EF-Ts and that required for eukaryotes is eEF.
      • Translocation is the movement of ribo- some on mRNA. It requires EF-G in prokaryotes and eEF, in eukaryotes.

• • • Chain termination: The termination of polypeptide is signalled by one of the three terminal codons in the mRNA. The three terminal codons are UAG (amber), UAA (ochre), and UGA (opal).
      • AGTP dependent factor known as release factor is associated with the termination codon. It is eRF1 in eukaryotes and RF1 ND RF2 in projaryotes.
      • At the time of termination, the terminal codon immediately follows the last amino acid codon. After this, the polypeptide chain, tRNA, and mRNA are released and the sumunits of ribosomes get dissociated.
      • Gene expression is the mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism.
      • In the translation unit, mRNA has some sequences that are not translated and are referred to as untranslated regions (UTR).
      • The UTRs are present at both 5′ end (before the start codon) and 3′ end (after the stop codon).
      • They are required for efficient translation process.

Regulation Of Gene Expression

• Constitutive genes are those genes that are constantly expressing themselves in a cell because their products are required for normal cellular activities. For example, genes for glycolysis and ATPase.
• Non-constitutive genes are not always expressing themselves in a cell. These are called luxury genes. These are switched on or off according to the requirement of cellular activities. For example, genes for nitrate reductase in plants and lactose system in E. coli. This provides maximum functional efficiency to the cell. Such regulated genes, therefore, are required to be switched on and off when a particular function is to begin or stop.
• This regulation can be achieved by any one of the following two processes:
  • Induction: This is a process of gene regulation where the addition of a substrate stimulates or in- duces the synthesis of enzymes needed for its own breakdown.
  • Repression: In this process of gene regulation, the addition of end product stops the synthesis of enzymes needed for its formation. This phenomenon is also known as feedback repression.

Operon

• Francois Jacob and Jacques Monod (1961) proposed a model of gene regulation, known as the operon model.
• Operon is a coordinated group of genes such as structural gene, operator gene, promoter gene, and regulator gene which function or transcribe together and regulate a metabolic pathway as a unit.

Inducible Operon (Lac Operon)

• In E. coli, the breakdown of lactose requires three enzymes.
• These enzymes are synthesized together in a coordinated manner and the unit is known as lac operon.
• Since the addition of lactose itself stimulates the production of required enzymes, it is also known as inducible system.

• Lac operon consists of the following genes:
  • Structural genes: These genes code for the proteins needed by the cell which include enzymes or other proteins having structural functions. In lac operon, there are following three structural genes:
    • lac a-Gene coding for enzyme transacetylase
    • lac y Gene coding for enzyme permease
    • lac z-Gene coding for enzyme ẞ-galactosidase
  • Operator gene (o): It interacts with a protein molecule (the regulator molecule), which pro- motes (induces) or prevents (represses) the transcription of structural genes.
  • Promoter gene (p): This gene is the recognition point where RNA polymerase remains associated.
  • Regulator gene (i): This is generally known as inhibitory gene (1). This gene codes for a protein called the repressor protein. It is an allosteric protein which can exist in two forms. In one form, it is paratactic to an operator and binds with it, and in the other form, it is paratactic to the inducer (such as lactose).
• The operon is switched off and on.
• The transcription or function of lac genes or structural genes depends on the operator gene.
• When the repressor protein produced by inhibitory (i) gene or regulatory gene binds to the operator (o) gene, RNA polymerase gets blocked. There is no transcription and the operon model remains in “switched off” position.
• On the other hand, when an inducer such as lactose is added, the repressor protein (produced by gene i) gets bound to it.
• The repressor, therefore, gets released from the opera- tor.
• RNA polymerase is now permitted to act.
• Hence, the transcription of lac genes now takes place and the operon model is in “switched on” position.
• All three genes are transcribed by RNA polymerase to form a single mRNA strand.
• Each gene segment of mRNA is called cistron and, therefore, the mRNA strand transcribed by more than one gene is known as polycistronic.

Tryptophan Operon-Repressible Operon System

• Operon model can also be explained using feedback repression.

• In tryptophan (trp) operon, three enzymes are necessary for the synthesis of amino acid tryptophan.
• These enzymes are synthesized by the activity of five different genes in a coordinated manner.
• The addition of tryptophan, however, stops the produc- tion of these enzymes.
• Thus, the system is known as repressible system.
• In this system, there are five structural genes: trp A, trp B, trp C, trp D, and trp E. Three enzymes are needed for the synthesis of tryptophan-an amino acid.
• Regulatory gene (R) produces repressor protein which is known as apo-repressor because it does not get bound to the operator directly.
• Hence, the operator gene remains in “switched on” po- sition.
• Tryptophan, when added, binds to the apo-repressor and is called co-repressor.
• This apo-repressor and co-repressor complex (acti- vated repressor) now binds to the operator gene and blocks the function of RNA polymerase.
• Thus, transcription would not occur and tryptophan operon would be in “switched off” position.
• Feedback repression is functional when there is no further need of the end product and, hence, there is no requirement of continuation of this anabolic pathway.
• This operon stops the process.
• In prokaryotes, the control of the rate of transcriptional initiation is the predominant site for the control of gene expression.

Regulation of Gene Expression in Eukaryotes

• In eukaryotes, the regulation of gene expression can be exerted at four levels:
  • Transcriptional level (formation of primary transcript)
  • Processing level (regulation of splicing)
  • Transport of mRNA from nucleus to the cytoplasm
  • Translational level
• In eukaryotes, functionally related genes do not represent an operon but are present on different sites in chromosomes.
• Here, the structural gene is called split gene. It is a mosaic of exons and introns, i.e., base triplet-amino acid matching is not continuous.
• The Britten-Davidson gene battery model is most popular for eukaryotic genes.
• It proposes the occurrence of five types of genes: producer, receptor, integrator, sensor, and enhancer-silencer.
• Exons are the coding part of cistron which forms RNA.
• Introns are the non-translated part of DNA called IVS (intervening sequences) or spacer DNA.
• An intron begins with GU and ends up with an AG.
• However, the entire split gene is transcribed to form a continuous strip of hnRNA.
• This removal of non-coding intronic part and the fusion of exonic coding parts of RNA is called RNA splicing. About 50%-90% of primary transcribed RNA is discarded during processing.

DNA Fingerprinting

• The chemical structure of everyone’s DNA is the same.
• The only difference between people (or any animal) is the order of the base pairs.
• There are so many millions of base pairs in each person’s DNA that every person has a different sequence.
• Using these sequences, everyone can be identified solely by the sequence of their base pairs.
• However, there are so many millions of base pairs due to which the task would be very time consuming.
• Instead, scientists are able to use a shorter method, because of repeating base patterns in DNA (satellite DNA).
• These patterns do not, however, give an individual “fingerprint.” But these are able to determine whether two DNA samples are from the same person, related people, or non-related individuals.

VNTRS, RFLP, SSR, AND RAPD

• Each strand of DNA has stretches that contain genetic information responsible for an organism’s development (exons) and stretches that, apparently, supply no relevant genetic information at all (introns).
• Although the introns may seem useless, it has been found that they contain repeated sequences of base pairs.
• These sequences are called variable number tandem repeats (VNTRS).
• VNTRS, also called minisatellites, were discovered by Alec Jeffreys et. al. of UK.
• These consist of hypervariable repeat regions of DNA having a basic repeat sequence of 11-60 bp and flanked on both sides by restriction sites.
• The number of repeats shows a very high degree of polymophism and the size of VNTR varies from 0.1 kb to 20 kb. The length of minisatellites and the position of restriction sites is different for each person.
• Therefore, when the genomes of two people are cut using the same restriction enzyme, the length and number of fragments obtained are different for both.
• This is called restriction fragment length polymorphism (RFLP).
• These fragments, when separated by gel electrophoresis, and obtained on a Southern blot, constitute what is called DNA fingerprint.
• The father of DNA fingerprinting is Alec Jeffreys while the Indian experts Lalji Singh and V.K. Kashyap are known as the father of Indian technique.

Methodology of DNA Fingerprinting

The Southern blot is one way to analyze the genetic patterns which appear in a person’s DNA. It was devised by E. M. Southern (1975) for separating DNA fragments.

• Isolating the desired DNA: It can be done either chemically (by using a detergent to wash the ex- tra material from the DNA) or mechanically (by applying a large amount of pressure in order to “squeeze out” the DNA).
• Cutting the DNA into several pieces of different size: This is done using one or more restriction enzymes.
• Sorting the DNA pieces by size: The process by which size separation is done is called gel electrophoresis. The DNA is poured into a gel, such as agarose, and an electric charge is applied to the gel, with the positive charge at the bottom and the negative charge at the top. Because DNA has a slightly negative charge, the pieces of DNA will be attracted towards the bottom of the gel. The smaller pieces, however, will be able to move more quickly and, thus, further towards the bottom than the larger pieces. The different-sized pieces of DNA will, therefore, be separated by size, with the smaller pieces towards the bottom and the larger pieces towards the top.
• Denaturing the DNA fragments, so that all of the DNA is rendered single-stranded: This can be done either by heating by chemically treating the DNA in the gel using alkali.
• Blotting the DNA: The gel with size-fractionated DNA is applied to a sheet of nitrocellulose paper or nylon membrane, and then baked to permanently attach the DNA to the sheet. The Southern blot is now ready to be analyzed.

In order to analyze a Southern blot, a radioactive genetic probe is used in a hybridization reaction with the DNA in ques- tion. If an X ray is taken of the Southern blot after a radioactive probe has been allowed to bind with the denatured DNA on the paper, only the areas where the radioactive probe binds will show themselves on the film (autoradiography). This allows researchers to identify, in a particular person’s DNA, the occurrence and frequency of the particular genetic pattern contained in the probe.

Practical Applications of DNA Fingerprinting

• Solving cases of disputed paternity and maternity: Because a person inherits his or her VNTRS from his or her parents, VNTR patterns can be used to establish paternity and maternity.
• Criminal identification and forensics: DNA isolated from blood, hair, skin cells, or other genetic evidence left at the scene of crime can be compared, through VNTR patterns, with the DNA of a criminal suspect to determine guilt or innocence.

• Personal identification: The notion of using DNA fingerprints as a sort of genetic bar code to identify individuals has been discussed, but this is not likely to happen anytime in the near future. The technology required to isolate, keep on file, and then analyze millions of very specified VNTR patterns is both expensive and impractical.

Genomics

• The application of DNA sequencing and genome mapping to the study, design, and manufacture of biologically important molecules is known as genomics.
• It is comparatively a more recent branch in the field of biology.
• The term genomics was introduced by Thomas Roder- ick.
• In genomics, the function of gene is identified by using the technique of reverse genetics.
• The study of genomics may be classified into three classes:
  • Structural genomics: It involves the mapping and sequencing of genes.
  • Functional genomics: It involves the identification of function of a particular gene.
  • Application genomics: It involves the use of genomics information for crop improvement, etc.
• The complete genomics of Arabidopsis and rice have been worked out. These have 130 million bp and 430 million bp, respectively.

Human Genome Project

• The Human Genome Project (HGP) was the international, collaborative research program whose goal was the complete mapping and understanding of all genes of human beings.
• All genes together are known as genome.
• The HGP as “mega project” was a 13 year project co- ordinated by the US Department of Energy and the Na- tional Institute of Health.
• An International HGP was launched in the year 1990 and completed in the year 2003.
• The International Human Genome Sequencing Consortium published the first draft of the human genome in the journal Nature in February 2001.
• Human genome is said to have approximately 3 × 10 bp, and the cost of sequencing required is US$ 3 per bp. The total estimated cost of the project would be approximately US$ 9 billion. In human ge- nome, 20,000 to 25,000 genes are present; out of them, the smallest gene is testis-determining fac- tor (TDF) gene with 14 bp and the largest gene is Duchenne muscular dystrophy gene with 2400 × 103 bp.

Goals of HGP

Following are the important goals of the HGP:

• Identification of all, approximately, 20,000-25,000 genes in human DNA.
• To determine the sequence of 3 billion chemical base pairs that make up human DNA.
• To store this information in databases.
• To improve tools for data analysis.
• To transfer related technologies to other sectors, such as industries.
• Bioinformatics, i.e., close association of HGP with the rapid development of a new area in biology.
• Sequencing of model organisms-Complete genome sequence of E.coli, S. cerevisiae, C. elegans, D. melanogaster, D. pseudoobscura, Oryza sativa, and Arabidopsis was achieved in April, 2003.
• Address the ethical, legal, and social issues (ELSI) that may arise from the project.

Methodologies

The HGP techniques include the following:

• Sequence tagged site (STS): It is a short DNA segment that occurs only once in a genome and whose ex- act location and order of bases are known. STSS serve as landmarks on the physical map of a genome. These are also called expressed sequence tags (ESTs). Genes that are expressed as RNA are referred to as ESTS.
• Sequencing the whole set of genome that contained all the coding and non-coding sequences and later assigning different regions in the sequence with functions is known as sequence annotation.
• The employment of restriction fragment length polymorphism (RFLP).
• Yeast artificial chromosomes (YACs).
• Bacterial artificial chromosomes (BACs).
• Polymerase chain reaction (PCR).
• Electrophoresis
  • For sequencing, the total DNA from a cell is isolated and converted into random fragments of relatively smaller sizes (recall DNA is a very long polymer, and there are technical limitations in sequencing very long pieces DNA) and cloned in suitable host using specialized vectors.
  • Cloning results in the amplification of each piece of DNA fragment so that it can be subsequently sequenced with ease.
  • The commonly used hosts were bacteria and yeast, and the vectors were called as BAC and YAC, respectively.

• The fragments were sequenced using automated DNA sequencers that worked on the principle of the method developed by Frederick Sanger. These sequences were then arranged based on some overlapping regions present in them. This required the generation of overlapping fragments for sequencing. The alignment of these sequences was humanly not possible. Therefore, specialized computer-based programs were developed. These sequences were subsequently annotated and were assigned to each chromosome. The sequence of chromosome-1 was completed only in May, 2006. (This was the last of the 24 human chromosomes-22 autosomes and X and Y–to be sequenced.)

Salient Features of Human Genome

Some salient observations drawn from the HGP are as follows:

• The human genome contains 3164.7 million nucleotide bases.
• On an average, a gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases.
• The total number of genes is estimated at 30,000- much lower than the previous estimates of 80,000- 1,40,000 genes. Almost all (99.9%) nucleotide bases are exactly the same in all people.
• The functions are unknown for over 50% of discov- ered genes.
• Less than 2% of the genome codes for proteins.
• Repeated sequences make up very large portion of the human genome.
• Repetitive sequences are stretches of DNA sequences that are repeated many times, sometimes hundred to thousand times. They are thought to have no direct coding functions, but they shed light on chromosome structure, dynamics, and evolution.
• Chromosome-1 has the highest number of genes (2968) while Y-chromosome has the fewest (231).
• Scientists have identified about 1.4 million locations where single-base DNA differences (SNPs or single nucleotide polymorphism, pronounced as “snips”) occur in humans. This information promises to revolutionize the process of finding chromosomal locations for disease-associated sequences and tracing human history.

Future Thrust of HGP

Francis Collins, the director of the public funded genome project, predicted the following progress in the HGP:

By 2010

• Scientists will have developed accurate predictive tests for at least a dozen common diseases so that preventive measures may be taken in advance.
• Infertility specialists will be using sophisticated techniques of pre-implantation genetic diagnosis to screen embryos for genetic disorders and designer babies.
• Medicines will be making use of gene therapy.

By 2020

• Doctors will have “designer drugs” to treat almost every disease.
• Doctors will test patient’s genetic make-up before pre- scribing drugs.
• Doctors will be able to change the genetic make-up of a living person by germ-line therapy.
• The treatment of diseases such as cancer, schizophrenia, and depression will have transformed.

By 2030

• Gene-based healthcare will have completely developed.
• Most medical researches will be carried out using computer models rather than the living tissues of animals.
• Average life span will rise up to 90 years.

The Indian Scenario

• The Indian gene center accounts for 160 species out of 2400 species in all of the 12 megagene centers.
• Though the plant genetic resources activities got in- tensified in India in the first half of the century, these received the required impetus after the creation of NBPGR in 1976 which has headquarters at IARI (In- dian Agricultural Research Institute), New Delhi, and several regional sub-centers at different agroclimatic zones such as Jodhpur (arid areas), Trichur (humid tropical zone), and Shillong (eastern sub-tropical/sub- temperate zone).
• On March 31, 1996, NBPGR held 15,4,533 accessions of various agri-horticultural crops.
• National Facility for Plant Tissue Culture Repository (NFPTCR) was established in 1986 by the department of biotechnology at NBPGR for the conservation of germplasm of vegetatively propagated plants.

Summary

• Nucleotide monomers constitute a polymer called nucleic acid. It is of two types: RNA and DNA.
• While DNA is the store house of information, RNA helps in the transfer and expression of information.
• As DNA is structurally and chemically more stable, it is a better genetic material, although both DNA and RNA serve as genetic material.
• RNA was the first to evolve, and DNA was derived from it.
• Bases in two DNA strands show hydrogen bonding (A=T, G=C) and follow Chargaff’s rule, so that both the strands are complementary and DNA replication is semi-conservative.
• The segment of DNA that codes for RNA is known as gene. During transcription, one DNA strand acts as template which directs the synthesis of complementary RNA.
• In prokaryotes, transcription and translation are continuous processes. In eukaryotes, the genes are split. Exons are interrupted by introns. Introns are removed and exons are joined to produce functional RNA.
• The mRNA contains genetic code in combination of three (triplet code) to code for an amino acid. This genetic code is read by tRNA which acts as an adapter molecule.
• There is specific tRNA for each amino acid. Each tRNA binds to amino acid at one end and with codons by H-bonding at the other end.
• Translation occurs at ribosome. Here ribozyme (rRNA enzyme) acts as catalyst which helps in peptide bond formation. The process of translation has evolved around RNA, which shows that life began around RNA.
• Since transcription and translation are energetically very expensive, they are tightly regulated. For example, lac operon, which is regulated by the amount of lactose in medium, i.e., the regulation of enzyme synthesis by its substrate.
• HGP is aimed at sequencing every base in human genome. DNA fingerprinting is used for this which is based on the principle of polymorphism in DNA sequence.

 

Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: RNA polymerase is of three types in eukaryotes for the synthesis of all types of RNAs.

Reason: RNA polymerase consists of six types of poly- peptides along with rho factor which is involved in the termination of RNA synthesis.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 2. Assertion: 5S rRNA and surrounding protein complex provides binding site of tRNA.

Reason: tRNA is soluble RNA with unusual bases.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 3. Assertion: Operator gene is functional when it is not blocked by repressor.

Reason: Regulator gene produces active protein only which acts on operon system in E. coli.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 4. Assertion: Peptidyl transfer site is contributed by larger subunit of ribosome.

Reason: The enzyme peptidyl transferase is contributed by both 23S and 16S ribosomal subunits.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 5. Assertion: Teminism is unidirectional flow of information.

Reason: It requires DNA dependent RNA polymerase enzyme.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 6. Assertion: In bacterial translation mechanism, two tRNA are required by methionine.

Reason: AUG codes for methionine and it shows non- ambiguity also.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 7. Assertion: Nutritional mutant strain of pink mold is auxotroph.

Reason: It is not able to prepare its own metabolites from the raw materials obtained from outside.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 8. Assertion: In DNA fingerprinting, hybridization is done with molecular probe.

Reason: Molecular probe is small DNA segment synthesized in laboratory with known sequence that recognizes complementary sequence in RNA.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 9. Assertion: cDNA libraries are important to scientists in human genomics.

Reason: cDNA is synthetic type of DNA generated from mRNA.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: SNP (pronounced “snips”) are common in human genome.

Reason: It is minute variation that occurs at a frequency of one in every 300 bases.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 11. Assertion: A single strand of mRNA is capable of forming a number of polypeptide chains.

Reason: Termination codons occur in mRNA.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 12. Assertion: Chromosomal aberrations are caused by a break in the chromosome or its chromatid.

Reason: Duplication, deficiency, transversion, and trans- locations are the causes of chromosomal aberrations.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 13. Assertion: The lac operon model is applicable to E. coli.

Reason: E. coli. lacks a definite nucleus.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 14. Assertion: Amber codon is a termination codon.

Reason: If in mRNA, a termination codon is present, protein synthesis stops abruptly whether it is completed or not.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 15. Assertion: Watson and Crick provided experimental proof of the semi-conservative nature of DNA replication.

Reason: RNA polymerase binds nucleotides in replication.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 16. Assertion: The mRNA attaches itself to the ribosome via its 3′ end.

Reason: The mRNA has nucleotide and bases of lagging sequence.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 17. Assertion: Replication and transcription occur in the nucleus but translation occurs in the cytoplasm.

Reason: mRNA is transferred from the nucleus into the cytoplasm where ribosomes and amino acids are available for protein synthesis.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 18. Assertion: Cancer cells are virtually immortal until the body in which they reside dies.

Reason: Cancer is caused by damage to genes regulating the cell division cycle.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Digestion And Absorption

Nutrition

The sum total of the processes starting from taking the food up to its utilization is called nutrition. Depending upon the mode of nutrition, organisms can be classified as autotrophs and heterotrophs. Euglena is mixotrophic (both autotrophic and saprophytic).

Animals Nutrients

• Nutrients may be organic or inorganic in nature.
• The organic constituents of nutrients are carbohydrates, lipids, proteins, and vitamins, and inorganic constituents are minerals and water.
• Carbohydrates, lipids, and proteins are macronutrients or proximate principles of food because these constitute energy sources for the production of heat and different organic functions.
• Minerals, vitamins, and water are micronutrients or protective principles of food because although these do not provide energy, yet their deficiencies are related to specific abnormalities in man.
• About 21 minerals (for example, sodium, potassium, calcium, sulfur, phosphorus, magnesium, and chlorine) or macroelements are known to be essential for human nutrition; they are required as 100 mg per day.
• Trace elements or microelements (for example, iron, iodine, zinc, manganese, cobalt, copper, molybdenum, etc.) are required in very small amounts.
• Altogether 20 vitamins are thought to be required in small amounts in human nutrition.

Types of Nutrition in Animals: Based on the methods of procurement or collection of food, heterotrophic nutrition is of three major types:

1. Holozoic Nutrition: When whole plants (or their parts) or whole animals (or their parts) or both are consumed either in solid or liquid form, through the mouth.

It can be classified in the following groups

• Herbivores: Feeding on plants, for example, rabbit, goat, cow, etc.
• Carnivores: Feeding on other animals, for example, tigers, lions, etc.
• Omnivores: Feeding on all types of food, for example, humans, cockroaches, etc.
• Detrivores: Feeding on detritus or organic remains, for example., earthworms, etc.
• Scavengers: Feeding on carrions (carrion eaters), for example., hawks, vultures, etc.
• Insectivores: Eating insects, for example, common bats, etc. Cannibals: Eating other members of one’s own species, for example, many snakes, etc.
• Frugivorcs: Feeding on fruits, for example, parrots, etc. Sanguivores: Taking meals of blood, for example, leech, female mosquito, etc.
• Coprophagous: Eats its own fecal matter, for example, rabbit.

2. Saprozoic Nutrition: When decaying organic materials of plant or animal origin are consumed.

The organisms following such nutrition secrete digestive enzymes directly onto their food and therefore the food is digested outside the body, for example, fungi, bacteria some protozoans, etc.

3. Parasitic Nutrition: When a living organism feeds on another living organism and causes harm to it, for example., tapeworm, malarial parasite, etc.

Read and Learn More NEET Biology Notes

Digestive System Of Mammals

Digestion is the catabolic process in which the non-diffusible complex of organic compounds converts into diffusible simple compounds by hydrolytic enzymes. The digestive system comprises (1) an alimentary canal and (2) acessary digestive glands.

Alimentary Canal: It is a coiled muscular tube about 6-9 m long, extending from mouth to anus. It is divided into three parts:

1. Foregut Or Stomodeum: Ectodermal in origin, it includes a mouth and buccal cavity.
2. Midgut Or Mesenteron: Endodermal in origin, it includes pharynx to rectum.
3. Hindgut Or Proctodeum: Ectodermal in origin, it includes the anal canal and anus.

Path Of Food From Mouth To Anus: Mouth → Buccal cavity → Oropharynx → Oesophagus → Stomach → Small intestine → Large intestine → Anal canal → Anus.

Parts Of The Alimentary Canal

Mouth: The mouth is an opening bounded by upper and lower lips. Lips are attached on the inner side with gums by a thin transparent fold called the labial frenulum of the mesentery.

• The motility of lips is due to orbicular oris muscles.
• Whales and platypuses have immovable lips.
• In men, the middle part of the upper lip is depressed and is called filtrum (a beauty sign).
• The lip of a rabbit has a cleft called a hare lip.

Buccal Cavity

• The buccal cavity is divided into (1) vestibule and (2) oral cavity. The space between lips and teeth is called a vestibule.
• The roof of the oral cavity is made up of a palate. On the anterior side, the hard palate (maxilla, premaxilla, and palatine bone), and on the posterior side, the soft palate is present.
• The mucus epithelium has thick transverse folds called palatine rugae.
• The terminal part of the soft palate hangs in the throat called the uvula.
• On the sides of the uvula, tonsils are present which are made up of lymphatic tissue.
• The floor of the oral cavity is occupied by a muscular tongue.

Tongue

• The tongue is a voluntary muscular and glandular structure.
• It is attached to the floor of the buccal cavity by a fold called the lingual frenulum of the tongue.
• An inverted V-shaped furrow termed sulcus terminalis divides the upper surface of the tongue into the anterior oral part and posterior pharyngeal part.
• The apex of the sulcus terminalis projects backward and is marked by a small median pit, named foramen cecum.
• Foramen cecum is an embryological remnant and marks the site of the upper end of the thyroglossal duct.
• The upper surface of the oral part of the tongue has small projections called papillae on its surface. These are:

Filiform Papillae: Smallest, most abundant, and have no taste buds.

Fungiform Papillae: These appear as red dots on the tongue and contain taste buds.

Foliate Papillae: Leaf-like, absent in humans.

Circumvallate Papillae: Largest in size, knob-like, and contain taste buds.

Functions Of Tongue

• It helps in the mastication of food.
• It helps in the cleaning of the buccal cavity.
• Helps in speech articulation.
• The sweat glands of dogs are present on the tongue (panting of dog); so it helps in thermoregulation.

Salivary Glands: Four pairs of salivary glands open in the buccal cavity:

Parotid: Largest—present below and in front of ears—Stenson’s duct.

Submaxillary: Medium-sized —present at the angles of the lower jaw—Wharton’s duct.

Sublingual: Smallest—located below the tongue__ Rivinus duct.

Infraorbital: Absent in man, otherwise present below eyes, for example., rabbit.

• Daily secretion of saliva is 1.5 L (pH of saliva is 6.7) and has salivary amylase (ptyalin), maltase, and lysozyme.
• Salivary glands are stimulated to secrete saliva by parasympathetic innervation; on the other hand, sympathetic nerves cause reduced secretion of saliva leading to the drying of the mouth.
• Cl- ions are required for the activation of salivary amylase.
• Viral infection of salivary glands (mainly parotid) causes mumps disease.
• Food mixed with saliva in the buccal cavity is called bolus.

Teeth: Teeth of human beings are of the following types:

1. Thecodont: Part of teeth present in the bony socket, called alveoli.

2. Heterodont: Different sizes and shapes of teeth are called heterodonts.

Teeth present on the upper and lower jaws are

• Incisors: For cutting; have a single root.
• Canines: For tearing; has a single root.
• Premolars: Upper premolars have two roots; lower premolars have one root.
• Molars: For chewing, the upper molars have three roots and the lower molars have two roots.
• Canines: Well-developed in carnivores. In rabbits, canines are absent and this space is called diastema.

3. Diphyodont: Teeth appear twice in life.

• Milk teeth (deciduous teeth/lacteal teeth)
• Permanent teeth (remains throughout life)

Classification Of Teeth According To Position

1. Acrodont: These teeth arise from jaw bones, for example, reptiles, amphibians, and fishes.
2. Pleurodont: Teeth fixed to the lateral surface of the jaw ridge, for example, fangs of snakes.
3. Thecodont: Embedded in sockets, for example, mammals and crocodiles.

Classification Of Teeth According To The Arrangement Of Enamel And Dentine

1. Bunodont: Small cusps, for example, human.
2. Lophodont: Long transverse dentine covered by enamel, for example., elephants.
3. Selenodont: Crescent-shaped cusps, for example, sheep/ cattle.
4. Secodont: Pointed cusps, for example, carnivores.

Structure Of Teeth: A tooth is made up of three parts:

1. Crown: It is the outer part of the tooth, exposed outside gums.
2. Neck: It is the middle part of the tooth which is embedded inside the gums.
3. Root: It is the part of the tooth that is inserted inside the socket of the jaw bone (alveoli).

Dental Formulae:

In a human being, 20 teeth grow twice during a lifetime, i.e., diphyodont \(\frac{2102}{2102}\) (premolars and last molars absent), and 12 teeth appear only once in life, i.e., monophyodont.

Additional Points Regarding Teeth

• Enamel (secreted by ameloblast): The hardest substance of the body—ectodermal in origin.
• Dentine (secreted by odontoblasts): Main part of tooth—mesodermal in origin.
• Caries: Decay of teeth due to degeneration of the enamel and formation of cavities.
• Pyorrhea: Infected gums and tooth sockets.

Pharynx: The pharynx is a common passage in swallowing food and breathing.

• The gullet is the aperture which leads into the esophagus air.
• The glottis is the structure that allows air to enter into the trachea.
• Epiglottis is the structure that prevents entry of food into the windpipe during swallowing in mammals.

Tonsils: The lymphoid tissues of the pharynx are called tonsils.

• Tonsils are of the following types:
• Nasopharyngeal/pharyngeal tonsil/adenoids
• Palatine/faucial tonsils
• Lingual tonsils

These are arranged in a ring-like manner called Waldeyer’s ring.

Oesophagus: It is a long (25cm) and thin tube that pierces the diaphragm and enters the abdominal cavity. Oesophagus is characterized by:

• Absence of visceral peritoneum. Its outermost fibrous (non-coelomic) covering is called tunica adventitia.
• Absence of digestive glands.
• It has mucus-secreting goblet cells.
• Presence of mucous membrane formed of non-keratinized stratified squamous epithelium.
• Presence of voluntary (anterior one-third) and involun¬ tary (posterior two-thirds) muscle fibers.

Stomach: The stomach is oval and pouch-like, divisible into cardiac,fundic, and pyloric parts

• The cardiac sphincter is present at the opening of the esophagus into the cardiac stomach and prevents the regurgitation of food into the esophagus.
• The pyloric part opens into the small intestine and the opening is guarded by the pyloric sphincter.
• The wall of the stomach has three layers of muscles, the outermost longitudinal layer, the middle circular layer, and the innermost oblique layer.
• Mucosa has folds called gastric rugae and cardiac, fundic, and pyloric glands.
• Only fundic glands secrete gastric juice. These contain neck cells (secrete mucus and are present in all three types of glands).
• Oxyntic or parietal cells secrete HCl and Castle’s intrinsic factor for absorption of B₁₂.
• HCl of gastric juice converts Fe³⁺ into Fe²⁺ which makes the absorption of iron possible.
• Non-secretion of HCl (achlorhydria) or gastrectomy can lead to iron deficiency (anemia).
• Peptic cells chief cells or zymogenic cells release large quantities of pepsinogen.

Mucus cells: A large amount of mucus is secreted, Semisolid food, mixed with gastric juices in the stomach, is known as chyme (it is highly acidic).

Compound stomach

• The stomach of ruminants is known as the compound stomach.
• It has four well-defined chambers or compartments, viz., rumen, reticulum, omasum, and abomasum.

• Rumen is the first and the largest chamber mainly meant for the storage of food.
• In camel and deer, omasum is absent, and water cells project from rumen. Digestion of cellulose takes place by fermentation, with the help of symbiont bacteria and protozoans.
• The abomasum is the true stomach, which secretes gastric juices.
• Rabbits eat their own feces (coprophagy) to complete the digestion of cellulose. It is taken as pseudo-rumination.

Small Intestine: It consists of three parts: (1) duodenum, (2) jejunum, and (3) ileum.

• The first part is the duodenum, 25 cm long, C-shaped in humans, and has an opening of the hepatopancreatic duct (bile duct + pancreatic duct).
• A small swelling is present at the opening of the hepatopancreatic duct and is called the ampulla of Vater or hepatopancreatic ampulla and the opening is regulated by the sphincter of Oddi.
• The next part ofsmall intestine is jejunum and ileum.
• The wall of the intestine has thin layers of longitudinal and circular muscles.
• Mucosa has folds plicae circulars (folds of Kerkrings or valvulae conniventes) and villi toward the lumen of the intestine.
• Epithelial cells lining the villi have microvilli which further increase the absorptive area.
• Intestinal glands or crypts of Leiberkuhn have epithelial cells (secrete mucus), path cells (secrete digestive enzymes), and argentaffin cells (probably secrete hormones).

