Bhavani

Stars And The Solar System Abstract

• The stars, the planets, the Moon, and many other objects in the sky are called celestial objects. They all form part of the universe.
• The Moon revolves around the Earth, and the Earth along with the Moon revolves around the Sun.
• Stars and the Solar System
• Stars are the celestial bodies which emit light of their own. The Sun is also a star.
• The distance between the stars and the Earth is expressed in a unit called a light year.
• A light year is a distance that light travels in one year.
• When seen from the Earth, stars appear to move from east to west.
• Pole Star is situated close to the direction of the axis of the Earth’s rotation; hence, it appears to be stationary when seen from the Earth.
• Groups of stars that appear to form recognizable shapes/patterns in the sky are called
  constellations.
• The most common constellations are Ursa Major, Orion, and Cassiopeia.
• The Sun, the planets revolving around it and many other celestial bodies comprise the solar system.
• A celestial body revolving around another celestial body is called a satellite. There are two types of satellites: natural and artificial.
• The shape of the Moon changes every day. Different shapes of the moon visible from the Earth are known as the phases of the moon.
• The phases of the Moon occur because only the part of the Moon which reflects sunlight is visible to us. The back of the moon is never visible.
• Moon is the only natural satellite of the Earth.
• Artificial satellites are man-made objects launched into the space. Artificial satellites are used for weather forecasting, communication, navigation, transmitting radio and television signals, and military.

Read and Learn More UP Board Notes for Class 8 Science

Stars and The Solar System Important Terms and Definitions

Asteroids: Numerous small bodies (mainly rocks, metals, etc.) that lie in the large gap existing between the orbits of Mars and Jupiter are known as asteroids.
Comets: The celestial bodies that revolve around the Sun and have very long periods of revolution are known as comets.
Meteors: Small objects that occasionally enter Earth’s atmosphere are known as meteors.
Meteorites: Meteors which reach the Earth are called meteorites.

The Moon

Moon is the natural satellite of the Earth. The surface of the Moon is dusty and infertile. There is complete absence of water and atmosphere on the Moon. Hence, there is absolutely no life on the Moon. The day when the entire Moon is visible, it is known as a full Moon day. The day when the Moon is not visible is known as the new Moon day. The day when only a small portion of the Moon appears in the sky is known as the crescent Moon day. The time period between one full moon day to the next full moon day is slightly longer than 29 days. This period is called a month.

Stars and The Solar System Activity 1

Aim: To observe the shape of the Moon for a few nights
Procedure:

1. Observe the Moon continuously for several nights, preferably from one full Moon to the next.
2. Make a sketch of the Moon every night in your notebook from the day of the full Moon.
3. Also, note the part of the sky (east or west) in which the Moon is seen.

Observations:

• The Moon appears to be perfectly round on the full Moon day.
• After the full Moon day, the size of the Moon decreases every day.
• On the new Moon day, the Moon is not visible even if the sky is clear.
• After the new Moon day, the size the Moon starts increasing each day and the full Moon appears again in another fifteen days.

Conclusion: The crescent-shaped Moon keeps on increasing every day until the full face of the Moon is visible on the fifteenth day. After the full Moon, the size and brightness of the Moon goes on decreasing each night. Subsequently, in another fifteen days, the Moon becomes invisible. This is known as new Moon day.

Stars and The Solar System Activity 2

Aim: To show the positions of the Moon on its orbit and its corresponding phases
Precaution: If you perform this activity in the morning, the white portion of the ball should face east. If this activity is performed in the afternoon, the white portion of the ball should face west. In each case, the line dividing the white and black portions should be kept vertical.

Procedure:

1. Take a big ball or a pitcher. Paint half of it white and the other half black.2. Go out into
2. The playground with two of your friends. Draw a circle of radius about 2 m on the ground. Divide the circle into eight equal parts as shown in the figure given here.

1. Stand at the center of the circle. Ask your friend to hold the ball at different points of the circle. Ask her to keep the white portion of the ball towards the Sun.
2. Standing at the center of the circle, observe the visible white portion of the ball while your friend stands at the points on the circle marked earlier. Draw the shape of the white portion as you see it.
3. Compare your drawings with the different phases of the Moon as shown in the figure given below.

Observation:
On the new Moon day, the Moon lies between the Sun and the Earth. However, on the full Moon day, the
Earth comes between the Moon and the Sun.

Conclusion:
After the new Moon day, the size of the illuminated part of the Moon, visible from the Earth, increases each day. However, the Sun-lit part of the Moon visible from the Earth decreases in size every day after the full Moon day.

Stars and The Solar System Objective Type Questions

A. State whether the following statements are true or false.

1. The shape of the Moon changes every month rather than daily.
2. Sound cannot be heard on the Moon.
3. There is no human life existing on the Moon; however, the growth of plants and other microorganisms is possible on its surface.The Moon produces light on its own.
4. After the full moon day, the visible part of the Moon starts decreasing.

Answers:

1. False
2. True
3. False
4. False
5. True

B. Multiple-Choice Questions

Question 1. Who was the first person to land on the Moon?

1. Galileo
2. Neil Armstrong
3. Isaac Newton
4. Edwin Aldrin

Question 2. When the Moon is not visible at night on a clear sky, what is the day called?

1. Full Moon day
2. Eclipsed Moon day
3. Crescent Moon day
4. New Moon day

Question 3.What is the reason that it is possible to observe phases of the Moon from the Earth?

1. The Moon revolves around the Earth.
2. The Moon’s axis is tilted.
3. The Moon spins on its axis.
4. The Moon’s distance from Earth changes at a particular rate.

Answers:

1. Neil Armstrong
2. New Moon
3. The Moon revolves around the Earth

C. Fill in the blanks.

1. The day when only a small portion of the Moon appears in the sky is called the_____ Moon day.
2. The day of the full Moon is popularly known as_____while the day of New Moon is popularly called____
3. The surface of the Moon is dry, rocky and _____

Answers:

1. Crescent
2. Purnima, Amavasya
3. Infertile/Barren

Stars and The Solar System Short Answer Type Questions

A. Why does the shape of the Moon change every day?

Answers: The Moon appears bright and shiny because it reflects the sunlight that falls on its surface. The Moon along with the Earth revolves around the Sun. As a result, the relative position of the moon keeps changing every day.

B. Why there is no life on the Moon?

Answers: The two essential factors, which are required for the survival of living beings are air and water. These are not available on the Moon’s surface. Hence, no life exists on the Moon.

Stars and Constellations

Answers: Stars emit their own light. Generally, stars are very far (many millions of kilometers) away from the Earth. This is why they appear too small in the sky. The Sun is the star closest to the Earth. This is why it appears to be very big while other stars seem so small, almost like dots or points in the sky. A group of stars forming a certain pattern with a recognizable shape is called a constellation. The shape of a constellation never changes. Like stars, constellations also appear to move from east to west.

Stars and The Solar System Activity 4

Aim: To show that if the stars appear to move from east to west, the Earth rotates from the west to the east
Procedure:

1. Stand in the center of a big room.
2. Now, start rotating and observe the direction in which the objects in the room appear to move.

Observation: The objects in the room appear to be moving in the direction opposite to our motion.
Conclusion: If the stars appear to move from east to west, the Earth rotates from west to east.

Stars and The Solar System Activity 5

Aim: To show that the Pole Star lies close to the axis of the rotation of the Earth
Procedure:

1. Take an umbrella and open it. Make about 10-15 stars out of white paper. Paste one star at the position of the central rod of the umbrella and others at different places on the cloth near the end of each spoke as shown in the figure given below.
2. Now rotate the umbrella by holding its central rod in your hand. Observe the stars on the umbrella.

Observation: The star situated at the central rod of the umbrella does not appear to move.
Conclusion: The Pole Star lies close to the axis of the rotation of the Earth, thus, it does not appear to move.

Stars and The Solar System Activity 6

Aim: To show that a constellation appears to move in the sky from east to west
Procedure:

1. Locate a constellation (e.g. Ursa Major) in the sky.
2. Observe this constellation for a few hours.

Observation: The shape of the constellation remains the same. However, the constellation appears to be moving.
Conclusion: The constellation appears to move in the sky from east to west.

Stars And The Solar System Activity 7

Aim: To locate the Pole Star in the sky
Precaution: This activity should be performed on a clear Moonless night
during summer at around 9 pm.

Procedure:

1. Look towards the northern part of the sky and identify Ursa
   Major.
2. Look at the two stars at the end of Ursa Major.
3. Imagine a straight line passing through these stars as shown in the
   figure given below.
4. Extend this imaginary line towards the north direction. (About
   five times the distance between the two stars).
5. This line will lead to a star.

Observation: The imaginary line will lead to a star, that is not too bright. This is the pole star.

Stars And The Solar System Activity 8

Aim: To show that the Ursa Major constellation moves around
the Pole Star
Procedure:

1. 1. During a summer night, observe Ursa Major 3-4 times at an interval of 2 to 3 hours.
2. Locate the Pole Star each time.
3. Compare your observations with those in the figure given below.

Observation Ursa Major appears to move from east to west.
Conclusion: The Ursa Major constellation moves around the Pole Star.

Stars and The Solar System Objective Type Questions

A. State whether the following statements are true or false.

1. The North Star which indicates the north direction is also called the Pole Star.
2. Stars always seem to move in the same direction as the axis of Earth’s rotation.
3. Stars are present in the sky at night only.

Answers:

1. True
2. False
3. False

B. Multiple Choice Questions.

Question 1. Which of these constellations resembles the shape of a hunter’s body?

1. Orion
2. Ursa Major
3. Leo Major
4. CassiopeiaAnswers: 1) Orion

Question 2. Which of these stars is located closest to the Earth?

1. Pole Star
2. Alpha Centauri
3. Sirius
4. Sun
   Answers: 4) Sun

Question 3. Which is the brightest star in the sky?

1. Alpha Centauri
2. Sun
3. Sirius
4. Pole Star
   Answers: 1) Alpha Centauri

C. Fill in the blanks.

1. The distance between the Earth and the Sun is measured in_____.
2. There are stars present in the sky.

Answers:

1. Light years
2. Infinite/Countless

Stars and The Solar System Short Answer Type Questions

A. Do all the stars of a constellation lie really close together?

Answers: The different stars that form a constellation are not related and lie at widely varying distances from the Earth. However, they all appear to our eye as close together but are not so.

B. Why are stars not visible during the day?

Answers: During the day, the light of the Sun is very bright and the brightness of stars becomes relatively dim in comparison to that of the Sun. Hence, they are not visible during the day.

C. How can you identify Ursa Major?

Answers: The constellation Ursa Major is seen during summer. It is also called the Great Bear or the Saptarishi. There are seven prominently shining stars in the constellation which are arranged in the shape of a question mark. The Pole Star lies at the tail of this constellation.

Solar System: Planets and Other Members

The solar system comprises the Sun, the planets, and various other members such as asteroids, comets, meteors/meteorites, and artificial satellites. There are eight planets in the solar system. These are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. All planets revolve around the Sun in a fixed path called orbit. The time taken by a planet to complete one revolution is called its period of revolution. In addition to the Sun and the eight planets, many other celestial bodies form the part of the solar system.

Stars and The Solar System Activity 9

Aim: To confirm that planets move in their own orbits
Procedure:

1. 1. Go out into the playground with four or five friends. Draw four circles of radii 1 m, 1.8 m, 2.5 m, and 3.8 m, all having a common center as shown in the given figure.

1. Ask one of your friends to stand in the centre and represent the Sun. Your other four friends may represent Mercury, Venus, Earth and Mars.
2. Ask your friends to move around the Sun in anti-clockwise direction in their own orbits and observe whether they collide with one another or not.

Observation: They do not collide because they are moving in their own fixed orbits.

Conclusion: Planets move in their own fixed path called orbits.

Stars and The Solar System Activity 10

Aim: To observe the planet Venus
Procedure:

1. Find out from a newspaper or from an almanac the time when Venus is visible in the sky.
2. Observe Venus either 1-3 hours before sunrise or 1-3 hours after sunset.

Observation: Venus can be easily recognized by its brightness. However, it cannot be seen very high in the sky.
Conclusion: Venus is the brightest planet in the night sky. It rotates from the east to the west. Since Venus is close to the Sun, it can also be seen just before sunrise and just after sunset.

Stars and The Solar System Objective Type Questions

A. State whether the following statements are true or false.

1. Until 2006, Pluto was also a planet (ninth planet) in the solar system.
2. Mercury and Venus have rings around them, which can be seen through a telescope.
3. The outer planets have very few moons.
4. Saturn is the least dense among all the planets.
5. The appearance of a comet is a natural phenomenon and it is not an indicator of any forthcoming disaster.

Answers:

1. True
2. False
3. False
4. True
5. True

B. Multiple-Choice Questions

Question 1. Which of these planets is also called the morning star or the evening star?

1. Mercury
2. Venus
3. Mars
4. NeptuneAnswers: 2) Venus

Question 2. Which planet in the solar system appears yellowish in color?

1. Jupiter
2. Uranus
3. Satum
4. MercuryAnswers:  3) Saturn

Question 3. The axis of the Earth is inclined to its orbital plane at what angle?

1. 23.5°
2. 25.5°
3. 60.5°
4. 66.5°Answers: 4) 66.5°

Question 4. Which of these celestial bodies is visible only through large telescopes?

1. Meteors
2. Meteorites
3. Asteroids
4. CometsAnswers:  3) Asteroids

Question 5. Which of these is the first Indian satellite launched from the Earth?

