Malleswari

Crop Production And Management Concepts

• Types of Crops
• Basic Practices of Crop Production
• Food from Animals

Chapter In A Nutshell

• The process of growing, cultivating, and harvesting crops is known as agriculture.
• Plants of the same kind that are cultivated in fields on a large scale for food, clothing or commercial purposes are known as crops.
• Several crops like rice, wheat, maize, cotton, vegetables, etc., are grown on a large scale for food, clothing, and commercial purposes.
• On the basis of the season in which they grow, crops grown in India are classified into Kharif and rabi crops.
• Kharif crops are planted in the rainy season and harvested in October. A few examples of Kharif crops are paddy, maize, cotton, etc.
• Rabi crops are planted in the winter season and harvested in March or April. A few examples of rabi crops are wheat, mustard, pea, gram, etc.
• For growing crops, farmers perform several activities known as agricultural practices. They include:
• Preparing the soil: It involves plowing, leveling, and applying fertilizers. Plowing is the process of loosening and turning the soil with a plow, hoe, etc. A wooden leveler is used for pressing the soil and fertilisers or manures are then applied to the soil.
• Selection of good quality seeds: Good quality seeds are clean and healthy seeds of a good variety.
• Sowing of seeds: Planting seeds into the soil is called sowing. Seeds can be sown by broadcasting or scattering: using seed drills or by transplantation. In transplantation, seeds are sown in a nursery. Small plants that grow out of these seeds are transferred to the fields.
• Adding manures and fertilizers to the soil: Manures are organic substances obtained by the decomposition of dead plants and animal wastes. Manures are not nutrient specific. Besides adding nutrients to the soil, manures also improve the texture of the soil and add beneficial soil
• organisms to the soil. Fertilisers are chemicals, which increase the fertility of soil by providing specific nutrients to the soil. Examples of fertilizers are urea, ammonium phosphate, NPK (nitrogen, phosphorous, and potassium), etc.
• Irrigation: It is the artificial supply of water to the soil at regular intervals and in a regular quantity as per the need of the crop. It can be done by traditional or modem methods. Traditional methods of irrigation include canal irrigation, furrow irrigation, chain pumps, etc.
  They are cheaper but often lead to a waste of water. Sprinkler irrigation and drip irrigation are j modem methods of irrigation and help in saving water.
• Weeding: Weeds are unwanted plants, which grow along the main crops and compete with them for sunlight nutrients, water, and space. Removal of unwanted plants from a field is called weeding. It can be done manually with a hoe or by using chemicals known as weedicides.
• Harvesting: Cutting and gathering of crops upon maturity is called harvesting. It is done manually; using a sickle or with the help of a machine called a harvester.
• Threshing: Harvested crops are separated from the plant bulk by beating the bulk on floors or with the help of machines. This process is called threshing.
• Winnowing: Grains are separated from the chaff by a process called winnowing.
• Storage: Harvested and cleaned grain must be stored in a safe and dry place. Stored grain may; be sprayed with pesticides to keep away pests. The grain is usually stored in jute bags or metallic; bins.
• Large-scale storage of go-as is done in silos and granaries.
• With the increase in population, several techniques are used for increase ng crop yield.
• Mixed cultivation and crop rotation is used so that the nutritional needs of one crop are fulfilled by another crop.
• Atmospheric nitrogen is fixed into the soil by nitrogen-fixing bacteria called Rhizobium, which is present in the root nodules of leguminous plants.
• The nitrogen cycle is the process of the circulation of nitrogen between the soil, organisms, and the atmosphere.
• Rearing of animals on a large scale for products such as milk, meat, eggs, fiber, hide, honey, etc., is known as animal husbandry.