• In the duodenum, Brunner’s glands are also present (located in the submucosa) which secrete mucus.
• Diffused patches of lymphoid tissues are present throughout the small intestine and are aggregated in the ileum to form Peyer’s patches.
• Intestinal villi are mainly concerned with absorption.
• The main function of intestinal villi is to provide a large surface area for absorption.
• The absorption of digested food mainly occurs in the small intestine mainly the ileum.
• The lymph capillary present in a villus is called a lacteal. It is concerned with the absorption of digested fat.
• The distal end of the ileum is expanded to form a small dilated spherical sac called sacculus rotundus in rabbits. The ileum opens into the cecum through an ileocecal valve.
• Half liquid food mixed with bile, pancreatic juice, and succus entericus in the intestine is called chyle (highly alkaline).

Food + Bile + Pancreatic + Intestinal juice = Chyle

Large Intestine

• It is about 1.5 m long and consists of three parts: (1) caecum, (2) colon, and (3) rectum.
• A blind pouch of the cecum is a vermiform appendix.
• These parts help in the digestion of cellulose in herbivores (rabbit).
• The wall of the colon has sac-like haustra. Histologically, the wall of the colon has three bands of longitudinal muscles called taenia coli.
• Another characteristic of the colon surface is the presence of small fat-filled projections called epiploic appendages.
• The colon part is divisible into ascending, transverse, descending, and sigmoid colon.
• The ascending colon is the smallest and is without mesentery.
• The last part of the rectum is the anal canal having a strong sphincter. It opens outside by the anus.
• In certain conditions (such as persistent constipation), rectal veins can get distended or enlarged due to weakening valves (varicosity). It leads to swollen areas called hemorrhoids (piles).

Histology Of Alimentary Canal: The alimentary canal consists of four basic layers. From the outer surface toward the inner side of the tire lumen (cavity), the layers are as follows:

Visceral Peritoneum (serous membrane or serosa): It is the outermost layer made up of squamous epithelium. It is continuous with the mesentery.

Muscular Coat: It is composed of outer longitudinal and inner circular muscle fibers.

• In the stomach, an additional layer of oblique muscle fibers is found in the circular muscle fibers.
• These muscle fibers are unstriped (smooth). The muscular coat also contains the major nerve supply to the gastrointestinal tract—the mesenteric plexus (plexus of Auerbach), which consists of fibers from both au autonomic divisions and mostly controls the peristaltic movements in the alimentary canal.

Submucosa: It consists of loose connective tissues richly supplied with blood and lymphatic vessels and in some areas with glands.

• Meissner’s plexus (submucosal plexus) is present between the muscular coat and the mucosa which is part of the autonomic nerve supply to the smooth muscles and secretory cells of mucosa glands.
• It controls various secretions of the alimentary canal and movements of the mucosa.

Mucosa (mucous membrane): It is so named because it secretes mucus to lubricate the inner lining of the gut. It is composed of three layers:

• The thin muscularis mucosa lies next to the submucosa. It consists of outer longitudinal and inner circular muscle fibers, both arc unstriped.
• Lamina propria, the middle layer of mucosa, consists of loose connective tissues, blood vessels, glands, and some lymphoid tissues.
• The innermost layer is the epithelium, which forms gastric glands in the stomach, and villi and intestinal glands in the small intestine. In the upper one-third of the esophagus, both Auerbach and Meissner’s plexuses are absent.

Digestive Glands

Liver

• Largest digestive gland. It lies in the upper right side of the abdominal cavity just below the diaphragm.
• The liver is divided into two main lobes: right and left.
• The right lobe is differentiated further into the right lobe proper, a quadrate lobe, and a caudate lobe on the posterior surface.
• The liver is surrounded by Glisson’s capsule’, its trabeculae divide liver lobes into hexagonal lobules.
• Polyhedral hepatocytes arc arranged in cords around a central venule.
• Portal triads contain the hepatic artery, portal venule, bile ductule, and lymphatics.
• Blood sinusoids are present. Kupffer cells are present in sinusoids and are phagocytic in nature.
• Gail’s bladder is situated on the inferior surface of the right lobe. It is 8 cm long and 2 cm wide.
• Bile is secreted by hepatocytes into the bile canaliculi, a series of narrow spaces between adjacent liver cells.

• The canaliculi drain via bile ductules into bile ducts, which run in portal tracts; the bile ducts themselves discharge into the right and left hepatic ducts which unite to form the common hepatic duct at the hilum of the liver.
• Gall bladder has the capacity of storing 30 to 50 mL of bile. It fills and empties via the cystic duct which joins the common hepatic duct to form the bile duct; this in turn empties into the duodenum through the ampulla of Vater (hepatopancreatic ampulla).
• At the point of its entry into the duodenum, the bile duct and adjacent pancreatic duct join each other.
• The sphincter of Boyden surrounds the opening of the bile duct into the pancreatic duct. The sphincter of Oddi surrounds the ampulla of Vater.

Functions Of Liver

1. Secretion of bile: Daily secretes 700-1000 mL. In the gall bladder, electrolytes and water are reabsorbed concentrating bile approximately 10 to 15 times.
   • The bile contains bile pigments bilirubin and biliverdin, bile salts sodium glycocholate, sodium taurocholate, and sodium bicarbonate.
   • Sodium bicarbonate is mainly responsible for the alkaline nature of bile (pH = 8.0).
   • Cholesterol is insoluble in water but its association with bile acid and phospholipid makes it soluble.
   • In the subjects, who regularly produce bile, supersaturated with cholesterol, the cholesterol may be deposited in the gall bladder as gallstones (cholelithiasis).
   • Inflammation of the gall bladder is called cholecystitis.
2. Storage of fat.
3. Urea synthesis.
4. Erythropoiesis (during embryonic period only).
5. Breakdown of RBCs.
6. Iron is stored as ferritin.

Pancreas (Heterocrine Gland)

• A racemosely branched gland situated between the stomach and duodenum.
• The pancreas consists of acini (which secretes digestive enzymes) and islets of Langerhans (which secretes insulin and glucagon hormones).
• The pancreas has two ducts with it. The first is the duct of Santorini which is accessory or non-functional, opening directly into the duodenum and the other is the duct of Wirsung which is functional and combines with the bile duct to form a common hepatopancreatic duct.

Mobility Of Alimentary Canal

• The alimentary canal undergoes regular contraction for proper digestion and absorption of food.
• Food enters into buccal cavity where it is mixed with saliva.
• Food is masticated with the help of teeth. The salivary enzyme ptyalin (salivary amylase) causes chemical breakdown of food (i.e., carbohydrates).
• Smaller food particles are held together by the mucin of saliva forming the food bolus which is then swallowed.
• The food bolus gets swallowed, i.e., enters the esophagus with the coordinated activity of the tongue, soft palate, and pharynx.
• Waves of contraction or peristaltic waves in the esophagus push it downward.
• As the food reaches the end of the esophagus, the cardiac sphincter, regulating the opening of the esophagus into the stomach, relaxes to allow the entry of food into the stomach.
• If the sphincter fails to open up properly, it leads to the accumulation of food in the lower part of the esophagus called cardia achalasia.
• In the stomach, mechanical churning of the food occurs by waves of contractions passing from the cardiac to the pyloric end, and mixing of food with gastric juice also occurs.
• During this activity, cardiac and pyloric sphincter remains closed.
• If the cardiac sphincter remains open, acidic gastric contents may escape into the esophagus, leading to a heart bum condition due to a burning sensation in the esophagus.
• From the stomach, acidic chyme enters the small intestine where digestion is completed followed by the absorption of digested food.
• From the small intestine, the chyle enters the large intestine and is finally egested out.
• Movements in the alimentary canal are caused by myenteric plexus, as well as hormones such as motilin, villikinin, gastrin, etc. It shows peristaltic movements.
• The peristaltic movements involve contraction and relaxation resulting in wave-like movement.
• Contraction is due to the contraction of circular muscles and relaxation of longitudinal muscles. Relaxation is caused by the simultaneous contraction of longitudinal muscles and the relaxation of circular muscles.
• Peristaltic movements start from the esophagus.
• The churning movements of the stomach are also peristaltic movements which become powerful as they proceed toward pylorus.
• In the large intestine, peristaltic movements are moderately weak.

Digestion And Gastrointestinal Secretions

Digestion Of Carbohydrates

• The diet of most of the animals including man consists of carbohydrates.
• Depending upon the complexity, carbohydrates are of three types: polysaccharides, disaccharides, and monosaccharides.
• During the process of digestion, both poly-and disaccharides are broken down into monosaccharides, and in this form, they can be absorbed into the body.
• Some of these complex carbohydrates are starch and cellulose present in cereal grains, potatoes, fruits, and tubers, sucrose present in cane sugar, lactose present in milk, etc.
• Enzymes that act- on carbohydrates are collectively known as carbohydrases. In the mouth cavity, the food is mixed with saliva.
• It contains an enzyme called salivary amylase or ptyalin.
• Salivary amylase acts on starch and converts it into maltose, isomaltose, and small dextrins or limit dextrin’ (disaccharides).

• Chewing and mastication of food increase the action of salivary amylase on starch by increasing the surface area of food on which the enzyme acts.
• Cooking of food also helps the action of salivary amylase by causing breaches in the cellulosic cell walls.
• The amylase can, thus, enter into the food and digest starch.
• About 30 % of the starch present in food is hydrolyzed in the mouth itself.
• The action of salivary amylase continues for sometimes even in the stomach but soon. HCl present in the gastric juice destroys all the enzymes.
• Salivary amylase is absent from the saliva of many mammals such as cows and buffaloes; and predatory carnivores such as lions and tigers. However, pigs have got amylase in their saliva.
• Pancreatic juice and intestinal juice also contain carbohydrate-digesting enzymes.
• Pancreatic juice contains pancreatic amylase that acts on starch to digest it into maltose, isomaltose, and dextrin.
• Intestinal juice contains a number of carbohydrates such as maltase, isomaltase, sucrase, and lactase.
• Maltase and isomaltase act on maltose, isomaltose, and dextrin and convert them into glucose; sucrase acts on sucrose to convert it into glucose and fructose, and lactase acts on lactose to convert it into glucose and galactose.

• Human beings can digest lactose present in the milk. But with advancing age, they also cannot digest milk. This is because less lactase is produced. In them, lactose remains undigested and gets fermented in the intestine producing gases and acids. This results in flatulence, intestinal cramps, and diarrhea. So these persons must consume curd or yogurt (sweetened curd) as lactose is fermented to lactic acid in them.
• Galactosemia: Galactosemia is a metabolic disorder due to the absence of the enzyme uridyl transferase. As a result, galactose will accumulate leading to mental retardation.
• It can be prevented in galactosemic children by giving them a milk-free diet.

Digestion Of Proteins

• Proteins are complex organic compounds made up of single units called amino acids.
• So in the process of digestion, all proteins are broken down into amino acids.
• Enzymes that hydrolyze proteins are collectively known as proteases or peptidases.
• Many of these enzymes are secreted in their inactive font or proenzymes as their active form would hydrolyze cellular and extracellular proteins of the organism itself.
• Inactive enzymes are converted to their active form only at the site of action.
• Saliva as such does not contain any protein-digesting enzyme, but it can denature the uncooked natural proteins, like the ones present in raw egg, unboiled milk, or uncooked germinating seeds.
• The action of gastric juice: The gastric glands of the stomach produce light-colored, thin, and transparent gastric juice.
• It contains water, hydrochloric acid (0.3%), and inactivated enzymes, prorennin and pepsinogen.
• The presence of MCI makes the medium highly acidic (pH = 1 or 2) so that pepsin can act on proteins to convert them into peptones and proteoses. However, there is no pepsin in invertebrates.
• Both prorennin and pepsinogen are converted to their active forms in the presence of HCl.
• Pepsin and rennin can also perform the same function once they are formed. HCl helps to kill bacteria and other harmful organisms that may be present along with the food.
• Rennin acts on the casein protein of milk and converts it into paracasein which in the presence of calcium ions forms calcium paracaseinate (curdling of milk).
• Adult humans do not produce renin.
• The function of renin is then taken over by pepsin and other milk-coagulating enzymes in them. These reactions are summarized below:

• Pepsin can even digest collagens of connective tissue fibers, but it cannot act on the keratins of horns, hair, skin, or nails.
• The action of pancreatic and intestinal juice: Both pancreatic juice and intestinal juice (succus entericus) are poured into the small intestine.
• Pancreatic juice contains trypsinogen, chymotrypsinogen, procarboxypeptidases, lipases, amylases or amylopsin, DNAases, and RNAases.
• All these enzymes of pancreatic juice can act only in the alkaline medium.
• This change in the medium of food, from acidic to alkaline, is done by the bile juice. Therefore, bile juice acts on the food before the action of pancreatic juice. All these actions are given below:

• In predatory animals, trypsin can hydrolyze fibrinogen of blood into fibrin leading to blood coagulation. However, it is unable to bring about coagulation of milk. Also, trypsin cannot hydrolyze keratins.
• Chymotrypsinogen (inactive) is activated to chymotrypsin by trypsin itself. Chymotrypsin is another important milk-coagulating enzyme and can hydrolyze casein into paracasein which then coagulates to form calcium paracaseinate. However, unlike renin, it acts in an alkaline medium. Chymotrypsin can act on other proteins also.
• Carboxypeptidase hydrolyzes the terminal carboxyl group from peptide bonds to release the last amino acid from the peptides thus making the peptide shorter.
• Intestinal juice or succus entericus contains two protein-digesting enzymes aminopeptidases and carboxypeptidases which hydrolyze the terminal amino group from peptide bonds to release the last amino acid from the peptides thus making the peptide chain shorter.
• Dipeptidase acts on dipeptides to release individual amino acids.
• Enterokinase is also released which activates trypsinogen to trypsin.

Digestion Of Fats

• Fat digestion starts only when the food reaches the small intestine.
• It starts with the action of bile juice from the liver.
• Bile juice contains bile salts that break down the bigger molecules of fat globules into smaller droplets by reducing the surface tension of fat droplets. This process is known as emulsification of fats.
• Lipase is the chief enzyme that acts on emulsified fats.
• It is present both in pancreatic juice and intestinal juice.
• Pancreatic lipase (steapsin) is the strongest lipase.
• Lipase converts emulsified fats into diglycerides and monoglycerides releasing fatty acids at each step.
• At the end of digestion, all fats are converted into fatty acids, glycerol, and monoglycerides.

Digestion Of Nucleic Acids

• Nucleic acids are digested in the small intestine with the help of pancreatic and intestinal juices.
• Pancreatic juice contains two nucleases: DNAase and RNAase. Intestinal juice contains nucleotidase and nucleosidase.

Absorption

Almost no absorption takes place in the mouth and esophagus. Water, alcohol, simple salts, and glucose are absorbed in the stomach. In the small intestine, absorption of all digested materials takes place by active, passive, and facilitated transport.

• Glucose, sodium, and amino acids are absorbed actively.
• The absorption of glucose or amino acids involves carrier-mediated transport which binds glucose/amino acid at one site and Na⁺ at the other site. Therefore, the movement of glucose/amino acid is coupled to the concentration gradient of Na⁺ (Co transport).
• Na⁺ moves along the concentration gradient while glucose/amino acids are moving against a concentration gradient.
• The rate of absorption of galactose is the highest.
• Fructose is absorbed by facilitated diffusion.
• The products of fat digestion, monoglycerides, fatty acids, and glycerol are first incorporated into water-soluble droplets called micelles (a combination of fatty acids, monoacylglycerols, and bile salts); reconstructed to triglycerides in the absorptive cells; and released into lymph in the form of protein-coated water-soluble fat droplets called chylomicrons.
• In the large intestine, only water is absorbed. Absorption of vitamin B₁₂ (cobalamine) in man requires a glycoprotein, called intrinsic factor (IF) secreted by the parietal cells of the stomach. Failure to absorb cobalamine causes pernicious anemia associated with a failure of RBC maturation (megaloblastic anemia) and neurological abnormalities.

Assimilation Of Food

The absorbed food materials are transported by blood and lymph. Lymph is finally transferred to the blood circulation. The blood transports absorbed food materials to different body cells where food materials become integral components of the living protoplasm and are used for energy, growth, and repair. This is called the assimilation of food.

• Amino acids are not stored but are taken up by the cells in connection with the synthesis of proteins.
• Proteins are used for growth, repair, etc. Excess amino acids can be converted into glucose and then to fat and are thus stored. This is an irreversible reaction.
• Amino acids can also be converted to glucose and used as fuel for the cell.
• During their conversion to glucose, the amino acids are deaminated (removal of amino groups —NH₂).
• The liver is the chief site for deamination, i.e., a process by which the amino group is removed from amino acids resulting in the production of ammonia.
• Ammonia is soon converted into urea, which is filtered from the blood in the kidney.
• The excess of monosaccharides (glucose, fructose, and galactose) is usually stored in the liver and muscle cells in the form of glycogen (glycogenesis).
• Whenever there is a deficiency of glucose in the blood, glycogen is converted into glucose (glycogenolysis).
• Muscle glycogen is utilized during muscle contraction. Glucose is utilized in the production of energy for various body activities.
• A considerable amount of glucose is converted into fat and stored as such.
• The fat is stored in the fat deposits of the body, such as subcutaneous layers, mesenteries, etc.
• The stored fat is a readily available source of fuel for the cells.
• Fat has important insulating properties in connection with the conservation of heat and maintenance of body temperature.
• Fat also plays a protective role as filling or packing material, between and around the organs.
• In the liver, phospholipids are formed which are returned to the blood, to be used by all the cells. In liver cells, fats are converted into amino acids and carbohydrates.
• Vitamins, salts, and water are also useful for various metabolic processes.

Egestion

Peristalsis gradually pushes the slurry of indigestible materials of the small intestine into the large intestine or colon. Approximately, 1500 mL of chyme normally passes through the large intestine each day. The colon absorbs most of the water, electrolytes, and ions from these contents. This is accomplished by the active pumping of sodium and water by osmosis from die chyme.

• The other function of the colon is to help in the excretion of excess salts from the blood.
• The population of Escherichia coli (bacterium), which is a resident species of the colon, lives on this undigested matter. This bacterium, in turn, produces vitamin B₁₂, vitamin K, thiamin, and riboflavin which are absorbed by the wall of the colon.
• Later on, the chyme is slowly solidified into coherent feces, which are about three-fourths water and one-fourth solid matter consisting of about 10-20% inorganic substance 30% dead bacteria, 10-20% fat, 2-3% protein, and 30% undigested roughage and dry constituents of digestive juices.
• Feces are given out through the anus by the process of defecation or egestion.
• The breakdown of bile pigments occurs, forming stercobilin pigment, which provides a brownish color to the feces.
• The foul smell of the fecal matter is due to skatole (3-methyl indole).
• Dark green mucilaginous material in the intestine of the full-term fetus is called meconium (includes residue from the swallowed amniotic fluid by the fetus and the residues of excretory products from intestinal mucosa and glands).

Cellulose Digestion In Ruminants

The minants ingest large quantities of fodder-rich food in cellulose or lignin. Neither substance can be digested directly by mammals, but the cellulose is attacked in the rumen by the enzymes produced by symbiotic bacteria. Sheep and cattle initially swallow food without masticating. In the rumen, food is mixed with water.

• The rumen has millions of bacteria of several species, some of which attack the cellulose walls of the fodder liberating the cell contents of the food.
• These bacteria also break down carbohydrates and proteins into simple substances.
• In the rumen, the main products of fermentation are acetic, propionic, butyric, and other acids, along with small quantities of other substances such as ethanol.
• Besides this, the symbiotic organisms in the rumen synthesize much more riboflavin and pantothenic acid than is normally procured through diet.
• In addition to bacteria, a number of protozoan ciliates have also been observed in the rumen. Some of these can break down cellulose.
• The stored carbohydrates and proteins within these ciliates are believed to serve as reserves for some time when fodder is not available.
• Cellulose breakdown by bacteria also takes place in the stomach of kangaroo (Setonix brachytirus), and sloth (Choloepus). In kangaroos, the stomach is larger, and in the sloths, there is a compound stomach.
• Cellulose-splitting bacteria are present mostly in the cecum and ventral colon of horses and in the cecum of rabbits. The rabbits, like other lagomorphs and many rodents, have developed a habit of ingestion or pseu domination.
• When a rabbit eats fresh food, it directly reaches the cecum, remains there for one or two days undergoing fermentation, and is then expelled as soft feces.
• These are then again eaten and this time they reach the cardiac stomach, but the freshly eaten food goes straight into the cecum as usual.
• After digestion and absorption, the twice-swallowed food is excreted as hard fecal pellets without letting it in through the cecum.

Nutritional Requirements Of Humans

Carbohydrates are used primarily as sources of chemical energy, to be either metabolized immediately as glucose or stored as glycogen. The synthesis of glycogen is called glycogenesis. The liver can store enough glycogen to maintain blood glucose levels for several hours. Under acute starved conditions, the liver cells begin to convert amino acids and glycerol (digestive products of fat molecules) into glucose. Such production of new glucose is known as gluconeogenesis.

• Proteins are used as structural components of tissues as channels, transporters, regulatory molecules, and enzymes.
• Proteins can also be utilized as energy sources when broken down into amino acids.
• Out of the 22 amino acids identified so far as the constituents of proteins, eight (10 in children) cannot be synthesized in the human body.
• These must be provided in the diet from outside and are designated as essential amino acids.
• Lipid (fat) molecules are especially suitable as concentrated energy reserves. The fat cells of adipose tissues are often called the fat depot of the body.
• Triglycerides are used as fuel. The human body is able to synthesize most of the lipids in enough quantity, except three polyunsaturated fats, such as linoleic, linolenic, and arachidic acids.
• These essential fatty acids must be provided to the human body through diets.

Calorific Value

The amount of heat liberated from the complete combination of 1g of food in a bomb calorimeter (a dosed metal chamber filled with O₂) is its gross calorific value GCV) or gross energy value. The actual amount of energy liberated in the human body due to the combustion of 1g of food is the physiologic calorific value (PCV) of food.

• The energy requirements of animals and the energy content of food are expressed in terms of a measure of heat energy because heat is the ultimate form of all energies. This is often referred to as calorie (cal) or joule (J). which is the amount of heat energy required to raise the temperature of 1g of water through 1°C.
• Since this value is a tiny amount of energy, physiologists commonly use kilocalories (kcal) as a unit of measure (1 keal = 1000 eal) or kilojoule (kJ). One kilocalorie is the amount of energy required to raise the temperature of 1 kg of water through 1°C.
• Nutritionists traditionally refer to kcal as Calorie or Joule (always capitalized).

Minerals And Vitamins

Both minerals and vitamins occur as small molecules and mostly do not require digestion. Minerals are ingested as salts dissolved in water or as part of organic compounds (food). Still, a few of the minerals are absorbed with the aid of digestive juices (like bile) and gastric juices. Of the 21 essential minerals required by man, some are important for maintaining fluid balance, whereas others help to regulate metabolism by acting as a component of enzymes.

• Vitamins are essential for normal metabolism, growth, and sound health.
• Humans can synthesize vitamin A (retinol) with the help of plant pigment, carotene, which is available in yellow and green leafy vegetables.
• Vitamin A forms the retinal pigment of human eyes such as rhodopsin of rod cells and iodopsin of core cells.
• Humans can also synthesize vitamin D (calciferol) in their skin in the presence of ultraviolet rays of light. Although most animals can synthesize vitamin C from glucose, humans cannot; hence, they require it in their diet. Vitamin Bl7 is a newly discovered vitamin with anticancerous properties.
• Chemical nature: Vitamins differ greatly in their chemical nature. Generally, they have nothing in common. Some vitamins are alcohol, some arc proteins, some arc quinone, and some are sterol.
• Provitamins: These are compounds that are changed into vitamins in our body. Examples arc:
• Ergosterol present in food is changed in the skin into vitamin D in the presence of sunlight.
• Carotene, a pigment present in carrots is changed in the liver and in the small intestine into vitamin A.
• Vitamers: This term is used for different forms of the same vitamin. Vitamers are also called isotopic vitamins. Examples are:
  • Vitamin A has vitamins A₁ and A₂ (vitamins).
  • Vitamin D, has vitamin C, D₂, and D₃ (vitamins).
• Vitamin poisons: These are compounds that replace vitamins in our body, for example, antibiotics. Sulfa drugs replace vitamin C; tetracycline or ampicillin replace vitamin B complex.

Nutritional Deficiencies And Disorders

Deficiencies of nutrients such as vitamins, minerals, and proteins, in the food are related to specific disorders, diseases, and abnormalities in humans. Impairment of health due to improper intake of food or nutrients results in the effect recognized as malnutrition. Malnutrition is a term that covers problems of both undernutrition and overnutrition. Two types of diseases due to inadequate nutrition are described.

Differences between kwashiorkor and marasmus:

• An individual or a group of individuals may be undernourished due to non-availability of food, and hence, deficiency of minimum required food and nutrients.
• In this situation of undernutrition, the intake of food is too insufficient to meet the needs for metabolic energy. Consequently, the individual shall have to make up the shortfall by metabolizing some molecules of its own body.
• Excess intake of food and nutrients may cause a great deal of harm to the body. The excess nutrients are stored as increased body mass. Such a situation is attributed to overnutrition. Excess intake of saturated fats such as butter, ghee, vegetable oils, red meat, eggs, etc., often leads to hypercholesterolemia, a condition in which blood cholesterol content becomes abnormally high, ultimately leading to cardiac disorder.
• Deposition of cholesterol on the walls of blood vessels stiffens the blood vessels and increases blood pressure. Besides, excessive intake of calories (sugar, honey, ghee, etc.) may produce overweight and obesity (excessive accumulation of fat in tissues), which is the most common form of overnutrition.
• Very high intakes of minerals and fat-soluble vitamins (obtained from food sources alone) can be toxic. This is because they are stored in the body. With the exception of folic acid (women of childbearing age), people who have well-balanced diets that supply enough energy do not usually need to take dietary supplements.
• But if they do decide to take supplements, then they should follow the advice on the label to reduce the risk of an overdose.

Disorders Of the Digestive System

• Inflammation Of The Intestinal Tract: The most common bacterial viral infection may be caused by intestinal parasites such as tapeworms, roundworms, threadworms, hookworms, and pinworms.
• Jaundice: The Liver is affected, and skin and eyes turn yellow due to the deposition of bile pigments.
• Vomiting: This reflex action is controlled by the vomiting center in the medulla. A feeling of nausea precedes vomiting.
• Diarrhea: It reduces the absorption of food, due to abnormal frequency of bowel movement.
• Constipation: Due to irregular bowel movement, feces are retained within the rectum.
• Indigestion: Feeling of fullness as the food is improperly digested. It is due to inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.
• Belching: It occurs usually when the stomach is over-dilated, air rises, and is expelled through the mouth producing a burping sound.
• Obesity: It is a condition when fat storage in the body increases, leading to its abnormal deposition in the subcutaneous layer. It is caused when energy input exceeds energy output in the body. 8.3 calories of energy from surplus food increases 1g of fat deposition in the body causing obesity.
• Flatus: It is an accumulation of gases in the gastrointestinal tract that are expelled through the anus producing a characteristic sound.
• Hepatitis: It is a condition of inflammation of the liver caused by infection of bacteria, viruses, or protozoa. It may cause cirrhosis.
• Colitis: It is a disorder in which inflammation of the colon and rectum, loose motions, bloody feces, and dehydration occur. It is caused by the infection of protozoans such as Entamoeba.
• Appendicitis: The wall of the vermiform appendix ruptures and bacteria are released in the coelomic cavity. Severe infection may cause death. By surgery, the appendix is removed.
• Hernia: It is a protrusion of the intestine into the inguinal canal and may extend into the scrotal sac.
• Mumps: It is a viral infection in parotid salivary glands. The throat region swells and fever develops.
• Nausea: It refers to the discomfort which leads to vomiting. It may be caused by distension of the stomach or of the gastrointestinal tract.
• Vomiting: It is a condition developed by reverse peristalsis caused by harmful substances in the stomach irritation in the pharynx or disturbance in semicircular canals. Food substance comes out through the mouth. Vomiting center is located in the medulla oblongata.
• Tonsillitis: The pharyngeal tonsils (lymphoid tissues) become infected with microorganisms. It causes pain in the throat followed by fever.

Summary Of Digestion:

Gastrointestinal Hormones In Mammals:

Summary Of Chemical Digestion Of Food:

Summary Of Human Vitamins:

Digestion And Absorption Points To Remember

1. Assimilation: Utilization of absorbed material by the cell.
2. The hunger center is in the hypothalamus.
3. The Satiety center is also in the hypothalamus.
4. Heartburn has nothing to do with the heart. It is caused by the regurgitation of acid from the stomach into the esophagus.
5. Splanchnology is the study of the viscera.
6. NIN: National Institute of Nutrition, Hyderabad.
7. Anorexia: Loss of appetite.
8. Spoilt hay of sweet clover Melilotus indica (fodder and green manure) contains a substance called dicumarol. Dicumarol prevents the action of vitamin K as it is antagonistic to it.
9. What destroys the vitamins? Overcooking and excessive boiling, medicines such as aspirin, antacids, and diuretics lead to iron deficiency—anemia.
10. Tea/coffee inhibits the absorption of iron from the diet. Prolonged consumption of tea or coffee after meals can lead to iron deficiency—anemia.
11. In the upper one-third of the esophagus, only skeletal muscles are found.
12. The chief seat of water absorption is the small intestine.
13. The liver produces proteins such as albumin, fibrinogen, and prothrombin, but does not produce globulin.
14. Poison glands of a snake are modified labial glands, homologous to parotid salivary glands.
15. Vomerine teeth of frogs kill the prey.
16. The tongue of a whale is not movable.
17. The gall bladder is absent in adult lamprey (jawless vertebrate), grain-eating birds, rats, whales, all the perissodactyla (odd-toed hoofed mammals such as horses), and some artiodactyla (even-toed hoofed mammals).
18. Alcoholics are short of vitamin C.
19. C-shaped duodenum is a characteristic of man.
20. During a high fever, one does not feel like taking meals because a high temperature shuts off the appetite center.
21. Bile is alkaline in humans, but acidic in cats and dogs.
22. Basal metabolic rate (BMR) is the minimum energy requirement for the maintenance of the body during rest or sleep. For a normal human adult, it is 1600 keal/day.
23. Routine metabolic Rate (RMR): It is the energy requirement of a moderately active person. RMR is 2800 keal/day for adult males and 2200 keal/day for adult females.
24. Entero-hepatic circulation: Of the total bile salts that enter the duodenum. 90-95% are reabsorbed actively from the terminal ileum in the portal vein and return to the liver, to be excreted again This is enterohepatic circulation. Approximately 93% of the cellular material is composed of C, H, and 0, and 2% is composed of N, P, Cl and S, I, F, B, are present in traces.
25. Choloretics are the substances that increase bile secretion from the liver for example, bile salts.
26. Cholagogues are the substances that cause the contraction of the gall bladder.
27. Achalasia cardia condition is characterized by failure of the cardiac sphincter to relax completely on swallowing, causing food accumulation in the esophagus and proximal esophagus to dilate.
28. Achloroydria means a lack of HCl secretion in the stomach. The capacity of the human stomach is 1.5-1.7 L.
29. Elephant tusks are modified incisor teeth.
30. Tusks of walrus are modified canines.
31. The teeth of sloths and armadillos have no enamel.
32. Spiny anteaters, scaly ant eaters, and some whales are toothless.

Comparative Study Between A Rabbit And A Human:

 

Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: Gastrectomy causes iron deficiency anemia.

Reason: Hydrochloric acid secreted by oxyntic cells converts ferric into ferrous and iron is absorbed as ferrous ions.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Question 2. Assertion: Cholagogues are substances that cause the contraction of the gall bladder.

Reason: These substances cause a release of CCK-PZ from the duodenum

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Question 3. Assertion: Aptyalism patients have higher than normal incidences of dental caries.

Reason: Aptyalism is caused by the action of the parasympathetic nervous system.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: In humans, the duct of Wirsung from the pancreas combines with the bile duct before opening into the duodenum.

Reason: Blockage in the duct of Wirsung will hamper the endocrine function of the pancreas.

Answer: 3. If Assertion is true, but Reason is false.

Question 5. Assertion: In acute constipation, purgatives containing magnesium salts are generally used.