1. INSET
2. Aryabhatta
3. Kalpana-I
4. EDUSAT

Answers:  2) Aryabhatta

C. Fill in the blanks.

1. _______ is commonly known as Dhruv Tara.
2. The reflection of light from______ and_____ on Earth’s surface makes it appears blue-green when seen from space.
3. Mars has_____ small natural satellites.
4. The mass of Jupiter is nearly______times that of the Earth.
5. _____ are swarms of meteors seen at the time when the Earth crosses the tail of a comet.

Answers:

1. The Pole Star
2. Water, landmass
3. Two
4. 318
5. Meteor showers

Stars and The Solar System Short Answer Type Questions

A. Why is life possible on the Earth?

Answers: Environmental conditions such as appropriate distance from the Sun, the correct level of temperature, the presence of atmosphere and water, together make the existence and continuation of life possible on the Earth. Moreover, the presence of the ozone layer around the Earth prevents organisms from harmful ultraviolet radiation of the Sun.

B. What do you mean by inner planets and outer planets?

Answers: The four planets, Mercury, Venus, Earth, and Mars, which are nearer to the Sun, are called the inner planets. However, the planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus, and Neptune are much farther off from the Sun. These planets are called the outer planets.

Stars and The Solar System Textbook Exercises

Choose the correct answer in Questions

Question 1. Which of the following is NOT a member of the solar system?

1. An asteroid
2. A satellite
3. A constellation
4. A Comet

Answer: 3) A constellation

Question 2. Which of the following is NOT a planet of the Sun?

1. Sirius
2. Mercury
3. Saturn
4. Earth

Answer: 1) Sirius

Question 3. Phases of the Moon occur because

1. We can see only that part of the Moon which reflects light toward us.
2. Our distance from the Moon keeps changing.
3. The shadow of the Earth covers only a part of the Moon’s surface.
4. The thickness of the Moon’s atmosphere is not constant.

Answer: 1) We can see only that part of the Moon which reflects light toward us.

Question 4. Fill in the blanks.

1. The planet which is farthest from the Sun is______.
2. The planet which appears reddish in color is______.
3. A group of stars that appear to form a pattern in the sky is known as a______.
4. A celestial body that revolves around a planet is known as_______.
5. Shooting stars are actually not_______.
6. Asteroids are found between the orbits of_______and_____.

Answer:

1. Neptune
2. Mars
3. Constellation
4. Satellite
5. Stars
6. Mars, Jupiter

Question 5. Mark the following statements as true (T) or false (F):

1. The Pole Star is a member of our solar system.
2. Mercury is the smallest planet of the solar system.
3. Uranus is the farthest planet in the solar system.
4. INSAT is an artificial satellite.
5. There are nine planets in the solar system.
6. Constellation Orion can be seen only with a telescope.

Answer:

1. True
2. True
3. False
4. True
5. False
6. False

Question 6. In which part of the sky can you find Venus if it is visible as an evening star?

Answers: Venus is visible as an evening star in the western part of the sky

Question 7. Name the largest planet of the solar system.

Answers: Jupiter

Question 8. What is a constellation? Name any two constellations.

Answers: A groups of stars that appear to form recognisable shapes/patterns are called constellations. The Great Bear and Orion are two constellations.

Question 9. Draw sketches to show the relative positions of prominent stars in

(a) Ursa Major and (b) Orion

Question 10. Name two objects other than planets which are members of the solar system.

Answers: Asteroids and Meteors

Question 11. Explain how you can locate the Pole Star with the help of Ursa Major.

Answers: The Pole Star is located at the end of Ursa Major. A straight line from the last two stars when extended towards the north leads to the Pole Star.

Question 12. Do all the stars in the sky move? Explain.

Answers: No, all stars in the sky do not move. However, they appear to move from east to west due to the rotation of the Earth about its axis.

Question 13. Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?

Answers: Stars are situated very far (many millions of kilometers) away from the Earth. Such large distance cannot be conveniently expressed in kilometers. Hence, it is expressed in a unit called a light year. One light year is the distance traveled by light in one year. One light year is equivalent to 9.46 x 10¹2 km. If we say that a star is eight light years away from earth, it means that the light from the star will reach the earth in eight years.

Question 14. The radius of Jupiter is II times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?

Answers:

Let the radius of Earth = R units
200
Volume of Earth = 4/3xpx R³ cubic units
Radius of Jupiter = ||R units
Thus, volume of Jupiter = 4/3 × px (11R)³ cubic units
= 4/3 × px 1331R³ cubic units
X
Now, ratio of the volume of Jupiter and Earth = Volume of Jupiter/ Volume of Earth
:. 4/3 × px 1331 = R³/4/3 × px R³
= 1331/1 or 331: 1
So, Jupiter can accommodate 1331 Earths.

Question 15. Boojho made the following sketch of the solar system. Is the sketch correct? If not, correct it.

Stars and The Solar System Hots corner

Question 1. Venus is far away from the Sun as compared to Mercury. Still Venus is hotter than Mercury. Why is it so?

Answers: Venus has a thick atmosphere rich in carbon dioxide gas. Carbon dioxide is a greenhouse which traps the infrared radiations of the Sun resulting in an increase in the temperature of the planet. In contrast, Mercury has a thin atmosphere with a variety of gases. The atmosphere does not have greenhouse gases which can trap heat. Thus, Venus is much hotter than Mercury.

Question 2. Why do stars twinkle when planets do not?

Answers: Stars are located at a distance from the Earth; so they appear as dots or points in the sky. These points vibrate due to the atmospheric air currents. Hence, when seen from the Earth, they generally seem to twinkle or vibrate. However, planets are not located as far from Earth. They are relatively bigger in size and so their vibrations, due to atmospheric currents, are apparently not visible as twinkling.

Stars and The Solar System Practice Exercise

Objective Type Questions

A. Give one word for the following.

1. The branch of science that deals with the study of the universe
2. The unit of measurement used to measure distance in space
3. The name of our galaxy

Answers:

1. Astronomy
2. Light year
3. Milky way

B. State whether the following statements are true or false.

1. Venus is also called the morning star.
2. Mercury is the smallest planet of the solar system.
3. Aryabhatta is a natural satellite of the Earth.

Answers:

1. False
2. True
3. False

C. Circle the odd one out.

1. Orion, Great Bear, Ursa Major, Big Dipper
2. Earth, Venus, Moon, Jupiter

Answers:

1. Orion
2. Moon

D. Fill in the blanks.

1. It is not possible to hear any sound on the Moon because of the absence of______.
2. Asteroids are found between the orbits of_____ and______.

Answers:

1. Atmosphere
2. Mars, Jupiter

Stars and The Solar System Short Answer Type Questions

A. What is meant by an equatorial plane and orbital plane?

Equatorial plane refers to the plane of the equator of the Earth. Orbital plane refers to the plane in which the Earth revolves around the Sun.

B. What are the differences between a star and a shooting star?

C. Name a periodic comet. Why is it so-called?

Answers: Halley’s comet is a periodic comet. It is called so because it appears at regular intervals of time. Halley’s comet is seen after every 76 years.

D. Why is the Sun classified as a star?

Answers:

The Sun is classified as a star due to the following reasons:

1. It has its own source of energy.
2. It continuously emits large amounts of light and energy.
3. It has a life period.

E. State in which month/season the following constellations are seen:

1. Ursa major
2. Orion
1. Visible in April in the northern part of the sky.
2. Visible during the winter season in the northern part of the sky.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 18 Pollution of Air and Water

Sense Organs – The Eye and The Sense of Sight

Sense Organs Learning Objectives

After completing this chapter, you will be able to:

• Define a receptor and name various kinds of receptors;
• Describe the structure and functions of the eye;
• Trace the pathway of light through the eye to the retina and explain how light is focused for distant and near vision;
• Explain the causes and consequences of various defects of the eye and methods of their correction;
• Describe the structure and general functions of the outer, middle, and Inner ears;
• Explain how the ear serves as an organ for dynamic and static equilibrium.

The eyes, ears, tongue, nose, and skin are the major sense organs in our body, which are sensitive to the sense of light, sound, taste, smell, and touch, respectively. Each of these sense organs is directly connected to the brain. These sense organs have special cells called receptors which receive stimuli from the environment.

Sense Organs Some Key Terms

Stimulus: A physical event that affects an organism by activating its receptors to bring about a change in its activity. For example, mechanical stimuli (pressure and touch), chemical stimuli (taste and smell), and thermal stimuli (heat and cold).

Sensation: A general state of awareness of the stimulus.
Receptors: Sensory cells in tissues that receive stimulus from the environment. Some common specific receptors are:

1. Photoreceptors: respond to light rods and cone cells in the retina of the eye.
2. Chemoreceptors: respond to chemicals – taste buds on the tongue, smell receptors of the nose.
3. Mechanoreceptors: respond to touch – nerve fibers around the hair on the skin.
4. Thermoreceptors: respond to changes in temperature – receptors on the skin.
5. Photoreceptors: respond to sound/hearing – receptors in the ear.
6. Exteroceptors: specialized to detect sensory information from the external environment.

Sense Organs The Eye And The Sen Sight

We all respond to the light stimulus. Human beings have two eyes situated in deep sockets or orbits on the front side of the head. Each eye is in the form of a ball called an eyeball that measures about 2.5 cm (1 inch) in diameter. Each eyeball can be rotated with the help of six distinct sets of muscles.

Sense Organs Accessory structures of the eye

Eyebrows, eyelids, eyelashes, and the tear glands (the lacrimal apparatus) are the accessory structures of the eye.

                                                               

• The eyebrows protect the eyeball from foreign objects, perspiration, and direct rays of sunlight.
  Eyelids are folds of skin and muscles. The upper and lower eyelids protect the eyes from excessive light and foreign particles, cover the eye during sleep and spread lubricating secretions over the eyeballs. The upper eyelid is more movable than the lower one. Projecting from the border of each eyelid is a row of thick, short hairs called eyelashes. At the base of the hair follicles of eyelashes, sebaceous glands are found. These glands secrete a lubricating fluid into the hair follicles called sebum.
• Tear glands or lacrimal glands (collectively known as lacrimal apparatus) are a group of glands that manufacture and pour tears (L. lacrima: tears). A lacrimal gland is a compound gland located at the upper sideward portion of each eyelid
  Each lacrimal gland gives rise to 6 to 12 excretory lacrimal ducts that empty their secretion (tears) onto the surface of the conjunctiva of the upper eyelid. From here it passes to the lacrimal canals and then into the lacrimal sac. From here a
• Nasolacrimal duct conducts the secretion into the nasal cavity in the nose. This duct is responsible for the passage of medicine dropped in the eye to the nose or sometimes into the throat.
• Tear or lacrimal secretion is a watery fluid containing salts, some mucus, and a bacteriocidal enzyme called lysozyme.

Functions of tears:

1. After being secreted, it spreads over the surface of the eyeball and serves as a lubricant.
2. It cleans the front surface of the eyeball by washing away the dust particles.
3. It moistens the eyeball.
4. The enzyme lysozyme contained in the tears kills the germs.
5. Tears help in communicating emotions.

Structure Of The Eyeball

• The eyeball is a spherical structure, measuring about 2.5 cm in diameter. Structurally, the eyeball is composed of three layers – the fibrous tunic (anterior cornea + posterior sclera), the vascular tunic also known as uvea (middle layer of the eyeball) (choroid + ciliary body + iris), and the inner retina.

 

1. Sclerotic layer or sclera

• The sclerotic layer or sclera is the outer tough coat of the eyeball made up of mainly collagen fibers. It can be divided into two regions – the posterior region called the sclera, and the anterior region called the cornea.
• The sclera is a white coat of fibrous tissue visible through the conjunctiva, which covers all the eyeballs except the cornea. It is also known as the white of the eye. The sclera gives shape to the eyeball and also protects its inner parts.
• The cornea is a transparent, fibrous coat through which the iris can be seen. The outer surface of the cornea is covered by an epithelial layer which is continuous with the epithelial layer of the conjunctiva. The cornea receives nourishment from tears and aqueous humor.
• Sometimes, the cornea turns white and opaque and can be replaced by a donated eye.

2. Uvea

• The choroid layer is the middle layer of the eyeball and is composed of three parts – choroid, ciliary body, and iris.
• The choroid is a thin, dark brown membrane that lines most of the inner surface of the sclera. It contains several blood vessels and a large amount of pigment melanin. The choroid absorbs light rays so that they are not reflected within the eyeball. The numerous blood vessels nourish the retina.
• The choroid expands to form the ciliary body. It consists of circular muscles. The smooth muscles of the ciliary body help in changing the shape of the lens.
• The iris (arid: rainbow) is a clouded part of the choroid around the pupil. It is a clouded part seen through the cornea. There is a hole (round window) in the canter of the iris, known as the pupil. The light enters the eyeball through the pupil. The iris contains radial and circular muscles.

                                                                  

The contraction of these muscles constricts the pupil which regulates the amount of light entering the eyeball through the iris

• In the case of bright light, the circular eye muscles contract, and the size of the pupil is decreased (constriction).
• In case of dim light, the radial muscles contract, and the size of the pupil is increased (dilation).