Read and Learn More UP Board Notes for Class 8 Science

Types of Crops

• Until around 10,000 BC, human beings were nomads and went from one place to another in search of food. They ate raw fruits, vegetables, and meat of animals. It was much later that is culture was discovered and human beings started to grow ing crops. Crops can be classified in many different ways. Based on the type of food they provide, crops can be classified as cereals, fruits, vegetables, oilseeds and nuts, legumes, sugar crops, etc. Crops can also be classified on the basis of the season in which they grow. Crops grown in India are classified as Kharif and rabi crops. Kharif crops are planted in the rainy season and harvested in October. A few examples of Kharif crops are paddy, maize, cotton, etc. Rabi crops are planted in winter and harvested in March or April. A few examples of rabi crops are wheat, mustard, pea, gram, etc. Pulses and vegetables are generally grown in summer.

Crop Production And Management Objective Type Questions

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

1. Wheat is a rabi crop.
2. Crops can be categorized on the basis of the type of food they provide.
3. Gram is a Kharif crop.

Answers

1. True
2. True
3. False

B. Multiple Choice Questions.

1. In which category of crops would you place wheat and rice?

a) Rabi crops
b) Kharif crops
c) Fruits
d) Cereals

2. Which of the following is a Khar/From?

a) Linseed
b) Mustard
c) Groundnut
d) Pea

3. Which of following constitutes the food that early human beings ate?

a) Cereals
b) Raw fruits, vegetables, and meat
c) Milk products
d) None of these

4. Under which of the following type of crops would you categorise mustard?

a) Cereals
b) Fruits and vegetables
c) Oilseeds and nuts
d) Sugar crops

Answers

1. (d) Cereals
2. (c) Groundnut
3. (b) Raw fruits, vegetables, and meal
4. (c) Oilseeds and nuts

C. Fill in the blanks.

1. ___________crops are planted in winter and _________in March or April.
2. _______and are __________grown in summer.

Answers

1. Rabi, harvested
2. Pulses, Vegetables

Coproduction And Management Short Answer Type Questions

Question 1. Define rabi and Kharif crops with two examples of each.

Answer
Crops which are grown in the winter season and harvested in March or April are known as rabi crops. Examples of rabi crops are wheat and mustard. Crops which are grown in the rainy season and harvested in October are known as Kharif crops Examples of Kharif crops are paddy and cotton.

Question 2. Define agriculture.

Answer
The process of growing, cultivating, and harvesting crops is known as agriculture.

Question 3. What is staple food?

Answer
The food which forms the main part of our diet is called staple food, e.g. rice and wheat. They are grown on a large scale in vast fields as they are consumed in large amounts.

Basic Practices Of Crop Production

Farmers perform several activities for producing crops and these are known as agricultural practices. The basic steps of growing crops include the following step:

1. Soil preparation which includes plowing, leveling, and applying fertilisers. Plowing is the process of loosening and turning soil using a plow, hoe, etc. Soil is loosened and overturned to make it better ventilated and suitable for the growth of tiny organisms living in it. During dry season, Dloughing turns soil into big mud pieces or crumbs, which are broken down by a park. At times, a wooden leveler is used for pressing the soil
2. The process of planting seeds in the soil is called sowing. Before sowing good quality seeds must be selected. Seeds can be sown in fields by hand (broadcasting) or with the help of a seed drill.
3. Manure or art facial fertilisers are added to the soil to provide nutrients for the healthy growth of crops.
4. Water is supplied to the soil at regular intervals and n regular quantity. This is called inigation and it can be done by traditional or modem methods. Traditional methods of irrigation include canal irrigation, furrow irrigation, chain pumps, etc. They are cheaper but often lead to a waste of water. Sprinklers and drip irrigation are modem methods of irrigation and help in saving water.
5. Unwanted plants are removed from fields either manually or by using chemicals called weedicides. Examples of weedicides are linazine, dilation, etc.
6. Cutting and gathering of crops upon maturity is called harvesting. It is done manually using a sickle or with the help of a machine called a harvester.
7. Grains are obtained from the harvested crops by threshing and winnowing.
8. Threshing: Separation of grains from the plant bulk is called threshing. On a large scale, threshing is done using animals. In large farms, a machine called a combine harvester is used for both harvesting and threshing.
9. Harvested grains are stored in a safe and dry place before they are made available for consumption. Pesticides may be sprayed to keep away pests from stored grains. Grains are usually stored in jute bags or metallic bins. Large-scale storage of grains is done in silos and granaries. Grains stored at home can be protected from pests by putting dried Neem leaves in them.