Reason: The osmotic effect of Mg²⁺ in the intestinal lumen prevents water reabsorption from the intestine. Mg²⁺ increases the solute concentration in the intestinal lumen because Mg²⁺ is absorbed very slowly.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Anatomy Of Flowering Plants Introduction

The branch of botany dealing with the internal organization of plants is called anatomy. N. Grew laid the foundation of plant anatomy. He coined the term tissue. He is known as the father of anatomy.

Tissues

A group of similar or dissimilar cells that perform a common function and have a common origin is called tissue. Tissues are classified into two main groups: meristematic and permanent.

Meristematic Tissues

1. Mesistematic tissues consist of cells that retain the power of division.
2. The protoplasm within the cell is dense, the vacuole is smaller or absent.
3. These cells are isodiametric without intercellular spaces.
4. The nucleus is bigger in size.
5. These cells have thin cellulosic cell walls.
6. These are metabolically active cells with high surface area per unit volume and nucleocytoplasmic ratio.
7. Ergastic substances are absent.
8. Colorless proplastids are present in the cells.

Classification Of Meristems

On The Basis Of Origin And Development

1. Promeristems (Primordial Meristem): Promeristerms are a group of cells that represent the primary stages of meristematic cells. They are found at the apices of embryonic shoots and roots. They give rise to primary meristems.
2. Primary Meristems: They originate from pro meristems. They are found at shoot and root apices, at the apex of leaves, and in intercalary parts. They give rise to primary permanent tissues.
3. Secondary Meristems: They are not present from the beginning of the formation of an organ but develop at a later stage. They give rise to secondary permanent tissues. They develop from primary permanent tissues, for example, interfascicular cambium, cork cambium, and cambium in dicot roots.

On The Basis Of Position

1. Apical Meristem: These cells or tissues are found at the apices of the stem and root. Due to continuous division, the root and stem increase in length. The apical meristem helps the plants to grow in length.
2. Intercalary Meristem: The tissues are intercalated between permanent tissues. These are actually the parts of the apical meristem that get separated from it during the growth of the stem and root in length. The most characteristic example is the stem of grasses and Equisetum. Intercalary meristem is especially responsible for the increase in the length of the stems of grasses.
3. Lateral Meristem: These meristems are present along the lateral side of the stem and root. They divide in a tangential plane, giving rise to secondary permanent tissues on the inner and outer sides and leading to the increase in thickness or girth of the plant body, for example, intrafascicular cambium, inter¬fascicular cambium, and cork cambium.

Read and Learn More NEET Biology Notes

On The Basis Of Plane Of Cell Division

1. Mass Meristem: The cells divide anticlinally in all planes so that a mass of cells is formed. For example, the formation of spores, cortex, pith, endosperm, etc.
2. Plate Meristem: The cells divide anticlinally in two planes, so the plate-like area is increased. For example, the formation of epidemics and lamina of leaves.
3. Rib Or File Meristem: The cells divide anticlinally in one plane, so a row or column of cells is formed. For example, the formation of lateral roots.

On The Basis Of Function

1. Protoderm: They are the outermost meristematic cells. They form the skin or epidermis of the plant and epidermal tissue system.
2. Procambium: They are the innermost meristematic cells. They form the primary xylem, primary phloem, and cambium.
3. Ground Meristem: They form ground or funda¬mental tissues such as hypodermis, cortex, pith, pericycle, etc.

Shoot Apex Organization: Shoot apex is present immediately above the youngest leaf primordia. It consists of meristematic cells. The lateral branches of the stem and leaves are formed by the activity of shoot apex. Many theories have been put forward to explain shoot apex such as

Apical Cell Theory: It was proposed by Hofmeister and Nageli. According to this theory, a single apical cell leads to the development of the entire plant body. This theory is applicable to algae, as well as to most of the bryophytes and pteridophytes.

Histogen Theory: It was proposed by Hanstein. According to this theory, a shoot apex consists of the following halogens:

1. Dermatogen: The outermost layer, it forms the epidermis (skin) and epidermal tissue system.
2. Periblem: It gives rise to the tissues between the epidermis and the stele.
3. Plerome: It is the innermost layer and the central mass of cells that give rise to the central stele.

Tunica Corpus Theory: It was proposed by Schmidt (1924). It is based on the plane of division of the cells. According to this theory, the shoot apex consists of two distinct layers:

1. Tunica: It is mostly single-layered and forms epider¬mis. The cells of tunica are smaller than the corpus and divide mostly by anticlinal divisions.
2. Corpus: It represents the central core with larger cells. The cells divide periclinally. Sometimes tunica is multi-layered, only the outer layer forms epidermis, and the remaining layers with corpus form the cortex of the shoot.

Root Apex Organization: The Root apex consists of a mass of meristematic cells. It is not responsible for the formation of lateral roots. Root cap or calyptrogen is present due to which the root meristem becomes subterminal in position. If the root cap is independent in origin, it arises from dermatogen. Regarding the organization of root apex, the following theories have been put forward

Korper-Kappe Theory: It was proposed b.y Schuepp (1917). This theory is comparable with the tunica corpus theory of shoot apex. Korper means body and kappe means cap.

Quiescent Center Theory: It was proposed by Clowes (1956-5 8). According to this theory, the root apex consists of an inverted cup-like structure, the quiescent center. The cells of this region are with very low mitotic activity (quiescent). They have low amounts of RNA, DNA, and protein. They are surrounded by a layer of actively dividing cells which are responsible for the formation of the different structures of roots.

Permanent Tissue

Permanent tissues are composed of living or dead cells that are derived from the meristematic tissues but have lost their ability to divide. There are of three types: simple tissues, complex tissues, and secretory tissues.

Simple Tissues: They are made up of a single kind of cells performing similar functions. Simple tissues are mainly of three types: parenchyma, collenchyma, and sclerenchyma.

Parenchyma (Grew): These cells are found almost in all parts of plants such as roots, stems, leaves, fruits, and seeds. These cells are isodiametric, spherical, oval, or polygonal with intercellular spaces. These cells are living and with thin cellulosic cell wall.

Types Of Parenchyma

• Prosenchyma: Elongated parenchyma with tapering ends is called prosenchyma.
• Aerenchyma: The parenchyma which encloses the air cavity is called aerenchyma (hydrophytes)
• Chlorenchyma: The parenchyma containing chloroplasts is called chlorenchyma.
• Idioblast: Sometimes parenchyma stores secretory substances (orgastic substances) such as tannins, resins, and gum. They are called idioblasts.

Storage Parenchyma: Examples are fruits, and endosperm.

Collenchyma (Schleiden): Collenchyma has thickenings on the cell wall and in corners of intercellular spaces. They are not found in the roots and monocots. These cells form hypodermis in the stem and petiole. These are living mechanical tissues with high refractive index. The thickening material in the cell wall contains pectocellulose.

Types Of Collenchyma

• Angular Collenchyma: Angular walls thickened, for example, the stem of a marigold and tomato.
• Lamellate Collenchyma: Tangential walls thickened, for example, the stem of a sunflower.
• Lacunate Collenchyma: Lacunate thickening, intercellular spaces are present, for example, stem of cucurbita.
• Functions Of Collenchyma: They provide mechanical support, flexibility, and elasticity to the organs. Due to the peripheral position in stems they resist the bending and pulling action of wind. Collenchyma is especially useful for young plants and herbaceous organs where these act as important supporting tissues.

Sclerenchyma (Mettenius): Sclerenchyma has thickened secondary walls due to the deposition of lignin. At maturity they become dead. These cells have simple pits. They are of two types: Sclereids and sclerenchymatous fibers.

Sclereids: They may be spherical, oval, or cylindrical. They are lignified, and extremely thick-walled. So the lumen of the cells is almost obliterated. They are found in hard parts of the plant.

Types Of Sclereids

1. Brachysclereids (Stone Cells): Grittiness in fruits is due to stone cells, for example, pear, and sapota.
2. Macrosclereids (Rod Cells): Found in the seed coat of leguminous plants.
3. Osteosclereids (Prop Cells): Found in the leaves and seed coat of many monocots and the sub-epidermis of legume seed coats.
4. Astrosclereids (Star Cells): They are common in the stem and leaves of dicots, for example, tea leaves, petiole of lotus, etc.
5. Trichosclereids (Internal Hair): Long, hair-like branched sclereids. They are common in hydrophytes. These are also present in the aerial roots of Monstera.

Sclei Enchymatous Fibers: They are long and tapering at ends. These are thick-walled cells (lignified). These fully developed fiber cells are always dead. The length of fiber varies from 72 to 550 mm in angiosperms and 1 to 12 mm in gymnosperms.

The fibers are present in the hypodermis of monocot stems, in the pericycle of many dicots, in the secondary wood, and in the vascular bundle sheath in monocot stems, for example, jute, flax, hemp, etc. Living fibers are found in tamarix.

Function Of Sclerenchyma: The main function of sclerenchyma is to provide mechanical strength.

Complex Tissues: A group of more than one type of cells having a common origin and working together as a unit is called complex tissue. These are made up of different types of cells: xylem and phloem.

Xylem (Nageli) Or Hadrome (Haberlandt): Xylem is the chief water-conducting element. It consists of the following types of cells

Tracheids: They are elongated cells with pointed chisel-like ends. Their wall is tough, thickened, lignified, and thickened or may be annular, spiral, reticulate, scalariform, or pitted. Cells are dead at maturity and have bordered pits.

In pteridophytes and gymnosperms, wood mainly consists of tracheids (no vessels). In angiosperms, tracheids are associated with vessels. The main function of tracheids is the conduction of water. Tracheids are the most primitive type of conducting elements in the xylem.

Vessels: Vessels are also elongated and tube-like. They are formed from a row of cells placed end to end. The partition walls are either perforated or disappear altogether resulting in an elongated tube. Walls are thickened, and lignified, and may have annular, spiral, reticulate, or scalariform thickening.

Vessels are dead at maturity and without nuclei. In pteridophytes and gymnosperms, vessels are absent (non-porous wood). Sometimes primitive vessels are present in gnetum and ephedra (Gnetales).

Vessels are characteristic of angiosperms (porous wood). Vesselless angiosperm families are Tetracentraceae, Trochodendraceae, and Winteraceae. The main function is the conduction of water. Vessels are an advanced type of conducting element.

On the basis of distribution and size of vessels, porous wood is of two types:

1. Diffuse Porous Wood (Primitive): Vessels of the same size are uniformly distributed throughout the growth, for example, Pyrus, Betula.
2. Ring Porbus Wood (Advanced): Large vessels are formed in early wood when the need of water is great and small vessels are formed in latewood, for example, Quercus, and morus.

Wood Or Xylem Fiber: These cells are elongated and pointed at both ends. Cell wall is highly lignified having simple pits. The air commonly found in the secondary xylem. They may lie

1. Fiber Tracheids: Fiber such ns tracheids with bordered pits.
2. Libriform Fiber: They have extremely thick walls and simple pits. They provide mechanical support.

Wood Or Xylem Parenchyma: They are living parenchymatous cells associated with xylem. They may occur as axial parenchyma or ray parenchyma. When parenchyma is diffused or not associated with vessels, they are called as apotracheal parenchyma and when parenchyma surrounds or is associated with vessels, it is called para tracheal parenchyma.

On The Basis Of Origin Xylem Is Of Two Types:

1. Primary Xylem: It is derived from procambium during the formation of the primary plant body. It is differentiated into protoxylem (formed first and consists of tracheary elements and xylem parenchyma) and metaxylem (formed later and consists of tracheary elements, xylem parenchyma, and fiber). The cells of metaxylem are bigger in size than the protoxylem.
2. Secondary Xylem: It is formed from cambium during secondary growth. It is well differentiated into two systems:
   • Axial Or Vertical System
     • Tracked elements (tracheids and vessels): For conduction of water
     • Xylem or wood fiber: For support
     • Xylem parenchyma: For storage of food
   • Ray Or Horizontal System: Ray parenchyma: For storage of food.

Phloem (Nageli) Or Bast Or Leptome (Haberlandt): The main function of phloem is the transport organic food materials from leaves to stems and roots in down word direction. Phloem consists of the following types of cells

Sieve Element: The sieve elements in angiosperms are sieve tubes which are cylindrical tube like cells with perforated cross walls called sieve plates. Sieve tubes are associated with companion cells, and they are without nuclei. In pteridophytes and gymnosperms, the sieve elements have sieve plates on their lateral walls and companion cells are absent. They are called as sieve cells.

The walls of sieve tube elements are made up of cellulose and pectic substances. The cytoplasm is confined to a thin peripheral layer. P-proteins are proteinaceous structures present in sieve tubes and are believed to be responsible for

1. The movement of materials and
2. The sealing of pores after wounding.

In old sieve tubes, at the end of the growing season, a callose plug (made of callose carbohydrate) is deposited on the sieve plate which inhibits the activity of the sieve tubes. In the spring season, the callose plug gets dissolved.

Companion Cells: They are elongated living parenchymatous thin-walled cells. They are associated laterally to sieve tubes and have dense cytoplasm and nuclei. Companion cells are absent in pteridophytes and gymnosperms. Both sieve tubes and companion cells are related ontogenetically because both develop from the same mother cell. So these are sister cells.

Phloem Or Bast Fiber: They are absent or fewer in phloem and abundantly found in secondary phloem. They are sclerenchymatous fibers associated with phloem. Phloem fibers of plants such as jute, flax, and hemp are rotted in water and extracted for making ropes and coarse textiles.

Phloem Parenchyma: They are parenchymatous living cells with cellulosic cell walls and nuclei. The main function is the storage of food. They are not found in monocotyledonous plants.

Types Of Phloem

On The Basis Of Position

1. External Phloem: It is of normal type and is present outside the xylem, for example, mostly angiosperms and gymnosperms.
2. Internal Or Intraxylary Phloem: It originates from procambium and is the primary phloem which occurs on the innerside of primary xylem in bilateral bundles, for example, members of Apocynaceae, Asclepiadaceae, Convolvtilaceae, Solanaceae etc.
3. Included Or Interxylary Phloem: It originates from cambium and is a secondary phloem that occurs in groups within the secondary xylem, for example, Leptadaenia, Salvadora, Chenopodiwn, Boerhaavia, Amaranthus, etc.

On The Basis Of Origin:

1. Primary Phloem: It develops from procambium and does not have radial differentiation or rays are absent. It is differentiated into protophloem (consists of sieve elements and parenchyma) and metaphloem (develops after protophloem and consists of sieve elements, parenchyma, and fiber). During primary growth, the protophloem elements are crushed by the surrounding tissues and disappear. This process is known as obliteration.
2. Secondary Phloem: It develops from cambium during secondary growth. It shows radial differentiation. It consists of two distinct systems such as
   • Axial or vertical system
     • Sieve Elements: Sieve tube and companion cells. For conduction of food.
     • Bast fiber: For support.
     • Bast Parenchyma: For storage of blood.
   • Ray Or Horizontal System
     • Ray Parenchyma: It is for the storage of food

Secretory Tissues: Secretory tissues perform special functions in plants. For example, secretion of resins, gums, oil, and later. These tissues are of two types laticiferous and glandular tissues.

Laticiferous Tissues: They contain colorless, milky, or yellow-colored juice called latex. These tissues are of two types

1. Latex Cells: They do not fuse and do not form a network. Plants having such tissues are called simple or non-articulated laticifers, for example, Calotropis (Asclep Fabaceae), Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).
2. Latex Vessels: They are formed due to the fusion of cells and form a network-like structure. Plants having such tissues are called compound or articulated laticifers, for example, Argemone, Pabaver (Papaveraccae), Sonchus (Compositae), Hevea, and Manihot (Euphorbiaceae).

Glandular Tissues: They include different types of glands which secrete oils, gums, mucilage, tannins, and resins. They may be

1. External Glands: Present as epidermal outgrowths. They are of many types
   • Glandular Hair: With a stalk and head, for example, tobacoo, Plumbago, Boerhaavia.
   • Stinging Hair: Secrete poisonous substances, for example, Urtica.
   • Nectaries: Secrete sugary substance; maybe extra-floral present on the stem, leaves, etc. Examples: Nepenthes, Catheranthus, or floral, for example, Corchorus, Thea, Polygonum, Jatropha.
   • Digestive Glands: Present in insectivorous plants, for example, Drosera, Nepenthes, etc.
2. Internal Glands
   • Oil Glands: Present in the mesophyll of leaves and cortex of stem fruit, for example, orange, lemon, etc.
   • Mucilage-Secreting Glands: Leaves of piper betel.
   • Gum, tannin, and resin-secreting glands or ducts are present in gymnosperms and angiosperms, for example, Pious resin ducts are schizogenous in origin.

Tissue System

The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “tissue system.” According to Sachs (1975), there are three major tissue systems in plants, namely the epidermal tissue system, the ground or fundamental tissue system, and the vascular tissue system.

Epidermal Tissue System: It consists of the epidermis and its associated structure such as hairs, trichomes cuticle, stomata, and bulliform cells. Mostly epidermis is single-layered parenchymatous but multilayered in Ficus, Nerium. The epidermis is mainly protective in nature.

In grasses, motor or bulliform cells are present in the upper epidermis. In grasses and Equisetum, silica is present in the epidermal cells. The epidermal cells containing cystoliths are called lithocysts.

Ground Or Fundamental Tissue System: It extends from the epidermis up to the center excluding vascular tissue. Ground tissues constitute the following parts

1. Cortex: It lies between the epidermis and the pericycle. It is further differentiated into the following parts:
   • Hypodermis: It is collenchymatous in the dicot stem and sclerenchymatous in the monocot stem. It provides strength.
   • General Cortex: It consists of parenchymatous cells. Its main function is the storage of food.
   • Endodermis (Starch Sheath): It is mostly single-layered and is made up of parenchymatous barrel-shaped compactly arranged cells. The inner and radial walls of endodermal cells have Casparian strips. In roots, thick-walled endodermal cells are interrupted by thin-walled cells just outside the protoxylem patches.
     • These thin-walled endodermal cells are called passage cells. The endodermis behaves as water water-tight dam to check the loss of water and air tight dam to check the entry of air in xylem elements.
2. Pericycle: It lies between endodermis and vascular tissue. It is parenchymatous in roots and sclerenchymatous or mixed with parenchyma in the stem. The pericycle cells just opposite the protoxylem are considered as seats for the origin of lateral roots. In dicot roots, the pericycle forms part of the cambium or the whole of the cork cambium.
3. Pith: It occupies the central part in the dicot stem and monocot root. It is mostly made up of parenchymatous cells. In the dicot root, the pith is completely obliterated by the metaxylem elements. In the dicot stem, the pith cells between the vascular bundles become radially elongated and are known as primary medullary rays or pith rays. They help in lateral translocation.

Vascular Tissue System: Vascular bundles found in the steer part constitute the vascular tissue system. Xylem. phloem and cambium are the major part of the vascular bundle. Vascular bundles may be of the following type:

1. Radial: When the xylem and phloem are arranged on different radii alternating with each other, for example, roots.
2. Conjoint: When xylem and phloem combine in the same bundles and are present on the same radius, for example, stem. Conjoint vascular bundles may be of the following types
   • Collateral: The xylem is towards the inner side and the phloem, is towards the outer side.
     • Open: Cambium is present between the xylem and phloem, for example, the dicot stem.
     • Closed: Cambium is absent between the xylem and phloem, for example, monocot stem.
   • Bicollateral: When the xylem has cambium and phloem on both sides, for example, members of Cucurbitaceae, Solanaceae, Apocyanaceae, etc.
3. Concentric: When one vascular tissue surrounds the other. They are of two types
   • Amphicribral Or Hadrocentric: The xylem is surrounded on all sides by phloem, for example, ferns.
   • Amphivasal Or Leptocentric: The phloem is surrounded on all sides by the xylem, for example, Yucca, and Dracaena.

Internal Structures Of Dicot And Monocot Plants

Anatomy Of Root: The three zones that can be distinguished in a root are

1. Epidermis (Epiblema/Rhizodermis): It is single-layered (uniseriate) and consists of tightly placed, thin-walled unscrutinized cells. This epidermis layer is called as epiblema, piliferous layer, or rhizodermis. Epiblema in younger roots bears unicellular root hairs (water-absorbing organs).
2. Cortex: It consists of thin-walled parenchymatous cells with intercellular spaces. In most monocots and some dicots, the cortex layer below the epidermis becomes suberized to form protective tissue called exodermis.
   • The cells of the cortex store food material (for example, carrots). The innermost layer of the cortex develops into endodermis. It is made up of closely packed living cells characterized by the presence of band-like thickening; made of lignin and suberin on their radial and transverse walls.
   • These bands or strips are called Casparian bands or strips. Some cells of endodermis lying opposite to the protoxylem remain thin-walled and are called passage cells which allow radial diffusion of water.
3. Vascular Bundles: Vascular bundles are radial and exarch. The center of the monocot root is occupied by parenchymatous cells called piths.

Some basic differences between a monocot and a dicot root are given.

Differences Between Dicot And Monocot Root

Anatomy Of Stem: The Primary structure of the diet stem consists of the following layers

• Epidermis: It is the outermost layer consisting of a single layer of closely arranged cells with cuticles (cutinized). It bears multicellular hairs.
• Cortex: It is differentiated into hypodermis, general cortex, and endodermis. Hypodennis is collenchymatous. The general cortex bundles consist of the phloem and xylem.
• Vascular Bundles: Vascular bundles are conjoint, collateral, or collateral, open and endarch, and are arranged in a ring (eustele).
• Pith: It is the central portion of the stem consisting of parenchymatous cells with narrow, radially elongated parenchymatous cells extending from the pith toward the periphery called medullary rays. The main function of pith is food storage.

The primary structure of a monocot stem consists of the following layers:

• Epidermis: It is the outermost layer and consists of compactly arranged parenchymatous cells which are usually covered with cuticles.
• Hypodennis: The cells of hypodermis are sclerenchymatous providing mechanical strength to the stem.
• Ground tissue: All the tissues internal to hypodermis represent the ground tissue. It is made up of parenchymatous cells rich in food reserves such as starch.
• Vascular bundles: They lie scattered in the ground tissue. Each vascular bundle is surrounded by a two- or three-layered sclercnchymatous sheath called bundle sheath. The vascular bundles are conjoint, collateral, closed, and endarch (atactostelc). Vessels are arranged in a V-shaped manner. Schizolysigenous water cavity or canal arc present below protoxylem.

Shows the transverse section of the dicot stem and monocot stem. The differences between dicot stem and monocot stem are given.

Anatomy Of Leaf: In the cross-section of a dorsiventral leaf (dicot), the following parts can be made out

Epidermis: The upper and lower surfaces are covered by the epidermis. The cells of the epidermis are parenchymatous and are closely packed together without any intercellular spaces. Mostly the stomata are restricted to the lower surface of the leaf such leaf is called hypostomatic. The outer walls of the epidermal cells are thickened and cutinized (cuticle) which prevents the loss of water.

Mesophyll: Between the two epidermal layers, there are numerous chlorenchyma cells that constitute the mesophyll. In dicots, there are two distinct layers of mesophyll, the palisade (the upper layer consisting of closely arranged column-shaped cells containing abundant chloroplasts) and spongy tissue (the lower layer of irregularly shaped cells containing fewer chloroplasts).

Vascular Bundles: Vascular bundles in the leaf are located in the midrib and the veins. Vascular bundles are conjoint, collateral, and closed. Bundles are surrounded by a compact layer of parenchymatous cells which is called bundle sheath. The xylem (protoxylem) is towards the upper epidermis (adaxial) and the phloem is on the lower side (abaxial). Like the dicot leaf, an isobilateral leaf (monocot) can also be differentiated into the following types of tissues

Secondary Growth

Secondary growth is the increase in girth thickness or diameter of the axis due to the formation of new tissues as a result of the joint activity of vascular cambium and cork cambium in stellar and extrasolar regions, respectively. It occurs in the roots and stems of gymnosperms and dicots. Secondary growth in the dicot stem is completed in the following steps:

Formation Of Vascular Cambium Ring

1. Intrafascicular Cambium: It is primary in origin, present in between the primary phloem and primary xylem.
2. Interfascicular Cambium: It is a true secondary meristem. It originates from the parenchyma cells of the medullary rays region. It lies in between the vascular bundles.
3. Vascular Cambium Ring: Both intrafascicular and interfascicular cambium join together and form cambium ring.

Cambium Cells Are Of Two Types:

1. Fusiform Initials: They form tracheids, vessels, fibers, and axial parenchyma in the secondary xylem and sieve tubes, companion cells, fibers, and axial parenchyma in the secondary phloem.
2. Ray Initials: These are isodiametric and form ray parenchyma and vascular rays.
3. The Periclinal division of the vascular cambium ring helps in the formation of secondary phloem tout side the vascular cambium) and secondary xylem firmer to vascular cambium). The amount of secondary xylem produced is H 10 times greater than secondary phloem.

Fate Of Primary Phloem And Primary Xylem: The primary phloem is crushed to death, known as obliteration. The primary xylem, being dead and lignified, is replaced in the pith region.

Formation Of Secondary Structures

1. Annual Rings: These are formed by the seasonal activity of vascular cambium. Cambium does not stay active uniformly throughout the year. In spring or summer, cambium is more active and forms large-sized xylem elements (vessels) which constitute spring or early wood.
   • In autumn or winter, cambium stays less active and cuts off small-sized xylem elements (vessels) and constitutes autumn wood or latewood. Both autumn and spring wood constitute a growth or annual ring. In one year, only one growth ring is formed. In successive years, numerous growth rings are formed. Thus, by counting the number of annual rings in the main stem at the base, we can determine the age of a tree. This branch of science is known as dendrochronology.
   • Growth rings are distinct or sharply demarcated in the plants of temperate climates, for example, Shimla, Nainital, Mussoorie, etc., due to the presence of contrasting seasonal variations. Growth rings are not distinct or sharply demarcated in the trees of tropical climates (near the equator) for example, Calcutta, Bombay, and Madras, due to the absence of contrast¬ing seasonal variations.
2. Heart Wood And Sapwood: The young elements of the secondary xylem in the peripheral region constitute sapwood or alburnum. It is light in color and physiologically active. The water conduction takes place through sapwood.
   • Sapwood is converted into heartwood or duramen in the central region. It is darker in color—due to the deposition of tannins, gums, and resins, and physiologically inactive (almost dead). It provides mechanical support only
   • During the conversion of sapwood into heartwood, the most important change is the development of tyloses in the heartwood. Tyloses are balloon-like structures, that develop from xylem parenchyma.
   • These tyloses block the passage of xylem vessels, hence so also called tracheal plug. The heart wood is commercially used as wood. When the plant is made hollow, it does not die because the water conduction takes place through sapwood.
   • The heartwood is well developed in Moms alba (mulberry). The heartwood is absent in Populus and salix plants. The wood of Tectona grandis is termite-resistant. As a tree grows older, the thickness of heartwood increases and sapwood remains the same.
   • Heartwood is much more durable and resistant to microorganisms, insects, pests, etc. than sapwood. The wood of dicot trees is called porous or hardwood because it consists of vessels (pores). The wood of gymnosperms does not contain vessels (pores) and is known as soft or non-porous wood. Such wood consists of 90 to 95% tracheids and 5 to 10% of ray cells. Sapwood will decay faster if exposed freely to the air.
3. Formation Of Cork Cambium: Cork cambium or phellogen develops from the outer layer of the cortex. It produces a secondary cortex or phelloderm on the inner side and cork or phellem on the outer side. The cells of phellem are dead, suberized, and impervious to water.
   • Cork cells are airtight and used as bottle stoppers or corks. The bottle cork is prepared from the cork of Quercus suber (oak tree). The cells of the phelloderm are thin-walled, living, and store food. Phellem, phellogen, and phelloderm are collectively called periderm. The periderm is a secondary protective tissue.
   • Due to the pressure of the secondary xylem, the epidermis ruptures arid cortex is largely lost after two or three years of secondary growth. In the cork layer (bark), the lenticels are present which are meant for gaseous exchange.
   • In cork, lenticels have loosely arranged cells called complementary cells with intercellular spaces. For bottle corks, the cork is processed in such a manner so that lenticels come in the vertical direction.

Bark includes all the dead and living tissues outside the vascular cambium. Bark may be of two types:

• Scaly Bark: When develops in strips, for example, Eucalyptus, Psidium.
• Ring Bark: When develops in the form of a sheet or ring, for example, Betula (bhojpatra). The outermost layer of bark is dead and called as rhytidome. The bark of Betula was used as a substitute of paper in ancient times to write manuscripts.

Secondary Growth In Dicot Root: Vascular bundles in the dicot root are radial, exarch, and mostly triarch. Vascular cambium is formed secondary) from conjunctive parenchyma cells lying just below each phloem strand. Thus, the number of cambium strips formed equals the number of phloem strands. The cells of the pericycle lying outside the protoxylem also become meristematic to form part of strips of cambium.

These cambial strips join the first formed cambium strips to form a complete but wavy ring of vascular cambium. This cambium ring produces a secondary xylem on the inner side and a secondary phloem on the outer side.

In roots, the growth rings are not distinct, because there is no seasonal variation under the soil. From the outer layers of the pericycle arises the phellogen which cuts phellem (cork) or the outer side and secondary cortex or phelloderm toward the inner side.

Anatomy Of Flowering Plants Point To Remember

Examples of dicots with scattered vascular bundles are Podophyllum, Peperomia, Piper, and Popover. Examples of cortical vascular bundles are Nyctanthus, Kalanchoe, and Casuarina.

• Examples of medullary bundles are Mivabilis, Bougainvillea, Amaranthus, and Achyranthus. Examples of polystelic conditions are Primula and Dianthera.
• Anomalous or abnormal secondary growth occurs in Bougainvillea, Boerhaavia, Chenopodium, and Aristolchia.
• Some monocots show abnormal secondary growth by meristematic tissue which develops around vascular bundles, for example, Dracaena, Yucca, Agave, etc.  Virgin cork is the first formed periderm.

Wound Cork: It is the secondary meristem; formed below the injured cell, it forms a cork on the outer side and a callus below which heals the wound.

• Abnormal secondary growth in dicot root occurs in beetroot (Beta vulgaris) and sweet potato (Ipomoea batatas) by the formation of numerous accessory rings of cambium which cut more storage—parenchyma in secondary phloem and less secondary xylem.
• Homoxylous wood is the wood of vessels dicots, for example, Ranales (Winteraceae, Tetracentraceae Trochodendraceae).
• Heteroxylous wood is the wood of vessel-bearing dicots.

The latex of some plants is of great commercial importance such as

1. The source of commercial rubber is the latex of Hevea brasiliensis, Ficus elastica, Ciyptostegia, and Manihot glaziovii.
2. The source of chewing or chuckle gum is the latex of Achras sapota.
3. The source of the enzyme papain is the latex of Carica papaya.
4. The source of alkaloid opium is the latex of Popover somnific (poppy).

Polyderm is a special type of protective tissue that occurs in roots and underground stems of the members of Rosaceae and Myrtaceae. Its outermost layer is dead and suberized.

 

Anatomy Of Flowering Plants Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: In maize stem, endodermis is present between the general cortex and pericycle.

Reasoning: Eustele is present in maize Stem.

Answer: 4. If both Assertion and Reason are false.

Question 2. Assertion: In the Cucurbita stem, vascular bundles are conjoint, bicollateral, and either open or closed.

Reasoning: The outer and inner cambium are present and only the inner cambium is functional in the Cucurbita stem.

Answer: 4. If both Assertion and Reason are false.

Question 3. Assertion: Fusiform cells are elongated and tapering cells.

Reasoning: These cells form an axial system consisting of vascular rays.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: Septa less tracheids are absent in Trochodendron.

Reasoning: Heteroxylous wood is present in Trochodendron.

Answer: 3. If Assertion is true, but Reason is false.

Question 5. Assertion: According to Hanstein, there are three halogens in a monocot root.

Reasoning: In monocot roots, the outermost groups of initials form both root cap and dermatogen.

Answer: 4. If both Assertion and Reason are false.

Question 6. Assertion: The apical meristem is always protected.

Reasoning: A root cap is present above the meristem in roots.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 7. Assertion: The stem in herbaceous plants do not develop cracks during severe wind and is used to bond under these conditions.

Reasoning: Sclerenchyma is peripheral in position and provides flexibility to the herbaceous stems.

Answer: 3. If Assertion is true, but Reason is false.

Question 8. Assertion: The death of a companion cell leads to the death of a sieve cell also.

Reasoning: Both companion and sieve cells are phloem cells.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 9. Assertion: Dicot roots are mostly tetrach.

Reasoning: There occur four phloem bundles forming rays.

Answer: 4. If both Assertion and Reason are false.

Question 10. Assertion: Heartwood is not involved in the conduction function.

Reasoning: Tyloses and depositions of tannins, resins, and gums is common in duramen cells.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 11. Assertion: Vascular cambium appears wavy in dicot roots.

Reasoning: Vascular cambium is formed by conjunctive tissue in dicot roots which is found located inside the xylem and outside phloem strands.