3. Sense Organs Retina – part of the eye where rod and cone cells are located

• The retina is the third and the inner layer of the eye. It is located only in the posterior part of the eye. It is the light-sensitive layer. It contains light-sensitive cells called rods and cones.
• The rod cells are sensitive to dim light. They do not respond to color. Rods contain the pigment rhodopsin or visual purple. The rod cells are distributed throughout the retina.
• The cone cells are sensitive to bright light. The cones are responsible for color vision. Cones contain the pigment rodeos in. Cone cells are mostly confined to the yellow spot or fovea centralis or macula.The differences between rod cells and cone cells are given in Table                                           

Yellow spot – the area of the best vision

The yellow spot or macula lutea (Fovea centralis) is a spot located inside the eyeball near the Centre of the retina. It contains a maximum number of light-sensitive cells, especially cone cells. The rest of the retina has lesser cone cells and more rod cells. The yellow spot is the region of color vision and also the brightest, clearest and best vision.

Blind spot – the area of no vision

A blind spot is the region of the retina just below the yellow spot. Since there are no light receptors here, no image is formed here, i.e. blind spot is the area of no vision.

                                                        

clockwise through 90° and hold it at an arm’s length with the two symbols straight in front of your eyes. Close your left eye and concentrate on the cross with your right eye. Bring the book slowly towards you. Keep looking at the cross on the left: eventually, as the dot falls on the blind spot of your right retina, the image of the dot on the right will disappear.

Sense Organs Lens

• The main body of the eye is divided into two parts by a biconvex lens which is a transparent crystalline body. The lens lies just behind the pupil and iris. It is flatter at the front than at the back and is soft and slightly yellow in color. It contains transparent lens fibers and an elastic lens capsule made of glycoprotein. There is no blood vessel in the lens.
• The lens is held in position by the suspensory ligaments which attach it to the ciliary body.

The eyeball is divided into two chambers:

• the anterior chamber (aqueous chamber) and
• posterior chamber (vitreous chamber).
• The anterior chamber (between the lens and cornea) is filled with a fluid called aqueous humor. It is a thin and watery fluid. It keeps the lens moist and protects it from physical shock.
• The posterior chamber is called the vitreous chamber. It is a large cavity of the eyeball. It lies between the lens and the retina. It contains a jelly-like substance called vitreous humor. The vitreous humor maintains the shape of the eyeball by preventing the eyeball from collapsing and supporting the retina.

Sense Organs How Do We See – Working Of The Eye

1. Light rays enter the eye

The light rays enter the eye through the cornea and then pass through the pupil, aqueous humor, the lens, and the vitreous humor (all transparent structures), before reaching the retina.
The pupil adjusts to different intensities of light. In bright light, the pupil constricts (becomes narrow) and in dim light, it dilates (becomes wider) to allow more light.

2. Image formation on the retina

The formation of an image on the retina requires four steps – a. refraction of light rays, b. constriction of the pupil, c. accommodation of the lens, and d. convergence of the rays.

1. Refraction of light rays: The eye focuses an image by refracting, or bending the light rays using the cornea and the lens. An upside-down or inverted image is formed at the yellow spot on the retina. Most of the refraction of light occurs in the cornea due to its curved surface.
2. Constriction of the pupil: The iris contains radial and circular muscles. Radial muscle widens and circular muscle constricts the pupil. This adjustment of the size of the pupil regulates the amount of light entering the eye.
3. Accommodation of the lens: The ability of the eye to focus on different objects placed at a distance by adjusting the curvature of the elastic eye lens is called accommodation. This is brought about by ciliary muscles which change the focal length of the eye lens to focus on near or distant objects on the retina.

(1) In the distant vision (more than 6 meters), the ciliary muscles are relaxed and the lens becomes flatter or thinner due to the stretching of the suspensory ligaments.

 

                                                           

(2) In the near vision (less than 6 meters), the ciliary muscles contract pulling the choroid forward. towards the lens and tension is released on the suspensory ligaments. The lens becomes shortened, more convex, and thickened due to its elastic nature

In normal conditions, the ciliary muscles are relaxed and the lens remains stretched by the suspensory ligaments. Then, it is less convex and best suited for distant vision.

4. Convergence of the rays: We have two eyes

                                                                  

 

but we see only one image of an object. This is known as binocular vision or stereoscopic vision. While viewing objects, both eyeballs move toward the object. Thus, both images fall on the corresponding points (fovea centralis) of both retinas at the same time and overlap with each other. This is known as the convergence of the rays.

The eye can be compared with a camera. The major similarities and differences between the eye and the camera are given. A summary of various parts and functions of the eyes is given in Table

                                                                     

3. Photoreception by brain

The image formed at the retina stimulates photoreceptors. The light energy of the image formed at the retina produces chemical changes in the rods and cones. This generates nerve impulses that travel through the optic nerve from the retina to the visual area of the brain. At the cerebrum (brain), the sensation of sight is interpreted.

How are the eyes adapted to bright light and dark?

• When we are in dark or dim light for a long period, then the rate of photopigment (rhodopsin) formation by rod cells in the retina is much faster than the rate at which they are broken down. The pupil dilates to allow more light to enter through the retina to allow viewing of objects in dim light. This is called dark adaptation.
• When we are in a very bright light for a long period of time, the light-sensitive pigments of the rods are broken down, and the visual purple of the rods gets bleached, reducing their sensitivity. The cone cells in the retina are activated and it forms iodopsin pigment. The further pupil constricts and the size of the pupil is reduced to reduce the amount of light entering the eyes. This is called light adaptation.

Colour vision

• color vision is possible because of cone cells. When all the cone cells are stimulated, we see white color. When very few cone cells are stimulated, we see black color. Stimulation of separate types of cone cells produces red, green, or blue and their combinations.

Why cannot we distinguish color on a moonlit night?

• In dim light, like that during a moonlit night, colors cannot be distinguished because cone cells that detect colors do not work in dim light.

Sense Organs Common Defects Of The Eye And Their Correction

Short-sightedness or Myopia

• In this condition, light is focused in front of the retina, and a blurred image is formed for distant objects. In myopia, the near objects are seen clearly while the distant objects appear blurred. This happens because either the eyeball is elongated or the lens has become too thickened or curved

                                     

Long-sightedness or Hyperopia

• In this condition, the light is focused behind the retina (the image is formed behind the retina). As a result, the distant objects are seen clearly, while the near objects appear blurred. Long-sightedness results due to the shortening of the eyeball or the lens have become. too thin

                                         

Hyperopia (Hypermetropia) can be corrected by using a convex (converging) lens.

Astigmatism

• This is a more complicated defect in vision. In this, the surface of the cornea becomes irregular and therefore some of the light rays are focused while others are not. As a result, some parts of the object appear blurred while other parts appear clear

                                             

Astigmatism can be corrected by using a cylindrical lens that bends light rays in one direction only.

Presbyopia

• Astigmatism can be corrected by using a cylindrical lens that bends light rays in one direction only. Presbyopia In this condition, the lens loses its flexibility causing long-sightedness (the near objects cannot be seen clearly). This defect normally occurs in older people.

Presbyopia can be corrected by using a convex lens.

Night-blindness

• In this condition, there is difficulty in seeing in dim light (hence the name night blindness). This is because of the non-production of rhodopsin pigment in the rod cells, which function in dim light. In the absence of rhodopsin, these cells cannot function. Thus, there is a lack of normal night vision. It is most often caused due to deficiency of vitamin A.

Night blindness can be cured by having Vitamin A rich diet.

Colour blindness

• In this condition, a person is unable to discriminate between red and green colors. This is a genetic defect which you have already studied in the chapter on heredity and genetics (X-linked inheritance).

Colour blindness cannot be treated during the lifetime of an individual.

Cataract

• In this condition, the lens of the eye becomes opaque and as a result, the vision is cut down. Cataracts can be treated by surgical removal of lenses and using convex lenses in the spectacles which compensate for the removed lens. Nowadays, a small plastic lens is implanted behind the iris to correct the defect.

Squint

• In this condition, either the two eyes somewhat converge (known as cross-eye) or diverge (known as wide-eye). In both these conditions, a person may have double vision. Squint in the eyes can be treated by surgery or by suitable exercises.

Sense Organs The Ear – Senses Of Hearing And Balance

• Humans have two ears, one on each side of the head. Human ears are organs for the senses of hearing and balance. The ear is a miniature receiver, amplifier, and signal processing system.

Structure of ear

The human ear is divided into three parts

• External ear
• Middle ear
• Inner ear

1. The external or outer ear
   It consists of the externally visible part of the ear called the pinna (or by a thin membrane. auricle) and the internal part, the auditory canal. The auditory canal. the passage leading to the eardrum (also known as the tympanic membrane).

Hair and earwax in the ears

The auditory canal contains fine hairs, which filter the air. In addition, the upper wall of the canal contains modified sebaceous glands which secrete earwax. Earwax prevents the entry of foreign material into the ear.

2. Middle ear

The middle ear is a small air-filled cavity. It is separated from the external ear by the eardrum (tympanic membrane) and from the internal ear by thin bony

                                                          

• The partition which contains two small openings namely, an oval window and a round window.
  The middle ear contains three tiny bones called ear ossicles. These bones are named malleus (or hammer), incus (or anvil), and stapes (or stirrup). The handle of the malleus is attached to the internal surface of the eardrum (tympanum). Its opposite end is connected with the incus. The incus is the intermediate bone and it articulates with the head of the stapes.
• The flat part of the stapes fits into the oval window. Directly below the oval window is another opening called a round window. The round window is covered by a thin membrane.

Ear ossicles (malleus, incus, and stapes) transmit sound vibrations to the internal ear.

• The anterior wall of the middle ear contains an opening that leads directly into the auditory tube (also known as the Eustachian tube). This connects the middle ear with the throat. The Eustachian tube also helps in equalizing air pressure on both sides of the eardrum helping it for free vibration.
•  Throat infection can lead to an ear infection.
  This is because the Eustachian tube connects the middle ear with the throat. Thus, any infection in the throat can pass to the ear through the Eustachian tube.

3. Internal ear

The internal or inner ear is also called the membranous labyrinth. It has two main parts – cochlea and semicircular canals.

                          

(a) Cochlea: It is a hollow, spiral-shaped (coiled), chamber. It resembles a snail’s shell. It consists of a bony spiral canal that makes about 23%4 turns around a central bony core. Its inner spiral cavity contains three separate channels or canals that run parallel. These canals are separated by membranes. The median (cochlear) canal is filled with endolymph and the outer two canals are filled with a fluid called perilymph.

• The middle canal contains a spiral organ called the organ of Corti. The organ of Corti is the organ of hearing. It contains a series of nerve cells and hair cells that join the auditory nerve and help in hearing.
• The hair cells of the organ of Corti are very sensitive to sound. They can be damaged by exposure to high-intensity noises such as noise produced by engines of jet planes, loud music, etc.(b) Semicircular canals: The inner ear also contains three semicircular canals. These canals are arranged at right angles to each other in three different planes (one horizontal, two vertical) and are filled with endolymph. One end of each canal is swollen to form an ampulla. The ampulla contains sensory cells which help in the balance of the body
• while moving. The nerve fibers arise from these cells and join the auditory nerve.
• There is a short stem (vestibule) that joins the semicircular canals and cochlea. It contains two small sacs – the utriculus and sacculus. These also contain tiny hair-like sensory cells which help in the static balance of the body while at rest (i.e. position of the head when not in motion like standing).
• The external ear, middle ear, and cochlea help in hearing. The sacculus, utriculus, and semicircular canals help in the sense of balance. A summary of various parts of the human ear and their functions is given in the table.

                                          

Sense Organs Mechanism Of Hearing And Balance

Hearing

Pinna collects and amplifies sound waves which then pass along the auditory canal to the eardrum.

• Sound waves strike the eardrum (tympanum) and cause vibrations in its thin stretched membrane. The Eustachian tube equalizes air pressure on either side of the eardrum which allows a free vibration.
• The vibrations reach ear ossicles in the middle ear. Ear ossicles transmit vibrations from the eardrum to the denser fluid in the inner ear.
• The lever-like action of malleus and incus magnifies the vibrations of the stapes. The vibrating stapes transmit vibrations to the membrane of the oval window.
• Vibrations from oval window get transmitted to cochlea. This leads to vibration in the fluid of cochlear canals. Vibration of fluid in cochlear canals triggers movement of sensory hair cells of organ of Corti in cochlea.
• Movement of sensory hair cells is converted to a nerve impulse.
• Nerve signal or impulse is transmitted to brain via auditory nerve and this results in hearing. Balancing

The following parts of ear are involved with balance.

1. Static balance with respect to center of gravity: Sensory cells in vestibule
2. Dynamic balance (while the body is in motion): Sensory cells in semi-circular canals

Movement of fluid inside semicircular canals triggers sensory hair cells of ampulla. This sensation passes to nerve cells and then to brain. The three canals are at right angles to each other so that the brain can detect even slightest tilting of the body in any direction.

Sense Organs Summary

Receptors are the special cells that receive stimuli from the environment. Some common receptors are photoreceptors, chemoreceptors, thermoreceptors, and mechanoreceptors.

• Receptors are contained in sense organs. There are five major sense organs in humans eyes (vision), ears (sound), nose (smell), tongue (taste), and skin (pressure and touch). Each of these sense organs is directly connected to the brain.
• Each eye is in the form of a ball called eyeball that can be rotated by six distinct sets of muscles.
• Eyebrows, eyelids, eyelashes, and tear glands are the accessory structures of the eye.
• Lacrimal glands are a group of glands that manufacture tears. Tears serve as a lubricant and also kill the germs that enter the eye.
• The eyeball has three layers – the sclerotic layer, the choroid, and the retina.
• The sclerotic layer is the outer tough coat of eyeball that is divided into sclera (white coat of dense fibrous tissue) and cornea (transparent fibrous coat through which the iris can be seen).
• The choroid layer is the middle layer of the eyeball and is composed of three parts – choroid, ciliary body, and the iris. There is a hole in the center of iris called the pupil.
• The retina is the third and inner coat of the eye. It contains light-sensitive cells called rods (sensitive to dim light) and cones (sensitive to bright light).
• The lens focuses light rays from outside and a real, inverted image is formed at yellow spot. The yellow spot is the area of best vision, while blind spot is the area of no vision.
• In myopia, the image is formed in front of retina.It can be corrected by using a concave lens.
•  Ear is concerned with two functions – hearing and body balance.
• Set eardrum into vibration. It sets malleus, incus and stapes (ear ossicles) into motion. From stapes, vibrations move to oval windows, then to cochlear canal and sensory hairs in cochlea. From here the nerve impulse reaches the brain through the auditory nerve

5. Sense Organs Structured/Application/Skill Type Questions

1. The diagram given below depicts a defect of the human eye. Study the same and then answer the following questions.

1. Name the defect shown in the diagram.
2. Give two possible reasons for this defect.
3. Name the parts labeled I to iv.
4. Name the type of lens used to correct this eye defect.
5. Draw a labeled diagram to show how the above-mentioned defect is rectified using the lens named
above

2. The following diagram represents a part of the human ear. Study the same and then answer the following questions.

• Give the biological terms for malleus, incus, and stapes.
• Name the parts labeled I, ii, and iii in the diagram.
• State the functions of the parts labeled I and ii.
• Name the audio receptor region present in the part labeled i.

3. The diagram given below represents the vertical section of the human eye.

• Name the parts labeled I to xii.
• What is the function of the part labeled x?
• What would happen if part v is damaged or cut?

4. The diagram of the human ear is given below. Study the same and then answer the questions that follow.

• What role does the eardrum play in hearing?
• What common term is given to the parts labeled i, ii, and v?
• Would there be any difference if these three parts mentioned in Q(2) above were replaced by one big one? Why?
• Give the biological terms for the parts labeled iii and iv.
• Name the fluid which fills the parts mentioned in Q(4) above.
• State the functions of the ear.

5. The following diagram represents a defect of vision of the human eye.

• Name the defect.
• What is the effect of this defect on man?
• Mention two causes for this defect.
• How can this defect be rectified?
• Draw a neat labeled diagram to show how this defect can be rectified.
• What is the nature of the image that falls on the retina of a normal eye?

6. The diagram given below refers to the vertical section of the eye of a mammal. Label the parts i to x to which the guidelines point.

7. The following diagram given below refers to the ear of mammals.

1. Label the parts i to x to which the guidelines point.
2. Name the structure which

• converts sound waves into mechanical vibrations.
• converts vibrations into nerve impulses.
• Responds to changes in position.
• Transmits impulses to the brain.
• Equalizes pressure in the ear.

8. The diagram given below represents the parts of the human ear.

• Name the parts numbered i to viii.
• What is the function of the parts labeled ii and vii?
• Why is it harmful to use a part or any sharp object to remove the wax from the ear?

9. The diagram given below depicts a defect of the human eye. Study the same and then answer the following questions:

1. Name the defect shown in the diagram.
2. What are the two possible reasons that cause this defect?
3. Name the type of lens used to correct this defect.
4. With the help of a diagram showing how the defect shown above is rectified using a suitable lens.

10. Draw a diagram of the human eye as seen in a vertical section and label the part which suits the following functions/descriptions:

• The layer which prevents reflection of light.
• The structure alters the focal length of the lens.
• The region of distinct vision.
• The part which transmits the impulse to the brain.
• The outermost transparent layer in front of the eye lens.
• The fluid present in the anterior part of the eye in front of the eye lens.
• The structure is responsible for holding the eye lens in its position.
• The structure maintains the shape of the eyeball and the area of no vision.

Heredity and Genetics Learning Objectives

After completing this chapter, you will be able to:

• Define heredity and variation;
• Understand Mendel’s contributions to genetics;
• Describe Mendel’s laws of inheritance;
• Explain the chromosomal basis of sex determination in humans:
• Describe the pattern of inheritance of sex-linked genes;
• Identify the characteristics that follow X and Y sex-linked inheritance;
• Give examples of sex-linked inheritance of diseases, namely, haemophilia and colour blindness.

In the earlier chapters, you have already studied about the structure of chromosomes and what happens to them during meiosis. You have also studied that genes are the carriers of heredity located on the chromosomes. In this chapter, you will study about the fundamentals of genetics, which includes Mendel’s laws of inheritance and sex-linked inheritance of diseases.

Genetics, heredity and variations

‘Like begets like’, which means young ones resemble their parents, is the well-known dogma associated with heredity. Heredity is the cause of similarities between individuals of a species.

Transmission of genetically based characteristics from parents to the successive generation is called heredity or inheritance.

Though offsprings resemble their parents, they are not identical. They usually differ among themselves and also from their parents. Even twins show resemblance in many respects, yet they show differences in certain features. Differences in the genetic characteristics or traits among individuals of a species are called variations.

The science that deals with the study of mechanisms responsible for similarities and differences among closely related species is called genetics.

W. Bateson was the first one to coin the term genetics in 1905. It is derived from the Greek word genesis meaning to grow into or to become. In other words, genetics is the study of heredity and variations. However, Mendel is known as the ‘Father of genetics.

Some important terms used in genetics

• Chromosomes: Filamentous bodies present in thenucleus of a cell, composed of chromatin material (DNA-protein complex).
• Variation: Different genetic characteristics or traits produced in individuals of the same species.
• Gene: It is the basic unit of inheritance. It is a segment of DNA found on the chromosome, which controls the expression of a character. It is passed from the parents to the offsprings.
• Homologous chromosomes: A pair of corresponding chromosomes of the same size and shape, one from each parent.
• Alleles: Alleles are alternating molecular forms of a gene or a pair of matching genes occupying same position on a chromosome, affecting the same characteristic but in two different ways.
• Dominant allele: A super ruling allele that masks any phenotypic effect of a recessive allele paired
  with it.
• Recessive allele: An allele that cannot express fully or partially in heterozygous condition, in presence of the other allele.
• Homozygous condition: A condition in which a pair of homologous chromosomes carries identical (similar) alleles of a gene for a particular character.
• Heterozygous condition: A condition in which a pair of chromosomes carries non-identical (dissimilar) alleles for a particular character (i.e. one dominant and one recessive).
• Character: Any heritable feature is called a character.
• Traits: The alternative forms of a character are called traits.
• Genotype: The genetic constitution of an organism.
• Phenotype: Externally visible expression of al gene, which is an inherited feature in an
  individual’s appearance. For example, free or attached earlobes.
• Mutation: A sudden change in one or more genes or in the number and structure of chromosome in the progeny which had not existed in the parents.

Inheritance of traits -mendel’s contributions

We have been using the principles of inheritance for thousands of years. But as a field of science, genetics did not begin until 1866, the year Mendel published the conclusions of his famous experiments on the common garden pea plant.

Gregor Mendel was born in 1822 in a peasant family He then became a monk in an Augustinian monastery at Brunn, Austria. He had great interest in plant breeding and hybridization experiments of different plant varieties. In order to understand the principle of inheritance of features from one generation to the another, Mendel conducted a series of hybridization experiments over a period of eight years on the common garden pea plant, Pisum sativum.

He was the first to systematically study and explain the mechanism of transmission of characteristics from the parents to the offsprings, generation after generation. Mendel published his work in the annual proceedings of the Natural History Society of Brunn. Mendel is considered as the Father of Genetics or Modern Genetics. He was the first to introduce the concept of genes as the basic unit of heredity. Mendel called genes as factors.

Selection of garden pea as the experimental plant was based on Mendel’s meticulous and careful observations about its unique features, which are as follows:
Such an organism should have the following features.
1. Shorter lifespan so that a large number of generations
can be studied and examined.
2. Presence of contrasting variants features.
3. Ease of rearing or cultivation.

Since almost all these features were present in the garden
pea plant, Mendel selected it for his experiment.

Experimental plant

Mendel conducted his experiments on Pisum sativum, the garden pea plant, for the following reasons:

• Pea plants have several distinct varieties. All the varieties have sharp contrasting characteristics, such as colour and shape of seeds.
• A pea plant bears bisexual flowers with each flower having both the male and female parts.
• The structure of the flower is such that it completely encloses the reproductive organs until fertilization, which ensures self-pollination.
• In pea plants, due to self-fertilization, it is easy to get pure lines for several generations.
• As it is an annual plant, it is possible to study several generations within a short span of time.
• The flowers of pea plants are adequate in size and easy to handle.
• Each plant can produce large number of seeds in a single generation.

Mendel selected seven pairs of contrasting characters which are listed in Table.

Heredity and Genetics Mendel’s experiments

Mendel conducted his experiments in three stages.

Stage 1

Mendel selected seven pairs of contrasting characters. He self-pollinated the pea plant for several generations. He observed that seeds from tall plants produce only tall plants, and those from plants with purple flowers produce plants that always have purple flowers. He could, thus, obtain pure line (plants that bred true for the selected characteristic).

Stage 2

He crossed two plants showing contrasting expressions of a single trait. These were reciprocal crosses (crosses with contrasting features), for example, he crossed a true-breeding tall stem variety plant (which he called tall plant) with a true-breeding short stem variety plant (dwarf plant). Such a cross between two parents representing contrasting forms of a single character was called monohybrid cross and the offspring was called a hybrid. In other crosses, he took two or more traits into consideration for his experiments. Thus, the crosses in which two traits were taken into consideration were designated as dihybrid crosses. The crosses in which three traits or four traits were taken into consideration were termed as tri-hybrid and tetra–hybrid crosses, respectively. He performed the experiment by transferring pollen grains from the anther of the tall plant to the stigma
of the dwarf plant. Self-pollination was prevented

by removing all the stamens from the dwarf plant. The plants of parental generation were designated as P₁. The seeds from the dwarf plant were then collected and sown.

He allowed these plants to self fertilize. He found that all plants that grew from these seeds were tall plants. The plants in this generation were called F₁ generation or first filial generation.

Stage 3

The plants of F, generation were allowed to self-pollinate and the seeds were collected. When these seeds were sown, few plants were tall and few were dwarf, in the ratio of 3:1 (tall: dwarf). These plants were called
second filial generation (or F₂ generation).

Heredity and Genetics Monohybrid cross

A cross between two parents representing contrasting forms of a single trait or feature is called monohybrid cross.

In a cross between round and wrinkled seeds, Mendel cross-pollinated flowers of plants raised on the respective type of seeds (round and wrinkled). Before making such crosses Mendel ensured that all the plants involved in the crosses are of pure line. For this purpose, seeds of only those plants were used in the parental generation, which produced the desired trait for at least six generations.

Example 1 Monohybrid cross between tall and dwarf plant

Cross-pollination between tall and dwarf plants

For a monohybrid cross between tall and dwarf plants, Mendel performed the experiment by transferring pollen grains from the flower (anther) of a dwarf plant (tt) to the stigma of the previously emasculated flower of a tall plant (TT). Seeds obtained in F₁ generation were carefully observed and the observations were recorded.

In this experiment, Mendel observed that all the plants of F₁ generation were tall.

Self-pollination of F, generation plants

Mendel further planted the F₁ seeds and flowers were again allowed to self-pollinate to produce the F₂ generation plants. In the F₂ generation, it was found that tall plants and short plants were obtained in the ratio of 3:1 (3 tall: 1 dwarf). So, 3:1 is the phenotypic ratio of the monohybrid cross. In other words, three-fourth plants were tall and one-fourth were dwarf in F₂ generation (Fig. 3.4). Thus, the dwarf plant trait that disappeared in first generation reappeared in the second generation.

Mendel called the expressed trait of tallness as the dominant trait and repressed trait of both tallness and dwarfness characters were inherited in F₁ generation plants, but only the tallness trait was expressed.

The offsprings inherit two copies of traits which may be identical or different depending on their percentage. A single copy of the trait (in this case T) is enough to make tall plant while both copies of the trait have to be ‘t’ for the plant to be short. Thus, the traits like ‘T’ (expressed trait) are dominant traits

while traits like ‘t’ (repressed trait) are recessive traits. In the F₂ generation, it was found that there is one plant with genotype “TT”, two plants with genotype “Tt’ and one plant with genotype ‘tt.

So, 1: 2: 1 is the genotypic ratio of the monohybrid cross in F₂ generation. The traits of
tallness or dwarfness are inherited separately and are not mixed together.

Example 2 A cross between plants with terminal flowers and axial flowers.

Monohybrid ratio in above cases is as follows: Phenotypic (visible feature) ratio = 3:1 (three axial, one terminal) Genotype ratio=1:2:1 (one AA, two Aa, one aa)

Dihybrid cross

A dihybrid cross is the one in which two varieties of pea plants having two contrasting characters are crossed to study the inheritance of two pairs of traits simultaneously.

For example, two plants differing in two characters like seed shape (round/wrinkled) and cotyledon colour (yellow/green) were crossed together.

Cross between yellow round and green wrinkled pea plants

Mendel selected a pure line variety of peas for yellow round seeds and another for green wrinkled seeds. He crossed these plants and observed that all F₁ generation seeds had the features of only one type (yellow coloured and round shape seeds). This showed that:

• Yellow colour of seed was dominant over green which is a recessive trait.
• Round shape of seed was dominant over wrinkled shape.