Answer: 3. If Assertion is true, but Reason is false.

Question 12. Assertion: Velamen is hygroscopic in nature and absorbs environmental moisture.

Reasoning: Velamen is common in orchids which are epiphytes.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Morphology Of A Flowering Plant

Morphology deals with the study of forms and features of different plant organs such as roots, stems, leaves, flowers, seeds, fruits, etc. The body of a flowering plant can be divided into two fundamental parts:

1. An Underground Root System And
2. An Aboveground Shoot System. The Root System Grows Downwards Into The Soil Anchors The Plant Firmly In The Soil And Absorbs Water And Various Dissolved Minerals From It. The Shoot Supports Foliage Leaves And Helps In The Conduction Of Water And Mineral Substances From The Soil And Food Material.

Life Span Of A Flowering Plant

The Life Span Of A Flowering Plant Can Be Any Of The Following Forms:

Annual: The life cycle is completed in one season or a few weeks to a few months. Examples are wheat, maize, and Euphorbia prostrata.

Biennials: The life cycle is completed in two seasons: vegetative growth occurs in the first season and food is stored and reproduction occurs in the second season, for example, henbane. Radish, carrot, and turnip are biennials in colder areas and annuals in warmer areas.

Perennials: The plant lives for a few to many years and may bear flowers and fruits every year (polycarpic). Bamboo (Bambusatulda) and agave are monocarpic, i.e., flowering occurs only once in life lifetime.

Habit Of Plants

Herb: Stem is soft, less than 2 m in height.

Shrub: Perennial woody stem with medium height, tiunk is absent for example, Capparis, Rosa, etc.

Trees: Woody stems of great height; the stem is called a trunk. An unbranched stem is called caudex or columnar, for example, a palm. An erect stem with swollen nodes is called a culm, for example, bamboo.

Excurrent: The lateral branches of the trunk do not compete with the stem, for example, Finns, Casuarina, and Eucalyptus.

Deliquescent: The main stem or trunk disappears after some time and the crown is dome-shaped, for example, Dalbergia, Ficus benghalensis.

Read and Learn More NEET Biology Notes

Root

Roots develop from the radicle of the seed. They are non-green, underground, positively geotropic, and negatively phototropic. Roots usually do not bear buds, but buds are present for vegetative propagation in sweet potato (Ipomoea) and Indian redwood (Dalbergia). They do not bear nodes and internodes. They have unicellular root hair. Lateral roots arise endogenously, i.e., from the pericycle.

Types Of Roots: Roots Are Mainly Of Two Types

1. Tap Root: They develop from seed radicles. The primary root grows and gives rise to secondary and tertiary roots forming a tap root system. For example, dicots.
2. Adventitious Roots: They develop from any part of the plant body other than the radicle. For example, monocots. They are usually shallow surface feeders such as fibrous roots of grasses.

Parts Of Root

Root Cap: At the apex of the root, a smooth cap-gaped structure is present which is called as root cap. It is protective. Multiple root cap is found in trial roots of screwpine (Pandanus). In hydrophytes, the root cap is either absent or replaced by a root pocket, for example, Pistia, Lemna, and Eichhomia.

Zone Of Cell Formation Or Division: The cells of this region are in an active state of division and their number increases continuously. No vacuoles or small vacuoles are present. It extends to a few millimeters.

Zone Of Cell Elongation: Maximum growth in the cells occurs in this zone. A large central vacuole is present.

Zone Of Cell Maturation: The cells are differentiated into permanent tissues depending upon the functions they have to perform. Root hairs are also present in this zone. In hydrophytes, root hairs are absent because they absorb water through the general body surface.

Modifications Of Tap Roots

1. Storage/Fleshy Roots
   1. Fusiform: Example, radish (Raphamts sativus).
   2. Conical: Example, carrot (Daucus carota).
   3. Napiform: Example, turnip (Brassica rapa), beetroot (Beta vulgaris).
   4. Respiratory Root (Pneumatophores): Example, Avicennia. Sonneratia.
2. Respiratory Root (Pneumatophores): Example, Avicennia, Sonneratia.
3. Nodulated Roots: Example, Pisum sativum, Cicer arietinum.

Modifications Of Adventitious Roots

1. Storage Roots
   1. Tuberous: Example, sweet potato (Ipomoea batatas)
   2. Fasciculated: Example, Asparagus, Dahlia.
   3. Palmate: Example, orchis.
   4. Nodulose: Example, mango ginger (Curcuma amada)
   5. Beaded Or Moniliform: Example, Portulaca, Momordica.
   6. Annulated: Example, ipecac (Psychortai)
2. Adventitious Roots That Provide Extra Support
   1. Prop Roots: Example, old banyan tree (Ficus benghalensis)
   2. Stilt Roots: Example, sugarcane, maize.
   3. Climbing Roots: Example, Pothos, Piper.
   4. Buttress Roots: Example, Bombax.
3. Adventitious Roots With Special Function
   1. Respiratory Roots: Example, Jussiaea.
   2. Assimilatory Roots: Example, Tinospora, Trapa.
   3. Haustoria: Example, Cuscuta.
   4. Hygroscopic Roots: Example, orchids.
   5. Contractile Roots: Example, saffron (Crocus), Freesia.
   6. Root Thorns: Example, Pothos arinatus, Acanthorhiza.
   7. Foliar/Epiphyllous/Leaf Roots: Example, Btyophyllum, Bignonia, Salvinia.

Stem

Stem is formed by the prolongation of the plumule of the embryo. It is positively phototropic negatively geotropic and hydrotropic. It bears nodes and internodes. The leaf-bearing part of the stem is called the shoot. It has buds. A bud is a condensed immature or embryonic shoot having a growing point surrounded by immature leaves. Cabbage is the largest bud.

According To Nature, Buds Can Be:

• Vegetative: Form leafy shoots.
• Floral: Form flowers.
• Mixed: Form both vegetative and floral characters.

According To Position, Buds Can Be Lateral Or Terminal. Lateral Buds Are Of Four Types:

1. Axillary: Present in the axil of a leaf.
2. Accessory: Additional buds occur either on the side or above the axillary bud.
3. Extra-axillary: Developing on the node but outside the leaf base.
4. Adventitious: Formed from places other than nodes. These can be:
   • Foliar: Example, Biyophyllum, Begonia.
   • Radical: Example, Dalbergia, Ipomoea batata (sweet potato).
   • Cauline: Example, jackfruit.

Types And Modifications Of Stem

Aerial Stems (Epiterranean Stem): It may be reduced, erect, or weak.

1. Reduced: The stem is reduced to a disc, for example, radish, carrot, turnip.
2. Erect Stem: The stem is strong and upright, for example, maize, wheat, and mango. An erect stem with swollen nodes is called a culm (for example, bamboo).
3. Weak Stem: These are thin, soft, and weak and need support. These can be upright or prostrate. These are of the following types.
   • Creepers: The stem creeps on earth and the roots arise at the nodes, for example, grasses, strawberries, and Oxalis.
   • Trailers: The stem creeps on the ground but the roots do not arise at the nodes. They may be of the following types:
     • Prostrate Or Procumbent And Diffuse: Example, Evolvulus, Tribulus.
     • Decumbent: Example, Tridax.
     • Diffuse: Example, Boerhaavia.
   • Lianas (Stem Climber): Woody perennial climbers found in tropical rainforests are lianas. They twine themselves around tall trees to secure sunlight, for example, Hiptage, and Bauhinici vahlii (phanera).
   • Climbers: Plants are with long weak stems and have organs of attachment to climb objects. They may be:
     • Rootlet Climber: Example, Tecoma, Pothos, Piper betel (paan).
     • Hook Climber: In Bougainvillea, Duranta.
     • Tendril Climber: Tendrils are thread-like structures that help in climbing plants.
       • Entire Leaf: Leaf tendrillar, for example, Lathyrus sativus.
       • Leaflet: Leaflet tendrillar for example, Pisum.
       • Twiners: The stem body twines around the support without any special organ of attachment, for example, Cuscuta, Dolichos, and Quisqualis.

Sub-aerial Stems

1. Runner: It is an elongated, prostrate, aerial branch with long internodes and roots that strike at nodes, for example, Oxatis, grasses, Hydrocotyl
2. Sucker: It arises from the axillary bud of the underground part of the stem. The branch creeps below the soil surface grows obliquely upward and produces new shoots. Examples, are Mentha, Chrysanthemum, and Rosa.
3. Offset: Short horizontal branch producing a cluster of leaves above and a cluster of roots below, for example, Pistia, Eichhornia.
4. Stolon: It is a subterranean long lateral branch arising from the base of the stem, for example, Colocasia. It first grows obliquely upward and then bends down to touch the ground surface.

Underground Stems

1. Rhizome: It grows parallel or horizontally to the soil surface. It bears nodes, internodes, buds, and scaly leaves, for example, ginger, banana, turmeric, and ferns. It is of two types:
   • Rootstocks: It is upright or oblique with the tip almost reaching the soil surface, for example, Dryopteris.
   • Straggling: It is horizontal and branched. Branching may be
     • Racemose: The Axis is monopodial, for example, Saccharum, lotus.
     • Uniparous Cymose: Axis is sympodial, for example, Zingiber officinale (ginger), Curcuma domestica (turmeric), and Canna.
2. Tuber: It is the terminal portion of underground stem branches that are swollen on account of accumulation of food, for example, potato, Jerusalem artichoke (Helianthus tuberosus).
3. Corm: It grows vertically to the soil surface. It bears nodes, internodes, buds, and scaly leaves, for example, Colocasia, Gladiolus, Colchicum, Crocus, and Amorphophallus.
4. Bulb: Stem is reduced and disc-shaped. The bud is surrounded by many concentric leaves. The leaf bases are fleshy and edible, for example, onion, lily, and garlic. The bulb may be tunicated or scaly.
   • Tunicated (Layered Or Laminate): The bulb is covered with a dry membranous sheath of scales called a tunic. These bulbs may be again of two types:
     • Simple Tunicated: Onion, Tulipa, and Narcissus.
     • Compound Tunicated Bulb: Garlic.
   • Scaly Or Imbricate Or Naked Bulb: Tunic is absent, for example, lily.

Special Stem Modifications

• Phylloclade: It is a green flattened or rounded succulent stem with leaves either feebly developed or modified into spines, for example, Opuntia, or Casuarina.
• Cladode: Phylloclade with one intemode is called cladode, for example, Asparagus, and Ruscus.
• Thorn: It is the modification of axillary buds, for example, Bougainvillea, Duranta, Carrisa, Alhagi, etc. Thoms of Alhagi possess flowers and in Duranta, thorns bear smalt foliage leaves. Thoms of Carissa are terminal and branched.
• Stem Tendril: Example, Vitis, Passiflora.
• Bulbils: A condensed auxiliary bud (vegetative) is called a bulbil. It helps in vegetative reproduction, for example, Dioscorea, Glabba, Agave, and Oxalis.

Branching Of Stem: Branching is defined as the mode of arrangement of branches on the stem. It is of two types:

1. Lateral Branching: Branches are produced laterally from the main stem. It may be racemose or cymose.
   • Racemose Type: The main stem grows indefi¬nitely by the terminal bud and produces branches later in the acropetal succession, for example, Casuarina, Poly alt hia, etc.
   • Cymose Type: The growth of the main stem is limited and lateral branches produced by the main stem show more vigorous growth. It may be of the following types:
     • Uniparous Cyme: When one lateral branch is produced at a time. It has two distinct types: helicoid (for example, Saraca) and scorpoid (for example, Vine).
     • Biparous Cyme: When two lateral branches develop at a time, for example, Mirabilis, Datura.
     • Multiparous Cyme: When more than two branches develop at a time, for example, Croton, Euphorbia.
2. Dichotomous Branching: When the terminal bud gives out two branches of equal size in a forked manner, for example, Pandanus, or Hyphaene.

Leaf (Phylopopium)

Leaves are lateral, flat, green, and expanded part of plants that arise from nodes on the stem or branches. Usually leaf has a bud in its axil. The chief functions of leaf is photosynthesis and transpiration. All leaves of a plant are collectively called phyllome. The leaves are of the following types:

1. Cotyledonary Leaves: These are embryonic or seed leaves.
2. Cataphylls: These are scale leaves. These may store food also, for example, onions.
3. Hypsophylls or bract leaves.
4. Prophylls: The first formed leaves.
5. Floral Leaves: Include sepals, petals, or perianth.
6. Sporophylls: Bear spores; this is also used for stamens and carpels.
7. Foliage Leaves: The green leaves of the plant are called foliage leaves.

Parts Of A Leaf: A leaf consists of the following three parts: leaf base, petiole, and lamina.

1. Leaf Base (Hypopodium): Leaves are attached to the stem by the leaf base. In some plants, the leaf base becomes swollen and is called pulvinus which is responsible for sleep movement, for example, Cassia, Mimosa, Bean. In some plants, the leaf base expands into a sheath (sheathing leaf base), for example, grasses, and bananas (monocots). When the leaf base partially encloses the stem, it is called auriculate or semi-amplexicaul, for example, prickly poppy, Calotropis procera (Madar).
   • If it completely encloses the stem, it is called amplexicaul, for example, Sonchus, Polygonum.
   • In some plants, minute appendages arising from the leaf base are known as stipules. Leaves with stipules are called stipulate (for example, Rosa, Polygonum) and those without stipules are called exstipulate, for example, Ipomoea.
   • Types Of Stipules: Depending upon duration, they can be:
     • Caducous: Fall off before unfolding of leaf, for example, Michelle Champaca.
     • Deciduous: Fall off soon after unfolding of leaves, for example, Cassia tora and Dillenia indica.
     • Persistent: Remain attached to the leaf throughout life, for example, rose, pea, etc. On The Basis Of Structure And Relation To The Leaf, Stipules Are Classified As:
     • Free Lateral: These are free; and present on both sides of the leaf base, for example, Hibiscus rosasinesis.
     • Scaly: Small dry scales present on both sides of the legal base, for example, Desmodium, etc.
     • Intrapetiolar: These are situated between the petiole and axis, for example, Gardenia.
     • Foliaceous: Large, green-leafy structures, two in number, for example, pea (Pisum) and sweet pea (Lathyrus).
     • Tendrillar: One tendri liar stipule lies on each side of the petiole, for example, Smilax.
2. Petiole (Mesopodium): Petiole in Eichhornia becomes spongy and bulbous. In orange (citrus plants), the petiole becomes winged. Petiole is modified into ten¬drils in Clematis. In Australian acacia, the petiole is modified into a leaf-like sickle-shaped phyllode.
3. Lamina (Epipodium): The broad fiat part of the leaf is the lamina (leaf blade).

Types Of Leaf

• Simple Leaf: Leaf which may be entire or incised and the incisions do not touch the midrib, for example, mango, or banyan.
• Compound Leaf: The leaf blade is incised up to the midrib or petiole and thus divides it into two or more leaflets.

They Are Of Two Types:

1. Pinnately Compound Leaves: Rachis bears several lateral leaflets. These may be of the following types:
   • Unipinnate: They are of two types:
     • Paripinnate: Example, Cassia, Sesbania.
     • Imparipinnate: Example, Rosa, Tephrosia, Azadirachta,
   • Bipinnate: Example, Acacia, Mimosa, Delonix.
   • Tripinnate: Example, Morinaa, Melia, Azadirachta.
   • Decompound: Example, Daucus carota (carrot), Parthenium, Coriandrum.
2. Palmately Compound Leaves: It has no rachis and all the leaflets are joined to a common joint at the tip of the petiole. They may be of the following types:
   1. Unifoliate: Example, Citrus.
   2. Bifoliate: Example, Biginonia grandiflora, Princepia. Balanites, Hardwickia.
   3. Trifoliate Or Ternate: Example, Medicago, Aegle, Oxalis, Dolichos.
   4. Quadrifoliate: Example, Marsilea, Paris quadrifolia.
   5. Multifoliate: Example, Cleome, Bombax.

Venation In Leaves: The arrangement of veins on the lamina is called venation. It is of three types: reticulate, parallel, and furcate.

1. Reticulate Venation: The branches of veins form a network, for example, dicots. However, there are some dicots that show parallel venation, for example, Calophyllum, Eryngium, and Corymbium. It Can Be Of Two Types:
   • Pinnate Or Unicostate: for example, mango, banyan, China rose.
   • Palmate Or Multicostate:
     • Convergent, for example, Ztzyphus, Smilax and
     • Divergent, for example, castor (Ricinus), Luffa, and Vitis (grapevine).
2. Parallel Venation: The veins and veinlets remain parallel to each other, for example, in monocots. Some monocots that show reticulate venation are, for example, Smilax, Dioscorea, and Alocasia. This is Two Types:
   • Pinnate Or Unicostate Parallel Venation: Example Banana {Musa paradisiaca), canna.
   • Palmate Or Multicostate:
     • Convergent, bamboo, grass or
     • Divergent, for example, fan palm.
3. Furcate: The veins branch dichotomously but the finer branches do not form reticulum. It is common in ferns (for example, Adiantum). Among higher plants, it is seen in Cirencester.

Phyllotaxy: It is the mode of arrangement of leaves on the stem or its branches. It is of the following types:

1. Alternate: Single leaf arising at each node, for example, mustard.
2. Opposite: Leaves occurring in pairs at the node. They May Be:
   • Decussate: Leaves that stand at a right angle to the next upper or lower pair, for example, Ocinunn sanctum (sacred basil). Zinnia.
   • Superimposed: Leaves that stand parallel to the next upper or lower pair, for example, Psidium (guava), Eugenia jambolana (Jamun).
3. Whorled: Leaves occurring in more than two at each node, for example, Nerium, Alstonia.

Heterophyily: The occurrence of more than one type of leaves on the same plant is known as heterophyily. It is of three types:

1. Developmental Heterophyily: Leaves of different forms and shapes occur at different periods or places on the same plant, for example, mustard, Sonchus, and Eucalyptus.
2. Habitual Heterophyily: Leaves differ in their shape and incisions due to their habit or nature, for example, Artocarpus, Heterophyllus (jack fruit), and Ficus heterophylla.
3. Environmental Heterophyily: This type is found in aquatic plants where the submerged leaves differ from the floating and aerial leaves, for example, Sagittaria, Ranunculus aquatilis, and Limnophilia heterophylla.

Inflorescence: The arrangement of flowers and mode of distribution of flowers on the shoot system of a plant is called inflorescence.

Racemose (Indefinite) Inflorescence: The main axis of inflorescence does not end in a flower but continues to grow. The development of flowers is acropetal. The opening of flowers is centripetal. It is of the following types:

1. Raceme: Peduncle has bisexual and pedicellate flowers arranged acropetally, for example, larkspur, mustard, and radish.
2. Spike: Peduncle has bisexual and sessile flowers, for example, Achyranthes, and Adhathoda.
3. Corymb: The main axis is short. Lower flowers have longer pedicels than upper ones so that all the flowers are brought more or less to the same level, for example, Iberis, and Capsella.
4. Compound Corymb, for example, cauliflower.
5. Umbel: The main axis is reduced very much and all flowers appear to be arising from the same point. At the base of flowers, clusters of bracts form involucre, for example, hydrocodone, onion. Compound umbel, for example, coriander, carrot, Prunus.
6. Spadix: It is a spike with a fleshy axis and has both male and female flowers. It is surrounded by a large colored bract called a spathe, for example, Musa, palm, Colocasia, and Alocasia (characteristic of innocents).
7. Catkin: It is a pendulous spike that bears unisexual flowers, Morns, birch, oak, and Acalypha.
8. Capitulum Or Head: The main axis becomes flat and called a receptacle which bears many sessile and small florets. Peripheral florets called ray florets are pistillate or neuter and zygomorphic whereas disc florets are bisexual and actinomorphic, for example, sunflower, Zinnia, Cosmos (Asteraceae).
9. Panicle: Peduncle branched and branches have pedicellate flowers, for example, gulmohr, and Rhus.
10. Spikelet: It is a small spike. Flowers are produced in the axis of fertile glumes (bract), for example, wheat, and grasses (Poaceae).

Cymose (Definite) Inflorescence: The main axis ends in a flower. The development of flowers is basipetal and the opening of the flowers is centrifugal. It is of the following types:

1. Monochasial Or Uniparous Cyme: It is of two types:
   • Helicoid Cyme: Example, Atropa, datura, Begonia, Heliotropium.
   • Scorpioid Cyme: Example, Solanum nigrum, Ranunculus.
2. Dichasial Or Biparous Cyme: Example, Dianthus, Clerodendron.
3. Polychasial Or Multiparous Cyme: Example, Hamelia, Calotropis.

Special Inflorescence: These are of the following types:

1. Vertici Ilaster: A cluster of sessile or subsessile flowers borne on a dichasial cyme ending in a monochasial cyme (scorpioid) in the form of a condensed whorl on either side of the node. For example, Ocimum (Tulsi), and Salvia (Lamiaceae).
2. Cyathium: It looks like a single flower. In this cup-shaped involucre encloses a single female flower and a number of male flowers. Each male flower is represented by a single stamen, for example, poinsettia (Euphorbia pulcherrima).
3. Hypanthodium: Fleshy receptacle forming a hollow cavity with an apical opening. The flowers are developed on the inner wall of the hollow cavity. The male flowers are situated at the top near the opening. Below them gall flowers are situated which are sterile and at the bottom are situated female flowers with long styles, for example, Ficus (banyan, fig, guar).
4. Coenanthium: In Dorstenia, the receptacle becomes saucer-shaped and its margins arc slightly. The florets are arranged as similar to hypanthodium.

Flower

Flower is defined as a highly condensed and modified reproductive shoot. The following points can be mentioned to justify that Flower is a modified shoot.

1. Calyx, corolla, androecium, and gynoecium represent four whorls of sterile and fertile leaves borne at different nodes. Sometimes intemode between the calyx and corolla becomes elongated and called as anthophore, for example, Silene, and Dianthus.
   • The intemode between corolla and androecium is known as androphore, for example, Passiflora. The intemode between androecium and gynoecium is called as gynophore, for example, Capparis. When androphores and gynophores both are present in the same flower, they are jointly termed as gynandrophores, for example, Gynandropsis, Cleome. The prolongation of the thalamus beyond the carpel is known as carpophore, for example, Coriandrum, and Foeniculum.
2. In Mussaenda, sepals enlarge to form a leafy structure (foliaceous sepals).
3. Sometimes floral bud is transformed into vegetative buds or bulbils, for example, Agave.

Types Of Floral Characters

• Complete Flower: Calyx, corolla, androecium, and gynoecium are present.
• Incomplete Flower: Flower with one of the four whorls missing.
• Bisexual Flower: Both gynoecium and andoecium are present in the same flower.
• Unisexual Flower: Only androecium (staminate flower) or gynoecium (pistillate flower) are present in the flower.
• Monoecious Plant: When both male and female flowers are present on the same plant. Examples, are Cocos, Ricinus, Zea, Colocasia, and Acalypha.
• Dioecious Plant: When male and female flowers are present on separate plants. Examples are mulberry and papaya.
• Polygamous Plant: When unisexual (male or female), bisexual and neuter flowers are present on the same plant. Example, Polygonum, Mango.
• Monocarpic Plant: The plant that produces flowers and fruits only once in life. For example, pea, mustard, or all seasonal plants.
• Polycarpic Plant: The plant which produces flowers and fruits many times in life. Example, mango, and pear (mostly fruit trees).
• Achlamydeous Flower: Flowers are naked without sepals and petals. Example, Pipelaceae.
• Monochlamydeous Flower: Only one whorl is present (perianth). Examples, are Polygonaceae, and Liliaceae.
• Dichiamydeous Flower: Both whorls are present in a flower Example, most of the flowers.
• Hemicyclic Or Spirocyclic Flowers: Some of the floral parts are in circles and some are spirally arranged. Example, Ranunculaceae.
• Cauliflory: Production of flowers on old stems from dormant buds. Examples, are Artocarpus, and Ficus.

Symmetry Of Flower

• Actinomorphic Flower: When a flower can be divided into two equal halves by many vertical sections passing through the center. Example, Cruciferae, Malvaceae.
• Zygomorphic Flower: When a flower can be divided into two equal halves by only one vertical section passing through the center. For example, pea.

Position Of Floral Parts On Thalamus

• Hypogyny: Ovary is at the top and separable from thalamus. Flowers are hypogynous and the ovary is superior. For example, Malva, Brassica.
• Perigyny: Ovary is hail superior, half inferior. For example, rose.
• Epigyny: Calyx and corolla arise from the upper side of the ovary. The ovary is completely surrounded by and fused with the thalamus. The ovary is inferior and the flower is epigynous. Example, Aster, Luffa.

Bracts: Bracts are specialized leaves that arise from the axil of leaves. They are of the following types:

• Petaloid Bracts: Bracts look like petals (brightly colored). Example, Bougainvillea.
• Spathy Bract: This is a large bract enclosing an inflorescence. Examples are bananas, maize, and palms.
• Foliaceous Bracts: Bracts are leaf-like in appearance. Examples, are Adhatoda, and Gynandropsis.
• Involucre: They are green-colored and in one or more whorls a round or below the entire inflorescence. Examples are sunflower and coriander.
• Glumes: These are small dry, scaly bracts found in spikelet of Gramineae. For example, wheat.

Calyx: The lowermost whorl of a flower is called calyx. It is the non-essential whorl and consists of sepals. Sepals may be free (polysepalous) or fused (gamosepalous). Sepals are modified as follows:

• Pappus: Sepals are modified into persistent hairy structures called pappus which help in the dispersal of fruits. Examples are sunflower and Sonchus (Asteraceae).
• Leafy: In Mussaenda, one sepal is modified into a large leaf-like white structure.
• Spinous: In Trapa, the calyx is persistent and modified into two spines.

Corolla: Corolla is the second whorl of flower and consists of a number of petals which are usually brightly colored. The petals may be gamopetalous (fused) or polypetalous (free). Various forms of petals are:

• Cruciform: Four petals arranged like a cross. For example, members of Brassicaceae.
• Papilionaceous: The number of petals is five with the largest petal standard or vexillum enclosing two lateral wings which are free and in turn enclose the innermost keel (united petals). For example, pea.
• Rosaceous: Five or many small-clawed petals and spread regularly outward. For example, rose.
• Caryophyllaceous: Five free long-clawed with limbs spread at right angles to claws. Example, Diantnus.
• Tubular: Petals are like a tube, for example, disc florets of sunflowers.
• Infundibuliform Or Funnel Shaped: Petals are like a funnel. For example, Datura.
• Bilabiate (Two-Lipped): Upper and lower lips are formed by a fusion of petals. Example, Salvia, Ocimum.
• Ligulate Or Strap Shaped: Gamopetalous petals forming tongue-like structure. For example, ray florets of sunflower.
• Campanulate Or Bell-Shaped: Petals like bell. For example, Physalis.
• Rotate Or Wheel Shaped: Example, brinjal.

Aestivation: The arrangement of floral parts in a floral bud is known as aestivation. It may be of the following types:

• Valvate: When sepals or petals lie very close to each other, without overlapping. For example, mustard.
• Twisted or contorted: When one margin of the sepal or petal overlaps the margin of the next and the other margin is overlapped by the third one. For example, China rose.
• Imbricate: When both margins of one of the petals are covered by others and both margins of another one are external, and of the remaining partly internal, partly external. Example, Cassia, Caesalpinia.
• Quincuncial: When two are inner, two are outer, and one is partly outer and partly inner, Example, Ranunculus.
• Vexillary: The posterior one is the largest and almost covers the two lateral petals and the latter in turn nearly overlaps the two anterior petals. For example, pea (Papilionaceae).

Androecium: It is the third and male whorl of flower in which each stamen consists of filament, anther, and connective. When stamens are free, it is called polyandrous, for example, lily, mustard, and radish. A two-lobed anther is called vitreous (for example, pea) and a one-lobed anther is called monoecious (for example, members of Malvaceae). Attachment of filament to the anther is categorized as:

• Adnate: The filament runs along the back to the anther. For example, Michelia (Champa).
• Basifixed: The Anther is fixed to the filament by its base. For example, Datura.
• Dorsifixed: The Anther is fixed to the filament by its back and the other is immobile. For example, passion flower.
• Versatile: Anther is attached to the filament as in dorsifixed but is able to swing freely. For example, wheat and grasses.

Cohesion Of Stamens: The fusion of stamens among themselves is called cohesion. It is of the following types:

• Monadelphous: Stamens may be united by means of their filaments in one bundle. Examples are China rose, lady’s finger, and cotton (Malvaceae).
• Diadelphous: When the filaments are united into two bundles, the anthers remain free. Examples are peas, beans, and gram (Papilionaceae).
• Polyadelphous: When the filaments are united into more than two bundles but anthers are free. Examples are castor (Euphorbiaceae), and lemon (Rutaceae).
• Syngenesious: When anthers are united but the filaments are free. For example, sunflower (Compositae).
• Synandrous: When anthers as well as filaments of stamens are united throughout their whole length. For example, members of Cucurbitaceae.

Adhesion Of Statements: Fusion with other floral parts is called adhesion. It is of the following types:

• Epipetalous: When stamens are united to the petals. For example, China rose, Solatium, sunflower.
• Episepalous: When stamens are united to sepals. Example, Verbena.
• Epiphyllous (Epipetalous): When stamens are united to perianth (tcpal). For example, members of Liliaceae.
• Gynaiidrous: When stamens are attached to the gynoecium (carpel) either throughout their whole length or by their anthers only. For example, Calotropis (forming gynostegium).

Length And Arrangement Of Stamens

• Didynamous: Four stamens, two outer small and two inner long. Examples, are Ocimum, and Salvia (Labiatae).
• Tetradynamous: Six stamens, two outer small and four inner long. Example, mustard, and radish (Brassicaceae).
• Heterostemony: Stamens are of different lengths. Example, Cassia.
• Obdiplostemonous: Two whorls of stamens, outer whorl lying opposite to the petals (antipetalous) and inner whorl lying opposite to sepals (antisepalous). Examples, are Steilaria, Spergnla, and members of Rutaceae.
• Diplostemonous: Two whorls of stamens, outer whorl Tying opposite to sepals (antisepalous) and inner whorl lying opposite to petals (antipetalous). Example, Cassia.

Gynoecium: It is the female part of the flower comprising carpels bearing ovules. It consists of an ovary, style, and stigma. The gynoecium may be monocarpellary or polycarpellary.

Cohesion Of Carpels

• Apocarpous: Carpels are free (no cohesion). Example, Ranunculaceae.
• Syncarnous: Carpels of more than two and fused. For example, most of the plants.
• Number Of Locules: Ovary has locules or chambers and may be unilocular, bilocular, trilocular, tetralocular, or pentalocular (multilocular).

Placentation: The arrangement of ovules on the placenta within the ovary is called placentation. It is of the following types:

• Marginal: Placenta developing along the junction of two margins of the carpel in a one-chambered ovary. It is the characteristic feature of the family Leguminosae. Example, pea, gram.
• Parietal: Ovary is one-chambered and the placentae bearing the ovules develop on the inner wall of the ovary.

The number of placentae corresponds to the number of carpels. Examples are mustard, radish, and cucumber.

• Axile: The ovary is two to many chambered and placenta-bearing ovules develop from the central axis. Examples are tomato, orange, cotton, China rose, and lily.
• Free Central: Ovary is one-chambered and the placenta bearing the ovules develops all around the central axis. Example, Dianthus, Steilaria.
• Basal: The ovary is unilocular and the placenta develops at the base of the ovary on the thalamus and bears a single ovule. Examples are wheat, maize, aster, Zinnia, and sunflower. It is the most advanced arrangement.
• Superficial: Ovary is multilocular with numerous carpels as in the axile type of placentation, but the placenta develops all around the inner surface of the partition wall. For example, water lily. It is the most primitive.

Fruits

A fertilized and ripened ovary is called a fruit. The fruit consists of seed and pericarp (fruit wall). The pericarp develops from the wall of the ovary and is differentiated into epicarp, mesocarp, and endocarp. Seeds develop from ovules. In some plants, the ovary grows into fruit without fertilization. Such fruits are called parthenocarpic fruits. They are seedless, for example, banana, grapes, pineapple, oranges.

The fruit that develops from ovary is called the true fruit. Most of the fruits are true fruits. If any other floral part takes part in fruit formation, it is called false fruit (pseudocarp), for example, apple, or pear.