Self-pollination of F, generation plants

In the next step, Mendel self-pollinated these hybrids obtained in the F, generation. When these F₁ generation

seeds were cross-bred to raise the F₂ generation, the F₂ progeny showed four different kinds of phenotypes of seeds. It was observed that not only both the parental types (round seeds of yellow colour and wrinkled seeds of green colour) were present, but two new combination of traits (round seeds of green colour and wrinkled seeds of yellow colour) also appeared.

Thus, there were yellow round, yellow wrinkled, green round and green wrinkled seeds in the ratio of 9 : 3:3 : 1, respectively. Of these, two are of the parental P₁ types and two are new combinations or recombinants. The dihybrid ratio is, therefore, 9:3:3:1.

Interpretation of mendel’s observations

On the basis of the analysis of results of the monohybrid and dihybrid crosses, following conclusions can be drawn:

1. In a monohybrid cross, when a cross is made between the contrasting pair of a trait, only one of the traits appears in the F₁ generation.
2. The trait, which was not present in the offspring of a particular cross in F₁ generation, again reappears in the F₂ generation.
3. In a dihybrid cross, when combination of contrasting pairs of two traits were taken together, only one variety of each trait appears in the F₁ generation.
4. The other variety of each trait reappears in the F₂ generation on the same lines of the dihybrid cross.
5. However, two new combinations of the two contrasting pairs of traits in the F₂ generation also appear.

Heredity and Genetics Mendel’s laws of inheritance

Mendel postulated three laws of inheritance on the basis of his monohybrid and dihybrid experiments.

Law 1: Law of dominance

When two alleles of contrasting character of a hereditary trait are brought together by fertilization, only one is expressed, while the other is suppressed. The characteristic which is expressed, is called dominant or expressive characteristic, and the characteristic which is repressed (not expressed), is called recessive or suppressive characteristic. Recessive character expresses only when the pair consists of both (homozygous) recessive alleles.

This is Mendel’s first law of heredity law of dominance. It states that when two homozygous
individuals with one or more sets of contrasting characteristics are crossed, the characteristics which appear in the F₁ hybrids are dominant and those which do not appear in F₁ generation are recessive.

Law 2: Law of segregation

According to Mendel, each organism that reproduces sexually by producing gametes has two factors (genes) for a characteristic. Of these, one is inherited from the male parent and other from the female parent. These are alleles for a characteristic. If the alleles are similar, they are in homozygous condition. And, if the alleles are different, they are in heterozygous condition. All gametes produced by a homozygous individual will have similar alleles. Gametes of a heterozygous individual will be dissimilar (different/not alike).

On the basis of results obtained from monohybrid cross, Mendel formulated the second law of inheritance – law of segregation. It states that when a pair of allele is brought together in a hybrid, the members of the allelic pair remain together without mixing and separate or segregate from each other when the hybrid forms gametes. Since each gamete is pure for a characteristic, the law is also known as law of purity of gametes.

Law 3: Law of independent assortment

It states that, when a dihybrid organism forms gametes,

• Each gamete receives one allele from each allelic pair (or each characteristic), and
• The assortment of alleles of different characteristics during gamete formation is independent of their parental combinations.

Transmission of traits

All living organisms produce their own kind. So, there must be some common thing that makes an offspring similar to its parents. It is called trait or character that is passed from the parents to the offspring during sexual reproduction. These traits are transferred through genes located on their chromosomes.

What are genes?

A segment of DNA on a chromosome which codes for the synthesis of a specific protein is called gene. Genes are the units of heredity. They are located in a linear fashion on chromosomes. Thus, chromosomes are the carriers of genes. These are located within the cell nucleus.

In normal condition, each gene has two alternative forms of a character producing different effects. These alternative forms are called alleles. In humans, all chromosomes are present in pairs within

the cell nucleus. Therefore, all genes are also in pairs, i.e. each character has two alleles, each present on one chromosome of the pair. Two alleles of a pair are always located at the same position on the chromosome pair. The position is called a gene locus. Both the parents contribute equally to the DNA of progeny during sexual reproduction. That means, both parents contribute a copy of the same gene. During meiosis, a gamete receives only one chromosome of a pair. Hence, each gamete (germ cell) has only one allele of the pair of alleles for a character. When two germ cells combine, they will restore the normal number of chromosome in the progeny and ensure the stability of the DNA of the species. The characters thus pass from the parents to the offsprings in the form of genes on chromosomes. They are transmitted physically. Thus genes are the physical basis of heredity.

Dominant and recessive alleles

Out of the two alleles of a gene, the allele that masks any phenotypic effect of any recessive allele is called the dominant allele, while the allele that is masked is called the recessive allele. For example, the allele controlling the rolling of tongue is the dominant allele and the allele which controls the condition in which tongue cannot be rolled is the recessive allele.

Genotype and phenotype

The genetic constitution of an organism in which the genes are present in various combinations is called genotype. On the other hand, the externally visible expression of genes, which is an inherited feature in an individual’s appearance is called phenotype. For every phenotype, there may be two conditions, homozygous and heterozygous.

Homozygous condition: A condition in which a pair of a homologous chromosomes carries identical alleles on a gene locus for a specific trait.

Heterozygous condition: A condition in which a pair of homologous chromosomes carries non-identical alleles on a gene locus for a specific trait.

Pedigree analysis – From parents to children

In the pedigree analysis, a family’s history for a particular trait is collected. Then, this information is assembled into a family tree describing the interrelationship
of parents and children across generations. Thus, a pedigree is a family tree or a chart describing the inheritance of a particular character across generations. A pedigree analysis of a recessive trait (attached earlobes) is described in Figure 3.7.

In this case, the first born daughter in the third generation has attached earlobes, although her parents lack that trait (they had free earlobes). Thus, thepresence of attached earlobe phenotype is due to a recessive allele.

Sex determination in human beings – son or daughter?

Determining the sex of an individual at prenatal (before birth or during pregnancy) stage is called sex determination. In a number of organisms, one specific pair of chromosome plays a significant role in the determination of sex of the organisms. These chromosomes are named as sex chromosomes. In human beings, there are 23 pairs of chromosomes, out of which one pair is sex

chromosome. There are two types of sex chromosomes- X and Y. A female contains two X chromosomes (i.e. homomorphic), while a male contains one each, i.e. X and Y chromosomes (i.e. heteromorphic). Rest of the 22 pairs of chromosomes are exactly similar and are called the autosomes.

At the time of reproduction, during gamete formation these paired chromosomes separate. Thus, two types of gametes are formed in males (one containing X chromosome while the other containing Y chromosome). Half of the male gametes or sperms carry the X chromosomes and rest half have Y one X chromosome. 22 pairs of autosomes are equally distributed in both sperm and ovum, and hence they are similar in terms of autosomes. Thus, the female has 44 + XX chromosomes and male has 44+ XY chromosomes.

How is it determined if the child would be male or female?

• The sex of the offspring will be determined by the type of chromosome (X or Y) inherited from father.
• At the time of fertilization, when the sperm and the egg unite to form a zygote, each individual inherits one of the two possible combinations of
• A zygote (XX) with two X chromosomes (one from father and one from mother) develops into a girl while a zygote, (XY) with one X chromosome (from mother) and one Y chromosome (from father) develops into a boy.

Sex-Linked Inheritance

Most genes of an organism are located on autosomes. Genes of sexual characters are located on the sex chromosomes. However, genes of some characteristics which are not related to the sex of the organism are also located on the sex chromosomes. These are sex-linked characters because their inheritance is sex-linked. Inheritance of non-sexual characters or traits due to the presence of an allele on sex chromosomes is called sex-linked inheritance. These characteristics may be present either on X chromosome or Y chromosome or both.

X-linked inheritance

X-linked genes are present on that portion of the X chromosome for which there is no homologous region on the Y chromosome. Examples of inheritance of traits determined by X-linked genes include colour blindness and haemophilia. These diseases are caused by recessive alleles located on the X chromosome. Mostly males suffer from these disorders and females are rarely affected. A person suffering from colour blindness is unable to distinguish between red and green colours.

Haemophilia is a sex linked recessive disorder that slows down the process of blood clotting in an affected individual. It results in prolonged bleeding following an injury. It is rare to have haemophilia in females but they act as the carrier and may transfer the mutated
gene their sons.

Inheritance of colour blindness (or haemophilia)

Let us understand how colour blindness is an X-linked characteristic. X chromosomes having the two alternative alleles of the colour blind characteristic are indicated as:

Alleles
X-normal (dominant allele)
Xº – colour blind allele (recessive allele)
Possible genotypes of females
XX- normal female
XX° – carrier female (does not show
symptoms of the disease)
X’X’ – colour blind female
Possible genotypes of males
XY – normal male
X’Y – colour blind male
Now let us take two situations.
Situation 1: A cross between carrier female vs normal male

Parents
Carrier mother Normal father
XX°
XY
Punnet square
Sperms from
normal father
X
XX
XXX
X Y
XY

50% of the progeny are normal. One daughter (25% of the progeny) is the carrier of the defective allele. One son (25% of the progeny) is colour blind.

Situation 2: A cross between normal female and colour-blind male

Normal mother
XX
Colour blind father
Xº Y
Sperms from
colour blind father
X Y
XXX° XY
XXX° XY

XX° – Daughters-heterozygous dominant, carrier
of colour blindness but with normal vision.
XY – Normal sons

None of the child colour l but daughters are carriers of the defective alleles for colour blindness.

 From the given crosses, it can be concluded that,

• an X-linked recessive gene affects more males than females because males need only one
  copy of the defective allele to express the characteristic.
• an X-linked recessive characteristic can skip generations because males can receive an X-linked recessive only from their mothers and very few females are homozygous for the gene.

What would happen if the X-linked genes were dominant?

The pattern of inheritance of an X-linked domainant gene is different from the X-linked recessive one. In this case, both males and females are equally affected by the presence of the gene. Irrespective of sex, half the male and female offsprings are affected by the gene or trait. Inheritance of defective tooth enamel is an example of X-linked dominant inheritance.

Y-linked gene

The gene for hairy pinna is an example of Y-linked inheritance. Will it be expressed if it were in the recessive form? Of course, yes, the gene shall be expressed irrespective of it being dominant or recessive. Can you guess the reason? Y chromosome is present singly in the male genotype. Will females be carriers of this gene? The obvious answer is no as the human female genotype does not have a Y chromosome. The characteristics of a Y-linked trait are as follows.

• They are expressed only in males.
• They are always passed from father to son.

Heredity and Genetics Mutation

A sudden change in the amount, arrangement or structure of the DNA or chromosomes of an organism is called mutation. The mutation alters the hereditary material of an organism’s cell and may or may not be inherited cells. A mutation may results in a change in the genetic appearance of a characteristic in a population.

• Mutations occurring in gamete cells are inherited while those occurring in somatic cells (called somatic mutations) are usually not inherited.
• A mutation resulting from a change in the amount or arrangement of the DNA is known as chromosomal mutation.
• A mutation resulting from a change in the structure of gene in DNA is known as gene mutation.

Causes of mutation

Mutation can be caused by a variety of chemical substances such as mustard gas, caffeine, gamma rays and X-rays and sometimes even certain pesticides.

Sickle cell anaemia is an example of disease caused due to mutation. It is a blood disorder characterized by red blood cells that assume an abnormal, sickle shape due to the sudden change in DNA structure.

Genetic engineering

A technique in which the genetic constitution of an organism can be altered by introducing new genes or replacing existing genes into its chromosomes is known as genetic engineering. For example, insulin-producing genes are introduced in a bacteria to produce insulin.

Genetic counselling

This is a counselling of the parents/newly married couples in which they are advised to get them screened rany genetic diseases such as haemophilia, thalassemia, Down syndrome, etc.

Heredity and Genetics Summary

• The science that deals with the mechanisms responsible for similarities and differences among closely related members of the species is called genetics.
• Gregor Mendel is considered as the father of genetics. He postulated three laws of heredity on the basis of his experiments on the sweet pea plant.
• Mendel’s first law of inheritance: Law of dominance states that when two homozygous individuals with one or more sets of contrasting characteristics are crossed, the characteristics which appear in F, hybrids are dominant.
• Mendel’s second law of inheritance: Law of segregation states that when a pair of alleles is brought together in a hybrid, the members of the allelic pair remain together without mixing and separate or segregate from each other when the hybrid forms gametes.
• Mendel’s third law of inheritance: Law of independent
• assortment states that when a dihybrid organism forms gametes, each gamete receives one allele
  from each allelic pair (of each characteristic), and the assortment of alleles of different characteristics during gamete formation is independent of their parental combinations.
• Pedigree is a family tree or a chart describing the inheritance of a particular characteristic across generations.
• Sex determination in humans is based on combination of sex chromosomes. Females have two X chromosomes while males have one X and one Y chromosome.
• Inheritance of non-sexual characteristics through sex chromosomes is called sex-linked inheritance. It may be either X-linked recessive/dominant or Y-linked.
• Colour blindness and haemophilia are examples of X-linked inheritance mostly in human males.

Endocrine System Learning objectives

After completing this chapter, you will be able to:

• Discuss the role of endocrine system in maintaining homeostasis;ZZ
• Define the term hormone;
• Describe the location of the major endocrine glands in the human body,
• List the major hormones secreted by the following endocrine glands and discuss their functions:
• Adrenal gland
• pancreas
• Thyroid gland
• pituitary glan
• Explain the effects of hyposecretion and hypersecretion of major hormones on the body;
• Describe the mechanism of hormonal action and regulation or the feedback mechanism.
• All organs, tissues, and cells in our body work in coordination. There are two regulatory systems involved in control and coordination of our body, namely, nervous system and endocrine system.
• In the previous chapters, you have read about nervous system and the functions performed by it. There is one more system involved in coordinating

various body activities. This is known as the endocrine system. The endocrine system affects body activities by releasing chemical messengers, called hormones, into the bloodstream. The endocrine system consists of many glands; however, in this chapter, we will learn about adrenal, pancreas, thyroid, and pituitary glands only.

Endocrine System Adjusting to Changes – Homeostasis

• The activities in our body occur in a coordinated manner and have to be regulated at a proper time. If any step in the coordination misses out, then our body is severely affected.
• Let us consider an example to understand this. We tend to sweat a lot on a hot day. Evaporation of the  sweat cools down our body and helps to cope with the heat. We feel thirsty and drink more fluids on a hot day. Thus, our internal system adjusts itself to cope with the external stress and attempts to maintain a steady state of functioning.

The capacity of an organism to adjust itself to cope with the external changes and maintain a steady state of functioning is called homeostasis.

In our body, homeostasis is achieved by both the nervous and the endocrine systems, but in different ways, as given in

What are Hormones?

• Hormone means to set in motion or to spur on. Hormones are chemical secretions secreted by specific endocrine glands, which are carried by blood to the target organs elsewhere in the body to stimulate a specific activity. Thus, hormones are special chemicals that regulate the physiological or biochemical processes in our body.

Bayliss and Starling, two British scientists, coined the term hormone in 1905 while working on hormonal secretions.

Characteristics of Hormones

• Hormones are regulatory chemicals that are secreted by endocrine glands directly into the blood.
• Some hormones are chemically made of proteins or polypeptides (e.g. insulin) which are water soluble;

Endocrine and Exocrine Glands

Hormones are secreted by endocrine glands (Gk. endo: within or inside; crin: secretion). These are also called ductless glands because they do not have their own ducts, and their secretions are directly released into the tissue space next to them from where these are carried by the blood to the target organ. On the other hand, an exocrine gland secretes material directly into the duct. They have their own ducts which carry the secretions directly to the target organs.

 

a. An endocrine gland secretes hormones into the extracellular fluld.

b. An exocrine gland secretes material into a duct.

some are amines (e.g. adrenaline and noradrenaline) which are also water-soluble, and some are steroids which are derived from cholesterol, and other steroids (e.g. testosterone and progesterone) which are lipid-soluble.

• Hormones are produced in very minute quantities.
• They are biologically very active but are slower than nervous control.
• They act only on target organs or cells located away from their sources. Thus, they are produced by one organ but act on some other organ.
• They regulate the physiological process of the body by bringing about chemical changes. Thus, they bring about metabolic regulation in the body.
• They are destroyed soon after their action, and thus, they are not stored in the body.
• Steroid hormones They are steroids In nature. They diffuse through the cell membrane and bind to Internal receptors. Examples: Oestrogen, progesterone, androgens, cortisol, aldosterone, thyroid hormones, etc.
• Peptide or non-steroidal hormones These are amino acid derivatives, peptides, or proteins. They bind to receptors located on the surface of target cell. Examples: Glucagons, ADH, oxytocin, Insulin, somatotropin, prolactin, FSH, LH, TSH, etc.

Endocrine Glands

Major endocrine glands in our body are as follows:

1. Adrenal glands
2. Thyroid gland
3. Pancreas (pancreatic islets clusters)
4. Pituitary gland (anterior and posterior)
5. Parathyroid glands (four in humans)
6. Thymus gland
7. Pineal gland
8. Gonads

In addition, endocrine cells of stomach, duodenum, liver, kidney, placenta, etc. also secrete hormones.

In this chapter, we will study about thyroid, adrenal, pancreas, and pituitary glands only. Location of various endocrine glands and their secretions are given in

Adrenal Glands

In our body, two adrenal glands are present, one on top of each kidney; hence, they are also called suprarenal glands. Each adrenal gland has following two parts:

1. Adrenal cortex (on the outer periphery)
2. Adrenal medulla (inside central part)

 

a. Kidney

b.Cross section of adrenal gland

Adrenal cortex

Adrenal cortex secretes two main hormones – glucocorticoids and mineralocorticoids.

Endocrine System Glucocorticoids

Glucocorticoids are group of hormones such as cortisol, corticosterone, and cortisone. Of the three, cortisol is the major hormone. Glucocorticoids,

• Regulate the metabolism of proteins, fats (lipids), and carbohydrates in the body;
• Regulate the blood sugar level and ensure energy supply to the body;
• Adapt the body to external stress such as severe heat or cold, infections and burns, etc.

Apart from these, certain cortical hormones act as sex hormones causing premature sexual maturity in children.

• Hyposecretion of glucocorticoids causes Addison’s disease. person suffers from mental lethargy, nausea and vomiting, weight loss, and muscular weakness. Hypersecretion of glucocorticoids causes Cushing’s syndrome. In this condition, person becomes obese, has high blood sugar levels, osteoporosis, weakness, and salt and water retention.

Mineralocorticoids (Aldosterone)

Aldosterone is a major mineralocorticoid secreted by adrenal cortex. Aldosterone,

• Controls mineral metabolism by reabsorption of sodium in urinary tubules and maintains Na* and K* ratio in the extracellular and intracellular fluids;
• Regulates salt-water balance in the body.

Hypersecretion of aldosterone causes  increased sodium and decreased potassium concentration in the blood, leading to hypertension (high blood pressure).

Adrenal medulla

• Adrenal medulla secretes two major hormones – adrenalin (also known as epinephrine) and nor-adrenalin (norepinephrine). Adrenalin accounts for almost 80 percent of the total secretion of the adrenal medulla.
• Both adrenalin and noradrenalin together control emotions, fear, anger, blood pressure, heartbeat, respiration, and relaxation of smooth muscles. Adrenalin is also known as an emergency hormone as it prepares the body for fight-or-flight situations.
• Adrenalin increases heartbeat and blood supply to muscles and decreases blood supply to visceral organs. We have high levels of adrenalin while playing or running. Effect of adrenalin on some organs of our body is given in

Thyroid gland

The thyroid gland is a large endocrine gland located in front of the neck region just below the larynx. It has two lateral lobes, one on either side of the trachea. The two lobes are connected by a narrow mass of tissue called isthmus. The thyroid gland has a rich blood supply. Thus, thyroid gland can deliver large amounts of hormones in a short period of time, if necessary. Four small round parathyroid glands are embedded in the posterior surface of the thyroid gland.

 

 

a. Anterior suface

b. posterior surface

The thyroid gland secretes two hormones thyroxine and calcitonin.

a. Thyroxine

• It regulates basal metabolism by stimulating rate of cellular oxidation, resulting in energy production and maintenance of body temperature.
• It regulates general growth of the body, ossification of bones, and mental development.
• It regulates activities of the nervous system. Undersecretion (hyposecretion) as well as
  oversecretion (hypersecretion) of thyroxine result in an abnormal growth of the body.

Undersecretion of thyroxine (Hypothyroidism)

Undersecretion of thyroxine may cause,

1. Simple goiter: In this condition, the thyroid. gland of adults enlarges and becomes visible as a swelling in the neck. Undersecretion of thyroxine or insufficient amount of iodine in diet may cause simple goiter.

Simple goiter is common in people living in hilly areas because the soil of hilly areas is deficient in iodine. Thus, the food crops grown there have less iodine content.

2. Cretinism (in children): This is caused due to defective development or early atrophy
(degeneration) of thyroid gland. This condition is observed in children. Children suffering from cretinism have stunted growth (dwarfism), short club-like fingers, and deformed bones and teeth. Their abdomen becomes pot-bellied, and skin becomes rough, dry with scanty hair growth. Mental retardation f various degrees is also observed.

3. Myxoedema (in adults): The hypothyroidism in adults causes myxoedema. In this condition, facial tissues swell and look puffy. Other symptoms include slow heart rate, low body temperature, sensitivity to cold, dry hair and skin, muscular weakness and general lethargy.

Oversecretion of thyroxine (hyperthyroidism)

Oversecretion of thyroxine may cause exophthalmic goitre. A person suffering from this disorder shows increased metabolic rate, rapid heartbeat, protruding eyes and short breathing rate.

b. Calcitonin (Not included in the syllabus)

Calcitonin is another hormone secreted by thyroid gland.

• It regulates calcium and phosphate levels in the blood.
• It facilitates absorption of calcium released by bones.

Endocrine System Pancreas

Pancreas is a compound gland located posterior to the stomach and attached to the duodenal loop in the abdominal region. It secretes both enzymes (digestive juices) as well as hormones. It has two parts –

1. An exocrine (duct) part, which produces digestive juices, and
2. An endocrine (ductless) part, which secretes hormones.

Its endocrine part contains hormone-secreting cells called islets of Langerhans, which are scattered in the entire gland (islets: little islands). The islets of Langerhans in pancreas contain alpha, beta and delta cells that secrete glucagon, insulin and somatostatin hormones, respectively.

Insulin

Insulin is secreted by beta cells of islets of Langerhans.

• It regulates blood sugar level by regulating conversion of glucose into glycogen. Whenever there is an increase in blood glucose, insulin is secreted which induces absorption of glucose through cells. This glucose is burnt or stored as glycogen. This reduces the blood glucose level.
• It stimulates deposition of extra glucose as glycogen in the liver and muscles.
• Undersecretion (hyposecretion) of insulin causes diabetes mellitus or hyperglycemia. A person suffering from diabetes mellitus has high concentration of glucose in the blood and urine. The sufferer feels thirsty because of loss of water through excessive urination and becomes weak.
• Oversecretion (hypersecretion) of insulin causes insulin shock or hypoglycemia. In this condition, sugar level in bloodis severely reduced and in adverse condition brain may enter into a state of coma. The person may also becom onscious at frequent intervals.

Why is Insulin Injected and Not Taken Orally?

When required, insulin is injected into the body, and not taken orally because if taken orally, it will be digested by the protein-digesting enzymes in the digestive tract.

Glucagon

Glucagon is secreted by alpha cells of islets of Langerhans.

• It stimulates glycogen breakdown in the liver and converts some carbohydrates back to glucose.
• It increases sugar level in the blood.

Endocrine System Pituitary – the Master Gland

• The pituitary is a small gland about 1 cm in diameter (about size of a pea). It lies just below the hypothalamus (the midbrain) connected to it by a stalk-like structure called hypophyseal stalk.
• It is popularly known as master gland because it controls the functioning of all other endocrine glands. Since most hormones secreted by pituitary stimulate other glands to produce their hormones, they are called tropic hormones. Thus tropic

• hormones are those hormones that stimulate other endocrine glands to secrete their own specific hormones. For example, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are also called gonadotropic hormones because they regulate functioning of gonads (ovaries and testes) to produce sex hormones.
• Except for human growth hormone, melanocyte-stimulating hormone (MSH), and prolactin (PRL), all the other hormones are tropic hormones.

The pituitary is mainly divided into three lobes.

1. The anterior lobe
2. The posterior lobe
3. The intermediate lobe (almost absent in humans)

Hormones of the anterior pituitary

The anterior pituitary releases at least six different hormones some of which are as follows.

1. Growth hormone (GH) or somatotropic hormone (STH) or somatotropin

• It controls the overall development or growth of the body, muscles and bones.
• It also stimulates fat metabolism.
• It increases the rate of protein synthesis.
• Deficiency (hyposecretion) of GH in the childhood causes dwarfism. In this condition, the muscles and skeleton do not grow much and the body proportions are like a child’s.
• Oversecretion (hypersecretion) of GH in the childhood causes gigantism. In this condition, the child has very tall skeleton and proportionately large muscles The oversecretion of GH in an adult leads to overgrowth of the bones and jaw bone and bowing of the spine. This condition is called acromegaly.

 

a. Gigantism

b. Dwarfism

Thyroid-stimulating hormone (TSH)

• It controls the growth and functioning of the thyroid gland.
• It stimulates the thyroid gland to produce thyroxine.

Adrenocorticotropic hormone (ACTH)

• It regulates the activity of adrenal cortex.

Gonad stimulating (Gonadotropic) hormones

a. Follicle stimulating hormone (FSH)

• In males, it stimulates the process of spermatogenesis.
• In females, it stimulates the follicle cells in the ovaries to develop into mature eggs and also stimulates them to produce estrogen.

b. Luteinizing hormone (LH)

• In males, it stimulates the secretion of testosterone, which in turn influences the appearance of secondary sexual characteristics.
• In females, it stimulates the secretion of estrogen and progesterone, which in turn influence the process of ovulation, formation, and maintenance of corpus luteum and appearance of secondary sexual characteristics.

Tropic hormone

A hormone that stimulates other endocrine glands to produce/secrete their specific hormone.

Prolactin hormone (PRL)

• In females, it enhances mammary gland development and milk production.
• In males, it enhances the production of testosterone.

Hormones of the posterior pituitary

• The posterior pituitary stores two hormones, antidiuretic hormone (ADH) also called vasopressin, and oxytocin. Both these hormones are produced and released by hypothalamus (and not by the pituitary as given in most books). These hormones are transported to posterior pituitary and stored there.