Types Of Fruits

1. Simple Fruit: These are developed from the ovary of the single flower with or without accessory parts.
   • Dry Indehiscent Fruits: They do not split or burst. Seeds are liberated only by the destruction of the pericarp.
   • Dry Dehiscent Fruits: These fruits burst automatically and discharge their seeds.
   • Dry Schizocarpic Fruits: They are intermediate between dehiscent and indehiscent fruits. One-seeded indehiscent parts are called mericarps while dehiscent one-seeded are termed cocci.
   • Fleshy Or Succulent Fruits: Pericarp and associated structure become fleshy.
2. Aggregate Fruits: These fruits are formed from polycarpel lary, apocaipous ovary. Each carpel develops into a fruitlet and all fruitlets together form an aggregate fruit. An aggregate of simple fruits borne, by a single flower is otherwise known as etaerio.
3. Multiple Or Composite Fruits: These fruits develop from the entire inflorescence. Sorosis develops from the spike, spadix, or catkin inflorescence. Syconus develops from hypanthium inflorescence.

Seed

Morphologically, a ripened ovule is known as a seed. In other words, seed is A mature, integumented megasporangium. Seeds arc characteristic of spermatophytes (gymnosperms and angiosperms).

Parts Of Seed

Seed Coat: The outer, protective covering of the seed is called the seed coat, which develops from integuments of the ovule. In seeds developing from bitegmic ovules, there are two distinct layers in the seed coat. The outer layer is thick, hard, and leathery (developing from the outer integument) called the testa, whereas the inner layer is thick and papery (developing from the inner in-tegument) called legmen. In seeds developing from unitegmic ovules, there is a single-layered seed coat.

Embryo: The embryo is the most important part of the seed which represents a tiny future plant. The embryo has an embryonal axis or main axis called tigellum, to which one or two cotyledons (seed) are attached, depending upon whether the seed is monocot or dicot. The portion of the embryonal axis or flagellum below the point of attachment of cotyledons is called hypocotyl, which bears a radicle or future root at its tip. Similarly, the portion of the embryonal axis or flagellum above the point of attachment of cotyledons is called epicotyl, which bears plumule.

In some seeds (for example, legumes), the reserve food is stored in cotyledons, whereas in others (for example, cereals), there is a special nutritive tissue called endosperm. The seeds having endosperm are called endospermic or albuminous seeds, for example, cereals, castor, etc., whereas seeds in which the endosperm is fully consumed by the embryo and no endosperm is left are called non-endospermic or exalbuminous seeds, for example, gram, pea, cucumber, tamarind, etc. The reserve food materials in seeds may be carbohydrates (for example, wheat, rice) proteins (legumes) fats (castor, peanut, sunflower), etc. All structures inside the seed coat constitute the kernel.

Monocot Embryo: There is a single cotyledon called scutellum, which is attached to the mid-part of the embryonal axis on its lateral side. On the opposite side of the scutellum is present a tongue-shaped flap-like outgrowth called epiblast (for example, wheat) which represents reduced cotyledon. Further, there is a covering or sheath of the radicle called coleorhiza and a sheath of the plumule called coleoptile.

In Castor Seed (Ricinus Communis): There is a specific outgrowth called caruncle or strophiole, present over the hilum. It is formed by the proliferation of cells of the outer integument at the tip. Caruncle is somewhat spongy and helps in the absorption of water during the germination of seed.

Pcrispermic Seeds: Mostly nucellus is consumed after fertilization due to the absorption of food by the endosperm and embryo. The remains are of nucellus in the seed are called perisperm. Such seeds are called pcrispermic seeds, for example, Piper nigrum (black pepper).

Chalazosperimic Seeds: Chalazosperm is perisperm-like tissue in chalazal region. It is a substitute for endosperm, for example, Cynastrum.

Economic Importance Of Papilionaceae

1. Source of pulses (gram, arhar, mung, etc.)
2. Edible oil (soybean, groundnut)
3. Dye (Indigofera)
4. Fodder (sesbania, Trifolium)
5. Ornamentals (lupin, sweet pea)
6. Medicine (mulethi)

Economic Importance Of Solanaceae

1. Source of food (tomato, brinjal, potato)
2. Spice (chili)
3. Medicine (belladonna, ashwagandha)
4. Fumigalory (tobacco)
5. Ornamentals (petunia)

Economic Importance Of Liliaceae

1. Good ornamentals (tulip, Gloriosa)
2. Source of medicine {Aloe) and vegetables (Asparagus)
3. Colchicine (Colchicum autumnale)

Morphology Of Flowering Plants Points To Remember

Style is generally terminal but may be lateral, for example, Graminae, or mango. Gynobasic style arises from the base of the ovary, for example, Labiatae.

• Examples of endospermic dicot seeds are castor, papaya, and cotton.
• Examples of non-endospermic dicot seeds are gram, bean, pea, cucumber, and tamarind.
• Examples of endospermic monocot seeds are maize, rice, and wheat.

Examples Of Non-endospermic Monocot Seeds: are pothos (money plant), Vallisneria, Alisina, and Amotphophallns.

Defense Mechanisms In Plants

1. Thoms: Lemon, pomegranate, Duranta
2. Spines: Agave, Yucca
3. Prickles: Agave, cotton tree, rose
4. Stinging Hair: Laportea, Urtica clioica
5. Glandular Hair: Jatropha, Boerhavia, Tobacco
6. Stiff Hair: Guaphalium
7. Latex: Ficus, Nerium, Euphorbia
8. Alkaloids: Poppy, Datura
9. Geophilous Habit: Ginger, turmeric. Colocasia, onion
10. Myremecophily: Guava, mango, litchi
11. Mimicry: Cladium, Sansevieria

Anemochory is common in orchids and grasses. The jaculator mechanism in Ruellia and the fountain mechanism in Ecballiurn are related to autochory.

 

Morphology Of Flowering Plants Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1.

Assertion: In head inflorescence, florets are arranged centrifugally.

Reason: There are always two types of florets in head.

Answer: 4. If both Assertion and Reason are false.

Question 2.

Assertion: Staminal tube is present in Malvaceae.

Reason: It is due to a monetary condition.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3.

Assertion: The nest of Dischidia is a modified structure of the root.

Reason: Nest roots absorb water and food from humus-rich soil collected in the nest.

Answer: 4. If both Assertion and Reason are false.

Question 4.

Assertion: The lower feathery end of the tigellum is known as a radicle.

Reason: Tigellum bears two nodes on which one or two cotyledons develop.

Answer: 4. If both Assertion and Reason are false.

Question 5.

Assertion: There are five alae in the Pisitm sativum flower.

Reason: Both alae are covered by the largest petal.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 6.

Assertion: All floral whorls are supposed to be modified leaves.

Reason: A Flower is considered as a modified shoot bearing floral parts on its nodes.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

Question 7.

Assertion: Young leaves in Poinsettia are brightly colored to attract pollinators and achieve pollination.

Reason: It is only a bright color that can attract pollinators on all plants.

Answer: 3. If Assertion is true, but Reason is false.

Question 8.

Assertion: Schizocarpic fruits arc intermediate between dehiscent and indehiscent fruits.

Reason: These fruits split into single-seeded parts.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 9.

Assertion: Leaf in Opuntia functions for the storage of sugars.

Reason: Sugar is transported from leaves in Opuntia and gets stored in the stem.

Answer: 4. If both Assertion and Reason are false.

Question 10.

Assertion: Prop roots develop mostly from horizontal branches of main stem.

Reason: Adventitious roots may perform mechanical supporting functions, working as ropes of a tent.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

 

Principles Of Inheritance And Variation

Inheritance: Heredity And Variations

• Heredity is the transmission of genetic characters from the parents to the offsprings.
• It deals with the phenomenon of “like begets like.” For example, human babies are like human beings in overall characteristics.
• About 200 characters are found to be hereditary in man.
• Variations are common in sexually reproducing organ- isms.
• Asexually reproducing organisms are monoparental and, hence, exhibit no genetic variations.

Pre-Mendelian Ideas About Inheritance or Theories of Blending Inheritance

• The science of genetics arose with the rediscovery of Mendelism in 1900. Early philosophers, thinkers, and workers have presented various theories to explain the phenomenon of inheritance.
• These are called the theories of blending inheritance. Some of these theories are as follows:
  • Moist vapor theory (Pythagoras: 500 BC): Various body parts emit certain vapors, which get aggregated to form a new individual.
  • Reproductive blood theory (Aristotle: 384-322 BC): According to Aristotle, the menstrual fluid and semen are kinds of highly purified blood. Menstrual fluid provides inert substance for embryo formation and semen provides form and shape to embryo.
  • Preformation theory or homunculus theory (J. Swammerdam): According to this theory, the miniature form of individual is already present in the sperm or egg called “homunculus.” Fertilization is required to stimulate its growth.
  • Theory of pangenesis (Darwin, 1868): According to Darwin, each part of body produces minute particles called gemmules or pangenes, which aggregate to form gamete. On fusion, these give rise to a new individual.
  • Theory of epigenesis (K.F. Wolff): According to this idea, neither egg nor sperm had a structural homunculus but the gametes contained undifferentiated living substance capable of forming an organized body after fertilization. This suggested that many new organs and tissues which were originally absent develop structurally de novo due to mysterious vital force.
• The theory of pangenes was disproved by Weismann.
• A. Weismann proposed his theory of germplasm, ac- cording to which the changes which affect the germplasm are heritable and the changes which affect the somatoplasm are nonheritable.
• Objections to blending inheritance:
  • Unisexual traits
  • Skin color in humans
  • Atavistic character

Read and Learn More NEET Biology Notes

Genetics Terms And Symbols

Mendelian Inheritance

• Mendel was born on July 22, 1822. He worked on Pisum sativum (garden pea or edible pea) for 7 years by taking 7 pairs of contrasting traits.
• The results were read out in two meetings of the Natu- ral History Society of Brunn in 1865.
• His paper “Experiments on Plant Hybridization” was published in the fourth volume of “Proceedings of Natural Science Society of Brunn” in 1866.
• Mendel was the first to apply statistical analysis and mathematical logic.
• He selected 14 true breeding pea plant varieties.
• He died due to kidney disorder in 1884.
• Mendel selected the characters listed in Table 5.1, in pea plant, for carrying out hybridization experiments.
• Mendel failed to produce the same results in hawk- weed (Hieracium) and beans (Lablab). Detailed inves- tigation by S. Blixt on pea plant led to locate Mendel’s seven characters on four different chromosomes-1, 4, 5, and 7.
• However, Mendel’s work did not receive any recognition, it deserved, till 1900.
• Mendel’s work remained unnoticed and unappreciated for several years due to the following reasons:
  • Communication was not easy in those days and his work could not be widely publicized.
  • His concept of stable, unblending, and discrete units or factors for various traits did not find acceptance from the contemporaries.
  • His approach of using mathematical and statistical analysis to explain biological phenomena was totally new and unacceptable to many biologists of that time.
  • He could not provide any physical proof for the existence of factors. It was the rediscovery of his work by Hugo de Vries (a Dutch), Carl Correns (a German), and Erich von Tschermak (an Austrian botanist), independently in 1900, that brought Mendel to limelight. Correns raised the status of Mendel’s generalizations to laws.
• Selection of pea plant: The main reasons for adopting garden pea (Pisum sativum) for experiments by Men- del were as follows:
  • Pea has many distinct alternative traits (clear con- trasting characters).
  • The lifespan of pea plant is short.
  • Flowers show self (bud) pollination and, so, are true breeding.
  • It is easy to artificially cross-pollinate the pea flowers. The hybrids, thus, produced were fertile.

Mendel’s Work and Results

• Mendel made cross between parents having contrasting traits.
• Firstly he made monohybrid cross (cross between parents that differ from each other in one character) followed by dihybrid cross (cross between parents that differ from each other in two characters) and finally trihybrid cross.
• The F, hybrids were self-crossed to give rise to F2 gen- eration.
• Mendel also carried out reciprocal crosses and found that these gave the same result. (Reciprocal cross means opposite cross, i.e., the parent that provides male gamete in one cross provides female gamete in the second experiment and vice versa).
• The result of reciprocal cross proves that both gametes produce the same effect and it does not matter which parent provides male and which one provides female gamete.
• On the basis of his experimental crosses, he formulated four postulates.
  • Postulate 1: According to this postulate, characters are controlled by a pair of unit factors. The two factors are now called alleles or allelomorphic pair.
  • Postulate 2: If two dissimilar unit factors are present in an individual, only one expresses itself. The one which expresses itself is known as the dominant fac- tor, while the second which does not express at all is known as the recessive factor.
  • Postulate 3: According to this postulate, two contrasting alleles responsible for contrasting traits pre- sent in an individual do not get mixed and get separated from each other at the time of gamete formation by F1 hybrid. Due to their recombination, four combinations can be obtained in equal frequency.
• All three postulates are based on Mendel’s monohybrid cross or one-gene interaction.
• Law of dominance and law of segregation can be explained on the basis of monohybrid cross or one-gene interaction.
  • Law of dominance:
    • This law states that when two contrasting alleles for a character come together in an organism, only one is expressed completely and shows visible effect.
    • This allele is called dominant and the other allele of the pair which does not express and remains hidden is called recessive.
    • This law is not universally applicable. Plant height is controlled by two alleles dominant allele (T) and recessive allele (t). These two alleles can be present in three forms.
    • Mendel crossed two pea plants-one homozygous tall (TT) and another homozygous dwarf (tt).
    • He observed that all the F, progeny plants were tall; like one of the parents, none were dwarf.
    • He made similar observations for the other pair of traits and found that F, always resembled only one of the parents, and that the traits of the other parent were not seen in them.

• • Law of segregation or law of purity of gametes
    • This law states that both parental alleles (recessive and dominant) of F, generation separate and are expressed phenotypically in F2 generation. This law is universally applicable.
    • The F2 generation was produced by allowing the F, hybrid to self-pollinate, to find out segregation or separation.
    • It was observed that both dominant and recessive plants appeared in the ratio of 3: 1. Thus, F2 progeny shows both parental forms.
    • On the basis of F2 generation, following observations can be made.
      • An organism generally has two alleles for each character. These alleles may either be similar or dissimilar. An organism with similar alleles of a pair is called pure or true breeding for that character. If the organism contains dissimilar alleles of a pair, the organism is impure or hybrid.
      • An organism receives one of the two alleles from the male gamete and the other from the female gamete. The gametes fuse during fertilization and form a zygote. Zygote develops into an organism.
      • Each gamete (male or female) has only one allele of the pair. Thus, each gamete is pure for a trait. That is why this law is often called the law of purity of gametes.
      • Fusion between male and female gametes to produce a zygote is a random process.
    • Plants obtained in F2 generation show 3 (tall): 1 (dwarf) phenotypic ratio. Of these three tall plants, one is pure or homozygous dominant and the remaining two are heterozygous (tall in this case). There is only one plant that shows recessive character (dwarf in this case). Dwarf is pure or true breeding, being homozygous recessive.
    • Postulate 4
      • This postulate was made on the basis of dihybrid cross or two-genes interaction.
      • He postulated that the inheritance of one character is independent of the inheritance of another character.
    • On the basis of this postulate, Mendel proposed the “law of independent assortment.”
  • Law of independent assortment
    • The law of independent assortment states that when a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independent of the inheritance of another character.
    • Because of their independent assortment, besides the parental types, recombinants are also obtained.
    • In dihybrid cross, these combinations are obtained in the ratio of 9:3:3: 1. For example, Mendel crossed homozygous dominant round and yellow seeded plant (RRYY) with homozygous recessive wrinkled and green seeded (rryy) plant.
    • The F, hybrids were all heterozygous, showing yellow and round seeded plants.
    • This law is not universally applicable.

• If the phenotypic ratio of each pair of alleles (e.g., yellow and green color of seed) is considered, it shows 12 (=9+3) yellow seeded plants and 4 (= 3 + 1) green seeded plants.
• This comes to the ratio 3: 1, similar to the one obtained in the F2 generation of monohy- brid cross showing segregation.
• The same is true for another pair of alleles involved, i.e., round and wrinkled seeded plants. So, the results of each character are similar to the monohybrid ratio.

Summarized Account Of Mendel’s Experiments

Back Cross and Test Cross

Back Cross

• F1 hybrids are obtained by crossing two plants of parental generation.
• Mendel devised a cross where the F, hybrid is crossed with any one of the two parents, i.e., homozygous dominant and homozygous recessive.
• Thus, there will be two possibilities:
  • F1 hybrid (Tt) is crossed with homozygous dominant (TT).
  • F1 hybrid (Tt) is crossed with homozygous recessive (tt).
• Both these crosses collectively are called back cross. If F, is crossed with dominant parent, it is called out cross.

Test Cross

• Out of the two types of back crosses, a cross between F, hybrid (Tt) and its homozygous recessive parent (tt) is called test cross.
• This cross is called test cross because it helps to find out whether the given dominant F, phenotype is homozygous or heterozygous.
• A monohybrid test cross between F, tall plant (Tt) and its homozygous recessive parent (tt) will produce 50% heterozygous tall (Tt) and 50% homozygous recessive (tt), i.e., ratio 1: 1, for both phenotype and genotype.

• If a test cross with two characters, i.e., dihybrid test cross, is made, it gives four types of plants in the ratio 1:1:1:1.
• The phenotypes obtained are similar to those found in the F2 generation of dihybrid cross.
• Thus, a dihybrid test cross between F, yellow and round seeded plant (YyRr) and its homozygous recessive green and wrinkled parent (yyrr) will give the following combinations:
  • 1 yellow, round (YyRr); parental combination 25%
  • 1 yellow, wrinkled (Yyrr); recombinants 25%
  • 1 green, round (yyRr); recombirants 25%
  • 1 green, wrinkled (yyrr); parental combination 25%
• If this ratio is obtained, it will be confirmed that F, hybrid with dominant phenotype is in fact heterozygous.
• The parental combinations (50%) are equal to the frequency of recombinants (50%).

Trihybrid Cross

• Mendel crossed two pea plants, which differed in three characters, and observed independent assortment of genes in them.
• He crossed two pea plants pure in three traits viz., height of stem, form of seed, and color of cotyledon of seed.
• The plants crossed were homozygous tall, round, and yellow (TT RR YY) plant and dwarf, wrinkled, and green (tt rr yy) plant.
• All F, individuals produced were tall, round, and yellow (Tt Rr Yy). These are called trihybrids.
• On selfing trihybrids, F2 phenotypic ratio is 27:9:9 :9:3:3:3:1. The ratio for a trihybrid test cross is 1:1:1:1:1:1:1:1.

One-Gene Interaction (With Respect To Post-Mendelian Inheritance)

• Incomplete dominance
  • After Mendelism, a few cases were observed where F, phenotype was intermediate between dominant and recessive phenotypes.
  • The most common example of incomplete dominance is that of flower color in Mirabilis jalapa (Gulbansi or 4’0 clock plant), studied by Carl Correns.
  • Homozygous red (RR) flowered variety was crossed with white (rr) flowered variety.
  • F, offspring had pink flowers.
  • Thus, here one allele is incompletely dominant over the other so that intermediate phenotype is produced by F, hybrid with respect to the parents.
  • This is called incomplete dominance.
  • Incomplete dominance for flower color [red (RR), pink (Rr), white (rr)] is also known to occur in Antirrhinum majus (snapdragon or dog flower).
  • The phenotypic ratio and genotypic ratio in F2 generation are identical in case of incomplete dominance, i.e., 12:1
• Explanation of the concept of dominance
  • Every gene contains information to express a particular trait.
  • Diploid organisms have two copies of each gene. These are called alleles.
  • These two alleles may be identical or non-identical.

• • One of them may be different due to some changes it has undergone which modifies the information that particular allele contains.
  • Theoretically, the modified allele could be responsible for the production of
    • normal/less efficient enzyme or
    • a non-functional enzyme or
    • no enzyme at all.
  • In case (1), the modified allele is equivalent to the unmodified allele, i.e., it will produce the same phenotype/trait.
  • But if the allele produces a non-functional enzyme or no enzyme [cases (2) and (3)], the phenotype may be affected.
  • The unmodified (functioning) allele which represents the original phenotype is the dominant allele and the modified allele is generally the recessive allele.
  • Hence, the recessive trait is due to nonfunctional enzyme or because no enzyme is produced.
  • If the mutated allele forms an altered but functional product, it behaves as incomplete or co- dominant allele.
• Multiple allelism
  • Mendel proposed that each gene has two contrasting forms, i.c., alleles.
  • But there are some genes that have more than two alternative forms (alleles).
  • The presence of more than two alleles for a gene is known as multiple allelism.
  • Multiple alleles are present on the same locus of homologous chromosome.
  • These alleles can be detected only in a population.
  • A well known example to explain multiple alleles in human beings is ABO blood type.
  • Landsteiner discovered ABO system of blood groups. The fourth group, AB, was discovered by de Castello and Steini.
  • Bernstein showed that these groups are controlled by three alleles I^, 13, and 1°/i.
  • These alleles are autosomal and follow the Mendelian pattern of inheritance.
  • Alleles I^ and I produce a slightly different form of sugar while 1o does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three “I” gene alleles.
  • IA and IB are completely dominant over 1°, but when I and I are present together, they both ex- press their own types of sugar, thus, behaving as codominant alleles.

• • Other examples of multiple alleles are coat color in rabbit, eye color in Drosophila, and self-incompatibility in tobacco. The formula to find the number of genotypes for multiple allelism is (n/2)(n+1), where n is the number of alleles.
• Co-dominance
  • In co-dominance, the genes of an allelomorphic pair are not related as dominant and recessive- both of them express themselves equally in F, hybrids.
  • These follow the law of segregation and F2 progeny exhibits ratio 1 2 1. Heterozygous for sickle-cell anaemia (Hb^Hb), AB, and MN blood groups are examples of co-dominance of alleles.
• Lethal genes or lethality
  • A lethal gene usually results in the death of an individual when present in homozygous condition. The most striking example to explain lethal gene is sickle-cell anaemia (HbsHb).
  • Cuenot (1905) first reported that inheritance in the mouse body color did not agree with Mendelian inheritance, because the dominant allele for yel- low body color is lethal in homozygous condition.
  • The homozygous dominant gene carrying mouse died, proving that dominant gene is lethal in homozygous form.
  • This is called absolute lethality. In plants, it was first reported in Antirrhinum majus by E. Baur, where yellow leaved or golden leaved or aurea plant never breeds true. Thus, the ratio comes out to be 2: 1.

• Pleiotropic genes
  • The ability of a gene to have multiple phenotypic effects (as it influences a number of characters simultaneously) is known as pleiotropy.
  • The gene having multiple phenotypic effects is called pleiotropic gene.
  • It is not essential that the traits are equally influenced. Sometimes, the effect of the gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect).
  • Occasionally, a number of related changes are caused by a gene. These are together called syn- drome.
  • Some common examples in humans are cystic fibrosis, Marfan syndrome, and phenylketonuria, while in Drosophila, a single gene influences the size of wings, the character of balancers, the po- sition of dorsal bristles, eye color, the shape of spermatheca, fertility, and longevity.
  • In human beings, pleiotropy is exhibited by sickle-cell anaemia in heterozygous condition (Hb*Hbs).
  • In case of pea, the gene which controls starch synthesis also controls the shape of the seed.

Two-Genes Interaction (With Respect To Post-Mendelism)

• Genes usually function or express themselves singly or individually.
• But many cases are known where two genes of the same allelic pair or genes of two or more different allelic pairs influence one another.
• This is called gene interaction.

Non-Allelic Genetic Interactions

• Non-allelic genetic interactions are interactions between genes located on the same chromosome or on different but non-homologous chromosomes controlling a single phenotype to produce a different expression.
• Each interaction is typical in itself and the ratios obtained are different from the Mendelian dihybrid ratios.
• Some of these interactions of genes, which fall under this category and deviate from Mendel’s ratios, are explained here.
• Complementary genes
  • Complementary genes are two genes present on separate loci that interact together to produce a dominant phenotypic character; neither of them if present alone can express itself. It means that these genes are complementary to each other.

• • Bateson and Punnet have demonstrated that in sweet pea (Lathyrus odoratus), the purple color of flowers develops as a result of interaction of two dominant genes, C and P.
  • In the absence of dominant gene C or P or both, the flowers are white.
  • It is believed that gene C produces an enzyme which catalyzes the formation of necessary raw material for the synthesis of pigment anthocyanin and gene P produces an enzyme which transforms the raw material into the pigment.
  • It means the pigment anthocyanin is the product of two biochemical reactions; the end product of one reaction forms the substrate for the other.

• • Therefore, if a plant has ccPP, ccPp, CCpp, or Ccpp genotype, it bears only white flowers. Purple flowers are formed in plants having genotype CCPP, CCPp, CCPP, or CcPp.
  • From the checker board, it is clear that 9: 7 ratio between purple and white is a modification of 9:3:3:1 ratio.
• Duplicate genes
  • If the dominant alleles of two gene loci pro- duce the same phenotype, whether inherited together or separately, the 9:3:3:1 ratio is modified to 15: 1.
  • Example: The capsules of shepherd’s purse (Capsella) occur in two different shapestriangular and top-shaped. When a plant with triangular capsule is crossed with one having top-shaped capsule, in F1, only tri- angular character appears. The F, offspring by self-crossing produces F2 generation with triangular and top-shaped capsules in the ratio of 15: 1.
  • Two independently segregating dominant genes (A and B) have been found to influence the shape of the capsule in the same way. All genotypes having dominant alleles of both or either of these genes (A and B) will produce plants with triangular capsules. Only those with genotype aabb will produce plants with top-shaped capsules.

• Epistasis
  • A gene which masks (hides) the action of an- other gene (non-allelic) is termed as epistatic gene. The process is called epistasis. A gene whose effects are masked is termed as hypostatic gene.
  • Epistasis is of two types:
• Recessive epistasis
  • Here the recessive allele in homozygous condition masks the effect of dominant allele. For example, in mice, the wild body color is known as agouti (grayish); it is controlled by a gene say A which is hypostatic to recessive allele c.

• • Dominant allele C in the presence of a gives colored mice.
  • In the presence of dominant allele C, A gives rise to agouti.
  • So, CCaa will be colored and ccAA will be albino.
  • When colored mice (CCaa) are crossed with albino (ccAA), agouti mice (CcAa) appear in F1.
  • cc masks the effect of AA and is, therefore, epistatic.
  • Consequently, ccAA is albino.
  • The ratio 9:3:3: 1 is modified to 9:3:4.
  • The combination ccaa is also albino due to the absence of both the dominant alleles.
• Dominant epistasis
  • In summer squash, or Cucurbita pepo, there are three types of fruit colors-yellow, green, and white.
  • White color is dominant over other colors, while yellow is dominant over green.
  • The gene for white color (W) masks the effects of yellow color gene (Y).
  • So, yellow color is formed only when the dominant epistatic gene is represented by its recessive allele (w). When the hypostatic gene is also recessive (y), the color of the fruit is green.
    White fruit: W-Y-, W-y-
    Yellow fruit: wwY-
    Green fruit: wwyy

• • A cross between a pure breeding white summer squash (WWYY) with a pure breeding green summer squash (wwyy) yields white fruits in the F, generation. Upon selling of F1, the F2 generation comes to have white, yellow, and green fruits, respectively, in the ratio of 12:3: 1.

Polygenic Inheritance or Quantitative Inheritance

• Quantitative inheritance is controlled by two or more genes in which the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional polypeptide and full trait is shown when all dominant alleles are present.
• Genes involved in quantitative inheritance are called polygenes.
• Swedish geneticist, H. Nilsson-Ehle (1908), and East (1910) demonstrated the segregation and assortment of genes controlling quantitative traits. For example, kernel color in wheat and corolla length in tobacco.
• H. Nilsson-Ehle crossed red kerneled variety with white kerneled variety of wheat.
• The grains of F, were uniformly red but intermediate between the red and white of parental generation.
• When the members of F, were self-crossed among themselves, five different phenotypic classes appeared in F2 showing the ratio of 1:4: 6:4: 1.
  • Extreme red-1/16 (as red as to the parent of F1)
  • Deep red (dark red)-4/16
  • Intermediate red-6/16 (similar to F1)
  • Light red-4/16
  • White-1/16 (as white as to the parent of F1)
• Nilsson-Ehle found that the kernel color in wheat is determined by two pairs of genes-AA and BB.
• Genes A and B determine the red color of kernel and are dominant over their recessive alleles. Each gene pair shows Mendelian segregation.
• Heterozygotes for two pairs of genes (AaBb) segregate into 15 red and 1 white kerneled plants.
• But all red kernels do not exhibit the same shade of redness.
• The degree of redness was found to correspond with the number of dominant alleles.

Skin Color in Man

• The presence of melanin pigment in the skin deter- mines the skin color.
• The amount of melanin developing in the individual is determined by three (two also) pairs of genes. These genes are present at three different loci and each dominant gene is responsible for the synthesis of fixed amount of melanin.
• The effect of all the genes is additive and the amount of melanin produced is always proportional to the number of dominant genes.
• Subsequent studies after Davenport have shown that as many as six genes may be involved in controlling the skin color in human beings.
• The effect of all genes is additive. (The character is assumed to be fixed by three pairs of polygenes.)

• • The F1 progeny between an albino and a Negro individual, called mulatto, produces intermediate skin color.
  • In F2 generation, the colored offsprings exhibit different shades in the ratio 1:6:15:20:15:6:1.
  • The frequency distribution for skin color can be rep- resented either as a histogram or in the form of a bell- shaped normal distribution curve.
  • Looking at the histogram, it can be concluded that in polygenic inheritance, the extreme phenotypes are rare and the intermediate ones are more frequent.

Some other examples of quantitative traits are cob length in maize; human intelligence; milk and meat production; height in humans; and size, shape, and number of seeds and fruits in plants.

• Number of phenotypes for polygenes = 2n + 1
• Number of genotypes for polygenes=3″, where n represents pairs of polygenes

Chromosomal Theory Of Inheritance/Parallelism Between Chromosomes And Mendelian Factors

• The chromosomal theory of inheritance was proposed independently by Sutton and Boveri.
• The two workers found a close similarity between the transmission of hereditary traits and the behavior of chromosomes while passing from one generation to the next through the agency of gametes.
• They noted that the behavior of chromosomes is parallel to the behavior of Mendelian factors (genes).
• The salient features of chromosomal theory of inheritance are as follows:
  • Like hereditary traits, chromosomes retain their number, structure, and individuality throughout the life of an organism and from generation to generation. The two neither get lost nor mixed up. They behave as units.
  • Both chromosomes as well as genes occur in pairs in the somatic or diploid cells. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
  • A gamete contains only one chromosome of a type and only one of the two alleles of a trait.
  • The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization.
  • Homologous chromosomes synapse during meiosis and then separate or segregate independently into different cells. This establishes the quantitative basis for segregation and independent assortment of hereditary factors.
  • Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
  • Johannsen (1909) coined the term gene for Mendelian factor.
  • Following the synthesis of ideas, the experimental verification of the chromosomal theory of inheritance by T.H. Morgan and his colleagues led to the discovery of the basis for variations that sexual reproduction produced.
  • Thomas Hunt Morgan (1866-1945) is known as the father of experimental genetics. He was awarded the No- bel Prize of physiology in 1933 for his pioneer work in experimental genetics.

Drosophila melanogaster as Material for Experimental Genetics

• Fruit fly Drosophila is a tiny fly of size about 2 mm. It is found over ripe fruits like mango and banana.
• The fly is actually attracted to the yeast cells present on the surface of ripe fruits. Drosophila is more suitable than pea as experimental material because of the following reasons:
  • It can be easily reared and bred under laboratory conditions.
  • The fly has a short life span of about 2 weeks. It can be bred throughout the year so that numerous generations can be obtained in a single year instead of one as in case of pea.
  • A single mating produces hundreds of offsprings. Females are easily distinguishable from males by the larger body size and the presence of ovipositor (egg laying structure).
  • It shows a number of externally visible and easily identifiable contrasting traits.
  • It has a smaller number (four pairs) of morpho- logically distinct chromosomes.
  • Polytene chromosomes occur in the salivary glands of larva. These can be used to study different types of chromosome aberrations.

• • The fly has heteromorphic (XY) sex chromosomes in the male. The transmission of heteromorphic chromosomes can be easily studied from one generation to another.

Linkage (Exception To Law Of Independent Assortment)

• According to Mendel’s law of independent assortment, genes controlling different characters get assorted in- dependent to each other.
• This is correct if the genes are present on two different chromosomes. But if these genes are present on the same chromosome, they may or may not show independent assortment.
• If crossing-over takes place between these two genes, then the genes get segregated and will assort independent to each other. But if there is no crossing-over between these two genes, there is no segregation and, hence, only parental combination will be found in gametes.
• The tendency of some genes to inherit together (en bloc) is known as linkage.
• In 1906, Bateson and Punnet crossed two varieties of Lathyrus odoratus (sweet pea) and observed that the results do not agree with Mendel’s law of independent assortment.
• They formulated the hypothesis of coupling and re- pulsion to explain the unexpected F2 results of dihybrid cross between a homozygous sweet pea having dominant alleles for blue flowers (BB) and long pollen grains (LL) with another homozygous double recessive plant having red flowers and round pollen grains (bbll).
• Test cross ratio of 7:1:1:7 indicated that there was a tendency of the dominant alleles to remain together. Similar was the case with recessive alleles.
• It was called gametic coupling by Bateson and Punnet.
• Two dominant genes from one parent entered the same zygote more frequently than expected.
• The tendency of two dominant genes to remain together in the process of inheritance was called coupling.
• In another cross, they took a sweet pea plant with blue flowers and round pollens (BBII) and other plant with red flowers and long pollens (bbLL) and obtained the ratio of 177 1 by test crossing the F1 generation.