Antidiuretic hormone (ADH) or vasopressin

• It promotes reabsorption of water from the kidney tubules. Thus, it causes the kidney to form more and more concentrated urine.
• It constricts blood vessels with the rise in blood pressure.
• The deficiency of ADH causes diabetes insipidus. In this disease, a person urinates more frequently and a large amount of urine is produced each time. This results in the loss of water from the body and the person feels thirsty. In diabetes insipidus, the urine does not contain any sugar.

Oxytocin

• It stimulates vigorous contraction of the uterus during labor, leading to the childbirth.
• It also causes the release of milk from the breast of a nursing mother.

Hormones of intermediate lobe of pituitary

It secretes melanocyte-stimulating hormone (MSH), that stimulates skin to secrete melanocytes (melanin pigment).

Summary of major hormones secreted in human body, their source glands and principal functions are given in Table

Endocrine System Feedback Mechanism of Hormone Secretion

As we know, hormones are required for various functions in our body. To perform these functions, a

• particular hormone may be required in a particular amount at a particular time, for which a control system is required. A control system should have a feedback mechanism to prevent excessive reaction or over-reaction. For example, the hypothalamus produces thyrotropin-releasing hormone (TRH), which in turn stimulates the anterior pituitary to produce thyroid-stimulating hormone (TSH). The TSH activates the thyroid gland to
• secrete thyroxine. If the level of thyroxine in the blood is less than normal, it has a positive feedback effect on the hypothalamus and pituitary to produce more TRH and TSH, respectively. If the level of thyroxine is more than normal in the blood, a negative feedback effect is seen on the hypothalamus and the pituitary so that they produce less of TRH and TSH, respectively. Positive feedback systems are relatively rare in higher vertebrates as they lead to instability.
• Thyroid-stimulating hormone (TSH) and thyroxine regulate each other’s level just like the requirement-supply kind of situation. Such a system of opposing effects leads to proper control and balance in a system. The two opposing systems work in coordination and help the body to adjust its output accordingly. The ultimate. effect of such a feedback system is to maintain homeostasis.

Endocrine System Summary

• Hormones are chemical messengers secreted by endocrine glands and carried by blood or lymph to a target organ elsewhere in the body to stimulate a specific physiological change.
• Hormones (1) are secreted in minute quantity, (2) are specific chemical messengers, (3) regulate physiological processes by chemical means, (4) are secreted by ductless (endocrine) glands, (5) are poured directly into the bloodstream, and (6) their action is very rapid and they act on a specific target, away from the source.
• Adrenal glands are also called suprarenal glands. Each
• adrenal gland has an outer region called cortex and an inner region called medulla.
• Adrenal cortex secretes glucocorticoids and mineralocorticoids.
• Glucocorticoids regulate metabolism of proteins, fats and carbohydrates in the body and regulate blood
• pressure and heartbeat rate and are called emergency hormones.
• Mineralocorticoids (aldosterone) control reabsorption of sodium in kidney tubules.
• Adrenal medulla secretes adrenalin and noradrenalin hormones. Both these hormones together control emotions, fear, anger, blood
• Thyroid gland is situated in the neck region. It secretes two hormones – thyroxine and calcitonin.
• Thyroxine stimulates rate of cellular oxidation and basal metabolism.
• Calcitonin regulates calcium and phosphate levels in the blood.
• Pancreas produces Insulin from beta cells and glucagon from alpha cells of islets of Langerhans.
• Insulin regulates conversion of glucose to glycogen.
•  Hyposecretion of insulin causes diabetes mellitus.
• Glucagon regulates the conversion of glycogen and some non-carbohydrates back to glucose.
• Pituitary gland has three lobes – anterior lobe, posterior lobe, and intermediate lobe.
• Anterior pituitary secretes six main hormones –
• Growth hormone (GH) controls the overall growth of the body. Its hyposecretion causes dwarfism and hypersecretion causes gigantism in children.
• Adrenocorticotropic hormone (ACTH) controls the growth and function of the adrenal cortex.
• Thyroid-stimulating hormone (TSH) controls the growth and function of the thyroid gland. It stimulates thyroid gland to produce thyroxine.
•  Follicle-stimulating hormone (FSH) stimulates process of spermatogenesis in males and ovulation in females.
• Luteinizing hormone (LH) stimulates secretion of testosterone in males and estrogen and progesterone in females.
• Prolactin (PRL) causes mammary gland development and milk production in females.
• Posterior pituitary stores two hormones – antidiuretic hormone (ADH) and oxytocin.
• ADH controls reabsorption of water in the kidney tubules. Its deficiency causes diabetes insipidus.
• Oxytocin controls contraction of uterine muscles at the time of childbirth. It also helps in milk ejection from mammary glands.
•  Hormone production is regulated by a feedback mechanism.

Reproductive System  Learning Objectives

After completing this chapter, you will be able to:

• Define reproduction and differentiate between asexual and sexual reproduction;
• Illustrate male and female reproductive systems in humans and state the functions of each part;
• Draw labelled diagrams of male and female reproductive systems;
• Describe the histology of human testis and ovary;
• Describe the main events in the process of reproduction In humans, starting from the production of gametes to pregnancy and childbirth;
• Describe the process of exchange of nutrients and respiratory gases between an embryo and Its mother,
• Explain how twins are produced.

Reproduction is a process by which a living organism is able to produce more of its own
kind. Reproduction involves the transmission of genetic material from the parents to the children, thereby ensuring that characteristics not only of the species but also of the parents, are perpetuated. Reproduction ensures continuity of life and survival of a species on the earth. In this chapter, you will learn about the organs for reproduction, fertilization, menstruation, and development of embryo in human beings.

Types of reproduction

There are two types of reproduction in living organismis – asexual and sexual reproduction.
Asexual reproduction involves the production of offspring from a single organism without the fusion of gametes. It is a common process of reproduction in lower group of plants and lower group of animals.
Sexual reproduction is a type of reproduction in which both sexes, the male and the female, are generally involved. It is the production of offspring by the fusion of the genetic material contained in the sex cells or gametes. As a result of fertilization, the male and the female gametes unite to form a zygote, which develops into an organism. Most animals and higher group of plants multiply by sexual reproduction.

Sexual Reproduction in Humans

Humans reproduce sexually. The reproduction in humans can be studied in two parts –

• reproductive system in human beings, and
• fertilization, pregnancy and development of the embryo.

Reproductive System in Human Beings

The reproductive organs in human beings become functional after attaining sexual maturity. The period during adolescence in which the body attains sexual maturity is called puberty. Girls (females) attain puberty earlier than boys (males). In males, sexual maturity is attained at an age of 13-14 years. In females, onset of menstruation takes place around the age of 13 years. This age is known as the age of puberty. During sexual maturity, hormonal changes take place in males and females, and under the influence of these hormones, secondary sexual characteristics are developed.

Secondary sexual characteristics in males develop under the influence of testosterone, a hormone secreted by testes in males. These characteristics Include deepening of voice, widening of shoulders, appearance of beard and moustache, growth of axillary and pubic hair, enlargement of external genital organs and formation of sperms.

Secondary sexual characteristics in females include growth of axillary and pubic hair, widening of pelvis and hip, enlargement of breasts and onset of the menstrual cycle.

Primary reproductive parts and accessory reproductive parts

Primary reproductive parts include gonads (testes in males and ovaries in females). These are the sites of gamete production – sperms in testes and eggs in ovaries.

Accessory reproductive parts include all other structures that help in transfer of sex cells (gametes) and their fusion leading to fertilization and also growth and development of zygote till the birth of the baby.

Male Reproductive System

The male reproductive system consists of the following organs – a pair of testes, a pair of epididymis, a pair of sperm duct (vasa deferentia; singular: vas deferens), urethra, penis and accessory glands.

Testes

• Testes (singular: testis) are the male gonads. A pair of testes are present in a human male.
• These are present in a thin pouch made up of skin and connective tissue called scrotal sac or scrotum. Thus, testes are extra-abdominal. The scrotum is divided into right and left compartments by a muscular septum. One testis lies in each compartment.

• In the embryonic stage, the testes are contained within abdomen. They descend down just before birth.
• The high temperature of body does not allow maturation of sperms. Thus, the scrotum acts as thermoregulator, and it helps in maintaining the temperature at about 2-3°C lower than the body temperature. This temperature is suitable for the development of sperms.

Reproductive System Structure of testes

• Each testis is encased in a capsule of white fibrous connective tissue called tunica albuginea. This tissue extends internally as septa, dividing the testis into 15-20 lobules.
• Each testicular lobule has several highly coiled tubules called seminiferous tubules which are the sites of the maturation of spermatozoa. The process of maturation and differentiation of sperm inside the testes called spermatogenesis.
• Between the seminiferous tubules, clumps of interstitial cells (packing tissue), also called Leydig cells are present. These cells secrete the male sex hormone, testosterone. This hormone regulates the maintenance of primary and secondary sexual characteristics in males.

Epididymis

From each testis arises a network of ducts called efferent ducts. These open into a common tube-like structure. This single, 6 m long, highly coiled tube where sperms are stored, get concentrated and become physiologically (mature) active, is called epididymis. It remains attached to the testis and lies within the scrotal sac.

Vas deferens (Sperm ducts)

• Each epididymis continues from its lower end as a vas deferens. It enters the abdominal cavity through the inguinal canal, passes over the urinary bladder and joins the duct of seminal vesicle to form the ejaculatory duct. The ejaculatory duct opens into the urethra.
• The inguinal canal allows the descent of the testes along with their ducts, nerves and blood vessels from the abdomen to the scrotal sacs just before birth. Sometimes, because of high pressure in the abdomen, the intestine bulges into the scrotum through this inguinal canal and causes inguinal hernia.

Urethra

• The male urethra is about 15-20 cm in length and is differentiated into three parts – an anterior prostatic
• part which passes through the prostate gland, a middle membranous part, and a posterior penile part which passes through the copulatory organ, the penis.
• The male urethra functions as a passage for both semen and urine. The sperm ducts from each testis open into the urethra near its anterior end.

Penis

• Penis is the copulatory organ in males. It is a cylindrical, spongy and a highly vascular organ. It consists of erectile tissue permeated by blood sinuses. The urethra runs through it centrally and serves as a common passage for the exit of urine and semen. It is used to deposit spermatozoa in the female genital tract.
• During sexual excitement, the spongy tissue gets filled up with blood, as the arteries in it dilate. This condition is known as erection. Externally, the penis is covered by skin. The tip of the penis is soft and highly sensitive. It is called glans penis. It is covered by a loose retractile fold of skin called prepuce.

Accessory glands

There are three accessory glands in males. These include seminal vesicles, prostate gland and Cowper’s glands.

1. Seminal vesicles: A pair of seminal vesicles are present at the base of the urinary bladder. The function of the seminal vesicles is to secrete seminal fluid that acts as a medium of transport for the spermatozoa through the sperm duct. The seminal fluid is an alkaline viscous fluid which contains fructose, which provides energy to the sperms. This secretion forms about 60 per cent of the ejaculate. Sperms get active when mixed with seminal fluid.

2. Prostate gland: It is a single gland that surrounds the upper part of the urethra. The alkaline nature. of this fluid neutralizes the acid in the female tract which would otherwise inactivate and kill the sperms.

3. Cowper’s glands or lbourethra glands: These paired glands lie below the prostate gland and join the urethra at a short distance from that of the prostate gland. They secrete a white, viscous, alkaline secretion resembling mucus, which acts as a lubricant.

Spermatozoa and semen

The spermatozoa are minute gametes produced by the testes in males. They are immotile when stored in the epididymis but get activated and motile by the secretions from the accessory reproductive glands in males. The secretions of various accessory glands along with sperms form the semen.

The sperms are released in millions. In one ejaculation about 200,000,000 (2 x 10³) sperms are discharged. Sperms move with the speed of
2 mm/minute in the female tract.

Structurally, a human sperm has three main parts – head, neck and tail. The tip of a sperm is covered by a cap-like structure, acrosome, which helps the sperm to penetrate inside the egg during fertilization. Acrosome also releases an enzyme (hyaluronidase) which enables the entry of the sperm into the egg by dissolving the proteinaceous wall around the egg. The general structure of a sperm is shown in.

The nucleus of the sperm contains genetic material that is transferred to egg and mixes with the female nucleus during fertilization. The middle piece of the sperm contains mitochondria that provide energy for sperm penetration and sperm motility. The tail helps the sperm in moving forward through the liquid medium while swimming.

The course of sperm movement from their production in the testes to reach the urethra in penis is given in.

Reproductive System Female reproductive system

The female reproductive system consists of the following organs – a pair of ovaries, a pair of Fallopian tubes, uterus, vagina and external genitalia. Important functions of the main reproductive organs in the human female are listed in.

Ovaries

A pair of almond-shaped or ovoid-shaped ovaries lie in the lower part of the abdominal cavity, one on each side of the uterus. Each ovary is attached to the

Oogenesis

Eggs can be observed in various stages of development inside the ovary. The egg mother cells or oogonia are diploid cells that undergo meiosis to produce the haploid egg or ovum.

• During the process of development the egg gets surrounded by some special cells called
  follicular cells or nurse cells. Nurse cells provide nourishment to the developing egg cell.
• The egg cell along with its surrounding nurse cells is called a follicle.
• Towards the end of oogenesis, the follicle develops a fluid-filled cavity amongst the nurse cells and is called the tertiary follicle or Graafian follicle.
• Upon maturation, the follicle ruptures, the ovarian wall ruptures and the ovum is released out of the ovary. This egg is sucked by the funnel of the oviduct or Fallopian tube. The egg is in the secondary oocyte stage as the meiosis is not yet complete. It has a proteinaceous layer around it called the zona pellucida.

Fallopian tubes (Oviducts)

There are two oviducts or Fallopian tubes or uterine tubes in the human female reproductive system. Each oviduct is about 10-15 cm long. The proximal funnel-shaped end of each oviduct lies near the ovary and is called infundibulum. Its margin bears finger-like projections called fimbrae. Each infundibulum continues as a thin and coiled tube called oviduct or Fallopian tube. Both Fallopian tubes open into the uterus.

The egg released from the ovary is sent into the Fallopian tube by the infundibulum. The Fallopian tube transports the ovum from the ovaries into the uterus by cilia along the wall of funnel and peristaltic contractions of the muscles of the tube wall. Fertilization of ovum and sperm is taking place in Fallopian tube.

Uterus

The uterus is a hollow, pear-shaped, muscular, thick- walled organ located in the pelvic cavity between urinary bladder and rectum. Its upper broader portion is called corpus uteri (body of the womb) and the lower narrow portion is called cervix uteri (neck of the womb). The innermost wall of the uterus is called endometrium. It is richly supplied with blood vessels as implantation of the embryo takes place in the endometrium. Cervix is mainly a sphincter muscle that closes the lower end of the uterus where it joins the vagina.

Vagina

It is a muscular tube, about 7-10 cm in length. Vagina is the organ where the spermatozoa are deposited during coitus (act of copulation) by the penis. It serves as the birth canal during childbirth (parturition) and also acts as a duct for the passage of uterine secretions and menstrual flow. The opening of the vagina in young girls is partially closed by a thin membrane called hymen.

External genitalia (Vulva)

The external genitalia in human female (also called vulva) mainly consists of labia majora, labia minora and clitoris.

The labia majora (large lips) is a pair of thick folds of skin containing hair, sweat glands and sebaceous. glands. These folds lie one on either side of the vagina. The labia majora is equivalent to scrotum in males. The labia minora (small lips) are two small folds of mucous membrane situated on the inner side of labia majora. They are devoid of hair and glands. The two folds of labia majora and labia minora, comprise the vulva. The clitoris is a small erectile organ which

is highly sensitive and is located at the junction of the labia minora. The clitoris corresponds to the penis in males.

The vagina opens to the outside by an opening into the depression between the vulva called vestibulum. In a human female, the urethra and the genital duct have separate openings.

Reproductive System Menstrual cycle in human females

In a human female, the fertility period extends from the age of puberty, i.e. about 12-13 years up to menopause, i.e. 45-50 years. The stage of puberty is marked by the appearance of secondary sexual

a. Transverse section through an ovary

b. Scanning electron micrograph of egg surrounded by follicle cells

characteristics. Thus, puberty is a period in which reproductive system matures and becomes capable for reproducing. At the time of menopause, ovulation and menstruation stop and the reproductive organs decrease in size.

During each menstrual cycle, an ovum is matured and released once every 28 days. The period of menstrual cycle is counted from the day of onset of menstrual flow to the next onset after about 28 days. There are four phases of menstrual cycle as given below.

Menstrual phase

The menstrual cycle starts with the menstrual flow, during which the cellular lining of the uterus, with blood flow, is shed off. This process continues for 3-5 days. During menstrual phase, the ovary starts to process a new egg in follicle.

Follicular phase

From the 5th day up to the 13th day of the onset of menstrual cycle, growth and maturation of the Graafian follicle take place. Graafian follicle is the final stage in the maturation of an ovum inside the ovary. It consists of an ovum and a mass of nurse cells or follicular cells surrounding it. The Graafian follicle produces a hormone, oestrogen. This hormone stimulates the uterus to prepare itself to receive the ovum. The cells lining the uterus grow rapidly and develop a dense network of blood vessels.

Ovulatory Phase

In this phase, ovulation takes place. The Graafian follicle ruptures to release the ovum. The cells of the ruptured follicle form the corpus luteum that secretes the hormone, progesterone. The release of the ovum from the ovary is called ovulation. The ovum reaches the uterus via the Fallopian tube on the 13th or 14th day and remains = there up to the 16th day (for 48-72 hours).

Luteal Phase

If the ovum does not get fertilized by any sperm during the ovulatory period then it degenerates. At the end of the 28th day, this ovum is rejected along with the uterine lining. This marks the onset of the disintegration of the thickened endometrial lining of the uterus – the next menstrual cycle. The remnant corpus luteum develops of the Graafian follicle in the ovary turns into corpus luteum.

If the ovum gets fertilized while in the Fallopian tube, it starts dividing by repeated mitotic divisions called cleavage divisions. The egg is now an early embryo. It reaches the uterus within 5-7 days of fertilization and implants itself in the thick endometrium. The female is now pregnant and shall give birth to a baby after the development of embryo in 38-40 weeks. This is the gestation period.

What happens to the menstrual cycle if the ovum receives sperm?

If the ovum receives sperm, menstruation (and ovulation) cease for as long as the woman is pregnant. This is because level of progesterone increases by the corpus luteum (which persists in the ovary) and later by the placenta. Progesterone prevents egg maturation in the ovary.

Fertilization, pregnancy and development of the embryo

•  Fertilization: The sexual intercourse, also known as coitus, is the first step towards pregnancy.

• During intercourse, the semen containing sperms is ejaculated in the vagina. This is known as copulation. A single ejaculation may contains 2-4 hundred million sperms, only one will actually fertilize the egg.
• The spermatozoa are deposited high up in the vagina close to the cervix. They reach the top of the Fallopian tube within 5 minutes of their release due to their motility and contractions in the walls of uterus and Fallopian tube. Spermatozoa remain viable in the female genital tract for 24-72 hours.
• If the ovum receives a sperm during the ovulatory period, the two fuse to form a zygote. This act of fusion of the male gamete (sperm) and the fernale gamete (ovum) to form zygote is called fertilization. About 13-15 days of the menstrual cycle are most favourable for conception. Of the millions of sperms ejaculated in the vagina, the first one to reach the ovum fertilizes it. Only one sperm can fertilize the ovum. This is possible due to a protein coat formed around the ovum soon after the first sperm comes in contact with the ovum.

• Implantation: The zygote immediately begins to divide by rapid mitotic divisions called cleavage. The daughter cells produced by cleavage divisions get smaller and smaller progressively. Successive cleavages produce a solid mass of cells called the morula which appears like mulberry on the outside. It’s volume is the same as that of the zygote. With further development, this ball of cells becomes hollow and is called blastocyst. The cells of blastocyst are now differentiated into two types.
• Trophoblast: outer compact layer cells
• Inner Cell Mass (ICM): inner mass of cells located at one end of the hollow ball. Also called embryoblast cells.

The blastocyst is still enclosed in Zona Pellucida which was present around the ovum at ovulation. The blastocyst passes down to the uterus and fixes itself to the endometrium wall of the uterus. The fixing of blastocyst in the endometrium of the uterus is called

implantation and the female is said to be pregnant or in the stage of pregnancy. Implantation takes place about a week after fertilization.

How does the blastocyst implant in the uterine wall?

Site of implantation: Endometrium of the uterine wall Time of implantation: 5-7 days of fertilization

Process of implantation

• Trophoblast cells secrete enzymes that perforate the Zona pellucida and the blastocyst oozes out.
• The special trophoblast cells secrete enzymes that digest and liquefy the endometrial cells at a point creating a pit.
• Blastocyst starts burrowing itself into the pit.
• Eventually, the blastocyst gets completely buried in the endometrium.
• Blastocyst is oriented in such a direction that the ICM faces towards the wall.
• Trophoblast develops further to form a connection between the embryo and the mother called the placenta.

Placenta

The developing embryo is attached to the uterine wall by the placenta. Placenta is an association between maternal and foetal tissue meant for physiological exchange. Umbilical cord is a tough structure that serves as the blood-vascular connection between the foetus and placenta. It develops from embryonic cells.

The placenta is formed of fine finger-like processes called villi. There are two sets of villi. One set of villi is given out by the uterine wall which opens up in a pool or sinusoid of blood of mother while the other set is an extension from the embryo. These two sets of villi are interlocked but they do not open into each other. Thus, although the blood of the mother and the embryo are in close contact with each other, the two do not get mixed.

Oxygen and nutrients diffuse from the mother’s blood into the capillaries of the villi. From here they

are carried by the blood vessels in the umbilical cord to the foetus. Conversely, carbon dioxide and other wastes are carried back by foetal blood and transferred to the mother’s blood through the villi in placenta.

The villi from the embryonic side arise from the trophoblast cells and get lined by chorion and not allantois in humans, which is the outermost lining of the developing embryo.

Functions of placenta

• Nutrition: Placenta serves as a tissue through which oxygen and nutrients (glucose, amino acids and salts) are supplied from the maternal blood to the foetus.
• Excretion: It also transports carbon dioxide and excretory waste from the foetal blood to the maternal blood.
• All these substances move across the placenta by diffusion. Placenta is permeable to respiratory gases, nutrients and antibodies. It prevents harmful material from reaching the embryo. It does not allow the passage of germs from the mother to the embryo. However, certain exceptions like German measles virus and HIV can pass through the mother’s blood to the embryo.
• Endocrine: Placenta also produces two hormones – progesterone and oestrogen. Under the influence of these hormones, neither ovulation nor menstruation takes place till pregnancy continues. However, these phenomena are resumed after childbirth.

Foetal membranes

During development, the embryo gets surrounded by four special membranes which protect and nourish it. These are as follows:

•  Yolk sac: contains nutrients
• Allantois: acts as a waste bag

The first two are non-functional in humans as this work is done by placenta.

• Chorion: outermost membrane, takes part in placenta formation from the embryonic side.
• Amnion: a membranous sac filled with amniotic fluid, completely envelops the developing foetus.

Functions of amnion

• Acts as a shock absorber for the developing foetus and protects from any physical damage.
• Assists in regulation of foetal body temperature.
• Prevents adhesion (sticking) of skin of foetus to the surrounding tissues.
• Allows movements of the foetus in the mother’s womb, in a restricted manner.

Amniocentesis Embryonic cells are sloughed off in the amniotic fluid. These cells found in the amniotic fluid are studied by a technique called amniocentesis. In this method, some amount of amniotic fluld is taken by a surgical needle inserted into the amniotic sac through the abdomen. The embryonic cells are studied for any genetic abnormalities in the child months before birth, by 16-18 weeks of development.

Gestation and parturition

Parturition is the process of childbirth. The sequence of events that occur during childbirth is called labour.

What happens during childbirth?

• The uterus undergoes rhythmic contractions. The amnion bursts and the amniotic fluid is discharged.
• The uterus contracts powerfully, expelling the baby.
• The baby’s lungs start functioning and the baby takes its first breath.
• The umbilical cord is tied and cut.
• The lung bypasses close so that blood flows to the lungs.
• The placenta is discharged (after birth).
• The breasts start producing milk.

The period of complete development of the foetus from the beginning of the last menstrual period till birth of the baby is called gestation period. It is of about 280 days.

Test tube babies Some women are unable to have babies due to certain reasons. This problem can be overcome by the test tube baby technique.

In this technique, one or more ripe ova are sucked from a woman’s ovaries using a special syringe. These ova are placed in a dish containing sperms from her partner. During this time, sperm fertilizes the ovum to form zygote which divides to form an embryo. The embryo is then inserted into the woman’s uterus where there is a chance it will implant and develop into a baby.

The entire process is performed under optimum conditions in a laboratory. The general notion is that people in a laboratory work with test tubes, hence the name test tube baby.

How twins occur?

Usually, only one ovum is released by an ovary in every reproductive cycle. If this ovum gets fertilized, one baby is born. But sometimes more than one egg may be released and fertilized. Or an ovum may divide into two or more separate cells after fertilization. This is how twins, triplets, quadruplets, etc. are produced.

Identical twins

Identical twins are the result of a fertilized egg/zygote separating into two cells after the first cleavage division. Both of which continue to divide, so two identical embryos come from the same egg and sperm. Such twins look alike and are of same sex. Since identical twins develop from the same zygote, these are called monozygotic twins.

Non-identical or fraternal twins

Non-identical twins occur when two eggs are produced at the same time and each is fertilized by a different spermatozoa. These twins do not resemble each other physically. They can be of the same sex or different sexes. Since fraternal twins develop from two different zygotes, they are called dizygotic twins.

Reproductive System Summary

• Reproduction is a process by which a living organism is able to produce more of its own kind.
• In sexual reproduction, both male and female gametes are produced and the process of fertilization takes place. The humans reproduce sexually.
• The age of 13-14 years in males and 12-13 years in females is called puberty. At this age, sex organs get matured and several secondary sexual characteristics appear in them.
• The male reproductive system consists of – a pair of testes, a pair of epididymis, a pair of vasa deferentia, urethra, penis and accessory glands.
• The female reproductive system consists of a pair of ovaries, a pair of Fallopian tubes, uterus, vagina and external genitalia.
• The fusion of male gamete (sperm) and female gamete (egg) to form a zygote is called fertilization.
• The fixing of a fertilized egg in the form of mass of cells (blastocyst) in the uterine wall is called implantation.
• Placenta Is an association between maternal and foetal tissues meant for physiological exchange.
• Amnion contains amniotic fluid which surrounds the foetus in placenta and acts as a shock absorber.
• Twins are of two types – fraternal and identical twins.

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