 

• When two dominant or recessive genes come from different parents, they tend to remain separate. Hence, this ratio was called repulsion ratio.
• T.H. Morgan in 1910 showed that coupling and repulsion are two aspects of the same phenomenon called linkage.
• He suggested that two genes when present on the same chromosome are in coupling phase and when present on two different homologous chromosomes are in re- pulsion phase.
• Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked.
  • At first, he crossed yellow-bodied (y) white-eyed (w) female with brown-bodied (y) red-eyed (w*) male which produced F, with brown-bodied red- eyed female and yellow-bodied white-eyed male. In the F2 generation, obtained by intercrossing of F1 hybrids, the ratio deviated significantly from expected. He found 98.7% to be parental and 1.3% as recombinants.
  • In a second cross between white-eyed miniature- winged female (wwmm) with wild-red-eyed (w’) normal-winged male (m), the F, generation included red-eyed normal-winged female and white-eyed miniature-winged male. After inter- crossing the F, progeny, he found 62.8% parental and 37.2% recombinants.
  • On the basis of results, we can say that the strength of linkage between y and w is higher than that between w and m.
• According to Morgan, the degree or strength of linkage depends upon the distance between the linked genes in the chromosome..
• Linkage, therefore, may be defined as the tendency of two genes of the same chromosome to remain together in the process of inheritance.

Kinds of Linkage

• T.H. Morgan and his coworkers found two types of linkage:
  • Complete linkage
    • It is the linkage of genes on a chromosome which is not altered and is inherited as such from generation to generation without any cross-over.
    • In this type of linkage, the genes are closely associated and tend to remain together. Example: Male Drosophila and female silk- worm (Bombax mort).

• • • 100% parental combinations indicated that the gene for gray body color is completely linked with long wings.
    • In this dihybrid, F2 phenotypic ratio is 3:1 and test cross ratio is 1:1 (like a monohybrid). Another example is the inheritance of red eye and normal wing (PV/PV) with purple eye and vestigial wing character (pv/pv).
  • Incomplete linkage
    • The linked genes do not always stay together because homologous non-sister chromatids may exchange segments of varying length with one another during meiosis.
    • This is known as crossing-over.
    • The linked genes that have chances of separation by crossing-over are called incompletely linked genes and the phenomenon of their in- heritance is called incomplete linkage.
    • It produces both parental and recombinant types in variable ratio.
    • Bateson and Punnet studied Lathyrus odoratus and defined coupling and repulsion of dominant and recessive genes.
    • In the cis arrangement or coupling condition, the incomplete linkage ratio was 7:1:1:7 (14 parental, 2 recombinants).
    • In the trans arrangement or repulsion case, the ratio was 1:7:7:1 (parental 14, recombinants 2).
    • Example: In maize, incomplete linkage was observed by Hutchinson with respect to seed coat color and seed shape. The results showed that parental combinations of alleles (CS/CS and cs/cs) appeared in about 96% cases. The other two were new combinations (Cs/cs and cS/cs) that appeared in about 4% cases. Thus, in about 4% cases, crossing- over occurred between linked genes.

Crossing-Over And Recombination

• Crossing-over is a process that produces new combination of genes by interchanging segments between non-sister chromatids of homologous chromosomes.
• Crossing-over occurs between the homologous chromosomes at four-stranded or tetrad stage during pachytene of prophase 1 of meiosis 1.
• The condition where an individual heterozygous for two pairs of linked genes (AaBb) possesses two dominant genes on one member of the chromosome pair and two recessive genes on the other pair is said to be cis arrangement .

• If an individual heterozygous for two pairs of linked genes (AaBb) possesses one dominant and one recessive allele of each pair of genes on each member of the homologous pair of chromosomes, the arrangement is said to be trans arrangement.

• When two genes are located very close to each other in chromosomes, hardly any crossing-over can be detected.
• The linkage is broken down due to crossing-over.
• Crossing-over will be relatively more frequent if the distance between two genes is more.
• The frequency of crossing-over can be determined cytologically by counting the number of chiasmata.
• The details of crossing-over for two genes A and B and their alleles a and b on homologous chromosomes.

Crossing-Over Occurs at Four-Stranded Stage

• Neurospora (pink mould), an ascomycetous fungus, is used to demonstrate that crossing-over takes place at four-stranded stage.
• It has the following advantages as experimental organism:
  • It is haploid and there is only one allele at each locus. Hence, dominant-recessive relationship does not interfere with observations and analysis.
  • The products of single meiosis can be easily analyzed.
  • The products of meiosis occur in the form of “ordered tetrads,” i.e., the eight ascospores formed are linearly arranged in a sac-like structure called ascus.
• If genes A and B are located on the same chromosome and undergo independent assortment, the genotype of linearly arranged ascospores can be studied.
• If crossing-over takes place at two-strand stage, the ascospores would show Ab, Ab, Ab, Ab, aB, aB, aB, aB (i.e., 4 Ab+4 aB) arrangement [Fig. 5.24(a)]. If crossing-over takes place at four-strand stage, the ascospores would show AB, AB, Ab, Ab, aB aB, ab, ab (i.e., 2AB+2 Ab+2 aB + 2 ab) or 2: 4: 2 arrange- ment.
• Tetrad analysis has demonstrated the presence of such an arrangement and, thus, it is now confirmed that crossing-over occurs at four-stranded stage.

Factors Affecting Crossing-Over

• The distance between the genes is directly proportional to crossing-over.
• Cross-over decreases with age.
• X rays and temperature increase crossing-over.
• Centromere and heterochromatin positions decrease crossing-over.
• One cross-over reduces the frequency of other cross- over in its vicinity. This is called interference.

Chromosomal Mapping

• Crossing-over is important in locating the genes on a chromosome.
• The genes are arranged linearly on the chromosome.
• This sequence and the relative distances between various genes are graphically represented in terms of recombination frequencies or cross-over values (COV).
• This is known as the linkage map of chromosome.
• Distance or cross-over units are called centiMorgan (cM) or map unit.
• The term centiMorgan is used in eukaryotic genetics while the term map unit is used in prokaryotic genetics. Recombination frequency or

• Recombination frequency depends on the distance be- tween the genes.

• If the distance between the genes is less, the chances of crossing-over are less and, hence, recombination frequency is also less, and vice versa. So, recombina- tion frequency is directly proportional to the distance between the genes.
• In any cross, if recombination frequency is 5%, it means the distance between the genes is 5 map units. A.H. Sturtevant suggested that these recombination frequencies can be utilized in predicting the sequence of genes on the chromosome.
• On the basis of recombination frequency, he prepared the first chromosomal map or genetic map for Drosophila.

Sex Determination

Sex Chromosomes and Autosomes

• Sex chromosomes are those chromosomes which determine the sex of the individual in dioecious or unisexual organisms.
• The normal chromosomes, other than sex chromosomes, of an individual are known as autosomes.
• Sex chromosomes may be similar in one sex and dissimilar in the other.
• The two conditions are, respectively, called homomorphic (similar, e.g., XX and ZZ) and heteromorphic (dissimilar, e.g., XY and ZW).
• Individuals having homomorphic sex chromosomes produce only one type of gametes.
• These are, therefore, called homogametic. For exam- ple, male birds, human female, and Drosophila female.
• Individuals having heteromorphic sex chromosomes produce two types of gametes.
• These are termed as heterogametic. For example, female birds, human male, and normal Drosophila male.
• The factors that control the sex of an organism are under genetic control.
• Various mechanisms that lead to sex determination can be classified into the following four categories:
  • Chromosomal mechanism of sex determination
  • Non-allelic genetic sex determination-fertility factor (plasmid) in bacteria
  • Genic balance mechanism or X/A balance
  • Environmental mechanism of sex determination

Chromosomal Mechanism of Sex Determination

• According to this mechanism, there are certain chromosomes known as sex chromosomes, or heterosomes or idiochromosomes, which are responsible for sex de- termination.
• This mechanism may be of the following types:
  • XX-XY type
    • In most insects, plants, and mammals including human beings, females possess two homomorphic (isomorphic) sex chromosomes, i.c., XX.
    • Males possess two heteromorphic sex chromosomes, i.e., XY. The Y-chromosome is often shorter and heterochromatic (made of heterochromatin).
    • Despite the differences in morphology, the Y- chromosome pairs with the X-chromosome at a certain segment during meiosis.
    • It, therefore, carries a segment which is homologous with a segment of X-chromosome.
    • The remaining segment of Y-chromosome is non-homologous and carries only Y-linked or holandric genes, e.g., testis-determining factor (TDF).

• • • Human beings have 22 pairs of autosomes and one pair of sex chromosomes.
    • All the ova (haploid) formed by a female are similar in their chromosome type (22 + X). Therefore, females are homogametic.
    • The male gametes or sperms (haploid) produced by human males are of two types-(22 + X) and (22 + Y).
    • Human males are, therefore, heterogametic.
    • The two sexes produced in the progeny may have 50: 50 ratio.
    • This type of sex-determination was reported in plant Sphaerocarpus for the first time and is also found in plants such as Melandrium and Coccinia.
  • XX-XO type
    • In roundworms, Dioscorea, and some insects (true bugs, grasshoppers, and cockroaches), females have two sex chromosomes, XX, while males have only one sex chromosome, X.
    • There is no second sex chromosome.
    • Therefore, males are designated as XO.
    • Females are homogametic because they pro- duce only one type of eggs.
    • Males are heterogametic with half the male gametes carrying X-chromosome while the other half being devoid of it.
    • The sex ratio produced in the progeny is 1:1.

• • ZW-ZZ type (WZ-WW type)
    • In birds, fishes, silkworm, Fragaria elatior, and some reptiles, both sexes possess two sex chromosomes.
    • Unlike human beings, females contain het- eromorphic sex chromosomes while males have homomorphic sex chromosomes.
    • Because of having heteromorphic sex chromosomes, females are heterogametic and produce two types of eggs (A + Z) and (A + W).
    • The male gametes or sperms are of one type (A + Z). The sex ratio produced in the offspring is 1 : 1.

• • ZO-ZZ type
    • This type of sex determination occurs in butterflies, pigeon, ducks, and moths.
    • It is exactly opposite of the condition found in cockroaches and grasshoppers.
    • Here, females have odd sex chromosomes while males have two homomorphic sex chromosomes.
    • Females are heterogametic.

• • • They produce two types of eggs-with one sex chromosome (A + Z) and without the sex chromosome (A+0).
    • Males are homogametic, forming similar types of sperms (A + Z).
    • The two sexes are obtained in the progeny in the ratio of 1: 1 as both types of eggs are produced in equal ratio.

Arrhenotoky/Haploid-Diploid Mechanism

• This mechanism is found in honey bee. In honey bees, ants, and wasps, the egg, if fertilized, gives rise to female fly.
• The unfertilized egg develops parthenogenetically into male. So, female flies are diploid while male flies are haploid.

Non-Allosomic Genetic Sex Determination

The fertility factor of plasmid in bacteria determines sex.

Genic Balance or X/A Balance Theory of Sex Determination

• The genic balance theory of sex determination was given by C.B. Bridges. According to him, Y-chromosome plays no role in the sex determination of Drosophila; it is the ratio of the number of X-chromosomes to the set of autosomes which determines the sex of fly.
• It was concluded that the X/A ratio greater than 1 expresses superfemaleness, equal to 1 expresses femaleness, below 1 and above 0.5 expresses intersexes, equal to 0.5 expresses maleness, and less than 0.5 ex- presses supermaleness.

• Gynandromorphs: It is a sex mosaic (an individual with one-half of the body male and the other half female).
• Gynandromorphism is common in silk moth and Drosophila. It is developed due to the accidental loss of X-chromosome from a 2A+XX cell during mitosis.
• Gynander: A gynander may be male or female with patches of tissues of other sex on it.

Environmental Mechanism of Sex Determination

• The environmental mechanism of sex determination was observed by F. Baltzer in Bonnelia viridis (marine worm).
• In this organism, the sex is undifferentiated in larva.
• The larva that settle down in mud grow up into mature females, while those that settle down near the proboscis of a female and become parasites develop into males.
• It has been demonstrated that females secrete a certain hormone which induces sex in larva.
• Crepidula and Ophryortocha also show such mechanism.

Sex-Linked Inheritance

• Sex linkage was discovered by Morgan while working on the inheritance of eye color in Drosophila. He made three types of crosses:
  • Cross 1
    • White-eyed male (W) was crossed with red- eyed (W) female.
    • All flies of the F, generation were found to be red eyed.
    • F, flies were allowed to self breed.
    • In the F2 generation, both traits, red eye and white eye, appeared in the ratio of 3: 1 showing that white eye trait is recessive to red eye trait.
    • F1 generation consisted of only red-eyed flies. In F2 generation, all female flies were red eyed, 50% of the male flies were red eyed, and the remaining 50% were white eyed.

• • Cross 2
    • Red-eyed females of F, generation were crossed with white-eyed male.
    • It is similar to the test cross where hybrids are cross-bred with recessive parents.
    • Morgan obtained red- and white-eyed females as well as males in equal proportions-1 red- eyed female 1 white-eyed female: 1 red- eyed male: 1 white-eyed male.
    • The test cross indicated that white eye color was not restricted to the male fly.

• • Cross 3
    • White-eyed females were crossed with red-eyed males. It was the reciprocal of cross 1 and should have given the same result as obtained by Mendel. However, Morgan obtained a surprising result. All males were white eyed while all females were red eyed.

• • • Taking all crosses into consideration, Morgan came to the conclusion that eye color gene is linked to sex and is present on the X-chromosome.
    • The X-chromosome does not pass directly from one parent to the offspring of the same sex but follows a criss-cross inheritance, i.e., it is transferred from one sex to the offspring of the opposite sex.
    • In other words, in criss-cross inheritance, a male transmits his traits to his grandson through his daughter (diagynic), while a female transmits the traits to her granddaughter through her son (diandric).

Sex Linkage in Human Beings

Colorblindness and hemophilia (Bleeder’s disease) are two common examples of sex-linked diseases in human beings.

• Colorblindness
  • This is a human disease which causes the loss of ability to differentiate between red color and green color.
  • The gene for this red-green colorblindness is present on the X-chromosome. Colorblindness is recessive to normal vision.
  • If a colorblind man (XCY) marries a girl with nor- mal vision (XX), the daughters will have normal vision but will be carriers, while sons will also be normal.

• • If a carrier girl (heterozygous for colorblind- ness, XCX) now marries a colorblind man (XY), the offsprings will be 50% females and 50% males.
  • Of the females, 50% will be carriers for color-blindness and the rest 50% will be colorblind.
  • Of the males, 50% will have normal vision and 50% will be colorblind.

• Hemophilia (Bleeder’s disease)
  • A person suffering from this disease cannot synthesize a normal blood protein called antihemophilic globulin (AHG) required for normal blood clotting. Hemophilia-A is more severe.
  • Therefore, even a very small cut may lead to con- tinuous bleeding for a long time.
  • This gene is located on the X-chromosome and is recessive.
  • It remains latent in carrier females.

• • If a normal man marries a girl who is a carrier of hemophilia, the progeny will consist of 50% females and 50% males.
  • Of the females, 50% will be normal and the rest 50% will be hemophilia carriers.
  • Of the males, 50% will be normal and the rest will be hemophiliacs.
  • Hemophilia-B (Christmas disease): In this, plasma thromboplastin is absent. Inheritance is just like hemophilia-A.

Mutation

• Mutation is sudden, inheritable, discontinuous variation due to change in chromosomes and genes.
• Hugo de Vries (1901), one of the discoverers of Mendel’s laws, observed two distinct varieties of Oenothera lamarckiana (evening primrose).
• These differed in the length of stem, flower form, and the color and shape of leaves.
• These mutant varieties are now known to have been produced due to chromosomal aberrations.
• Seth Wright (1791) is considered to be the first to re- cord point or gene mutation.
• He noticed a lamb with unusually short legs.
• This short-legged breed of sheep was known as ancon breed.
• Darwin called this variation as sports.
• Bateson (1894) termed them as discontinuous or saltatory variations.
• The credit for starting the scientific study of mutations goes to Thomas Hunt Morgan (1910).
• He is known for his work on fruit fly, Drosophila melanogaster.
• He found white-eyed mutant of Drosophila. Since then, about 500 mutations have been observed by geneticists around the world.

Types of Mutations

Different classifications of mutations are known, each based on a definite criterion or character.

• On the basis of the agency involved
  • Spontaneous mutations: Such mutations occur at a frequency of 1 x 105 in nature. These are natural mutations and have also been called back- ground mutations.
  • Induced mutations: These have been observed in organisms due to specific factors such as radiations, ultraviolet light, or a variety of chemicals. The agents that induce mutations are called mutagens or mutagenic agents.
• On the basis of the type of cells in which mutations occur
  • Somatic mutations: These mutations occur in the somatic cells, i.e., body cells or the cells other than germinal cells. These mutations do not have any genetic or evolutionary importance. This is because only the derivatives or the daughter cells formed from the mutated cell will show mutation and not the whole organism.
  • Germinal mutations: These mutations occur in the gametes or germ cells and are also known as gametic mutations. Such mutations are heritable and, therefore, are of great evolutionary significance. If the mutations are dominant, these are expressed in the next generation, and if they are recessive, their phenotypic expressions remain suppressed.
• Forward and backward mutations
  • The most common type of mutation is the change from the normal or wild type to a new genotype (recessive or dominant).
  • Such mutations are called forward mutations.
  • An organism which has undergone forward mutation may again develop mutation which restores the original wild-type phenotype.
  • Such reversions are known as backward mutations or reverse mutations.
  • Mutations can occur at any stage during the life cycle of an organism.

Some Other Types of Mutations

• Gene mutation
  • It is alteration in the sequence of nucleotides in nucleic acids or any change in the sequence of triplet bases.
  • If gene mutation arises due to change in single base pair of DNA, it is called point mutation.
  • Gene mutation occurs by the following methods:
    • Frame-shift mutation (gibberish mutation)
      • Deletion: Removal of one or more bases from a nucleotide chain.
      • Insertion or addition: Addition of one or more bases to a nucleotide chain.
    • Substitution: The replacement of one base by another. It is of two types:
      • Transition: When a purine base (A or G) is substituted by another purine base or a pyrimidine base (T or C) is substituted by another pyrimidine base.
      • Transversion: The substitution of a purine base with a pyrimidine base or vice versa.
    • Tautomerization: The purines and pyrimidines in DNA and RNA may exist in several alternate forms or tautomers. Tautomerization occurs through the rearrangement of electrons and protons in a molecule. Tautomers show changed base pairing so as to cause change in sequence, e.g., AT to CG.
      • Nonsense mutation: Such mutation arises when a normal codon-coding for an amino acid-is changed into a chain- terminating codon (UAG, UAA, UGA) resulting in the production of an incomplete polypeptide.
        Nonsense mutations rarely go unnoticed because the incomplete or shorter protein formed is generally inactive.
      • Mis-sense mutation: It involves change in base in a codon, producing a different amino acid at the specific site in a polypeptide. In mis-sense mutation, the change in one amino acid is frequently compatible with some biological activity, e.g., sickle-cell anemia.
• Chromosomal mutation (chromosomal aberrations)
  • A change in chromosome morphology is called chromosomal aberration. Structural changes in chromosomes take place during meiosis. There are four types of chromosomal rearrangements:
    • Deficiency or deletion
      Deficiency occurs due to the loss of a terminal segment of chromosome. Deletion occurs due to the loss of an intercalary part of chromosome, e.g., cri-du-chat syndrome (short arm of chromosome 5 loses a part).
    • Duplication
      It occurs due to the addition of a part of chromosome so that a gene or a set of genes is represented twice, e.g., Barr eye in Drosophila.
    • Translocation
      It involves the shifting of a part of one chromosome to another non-homologous chromosome. So, new recombinant chromosomes are formed, as this induces faulty pairing of chromosomes during meiosis. An important class of translocation having evolutionary significance is known as reciprocal trans- location or segmental interchanges, which involves the mutual exchange of chromo some segments between non-homologous chromosomes, i.e., illegitimate crossing- over. Chronic myelogenous leukemia (CML) occurs due to the translocation of segment of long arm from chromosome 22 to chromosome 9. Chromosome 22 is called Philadelphia chromosome.
    • Inversion
      Inversion is change in the linear order of genes by the rotation of a section of chromosome by 180°. It occurs frequently in Drosophila as a result of X-ray irradiation. It may be of two types:
      • Paracentric: It is inversion without involving centromere. (Inverted segment does not carry centromere.)
      • Pericentric: It is inversion involving centromere.
• Genomatic mutation or numerical changes in chromosome number: It is of two types:
  • Aneuploidy: In aneuploidy, any change in the number of chromosomes in an organism will be different than the multiple of basic set of chromosomes. It results due to the failure of segregation of chromatids during cell division cycle. There will be following two possibilities:
    • Hypoploidy: This arises due to the loss of one or more chromosomes or pairs of chro- mosomes. Thus, the following conditions are likely to be produced:
    • Monosomy (2n-1): It is the result of loss of one chromosome for a homologous pair. Its other variant is double monosomy, i.e., (2n-1-1).
    • Nullisomy (2-2): It is the result of loss of a complete homologous pair of chromosomes.
  • Hyperploidy: This arises due to the addition of one or more chromosomes or pairs of chromosomes. The following conditions are, thus, likely to be produced:
    • Trisomy (2n+1): In this type, a single chromosome is added to the chromo- some set. Trisomics were obtained for the first time in Datura stramonium (Jimson weed) by A.F. Blakeslee and his cowork- ers (1924). In human beings, Mongolism or Down’s syndrome is due to the trisomy of chromosome 21 (2n+1 or 46 + 1), i.e., chromosome 21 is present three times. Others are the Patau syndrome, due to the trisomy of the 13th, and Edward’s syndrome, due to the trisomy of the 18th chromosome.
    • Tetrasomy (2n+2): It is the result of the addition of a complete homologous pair of chromosomes (i.e., two chromosomes).
  • Euploidy: In euploidy, any change in the number of chromosomes is the multiple of the number of chromosomes in a basic set or it occurs due to variation in one or more haploid sets of chromosomes. Accordingly, these may be haploid and polyploid.
    • Haploidy: In haploid, only one set of chromosomes is present. Haploids are better for mutation experimental studies, because all mutations, either dominant or recessive, can express immediately in them (as there is only one allele of each gene present in each cell).
    • Polyploidy: The failure of cytokines after the telophase stage of cell division results in an increase in the whole set of chromosomes in an organism. This phenomenon is known as polyploidy. Polyploids are of particular importance and are, therefore, discussed here. These fall into two major categories:
      • Autopolyploids: These have the same basic set of chromosomes multiplied more than twice, e.g., AAA (autotriploid) and AAAA (autotetraploid). Autopoly- ploids with odd number of chromosomes are seedless but show gigantism (large size) or gigas effect (more yield and more adaptability), but are odd numbered and, so, can only be propagated vegetatively. For example, banana and pineapple. Naturally occurring autotriploids are known in banana, grapes, sugar beet, tomato, and watermelons. Similarly, autotetraploids occur amongst apples, berseem, corn, etc.
      • Autopolyploids can also be produced ar- tificially by treating the seeds, seedlings, or axillary buds with an alkaloid called colchicine. It is extracted from the corm of Colchicum autumnale. The treatment produces doubling of chromosomes.
      • Allopolyploids: These are hybrids whose chromosome sets are derived from two different genomes.
        Let A and B be two different genomes. The two diploid organisms should have AA and BB chromosome sets. The autotetraploid can now be represented as
        AAAA while the allotetraploid is represented as AABB.
        Such polyploids are the result of doubling of chromosomes in F, hybrids derived from two different or related species.
      • The most common example of allopoly- ploidy is Raphanobrassica, developed by Russian geneticist G.O. Karpechenko (1927). A cross was made between Brassica oleracea (2n = 18) and Raphanus sativus (2n = 18). The F, hybrid produced was sterile, because the chromosome sets of both these plants were dissimilar and could not pair dur- ing meiosis. However, some fertile plants with 2n=18A+18 B (18 bivalents) were found amongst these.

Another example is a man-made cereal, Triticale, produced by (a) crossing Triticum durum (2n = 28) with Secale cereale (2n = 14) and then treating the F, hybrid with colchicine to obtain hexaploid triticale and (b) crossing Triticum aestivum (2n = 42) with Secale cereale (2n = 14) and then treating the F, hybrid with colchicine to produce octaploid triticale.

Mutagens

Mutations can be artificially produced by certain agents called mutagens or mutagenic agents. Following are two major types of mutagens:

• Physical mutagens
• Chemical mutagens

Physical Mutagens

• Radiations are the most important physical mutagens.
• H.J. Muller, who used X rays for the first time to increase the rate of mutation in Drosophila, opened an entirely new field in inducing mutations.
• So, Muller is considered as the father of actinobiology.
• The main source of spontaneous mutations is natural radiations coming from the cosmic rays of the sun.
• The spectrum of wavelengths that are shorter (i.e., of higher energy) than the visible light can be subdivided into the following two groups:
  • Ionizing radiations
  • Non-ionizing radiations
    • Physical mutations occur in small amounts in the environment and are known as background radiations.

Following are the biological effects of radiations:

• Effects of ionizing radiations: These radiations include X rays, y rays, a rays, and Brays. Ionizing radiations cause breaks in the chromosome. These cells then show abnor- mal cell divisions. If these include gametes, they may be abnormal and even die prema- turely. Different types of cancers may result due to radiations. The frequency of induced mutations is directly proportional to the doses of radiations.
• Effects of non-ionizing radiations: These radiations have longer wavelengths but carry lower energy. This energy is insufficient to induce ionization. Therefore, non-ionizing radiations such as UV light do not penetrate beyond the human skin. Thymine (pyrimidine) dimer formation is a major mutagenic effect of UV rays that disturbs DNA double helix and, thus, DNA replication.

Chemical Mutagens

• A large number of chemical mutagens are now known. These are more injurious than radiations. The first chemical mutagen used was mustard gas by C. Auer- bach et. al. during World War II.
• Chemical mutagens are placed into two groups:
  • Those which are mutagenic to both replicating and non-replicating DNA such as nitrous acid and
  • Those which are mutagenic only to replicating DNA such as acridine dyes and base analogs.
• Following are the effects of some chemical mutagens:
  • Nitrous acid: It is mutagenic to both replicating and non-replicating DNA. It acts directly by oxidative deamination on A, G, and C bases which contain amino groups. A is deaminated to hypoxanthine which is complementary to cytosine. G is converted to xanthine which pairs with C. Cytosine is converted to U which pairs with A.
  • Acridines: Acridines and proflavins are very powerful mutagens. These can intercalate between DNA bases and interfere with DNA replication, producing insertion or deletion or both of single bases, respectively. This induces frame- shift mutation or gibberish mutation, eg, thalassemia.
  • Base analogs: These have structures similar to the normal bases and are incorporated into DNA only during DNA replication. Base analogs cause mis-pairing and eventually give rise to mutations. Base analogs may be either natural or artificial. Natural base analogs include 5-methyl cytosine, 5-hydroxymethyl cytosine, 6-methyl purine, etc. The most commonly used artificial base ana- logs are 5-bromouracil and 2-aminopurine. 5-Bromouracil is a structural analog of thymine. It gets incorporated into the newly replicated DNA in place of thymine (T). 2-Aminopurine is an artificial base analog of adenine. It acts as a substitute of adenine (A) and can also pair with cytosine (C).

Cytoplasmic Inheritance

• Some self-replicating genes (DNA) are present in the cytoplasm (mitochondrial DNA and chloroplast DNA) also.
• These are called plasmagenes. All plasmagenes to- gether constitute plasmon (like genome).
• The inheritance of characters by plasmagenes is called extranuclear or extrachromosomal inheritance.
• The most important examples of extranuclear inheritance in eukaryotes are maternal inheritance and organelle inheritance.

Maternal Inheritance

• The amount of nuclear hereditary material contributed by the two sexes is almost equal but the cytoplasm in the egg is always much more than that in the sperm.
• So, in extranuclear inheritance, the contribution of fe- male parent is more.
• This is called maternal inheritance.
• The evidence of maternal inheritance is the coiling of shell in snails.

Genetic Disorders

Pedigree Analysis

• A record of inheritance of certain genetic traits for two or more generations presented in the form of a diagram or family tree is called pedigree.
• Parents are shown by horizontal line while their off- springs are connected to it by a vertical line.
• The offsprings are also shown in the form of a horizontal line below the parents and numbered with Arabic numerals.
• Pedigree analysis is the study of pedigree for the transmission of a particular trait and finding the possibility of absence or presence of that trait in homozygous or heterozygous state in a particular individual.
• It is useful for genetic counselors to advise intending couples about the possibility of having children with genetic defects such as hemophilia, colorblindness, alkaptonuria, phenylketonuria, thalassemia, sickle-cell anemia (recessive traits), brachydactyly, myotonic dystrophy, and polydactyly (dominant traits).
• Pedigree analysis indicates that Mendel’s principles are also applicable to human genetics with some modifications found out later such as quantitative inheritance, sex-linked characters, and other linkages.

Symbols Used in Pedigree Analysis

• Proband is the person from which case history starts. If it is male, it is called propositus; if it is female, it is called proposita.

Mendalian Disorders

Sickle-Cell Anemia

• It is an autosomal recessive disorder. In this disorder, the RBCs become sickle shaped under low O2 concentration.
• The affected persons die young.
• Other heterozygous individuals for this trait have normal phenotype and live long.
• The disease is due to the base substitution of the sixth codon in gene coding for the ẞ chain of hemoglobin.
• The middle base of a DNA triplet coding for the amino acid glutamic acid is mutated so that the triplet now codes for valine instead.
• The mutant hemoglobin molecule undergoes polym- erization under low O2 tension causing a change in the shape of RBC from biconcave disc to elongated sickle-like structure .

Thalassemia

Thalassemia is a recessive autosomal disease caused due to the reduced synthesis of a or ẞ polypeptide of hemoglobin. B-thalassemia is a major problem; individuals suffering from major thalassemia often die before ten years of age.

Phenylketonuria

Phenylketonuria is a recessive autosomal disorder (chromosome 12) related to phenylalanine metabolism. This disorder is due to the absence of a liver enzyme called phenylalanine hydroxylase. Due to the lack of this enzyme, phenylalanine follows another pathway and gets converted into phenylpyruvic acid.

This phenyl pyruvic acid upon accumulation in joints causes arthritis; if it hits the brain, it causes mental retardation known as phenyl pyruvic idiocy. Phenylalanine are also excreted through urine because of poor absorption by kidney.

Cystic Fibrosis

Cystic fibrosis is an autosomal recessive disorder common among Caucasian Northern Europeans. Persons suffering from this disease have extremely salty sweat. It is due to decreased Na and CI reabsorption in the ducts.

The disease is due to a gene present on chromosome 7. Due to a defective glycoprotein, thick mucus develops in pancreas and lungs and the formation of fibrous cyst occurs in pancreas.

Huntington’s Chorea

Huntington’s chorea is an autosomal dominant disorder. The gene responsible for this disorder is present on chromosome 4. The disease is characterized by the gradual degradation of brain tissue in the middle age and consequent shrinkage of brain.

Alzheimer’s Disease

Alzheimer’s disease is an autosomal recessive disease that results in mental deterioration (loss of memory, confusion, and anxiety) and, ultimately, the loss of functional capacities. The disease is due to the deposits of ẞ-amyloid, a short protein in brain which results in the degradation of neurons. It involves two defective alleles located on chromosomes 19 and 21. This disease is common in Down’s syndrome.

Myotonic dystrophy is due to a dominant autosomal mutant gene located on the long arm of chromosome 19. Mild myotonia (atrophy and weakness of the musculature of the face and extremities) is most common.

Other Mendelian Disorders

• Alkaptonuria (Garrod, 1908): Due to deficiency of oxidase enzyme
• Albinism (chromosome 11): Absence of tyrosinase
• Tay-Sach’s disease (chromosome 15): Absence of hexosaminidase B
• Gaucher’s disease (chromosomes 1): Due to the inhibition of glucocerebrosidase enzyme action which leads to accumulation of cerebroside

Other Abnormalities due to Autosomal Dominant Gene Mutation

• Polydactyly: Presence of extra fingers and toes
• Brachydactyly: Abnormal short fingers and toes

Abnormalities due to Sex-Linked (X-Linked) Recessive Gene Mutation

• Hemophilia A: Due to lack of antihemophilic globulin
  Hemophilia B: Due to lack of plasma thromboplastin
• Red-green colorblindness: Daltonism
  Protanopia: Red colorblindness
  Tritanopia: Blue colorblindness
  Deuteranopia: Green colorblindness
• Muscular dystrophy: Due to non-synthesis of protein dystrophin; deterioration of muscles at an early stage
• Lesch Nyhan syndrome: Deterioration of nervous system due to HGPRT (hypoxanthin guanine phosphoribosyl transferase) deficiency

Chromosomal Disorders

• Autosomal abnormalities (due to mutation in body chromosome)
  • Down’s syndrome: It occurs due to the trisomy of the 21st chromosome. The affected individual is short-statured with small round head, furrowed tongue, and partially open mouth. The palm is broad with characteristic palm crease and mental retardation. Physical and psychomotor develop- ment is retarded.
  • Edward’s syndrome: It occurs due to the trisomy of the 18th chromosome.
  • Patau’s syndrome: It occurs due to the trisomy of the 13th chromosome.
  • Cri du chat syndrome: It occurs due to deletion in the short arm of the fifth chromosome.
• Allosomal or sex chromosomal disorder
  • Klinefelter’s syndrome: It occurs due to the trisomy of the X-chromosome in male, resulting in a karyotype of 47 (44 + XXY). Individuals have long legs, sparse body hair, small prostrate gland, small testes, reduced mental intelligence, and enlarged breasts (gynaecomastia). Such individuals are sterile.
  • Turner’s syndrome: It is caused due to the absence of one of the X-chromosomes in females, i.e., 45 with chromosome complement 44 + XO. Such females are sterile with undeveloped breasts, short stature, reduced ovaries, and absence of menstrual cycle.
  • Super-female: AA+ XXX, AA + XXXX
  • Jacob’s syndrome or super-male: AA + XYY, also called criminal syndrome

Population Genetics

Hardy Weinberg’s equation is applied to know the distribution of traits and the frequency of autosomal dominant recessive gene distribution in the entire population.

p = Dominant gene/allele

q= Recessive gene/allele

p+q=1

(p+q)2= p2+q2+2pq=1

2/3(p2+2pq) = Frequency of dominant trait

1/3(92) = Frequency of recessive trait

Some Important Definitions

• The subject that deals with the inheritance as well as the variation of characters from parents to offspring is called genetics.
• Inheritance is the process by which characters are passed on from parent to progeny.
• Variation is the degree by which progeny differ from their parents.
• A true breeding line is one that, having undergone con- tinuous selfing, shows the stable trait inheritance and expression for several generations.
• Genes that code for a pair of contrasting traits are known as alleles.
• A cross between F, hybrid (Tt) and its homozygous recessive parent (tt) is called a test cross.
• If F, phenotype does not resemble either of the two parents and is in between the two, it is called incom- plete dominance.
• The presence of more than two alleles for a gene is known as multiple allelism.
• If F, phenotype resembles both the parents, it is called co-dominance.
• If more than one phenotype is influenced by the same gene, it is called pleiotropy.
• If two genes present on different loci produce the same effect when present alone but interact to form a new trait when present together, they are called comple- mentary genes.
• A gene that masks the action of another gene (non- allelic) is termed as an epistatic gene. The process is called epistasis.
• Polygenic inheritance is controlled by two or more genes in which the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional phenotype and full trait is shown when all dominant alleles are present.
• The tendency of some genes to inherit together is called linkage.
• Sex-limited traits are autosomal and found in both sexes but are expressed in one sex only.
• Sex-influenced traits are autosomal and appear more frequently in one sex than in the other.
• Mutation is sudden, inheritable, discontinuous varia- tion due to change in chromosomes and genes.
• If mutation arises due to change in a single base pair of DNA, it is known as point mutation.
• The failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
• The failure of cytokinesis after the telophase stage of cell division results in an increase in the whole set of chromosomes in an organism. This phenomenon is known as polyploidy.
• A record of inheritance of certain genetic traits for two or more generations presented in the form of a diagram or family tree is called pedigree.

Formula Chart

Summary

• Genetics is a branch of biology which deals with the principles of inheritance and variation.
• Mendelian inheritance (Mendelism)
  • Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as “factors.”
  • Dominant characters are expressed when factors are in heterozygous condition (law of dominance).
  • The characters never blend in heterozygous condition.
  • Recessive characters are only expressed in homozygous condition.
  • A recessive trait that was not expressed in heterozygous condition may express again when it becomes homozygous. Hence, characters segregate during the formation of gametes (law of segregation).
  • Mendel also studied the inheritance of two characters together and he found that the factors independently assort and combine in all permutations and combinations.
• The factors on chromosomes regulating the characters are called the genotype and the physical expression of the characters is called phenotype.
• Walter Sutton and Theodore Boveri noted that the behavior of chromosomes was parallel to the behavior of genes and used chromosome movement to explain Mendel’s laws.
• Mendel’s law of independent assortment is not true for genes that are located on the same chromosome (i.e., linked genes).
• Closely located genes assorted together, and distantly located genes, due to recombination, assorted independently.
• The frequency of recombination between gene pairs on the same chromosome is a measure of the distance between the genes.
• Mutation is defined as the change in the genetic material. A point mutation is a change of a single base pair in DNA. Some mutations involve changes in the whole set of chromosomes (polyploidy) or change in a subset of chromosome number (aneuploidy).
• Sickle cell anemia is caused due to change of one base in the gene coding for B-chain of hemoglobin.
• Inheritable mutations can be studied by generating a pedigree of a family.
• Down’s syndrome is due to the trisomy of chromo- some 21. In Turner’s syndrome, one X-chromosome is missing and the sex chromosome is XO.
• In Klinefelter’s syndrome, the condition is XXY.

 

Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: Mendel gave postulates such as “principles of segregation” and “principles of independent assortment” after studying seven pairs of contrasting traits in garden pea.

Reason: He was lucky in selecting seven characters in pea that were located on seven different chromosomes.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 2. Assertion: Test cross is the tool for knowing linkage be- tween genes.

Reason: Monohybrid test cross gives two phenotypes and two genotypes.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 3. Assertion: Marfan syndrome is caused by recessive mu- tant pleiotropic gene.

Reason: Gene mutation leads to more synthesis of fibril- lin proteins.

Answer. 4. If both Assertion and Reason are false, them mark (4).

Question 4. Assertion: In snapdragon, F, plants do not have red or white flowers.

Reason: It is intermediate inheritance with neither of the two alleles of a gene being dominant over each other.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 5. Assertion: en block inheritance of all genes located on the same chromosome may occur in some organisms.

Reason: Dihybrid test cross will have only two pheno- types.

Answer. 4. If both Assertion and Reason are false, them mark (4).

Question 6. Assertion: Morgan’s cross III was conducted in Drosophila to locate genes on chromosome for white eye color.

Reason: The cross was done between red-eyed hybrid female and white-eyed male.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 7. Assertion: Antlers in male deer are sex influenced traits.

Reason: These are controlled by autosomal genes which are influenced by the sex of bearer.

Answer. 4. If both Assertion and Reason are false, them mark (4).

Question 8. Assertion: One drumstick per nucleus is present in the neutrophil of normal female.

Reason: It is absent in the neutrophil of male.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 9. Assertion: Blood group phenotype is controlled by the presence or absence of antigens present on the surface coating of ABC.

Reason: These antigens are of three types and found in the oligosaccharides-rich head regions of a glycophorin.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: XX-XY type sex determination is found in Coccinia indica.

Reason: Male plant is produced only when Y-chromosome is present irrespective of the number of X-chromosomes.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 11. Assertion: Drosophila melanogaster is widely used in genetic research.

Reason: Drosophila melanogaster is a readily available insect.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 12. Assertion: In humans, the gamete contributed by the male determines whether the child produced will be male or female.

Reason: Sex in humans is a polygenic trait depending upon the cumulative effect of some genes on X-chromosome and some on Y-chromosome.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 13. Assertion: A father may be a hemophilic only if his mother is carrier.

Reason: A father cannot pass on a sex-linked gene to his son.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 14. Assertion: An organism with lethal mutation may not even develop beyond the zygote stage.

Reason: All types of gene mutations are lethal.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 15. Assertion: Cancer cells are virtually immortal until the body in which they reside dies.

Reason: Cancer is caused by damage to genes regulating the cell division cycle.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Reproductive Health Introduction

• According to the World Health Organization (WHO), reproductive health means total well-being in all aspects of reproduction, i.e., physical, emotional, social, and behavioral.
• Thus, a society with people who have physically and functionally normal reproductive organs and normal emotional and behavioral interactions among them in all sex-related aspects may be called reproductively healthy.

Reproductive Health: Problems And Strategies

The problems and strategies of reproductive health in human beings are explained as follows:

• Over-population
  • The main problem of India is its excess population, which is directly connected with reproductive health.
  • To achieve total reproductive health, some plans and programs were started.
  • Family planning program was initiated in 1951 and was periodically assessed.
  • These programs were popularly named Reproductive and Child Healthcare (RCH).
  • The major tasks carried out under these programs are providing facilities and support for building up a reproductive healthy society.
• Awareness about reproduction
  • Audio-visual and print media and government and non-government agencies are doing a good job in creating awareness among people about re- production in humans.
  • Parents, close relatives, friends, and teachers also have a major role in giving the above information.
• Sex education
  Sex education in schools also should be introduced and encouraged to provide right information about myths and misconceptions regarding sex-related aspects.
• Knowledge of growth of reproductive organs and sexually transmitted diseases (STDs)
  Proper information about reproductive organs; adolescence (period of rapid growth between childhood and adulthood); safe and hygienic sexual practices; STDs, e.g., AIDS; etc., would help to lead a reproductive healthy life.
• Birth-control devices and care of mother and child (pre-natal, natal, and post-natal care)
  • Fertile couples and people of marriageable age- group should know about available birth control devices, care of pregnant mothers, post-natal (after birth) care of the mother and child, importance of breast feeding, equal importance for the male and female child, etc.
• Prevention of sex abuse and sex-related crime
  Awareness of problems due to uncontrolled population growth, social evils such as sex abuse and sex-related crimes needs to be created so that people think and take up necessary steps to prevent them and, thereby, build up a reproductively healthy society.
• Information about reproduction-related problems
  • For successful action plans to attain reproductive health, we require good infrastructural facilities, professional expert knowledge, and material support.
  • These are necessary to provide medical help and care for reproduction-related problems such as menstrual problems, infertility, pregnancy, delivery, contraception, abortions, and STDs.
  • Implementation of better techniques and new strategies is also required to provide better care and help to people for reproductive health.
• Amniocentesis Meaning and use
  Amniocentesis is a fetal sex determination and disorder test based on the chromosomal pattern in the amniotic fluid surrounding the developing embryo.
  Procedure
  • Amniotic fluid contains cells from the skin of the fetus and other sources.
  • These cells can be used to determine the sex of the infant, to identify some abnormalities in the number of chromosomes, and to detect certain biochemicals and enzymatic abnormalities.

• • If it is established that the child is likely to suffer from a serious incurable congenital defect, the mother should get the fetus aborted.
  • Misuse of amniocentesis: It is being used to kill normal female fetus. Female feticide is illegal.
• Research in reproductive health area
  • It should be encouraged and supported to find out new methods.
  • “Saheli,” a new oral contraceptive for females, was developed by our scientists at the Central Drug Research Institute (CDRI) in Lucknow, In- dia.
• Medical facilities
  Better awareness about sex-related problems, pre-natal care of mother, medically assisted deliveries, and post-natal care of mother and infant decrease maternal and infant mortality. Small families, better detection and cure of STDs, and increased medical facilities for sex-related problems, etc., indicate improved reproductive health of male and female adults and children.

Read and Learn More NEET Biology Notes

Measures To Control Over-Population

• Education
  • People, particularly those in the reproductive age- group, should be educated about the advantages of a small family.
  • Mass media and educational institutions can play an important role in this campaign.
  • Posters showing a happy couple and two children with a slogan “Hum Do Humare Do” should be displayed. (Many couples have even adopted “one child norm.”)
• Marriageable age
  Raising the age of marriage is a more effective means to control the population. (Now the marriageable age of females is 18 years and that of males is 21 years.)
• Incentives: Couples with small families should be given incentives.
• Family planning: There are many birth control measures which can check birth rate.

Population Explosion And Birth Control

• Rapid increase in population over a relatively short period is called population explosion.
• Census gives information about the number of individuals present in a given region at a given time.

• It means every sixth person in the world is an Indian.
• The time required for a population to double itself is called doubling time.
• The present growth rate of approximately 1.6% per year for India is smaller than the peak of about 2.1% per year during 1965-1970.
• Population growth rate is indicated by (a) the annual average growth rate and (b) the doubling time.
• Growth rate depends on birth (fertility) rate, death (mortality) rate, migration, and age-sex ratio.
• The major reasons for this growth are as follows:
  • A rapid decline in death rate.
  • A decline in maternal mortality rate (MMR).
  • A decline in infant mortality rate (IMR).
  • Increase in the number of people reaching reproducible age.

Birth Control

Birth control is the process to prevent conception or pregnancy without interfering with the reproductive health of individuals.

• The characteristics of an ideal contraceptive are that it is (a) user friendly, i.e., comfortable and easy to use, (b) without side-effects, (c) reversible, and (d) completely effective against pregnancy.
• There are several methods of contraception-natural or traditional methods, barriers, IUDs, oral contraceptives, injectables, implants, and surgical methods.
• Couple protection is the process of bringing eligible couples under family planning measures. In India, it is over 55% at present and is voluntary in nature.
• In 2004, there were 60.79 lakh IUD insertions, 48.74 lakh sterilizations, 249.9 lakh condom users, and 87.54 lakh oral pill users.

Methods Of Birth Control

• Natural methods
  • These are methods which do not require any device or medicine. So, there are no side-effects; but the chances of failure are very high.
  • Natural methods are of three kinds-safe period, withdrawal, and breast feeding.
    • Safe period (rhythm method)
      • Ovalation occurs roughly about the middle of menstrual cycle.
      • Fertility period is up to 48 h after ovulation, when fertilization can occur.
      • Avoiding sex during the fertility period will naturally prevent conception.
      • Ovulation period can be known by the rise in body temperature by about 1°F; cervical mucus is slippery and can be drawn into a thread (Spinnbarkeit test) when stretched between two fingers.
      • Period prior to ovulation is safe.
      • Period after the fourth day of rise in body temperature (or last positive Spinnbarkeit test) is also considered safe.
      • It is, however, always better to avoid sex from day 10-17 of the menstrual cycle.
    • Withdrawal method (coitus interruptus)
      • The method is based on the withdrawal of penis from the vagina before ejaculation.
      • This method has high failure rate due to pre-ejaculatory release of sperms or failure to withdraw penis from the vagina before ejaculation.
    • Lactational amenorrhea
      • Just after parturition, there is a phase of amenorrhea or absence of menstruation.
      • It is also the phase of intense lactation.
      • Breast feeding the child fully prevents conception.
      • The method is, however, effective only up to a maximum period of 6 months.
• Barrier methods
  • These are mechanical devices which prevent the deposition of sperms into vagina and their pas sage into the uterus.
  • Further, they can be self-inserted by the user in complete privacy.
  • The common barrier methods are condoms, diaphragm, fem shield, and cervical cap.
    • Condom
      • It is a tubular latex sheath which is rolled over the male copulatory organ during sex.
      • The common brand provided by family welfare services is Nirodh.
      • It also provides protection against STDs including AIDS.

• • • Fem shield (female condom)
      • The device is a polyurethane pouch with a ring at either end.
      • The inner ring is smaller and present at the inner closed end.
      • The device covers the external genitalia as well as the lines the vagina.
      • Fem shield provides protection from STDs also.

• • • Diaphragm
      It is a tubular rubber sheath with a flexible metal or spring ring at the margin which is fitted inside the vagina.
    • Cervical cap
      • It is a rubber nipple which is fitted over the cervix and is designed to remain there by suction.
      • The device prevents the entry of sperms into the uterus.

• • • Vault cap
      It is a hemispheric dome-like rubber or plastic cap with a thick rim which is meant for fitting over the vaginal vault over the cervix.
• Chemical methods
  • These are contraceptives which contain spermicidal chemicals.
  • Chemical contraceptives are available in the form of creams (e.g., delfen), jelly (e.g., perceptin, volpar paste), foam tablets (e.g., aerosol foam, chlorimin T or contab), etc.
  • These commonly contain lactic acid, boric acid, citric acid, zinc sulfate, and potassium permanganate.
  • The contraceptives are introduced in the vagina prior to sex.
  • Sponge (today) is a foam suppository or tablet containing nonoxynol-9 as spermicide. It kills the sperm by disrupting the membrane. It is moistened before use to activate the spermicide. The device also absorbs the male ejaculate.
• Intra-uterine devices or IUDs (intra-uterine contraceptive devices or IUCDs)
  • These devices are inserted by doctors or expert nurses in the uterus through vagina. IUDs affect the motility of sperms within the uterus. IUDs can be
    • Non-medicated (e.g., Lippes loop)
    • Copper releasing (e.g., CuT, Cu7, and Multi- load 375): The copper ions suppress the motility and fertilization capacity of sperms.

• • • Hormone releasing (e.g., Progestasert and LNG-20)
      • Hormone releasing IUDs, in addition, make the uterus unsuitable for implantation and the cervix hostile to sperms.
      • IUDs are ideal contraceptives for females who want to delay pregnancy and/or space children. It is one of the most widely accepted methods of contraception in India.
• Oral contraceptives (oral pills)
  • Oral contraceptives are preparations containing either progestin (progestogen or progesterone) alone or a combination of progestogen and oestrogen (estrogen).
  • The pills are taken orally for 21 days in a menstrual cycle starting from the 5th day and ending on the 25th day.
  • However, it is advisable to restart the course after a gap of 7 days irrespective of the onset or non-set of menstruation during the pill-free days.
  • When a pill is missed, it should be taken when- ever one remembers, sometimes two at a time.

• • This helps in keeping the hormonal level optimum for contraception.
  • Hormonal pills act in the following four ways:
    • Inhibition of ovulation.
    • Alternation in uterine endometrium to make it unsuitable for implantation.
    • Changes in cervical mucus impairing its ability to allow the passage and transport of sperms.
    • Inhibition of motility and secretory activity of fallopian tubes.
    • Oral pills are of two types: combined and minipills.
    • Combined pills contain both oestrogen and progestin.
    • These are synthetic products.
    • Oestrogen is an ovulatory that inhibits FSH production. Progestin is anovulatory that inhibits LH production.
    • It protects the endometrial lining from the adverse effect of oestrogen.
    • The hormone also changes cervical mucus.
    • The most commonly used progestin is levonorgestrel or desogestrel.
    • The most common oestrogen is ethinyl oestradiol or menstranol.
    • In monophasic combined pills, both oestrogen and progestin are present in nearly the same amount, e.g., Mala D and Mala L.
    • In multiphasic combined pills, oestrogen is maintained at the same level throughout the 21-day course (0.03 mg) but the amount of progestin is increased (0.05 mg for the first six days, 0.075 mg for the next five days, and 0.125 mg for the last ten days), e.g., triquilar and orthonovum.
    • Minipills are progestin pills only (with no estrogen). They are taken daily without break.
    • Saheli-a non-steroidal preparation-is taken once a week after an initial intake of twice a week dose for 3 months.
• Injectable contraceptives (Depo-Provara)
  • Two types of progestin preparations are used singly: Depot-medroxy progesterone acetate (DMPA) 150 mg every 3 months or 300 mg every 6 months and norethisterone enanthate (NET EN) 200 mg every 2 months.
  • Cyclofem and mesigna are combined injectable contraceptives which are given once every month.
  • They contain progestin preparation as well as oestradiol.
• Implants
  • These are hormone containing devices which are implanted subdermally for providing long-term contraception.
  • Norplant is progestin only. The device comes with six small permeable capsules (34 mm × 2.4 mm each) with about 36 mg levonorgestrel.
  • These are inserted under the skin in a fan-shaped manner inside the upper arm or forearm through a small incision.

• • Suturing is not required. Norplant remains effective for about 5 years.
  • Implanon is a single rod-like device which is implanted through a wide bored needle. It contains 3-keto desogestrel. It remains functional for three years.

• Emergency contraception
  • It is the treatment for unprotected sex, sexual assault, missed pills, and other reasons which have a risk of pregnancy.
  • The drugs used for treating emergency contraceptions are called morning-after pills.
  • These are available in India under the Family Welfare Program since 2002-2003.
  • Two oral tablets to start and two tablets after 12 h provide relief.
  • Other morning-after pills are noral, norgynon, and ovidon.
  • An antiprogesterone pill (mifepristone) is a single-pill treatment.
  • Insertion of IUD within 5 days of unprotected sex prevents implantation.
• Surgical methods of family planning
  • Surgical methods are permanent methods of family planning where there is no need of replacement or augmentation but the reversibility is poor.
  • These are also called the terminal methods of family planning.
  • The methods are operative procedures which block the passage of semen in males and that of ova in females.
  • The techniques are also called sterilization procedures.
  • These are called vasectomy in males and tubectomy in females.
    • Vasectomy (L. vas-vessel, ektome-excision)
      • It is a surgical method of sterilization of males.
      • Vasa deferentia are blocked by cutting and occluding them so that sperms are
    • Conventional vasectomy (scalpel surgery):
      • Under local anesthesia, transverse 1 cm incision is made through the skin of the scrotum with the help of the scalpel over the area of vasa deferentia.

• • • • Each vas deferens is exposed and cut.
      • The two ends are separated and tied.
      • A gap of 1-4 cm is must between the two ends; otherwise reunion can occur.
    • No-scalpel vasectomy
      • Here, instead of scalpel, dissecting forceps and ringed forceps are required.
      • The skin is punctured and the vas deferens is taken out.
      • It is occluded by removal of 1-2 cm followed by ligation of ends.
      • Occlusion can also be achieved by heat and clips.
      • Vasectomy is a reversible procedure as the cut ends can be joined together to open to sperm passage.
    • Tubectomy (L. tubes-pipe, ektome-excision)
      • It is a surgical procedure of female sterilization where a portion of both the fallopian tubes is excised or ligated to block the passage of ovum through them. Tubectomy is performed by conventional transabdominal surgery, conventional laparotomy, and minilaparotomy.
      • In surgical procedures, the fallopian tubes are cut and the cut ends are tied to prevent reunion.
      • The procedure is reversible as the cut ends can be rejoined.
      • In laparoscopic procedure, sterlization is achieved by loop development and constricting the basal region of loop with the help of elastic ring either through a small incision in the abdomen or through the vagina.

Medical Termination of Pregnancy

• Medical termination of pregnancy (MTP) is voluntary or intentional abortion induced and performed to end pregnancy before the completion of full term.
• Worldwide, nearly 20% of the total pregnancies get aborted.
• The number of MTPs is 40-50 million/year.
• Therefore, MTPs play a significant role in the containment of population though these are not performed for this purpose.
• These are mainly meant for removing unsustainable pregnancies.
• Many countries do not have a law about MTPs because the latter involve emotional, ethical, religious, and social issues.
• However, in India, there is a proper act-Medical Termination of Pregnancy Act, 1971.
• It is mainly meant for preventing unnatural maternal deaths due to unsafe abortions (which is 8.9% of the total maternal deaths).
• The act has been amended in 2002.
• Under this act, the termination of pregnancy can be done up to 20 weeks, if pregnancy is likely to produce a congenitally malformed child, is a result of rape and contraceptive failure, or is likely to harm the mother.
• MTP is safe if it is performed up to 12 weeks (first trimester) of pregnancy.
• Misoprostol (prostaglandin) along with mifepristone (antiprogesterone) is an effective combination.
• Vacuum aspiration and surgical procedures are adopted thereafter.
• Abortions in the second trimester are risky.
• These are generally performed after testing the sex of the baby through amniocentesis or sonography.
• This has resulted in large-scale female feticide and complications due to unsafe abortions in the hands of untrained persons.
• To prevent such happening, the government has enacted a law-Prenatal Diagnostic Techniques (Regulation and Prevention of Misuse) Act, 1994-with amendments in 2003.
• It prohibits preconception and prenatal sex determination.

STDs

• The general term STD is applied to any of the large group of diseases that can be spread by sexual contact.
• The group includes conditions traditionally specified as venereal diseases (VD), such as chlamydia, gonorrhea, syphilis, and genital herpes.
• AIDS and hepatitis, which are STDs, can be contracted in other ways also.

Infertility

• Infertility (L. in-not, fertilis-fruitful) is failure to conceive even after 1-2 years of regular unprotected sex.
• The term is not a synonym of sterility which means complete inability to produce an offspring.
• Infertility can best be defined as relative sterility.
• It is of two types: primary and secondary.
• Primary infertility is the infertility found in patients who have never conceived.
• Secondary infertility is the infertility found in patients who have previously conceived.
• Infertility is caused by defects found in males, females, and in both of them.

Infertility in Males

• The semen of a fertile male is 2.5-5 ml per ejaculation with a sperm count of over 200-300 million, mostly motile, having proper fructose content and fluidity which is deposited high in the vagina.
• Any defect in the sperm count, sperm structure, and sperm motility of seminal fluid leads to infertility.
• Low sperm count is called oligospermia while the near absence of sperms is known as azoospermia.
• Low sperm motility is called asthenozoospermia while defective sperm morphology is termed as teratozoospermia.

Infertility in Females

• A fertile woman is the one who regularly ovulates once every cycle and passes the egg down the reproductive tract which develops conditions for the smooth pas- sage of sperms and the implantation of fertilized egg.
• The various causes of infertility in females are as follows:
  • Anovulation (nonovulation) and oligoovulation (deficient ovulation).
  • Inadequate growth and functioning of corpus luteum.
  • Ovum not liberated but remains trapped inside the follicle due to hyperprolactinaemia.
  • Chances of failure of fallopian tube to pick up ovum.
  • Noncanalization of uterus.
  • Defective uterine endometrium.
  • Fibroid uterus.
  • Defects in cervix.
  • Defective vaginal growth.

Assisted Reproductive Technologies

• More than two decades ago, in an experimental procedure called in vitro fertilization (IVF), doctors joined a woman’s egg and a man’s sperm in a glass dish on the laboratory table.
• For the first time, fertilization happened outside a woman’s body. Nine months later, the first test-tube baby was born.
• Today, assisted reproductive technology (ART) refers not only to IVF but also to several variations tailored to patient’s unique conditions.
• These procedures are usually paired with more conventional therapies, such as fertility drugs, to increase success rates.
• Almost one out of every three cycles of ART results in the birth of a baby.
• But ART procedures are invasive and expensive.
• Though no long-term health effects have been linked to children born using ART procedures, most doctors recommend reserving ART as the last resort for having a baby.
• Following is the list of important techniques which can benefit infertile couples.
  • IVF and ET
    IVF is the fertilization outside the body, in almost similar conditions as that in the body, which is followed by embryo transfer (ET). In this method, popularly known as test-tube baby method, ova from the wife/donor (female) and sperms from the husband/donor (male) are collected and induced to form zygote under simulated conditions in the laboratory. The zygote or early embryos are then transferred into the fallopian tube or uterus to complete their further development.
  • ZIFT
    In ZIFT (zygote intra-fallopian transfer), the zygote, formed in vitro, or early embryo up to 8 blastomeres, is transferred into the fallopian tube.
  • IUT
    In IUT (intra-uterine transfer), the embryos with more than 8 blastomeres are transferred into the uterus for further development. The embryos formed by in vivo fertilization can also be used for transfer to assist the females who cannot conceive.
  • GIFT
    GIFT (gamete intra-fallopian transfer) is the transfer of an ovum collected from a donor into the fallopian tube of the recipient who can pro- vide suitable environment for fertilization and further development.
  • ICSI
    ICSI (intra-cytoplasmic sperm injection) is an- other specialized procedure to form an embryo in the laboratory. In this method, a sperm is directly injected into the ovum.
  • AI
    Al (artificial insemination) is used for the cases of infertility which is either due to the inability of the male partner to inseminate the female or due to very low sperm count in the ejaculate.
    In this technique, the semen collected either from the husband or a healthy donor is artificially introduced either into the vagina or the uterus (IUI- intra-uterine insemination) of the female.
    ART requires extremely high precision handling by specialized professionals and expensive instrumentation. The infertility facilities are presently available only in a few centers in the country.
    Obviously, their benefits are affordable only to a limited number of people. Emotional, religious, and social factors are also involved in the adoption of these methods.
  • Adoption
    Our law also permits legal adoption. Adoption can benefit not only the people who are looking for parenthood but also many orphaned and destitute children in India, who would probably not survive till maturity, unless taken care of.
  • Surrogacy or use of a gestational carrier
    In this method, another woman carries embryo or a donor embryo. It is termed as surrogacy.

 

Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 79. Assertion: Population of India crossed 1 billion in May 2000.

Reason: It is the result of rapid decline in death rate, maternal mortality rate (MMR), and infant mortality rate (IMR) as well as an increase in the number of people in reproducible age.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 80. Assertion: Intra-uterine devices (IUDs) are very effec- tive contraceptive method.

Reason: IUDs do not allow sperms to enter uterus.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 81. Assertion: Surgical method blocks gamete transport and thereby prevents conception.

Reason: Surgical method used in males for this purpose is called vasectomy.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 82. Assertion: Saheli-the new oral contraceptive for fe- males contains a non-steroidal preparation.

Reason: It is “once-a-week” pill with very few side- effects and high contraceptive value.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 83. Assertion: In test-tube baby program, ova from wife/donor (female) and sperms from husband/donor (male) are collected and are induced to form zygote under simulated conditions in laboratory.

Reason: Embryos with more than 8 blastomeres are then transferred to fallopian tube (ZIFT) to complete its further development.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 84. Assertion: Surgical methods of contraception are prac-ticed to space two successive conceptions.

Reason: During surgical methods, ovaries from females or testes from males are removed.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 85. Assertion: Natural methods are based on menstrual cycle and the life of sperms.

Reason: Natural methods often fail to contracept.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 86. Assertion: Sexually transmitted diseases get transmitted from the infected to the normal person, only during sexual contact.

Reason: All sexually transmitted diseases can be cured by antibiotics.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 87. Assertion: In 1900, world population was 2000 million.

Reason: Indian population crossed 2000 million mark in May 2000.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 88. Assertion: Marriageable age of Indian females and males is 18 years and 21 years, respectively.

Reason: Under normal conditions, a girl child will re- lease around 450 ova in her lifetime.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Human Reproduction Introduction

• Human beings are unisexual.
• The growth, maintenance, and functions of gonads are regulated by gonadotropins secreted from the anterior lobe of pituitary gland.
• The organs that neither produce gametes nor secrete sex hormones but perform important functions in reproduction are called secondary sex organs.
• These include prostate, seminal vesicles, vas deferens, and penis in males; and fallopian tubes, uterus, vagina, and mammary glands in females.
• The characters that distinguish a male from a female externally are called accessory or external sex characters or secondary sex characters.

Male Reproductive System

• The male reproductive system of mammals consists of a pair of testes, several accessory glands, a duct system, and a mating organ called the penis. Testis is the primary male sex organ. It produces spermatozoa and secretes the male sex hormone testosterone.
• A human testis measures about 5 cm, 3 cm, and 2.5 cm, respectively, in length, thickness, and width. It is covered by thick, fibrous, connective tissue called tunica albuginea.
• In a man, both testes normally remain suspended in a pouch called scrotum outside the abdominal cavity. This keeps the testes at a low temperature than the body temperature (about 2°C less). This is essential for the maintenance and normal functioning of the spermatogenic tissue of testes.
• Testes descend in the scrotal sac when fetus is about 7 months old. This occurs under the influence of follicle-stimulating hormone (FSH) and testosterone.
• If testes fail to descend, then the condition is called cryptorchidism. It leads to sterility. Scrotum remains connected with the abdomen or pelvic cavity by the inguinal canal. Blood vessels, nerves, and conducting tubes pass through the inguinal canal.
• Cremaster muscles and connective tissue form spermatic cord and surround all structures passing through the inguinal canal. Cremaster muscles and dartos muscles of the scrotal sac help in the positioning of testes.
• When the outside temperature is low, these contract to move the testes close to the abdominal cavity/pelvic cavity. When the outside temperature is high, these relax moving the testes away from the body.
• In some seasonally breeding mammals, testes descend into the scrotum during the breeding season but ascend back into the abdomen in the non-breeding season. For example, rats and bats.

Read and Learn More NEET Biology Notes

Anatomy Of Testis

• Each testis contains numerous tiny, highly convoluted tubules, called seminiferous tubules. These constitute the spermatogenic tissue of the testis.
• Cells lining these tubules give rise to spermatozoa which are released into the lumen of the tubule.
• In between spermatogenic cells, Sertoli or sustentacular or nurse cells are present which provide nourishment to the developing spermatozoa and regulate spermatogenesis by releasing inhibin to check FSH overactivity.
• The other functions of Sertoli cells include the following:
• • Providing nourishment to the developing spermatozoa.

• • Absorbing the parts being shed by the developing spermatozoa.
  • Releasing anti-Müllerian factor (AMF) to prevent the development of Müllerian duct/oviduct.
  • Releasing androgen binding protein (ABP).
• Groups of polyhedral cells called interstitial cells, or the Leydig cells, are located in the connective tissue around the seminiferous tubules.
• They constitute the endocrine tissue of the testis. The Leydig cells secrete testosterone into the blood.
• Seminiferous tubules unite to form several straight tubules called tubuli recti which open into irregular cavities in the posterior part of the testis.
• Rete testis is a highly anastomosing labyrinth of cuboidal epithelium lined channels.
• Several tubes called vasa efferentia arise from it and conduct spermatozoa out from the testis. (Seminiferous tubule to vasa efferentia form intratesticular genital duct system).

• The extratesticular duct system consists of tubes which conduct sperms from the testes to the outside.
• It starts with vasa efferentia, which arise from each testis, and becomes confluent to form a folded and coiled tube called epididymis behind each testis.
• The epididymis consists of three parts: (a) Caput, (b) corpus, and (c) cauda. It stores the sperms temporarily.
• From cauda epididymis, a partially coiled tube called vas deferens ascends into the abdomen through the inguinal canal, passes over the urinary bladder, and receives the duct from the seminal vesicle behind the urinary bladder to from an ejaculatory duct.

• Before entering prostate, the ductus deferens dilates to form ampulla. The final portion of ampulla passes through the prostate to open into the urethra shortly after its origin from the urinary bladder.
• Urethra receives the ducts of the prostate and Cowper’s glands, passes through the penis, and opens to the outside.

Male External Genital Organ

Penis

• Penis is the copulatory organ of man. It is a cylindrical and erectile, pendulous organ suspended from the pubic region in front of scrotum.
• It remains small and limp (or flaccid) but on sexual arousal, it becomes long, hard, and erect- ready for copulation (or coitus or intercourse). Erect human penis, on an average, is about 15 cm long.

• The penial mass is itself encased in a fibrous sheath, called tunica albuginea. The interior of penis is mostly formed of three cylindrical cords of spongy, erectile (cavernous) tissue.
• Two of these cords are thicker and situated parallely on the right and left sides, forming the thick part of penis that remains in the front when the penis is limp, but becomes superio-posterior when it is erect.
• These two cords are called corpora cavernosa.
• The fibers of tunica albuginea surround both cords jointly and form a separate sheath around each cord. Some fibers form a partition, called septum penis, between these cords.
• The third, smaller cord forms that part of penis which remains inferio-anterior in erect penis. Urethra runs through this cord. Hence, this cord is called corpus urethrae or spongiosum.
• The extended part of corpus spongiosum is enlarged, forming a bulged, conical structure called glans penis. The surface of glans is formed of a thin, smooth, shiny, and hairless skin.
• The base line of glans is referred to as the neck of the penis. The loose skin of penis folds over and is retractile on glans. It is called foreskin or prepuce.
• At the tip of glans penis is a slit-like external urethral orifice or meatus by which urethra opens out and discharges urine or semen.
• Preputial glands, present in the skin of the penis neck, secrete a white sebaceous substance, called smegma. Microbial infection in smegma causes irritation.

Glands

• Seminal vesicles: These are paired, tubular, coiled glands situated behind the urinary bladder. They secrete viscous fluid that constitutes the main part of the ejaculate. Seminal fluid contains fructose, citric acid, inositol, and prostaglandins.
• Prostate gland: It is a chestnut-shaped gland and is a collection of 30-40 tubuloalveolar glands. It lies at the base of the bladder and surrounds the base of the ure- thra. It contributes an alkaline substance to the seminal fluid.
• The substance of prostate helps the sperms to become active and counteract any adverse effects of urine on sperms. The prostatic fluid provides a characteristic odor to the seminal fluid. Prostate gland secretes citrate ion, calcium, phosphate ion, and profibrinolysin.
• Bulbourethral glands or Cowper’s glands: The two bulbourethral glands are pea-sized structures lying adjacent to the urethra at the base of penis. These secrete a viscous lubricant.
• Duct system, accessory glands, and penis are secondary male sex organs. Their growth, maintenance, and functions are promoted by testosterone secreted by the Leydig cells.
• On the other hand, the growth, maintenance, and functions of seminiferous tubules and the Leydig cells are regulated, respectively, by FSH and interstitial cell stimulating hormone (ICSH) of anterior pituitary.

Semen

• Semen is a mixture of sperms and the secretions of seminiferous tubules, seminal vesicles, prostate gland, and bulbourethral glands.
• The average volume of semen in an ejaculation is 2.5-5 ml, with a sperm count (concentration) of 200-300 million. For normal fertility, at least 60% sperms must have normal shape and size, and at least 40% of them must show vigorous motility. When the sperm count falls below 20 million/ml, the male is likely to be infertile.
• Semen has a slightly alkaline pH of 7.2-7.7. The prostatic secretion gives semen a milky appearance, whereas fluids from the seminal vesicles and bulbourethral glands give it a sticky consistency.
• Semen provides transportation medium and nutrients to sperms. It neutralizes the hostile acidic environment of the male urethra and the female vagina.

Female Reproductive System

• The female reproductive system consists of a pair of ovaries, a duct system consisting of a pair of fallopian tubes (oviduct)-uterus, cervix, and vagina. The pair of mammary glands is accessory genital glands.

Ovaries

• Ovary is the primary female sex organ. It produces ova and secretes female sex hormones, estrogen, and progesterone that are responsible for the development of secondary female sex characters and regulate cyclic changes in uterine endometrium.
• Human ovaries are small, almond-like flat bodies about 3 cm in diameter.

Location

Ovaries are located near kidneys and remain attached to the lower abdominal cavity through mesovarium.

Structure

The free surface of the ovary is covered by a germinal epithelium made of a single layer of cubical cells. This epithelium is continuous with the mesothelium, called peritoneum. The epithelium encloses the ovarian stroma. The stroma is divided into two zones a peripheral cortex and an inner medulla. The cortex is covered by a connective tissue, called tunica albuginea, and it contains numerous spherical or oval, sac-like masses of cells, known as ovarian follicles. The medulla consists of loose connective tissue, elastic fibers, blood vessels, and smooth muscle fibers.

Ovarian Follicle

• Ovarian follicle carries a large, centrally placed ovum, surrounded by several layers of granular cells (follicular granulosa or discus proligerus or cumulus oophorus). It is suspended in a small cavity-the antrum.
• Antrum is filled with liquid folliculi. The secondary oocyte in the tertiary follicle forms a new membrane called zona pellucida. The follicle bulges on the surface of the ovary. Such a follicle is called mature Graafian follicle (after De Graaf, who reported them in 1672).

Corpus Luteum

• The ovum is shed from the ovary by rupturing the follicle. The release of ovum is called ovulation and occurs nearly 14 days before the onset of the next menstrual cycle.
• After the extrusion of the ovum, the Graafian follicle transforms into corpus luteum. Corpus luteum is filled with a yellow pigment, called lutein.
• Corpus luteum grows for a few days and if the ovum is fertilized and implantation occurs, then it continues to grow. But if the ovum is not fertilized, then corpus luteum persists only for about 14 days.
• It secretes progesterone. At the end of its functional life, it degenerates and gets converted to a mass of fibrous tissue, called corpusalbicans (white body). It remains as a scar in the ovary throughout the life of a female.

Fallopian Tubes (Oviducts)

• Oviducts are a pair of long (about 10 cm), ciliated, muscular, tubular structures that extend from ovaries to uterus. Each is suspended by mesosalpinx. Each fallopian tube is differentiated into four parts:
• • Infundibulum: The part of oviduct closer to the ovary is the funnel shaped infundibulum. Its edges possess finger-like projections called fimbriae. Fimbriae help in the collection of the ovum after ovulation. Infundibulum opens in the abdominal cavity by an aperture called osteum.
  • Ampulla: The infundibulum leads to a wider part of the oviduct called ampulla.
  • Isthmus: It is the middle, narrow, ciliated part of the oviduct.
  • Uterine part: It is the inner, narrow part which opens in the upper part of uterus. It is involved in the conduction of ovum or zygote towards the uterus by peristalsis and ciliary action. (Fertilization occurs at the junction of ampulla and isthmus.)
• Uterus: It is a large hollow, muscular, highly vascular, and pear-shaped structure present in the pelvis between the bladder and rectum. It is suspended by a mesentery-mesometrium. It is formed of three parts.
  • Fundus: It is the upper dome-shaped part above the opening of fallopian tubes.
  • Body: It is the middle and main part of the uterus.
  • Cervix: It is the lower, narrow part which opens in the body of the uterus by internal os and in vagina below by external os. It is formed of the most powerful sphincter muscle in the body.

Its wall is formed of outer peritoneal layer (called perimetrium); middle muscular myometrium made of smooth muscle fibers, and inner, highly vascular and glandular endometrium.

It is the site of fetal growth during pregnancy. It also takes part in placenta formation and helps in pushing the baby out during parturition.

Vagina

• Vagina is a long (7.5 cm), fibro-muscular tube. It extends backward in the front of rectum and cervix to the vestibule. It is a vascular tube internally lined by mucus membrane and is raised into transverse folds called vaginal rugae.
• In a virgin female, vaginal orifice is closed by a membranous diaphragm called hymen which becomes centrally perforated at puberty for the discharge of menstrual flow (or menses).
• Vagina acts both as copulation canal (as it receives sperms from penis during copulation) and as birth canal during parturition.

Female External Genital Organ

• Vulva: It is the external genitalia of females. It has a depression-the vestibule-in front of anus. A vestibule has two apertures-upper external urethral orifice of urethra and lower vaginal orifice of vagina.
• Mons pubis: It is a fleshy and fatty tissue covered by skin and pubic hair.
• Labia majora: It is a pair of fleshy folds which extend from mons pubis and surround the vaginal opening.
• Labia minora: It is another pair of tissue folds below the labia majora.
• Both labia majora and labia minora are provided with sebaceous glands.
• Hymen: It is a membrane that partially covers the vaginal opening. It gets torn during the first coitus.
• Clitoris: It is a tiny, erectile, finger-like structure present at the upper junction of the two labia minora above the urethral opening. The fold of skin that covers the clitoris is called prepuce. Clitoris is homologous to penis (as both are supported by corpora cavernosa).

Glands

Vestibular Glands

Vestibular glands are of two types-greater and lesser.

• Greater vestibular or Bartholin’s glands are a pair of small reddish-yellow glands on each side of vaginal orifice. These secrete alkaline secretion for lubrication and neutralizing urinary acidity.
• Lesser vestibular or paraurethral or Skene’s glands are small mucus glands present between urethral and vaginal orifices.

Mammary Gland

• Each mammary gland consists of 15-25 lobules of the compound tubuloalveolar type. These lobules secrete milk to nourish the newborn babies.
• Each lobe is separated from the others by dense connective and adipose tissue and represents a gland. From each lobe, excretory lactiferous ducts emerge independently in the nipple, which has 15-25 openings, each about 0.5 mm in diameter.

• However, the histological structure of mammary glands varies, depending on sex, age, and physiological state.

Path of Milk Ejection

Mammary alveolus → Mammary duct→ Ampulla → Lactiferous duct → Nipple

Events In Human Reproduction

Gametogenesis→ Insemination → Fertilization → Implantation → Gestation → Parturition

Formation Of Gametes

• Sexual reproduction requires the fusion of two haploid gametes to form a diploid individual. These haploid cells are produced through gametogenesis.
• As there are two types of gametes-spermatozoa and ova gametogenesis can be studied under two broad headings, namely spermatogenesis and oogenesis.
• Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova. Both spermatozoa and ova originate from primordial germ cells (PGCs), which are extragonadal in origin.
• In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm. Eventually, they migrate to the yolk sac endoderm and, ultimately, to the gonads of the developing embryo, where they undergo further development.

Spermatogenesis

• Spermatozoa are produced in the seminiferous tubules of testes. Spermatogenesis is the process of maturation of reproductive cells in the testes.
• Spermatogenesis includes two stages: (a) formation of spermatids and (b) metamorphosis of sperma- tids. Spermatids are formed in three phases, namely (a) phase of multiplication (mitosis), (b) phase of growth, and (c) phase of maturation (meiosis).
• During the phase of multiplication, the primordial germ cells divide repeatedly by mitosis to form diploid spermatogonia.

• During the phase of growth, spermatogonium enlarges in size to form primary spermatocyte and prepares to undergo maturation division.
• During the phase of maturation, the primary spermatocyte undergoes meiosis 1 giving rise to two haploid (n) secondary spermatocytes. The secondary spermatocytes undergo meiosis 2 resulting in the formation of four spermatids.
• The transformation of spermatid to sperms is termed as spermiogenesis. A spermatid is non-motile. It has organelles such as mitochondria, Golgi bodies, centrioles, and nucleus.
• During spermiogenesis, the weight of gamete is reduced along with the development of locomotory structures. The nucleus becomes compact forming the major part of head of spermatozoa.
• The Golgi complex of spermatid gives rise to acrosome. Acrosome forms a cap in front of the nucleus containing lytic agent. It dissolves egg membranes during fertilization.
• The acrosome of mammalian sperm produces sperm lysins. The two centrioles of spermatids become arranged one after the other behind the nucleus.
• The anterior one is known as proximal centriole. It is usually located on the neck of spermatozoan. During fertilization, it is introduced into the egg and is required for the first cleavage.

• The posterior centriole is known as distal centriole. It gives rise to the axial filament of the sperm. Mitochondria from different parts of spermatid get arranged in the middle piece around the axial filament. Mitochondria in the middle piece provide energy to the sperm for locomotion.
• A typical mammalian sperm is flagellated, consisting of four parts, namely head, neck, middle piece, and tail.
• The human sperm was first seen by Hamm and Leeu- wenhoek. Tail-less (non-flagellate), “amoeboid” sperm is found in the roundworm Ascaris.

Hormonal Control of Spermatogenesis

• Spermatogenesis is under the control of endocrine hormones. Hypothalamus produces gonadotropin releasing hormone (GnRH).
• It acts on anterior pituitary to produce gonadotropins, ICSH, and FSH. ICSH acts on the interstitial or Leydig’s cells which produce testosterone.
• Testosterone is essential for the formation of sperms, at least the spermiogenesis part by the Sertoli cells. Under the influence of FSH, the Sertoli cells develop ABP.
• The latter helps in concentrating testosterone in the seminiferous tubules.

• Excess of testosterone inhibits LH/ICSH (luteinizing hormone/interstitial cell stimulating hormone) by ante- rior pituitary and GnRH production by hypothalamus.
• The Sertoli cells also produce a glycoprotein called inhibin. Inhibin suppresses FSH synthesis by anterior pituitary and GnRH synthesis by hypothalamus.
• Thus, the normal release of testosterone is under negative feedback control.

Oogenesis

• Oogenesis is the process of maturation of reproductive cells in ovary. Oogenesis starts before birth. In 25-weeks-old female fetus, all oogonia are produced.
• Oogenesis is basically similar to spermatogenesis. It includes the phase of multiplication, the phase of growth, and the phase of maturation.
• During the phase of multiplication, the primordial cells in the ovary divide mitotically to form oogonia (egg mother cell). Each oogonium divides mitotically to form two primary oocytes.
• Primary oocytes undergo growth. The growth phase during oogenesis is comparatively longer.
• Primary oocytes begin the first step of meiosis 1 and proceed up to diakinesis.
• These oocytes resume their development at puberty. The primary oocyte (2n) completes meiosis 1 producing two haploid cells (n)-the larger one is secondary oocyte and the smaller one is first polar body.
• The secondary oocyte starts meiosis 2 and proceeds up to metaphase 2 only. Further development will start only after the arrival of spermatozoa.
• The entry of sperm restarts the cell cycle by breaking down MPF (M-phase promoting factor) and turning on APC (anaphase promoting complex). The completion of meiosis 2 results in the formation of functional egg or ovum and a second polar body.

 

Hormonal Control of Oogenesis

• In response to the production of GnRH, anterior pituitary secretes two hormones-FSH and LH .
• FSH stimulates follicular growth and maturation of oocyte. Granulosa cells of developing ovarian follicle produce estrogen.
• In the presence of high titer of both estrogen and LH, ovulation occurs. High concentration of estrogen inhibits secretion of both FSH and GnRH.

 

• This is negative feedback control. LH helps in converting ruptured Graafian follicle into corpus luteum.
• The latter secretes progesterone which prepares the uterus to receive fertilized ovum. High concentration of progesterone inhibits further release of LH from anterior pituitary and that of GnRH from hypothalamus.

Menstrual Cycle

• Menstrual cycle is the cyclic changes in the reproductive tract of primate (man, monkey, and apes) females. Menstruation is the periodic shedding of the endometrium of uterus with bleeding. In healthy women, menstruation occurs at intervals of about 28-29 days.
• Menarche is the starting of menstruation in girls. It occurs at about 13 years of age. Menstrual cycle consists of menstrual phase, proliferative phase (follicular phase), and secretory phase (luteal phase).
• The proliferative phase (5th to 14th day of menstrual cycle) consists of the growth of the endometrium of uterus, the fallopian tube, and the vagina. In ovary, a Graafian follicle secretes estrogen during this phase.
• Estrogen is the hormone active during the prolifera- tive phase. Ovum is released from the follicle near the end of the proliferative phase, i.e., 14th day or midway during menstrual cycle.
• Ovulation occurs under the influence of LH from pituitary. The subsequent 14 days in which corpus luteum is active make up the secretory phase.
• Progesterone secreted by corpus luteum is active during the secretory phase. Uterine endometrium and glands grow further during this phase.
• At the end of the secretory phase, corpus luteum degenerates into corpus albicans in the ovary, progesterone secretion falls, the overgrown uterine endometrium breaks down, and mensturation takes place.
• Menstrual cycle is controlled by FSH, LH, estrogen, and progesterone. Menstrual cycle and menstruation remain suspended during pregnancy and lactation. Menopause (climacteric) is the period of life when menstruation naturally stops.
• Menopause occurs in females at the age of around 45-50 years. The ability to reproduce is lost in females after menopause.

Estrous Cycle

• The estrous cycle consists of cyclic changes in the female reproductive system of non-primate mammals. There is no menstruation at the end of estrous cycle. The estrogen level in blood increases resulting in strong sex urge in the female. This is called the “period of heat.”
• The estrous cycles run only during breeding season these remain suspended in females during non-breed- ing season. The suspension of estrous cycles is called the state of anestrum.
• Animals that have only a single estrous during the breeding season are called monoestrous. For example, dog, fox, deer, bat, etc.
• Animals that have recurrence of estrous cycle during the breeding season are called polyestrous. For example, mouse, squirrel, cow, sheep, pig, horse, etc.

Events In Mammalian Reproduction

Fertilization

• Ovum is released in the secondary oocyte stage (arrested in metaphase 2). Due to ciliary current produced by fimbriae of oviduct, ovum is drawn in through ostium.
• It reaches ampulla the site of fertilization by the ciliary action of ciliated columnar epithelium lining of oviduct.

• A human sperm can live for many weeks in the male genital duct. Once ejaculated, the sperm can live alive only for 24-48 h outside the body. Sperms move in the liquid medium secreted by the female genital tract (1.5-3 mm/min).
• Prostaglandins of semen help in the movement of sper- matozoa and finally reach the ampulla portion of the oviduct.
• Capacitation of sperm occurs in the female genital system due to the following factors:
  • Removal of membrane cholesterol present over acrosome, weakening the membrane cover.
  • Dilution of decapacitation factors.
  • Entry of Ca2⁺ into sperms causing rapid whiplash motion of the tail.
• Fusion of gametes/syngamy: The various steps are as follows:
  • Acrosomal reaction: The sperms adhere to the surface of egg covers (agglutination). The acro- some starts releasing its hydrolytic enzymes (sperm lysins).
    It includes the following:

• • • Hyaluronidase: It dissolves the hyaluronic acid responsible for the cementing of follicle cells or granulosa cells.
    • Corona digesting enzyme (CDE): It dissolves corona radiata.
    • Zona lysin/acrosin: It digests zona pellucida. It involves zona pellucida compatibility reaction determined by the protein fertilizin over zona pellucida and antifertilizin in case of sperm.
  • The contact of acrosome stimulates the development of an outgrowth by the oocyte called the fertilization cone or the cone of reception.
  • As the sperm head gets in contact with the fertilization cone, it causes the opening up of Na* channel to cause the depolarization of membrane (fast block to check polyspermy) and Ca2⁺ wave inside the egg.
  • Sperm and egg membranes dissolve. Male pronucleus and proximal centriole of sperm enter the cytoplasm of egg and the rest part is left out.
  • Ca2⁺ wave causes the extrusion of cortical granules (cortical reaction) and the zona reaction makes zona pellucida impervious to the second sperm by destroying sperm receptors.
  • Cortical reaction and zona reaction constitute slow block to check polyspermy.
  • The entry of sperm causes the breakdown of metaphase promoting factor (MPF) and turns on anaphase promoting complex (APC). This results in oocyte completing its meiosis 2.
  • Male and female pronuclei approach each other and, finally, mixing up of paternal and maternal chromosomes (amphimixis) occurs resulting in the formation of zygote/synkaryon.

 

Embryonic Development

Embryonic development includes cleavage, blastulation, implantation, gastrulation, and organogenesis.

Cleavage

• The first cleavage is completed after 30 h of fertiliza- tion. Cleavage furrow passes from the animal vegetal axis as well as the center of zygote (meridional cleavage).
• It divides the zygote into two blastomeres (holoblastic cleavage). The second cleavage is completed after 60 h of fertilization.
• It is also meridional but at right angle to the first one. It is completed earlier in one of the two blastomeres, resulting in transient three-celled stage.
• The third cleavage is horizontal forming 8 blastomeres. It is slightly unequal. Thereafter the rate and pattern of cleavage are not specific.

Morula

• Cleavage results in solid ball of celled morula with 16 cells (occasionally 32 cells). Zona pellucida is still present as the outer cover. Morula undergoes compaction.
• The outer/peripheral cells are small/flat with tight junction while the inner cell mass is slightly large round and with gap junction.
• Morula descends slowly towards the uterus in 4-6 days and corona radiata detaches during this period.

Blastulation or Blastocyst Formation

• Endometrium secretes a nutrient fluid and its mucosal cells become enlarged with stored nutrients. As the morula enters uterus, it obtains enriched supply of nu- trients.
• The outer peripheral cells enlarge and flatten further. They form trophoblast or trophoectoderm. Trophoblast secretes a fluid into the interior. It creates a cavity called blastocoel.
• The inner cell mass now comes to lie on one side as embryonal knob.
• With the formation of blastocoel, morula is converted into blastula which is called blastocyst in mammals because of different nature of surface layer and eccentric inner cell mass.

 

• Due to the pressure of growing blastocyst, a slit is produced in zona pellucida. The growing blastocyst comes out. At times, it gets broken into two parts which then give rise to identical twins.
• Trophoblast cells in contact with the embryonal knob are called the cells of Rauber. The area of embryonal knob represents animal pole.
• The opposite side is embryonal pole. Soon embryonal knob shows rearrangement to form embryonal disc. The cells of trophoblast layer divide periclinally.
• This makes trophoblast two layered-outer syncyto trophoblast and inner cytotrophoblast. The two layers later form the chorion, amnion, and fetal part of placenta.

Implantation

• Implantation is the embedding of blastocyst into the endometrium of uterus.
• Blastocyst comes in contact with endometrium in the region of embryonal knob or embryonic disc. It adheres to the same.
• The surface cells of trophoblast secrete lytic enzymes which cause the corrosion of endometrial lining.

• They also give rise to finger-like outgrowths called villi. Villi not only help in fixation but also in the absorption of nourishment.
• Implantation causes nutrient enrichment, enlargement of cells, and formation of uterine part of placenta called decidua (L. deciduus-falling off).
• Decidua has three regions:
  • Decidua basalis (basal decidua, tunica serotina): It is the part of decidua underlying the chorionic villi and overlying the myometrium.
  • Decidua capsularis (decidua reflexa): It lies between the embryo and the lumen of uterus.
  • Decidua parietalis (decidua vera): It is the part of decidua that lines the uterus at a place other than the site of attachment of embryo.
• Trophoblast secretes a hormone called human cho- rionic gonadotropin (hCG). The detection of hCG in urine is the basis of pregnancy/Gravindex test.
• hCG maintains the corpus luteum beyond its normal life. It continues to secrete progesterone which pre- vents menstruation and maintains the uterine lining in nutrient-rich state.
• Progesterone induces cervical glands to secrete viscous mucus for filling the cervical canal to form a protective plug.
• Progesterone is also called pregnancy hormone as it is essential for the maintenance of pregnancy. The hormone is secreted by placenta as well.

Gastrulation

• Gastrulation is characterized by the movement of cells in small masses or sheets so as to form primary germinal layers. There are three primary germinal layers: (a) endoderm , (b) ectoderm, and (c) mesoderm.
• The cell movements that occur during gastrulation are called morphogenetic movements since they lead to the initiation of morphogenesis. The product of gastrulation is called gastrula.

Formation of Primary Germinal Layers

• The cells of inner cell mass or embryonal knob get rearranged to form a flat embryonic or germinal disc. The latter differentiates into two layers-outer epiblast of larger columnar cells and inner hypoblast of smaller cuboidal cells.
• Gastrulation begins with the formation of primitive streak on the surface of the epiblast.
• Cross-section through the cranial region of the streak after 15 days showing the invagination of epiblast cells. The first cells to move inward displace the hypoblast to create a definitive endoderm. Primitive node

• Once the definitive endoderm is established, inwardly moving epiblast forms mesoderm.
• Cells remaining in the epiblast form ectoderm. Thus, epiblast is the source of all germ layers in the embryo.

Placenta

• Placenta is an organ that connects the fetus and uterine wall.
• It is contributed by both maternal as well as fetal part although there is no blending of the maternal and fetal blood supplies. Placenta acts as an ultrafilter. Soluble inorganic and organic materials; nutrients; hormones; and antibodies against diphtheria, small pox, scarlet fever, measles, etc., can pass from the mother to the fetus.

• Placenta acts as an endocrine gland and synthesizes large quantities of proteins and some hormones, such as hCG, chorionic thyrotropin, chorionic corticotropin, chorionic somatomammotropin, estrogens, and pro- gesterone.
• hCG stimulates corpus luteum to secrete progesterone until the end of pregnancy. In addition, it secretes re- laxin that facilitates parturition by softening the con- nective tissue of symphysis pubica.
• The metabolic activity of the placenta is almost as great as that of the fetus itself. The umbilical cord con- nects the fetus to the placenta.
• During the first trimester (first 3 months) of pregnancy, the basic structure of the baby is formed.
• This involves cell division, cell migration, and the differentiation of cells into many types found in the baby. During this period, the developing baby-called fetus is very sensitive to anything that interferes with the steps involved.
• Virus infection of the mother, e.g., by rubella (German measles) virus or exposure to certain chemicals, may cause malformations in the developing embryo. Such agents are called teratogens (monster forming).
• After 3 months, all the systems of the baby have been formed, at least in a rudimentary form.
• From then, the development of fetus is primarily a matter of growth and minor structural modifications.
• The fetus is less susceptible to teratogens.

Parturition

• The gestation period of a human is about 38 weeks/ 266 days followed by birth. The process of giving birth to a baby is called parturition.
• It starts with the rise in estrogen/progesterone ratio and increase in the level of oxytocin secretion by both— mother and fetus.
• It includes three stages.
  • Dilation stage:
    • Uterine contraction starts from the top and occurs at long intervals (once every 30 min). This forces the baby to push its head against the cervix.
    • As a result, the cervix gets dilated with va- gina also showing similar dilation. The di- lation of cervix increases the stimulus for oxytocin secretion, further increasing the strength and frequency of contractions (1- 3 every minute).
    • With continued powerful contractions, the amnion ruptures and the amniotic fluid flows out through vagina.
  • Expulsion stage
    • With further increase in the intensity of uterine and abdominal contraction, the baby comes out through the cervix and vagina with head coming out first.
    • It may take 20-60 min. The umbilical cord is cut. The infant’s lungs expand and breathing begins. This requires a major switch-over in the circulatory system.
    • Blood flow through the umbilical cord ductus arteriosus and foramen ovale ceases; the adult pattern of blood flow-through the heart, aorta, and pulmonary arteries-begins.
    • In some infants, the switch-over is incomplete and blood flow through the pulmonary arteries is inadequate. Failure to synthesize enough nitric oxide is one cause.
  • After birth: Within 10-15 min after delivery, the placenta and the remains of the umbilical cord, called “after birth,” are expelled out.

Lactation

• Although estrogen and progesterone are essential for the physical development of breasts during pregnancy, a specific effect of both these hormones is to inhibit the actual secretion of milk. Conversely, the hormone prolactin has exactly the opposite effect on secretion- promotion of milk secretion.
• This hormone is secreted by the mother’s anterior pi- tuitary gland and its concentration in her blood rises steadily from the fifth week of pregnancy until the birth of the baby at that time it has risen 10-20 times the normal non-pregnant level.
• In addition, placenta secretes large quantities of human chorionic somatomammotropin, which probably also has lactogenic properties, thus, supporting the prolactin from the mother’s pituitary during pregnancy.
• The fluid that is secreted in the last few days before and in the first few days after parturition is called colostrums. It contains essentially the same concentrations of proteins and lactose as milk but almost no fat.

Ejection (or “Let-Down”) Process in Milk Secretion

• Milk is secreted continuously into the alveoli of the breasts, but it does not flow easily from the alveoli into the duct system and, therefore, does not continually leak from the breast nipples.
• Instead, milk must be ejected from the alveoli into the ducts before the baby can obtain it. This is caused by a combined neurogenic and hormonal reflex that in- volves the posterior pituitary hormone oxytocin.
• When the baby suckles, sensory impulses are trans- mitted through somatic nerves from the nipples to the mother’s spinal cord and then to her hypothalamus, there causing nerve signals that promote oxytocin secretion; at the same time they cause prolactin secretion.
• Oxytocin is carried in the blood to the breasts, where it causes myoepithelial cells (that surround the outer walls of the alveoli) to contract, thereby, expelling milk from the alveoli into the ducts.

 

Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

(1) If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

(2) If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

(3) If Assertion is true but Reason is false, then mark (3).

(4) If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: Scrotum provides optimum temperature conditions for spermatogenesis.

Reason: Dartos and cremaster muscles in scrotum contract and relax involuntarily in response to temperature.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 2. Assertion: The process of reproduction does not suffer if one ovary is removed.

Reason: The other ovary enlarges to take over the function of the missing ovary too.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 3. Assertion: “Nothing lives forever, but life continues.”

Reason: Death keeps the population growth under check.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 4. Assertion: Placenta is connected to the fetus by an umbilical cord.

Reason: Fetal components of placenta are derived from endometrium.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 5. Assertion: Placenta is contra-deciduate and even the fetal placenta is absorbed in mole.

Reason: Mole’s egg contains abundant yolk in ooplasm.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 6. Assertion: Polar bodies have small amount of cytoplasm.

Reason: It is formed by unequal mitotic division.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 7. Assertion: Ovulation takes place when the blood level of luteinizing hormone is high.

Reason: Leutinizing hormone is responsible for ovulation.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 8. Assertion: Umbilical cord contains 100% fetal blood.

Reason: It has single umbilical artery and single umbilical vein.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 9. Assertion: The activation of sperm is called capacitation.

Reason: Capacitation takes about 5-6 h.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: Before fusion, spermatozoa have to penetrate egg membrane,

Reason: The activated spermatozoa undergo acrosomal reactions and release sperm lysin.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 11. Assertion: In post natal life, oocyte development occurs in mature follicle,

Reason: After ovulation, the Graafian follicle transforms in corpus luteum.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 12. Assertion: Placenta is combined structure of fetal tissue and maternal tissue.

Reason: Placenta formation is completed before 6 weeks.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 13. Assertion: Seminal vesicle is known as the accessory sex organ of males.

Assertion: Seminal vesicle conserves sperm energy and provides fuel to sperm.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 14. Assertion: Testes are retroperitoneal organ in man.

Reason: Peritoneal layer covers the testes on the dorsal side.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 15. Assertion: Cervix contains the most weak sphincter muscle in the body.

Reason: Cervix opens into fallopian by os external.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 16. Assertion: In ovarian cycle, corpus luteum is exocrine gland.

Reason: It secretes pheromones.

Answer. 4. If both Assertion and Reason are false, then mark (4).