Crop Production and Management Activity 1

Aim: To show that damaged seeds would float on top of water Procedure:

1. Take a beaker and fill half of it with water.
2. Put a handful of wheat seeds into it and stir well.
3. Leave the beaker undisturbed for a few minutes.

Observation: Few seeds float on top of the water.
Conclusion: Damaged seeds are hollow from inside and thus, they float on top of water.

Crop Production and Management Activity 2

Aim: To show that manure or fertilisers are needed for good growth of crops
Procedure:

1. Germinate moong or gram seeds in three equal-sized containers.
2. In the first container, sow the seeds into the soil.
3. In the second container, add some cow dung to the soil.
4. In the third container, add urea to the soil.
5. Keep the three containers in a warm place and water them every day.
6. Observe after 7 to 10 days.

Observation: Growth of seeds in the second and the third container is better than in the first one. Conclusion: Manures and fertilisers help the seeds to grow- better.

Objective Type Questions

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

1. Soil preparation involves plowing, leveling, and applying fertilizers.
2. Water should be supplied to crops only when the soil seems dry and not at regular intervals
3. Tilled and plowed soil is better aerated and lets water and nutrients reach to the roots of the plants.
4. Weeds are unwanted plants, which grow alongside crops and must be removed for healthy growth of crops.
5. Cutting and gathering of crops upon maturity is called threshing.
6. A combined harvester is used for both harvesting and threshing.
7. Leguminous plants bear root nodules.
8. Winter crops are known as Kharif crops.
9. In India, special festivals are held commemorating the harvest.
10. Large-scale storage of grains is done in silos or granaries.

Answer

1. True
2. False
3. True
4. True
5. False
6. True
7. True
8. False
9. True
10. True

Crop Production and Management Multiple Choice Questions.

1. In which category of crops would you place cotton?

a) Rabi crops
b) Kharif crops
c) Fruits
Cereals

2. Which of the following is a manual method of sowing seeds directly into the field?

a) Using seed drills
b) Broadcasting
c) Transplantation
d) None of these

3. Which of the following is a result of adding fertilizers to the soil?

a) Addition of specific nutrients to the soil
b) Improvement of soil texture
c) Addition soil organisms to the soil
d) None of these

4. Which of the following can be used for protecting stored grains at home?

a) Weedicides
b) Dried Neem Leaves
c) Dried Tulsi Leaves
d) None of these

Answers

1. (b) Kharif crops
2. (a) Addition of specific nutrients to the soil
3. (b) Broadcasting
4. (b) Dried Neem Leaves

Fill in the blanks.

1. A ______________may be used for sowing seeds
2. ________and______ irrigation are modern methods of irrigation.
3. A___________is used for harvesting crops.
4. ____________ are chemicals that kill or destroy weeds.
5. When crops in a field are supplied water at regular intervals of time, it is called_______
6. __________of crops take place due to lack of water.
7. The area of land on which crops are grown is known as a________
8. The downward movement of water through the pores of the soil is known as________
9. There are two types of fertilizers: and, artificial.
10. Rhizobium bacteria fixes__________into the so

Answers

1. Seed drill
2. Sprinkler and drip
3. Sickle
4. Weedicides
5. Irrigation
6. Shriveling
7. Crop field
8.  Percolation
9. Natural
10.  Atmospheric nitrogen

Crop Production and Management Short Answer Type Questions

Question 1. Before sowing seeds, soil is plowed and tilled. Why?

Answer Soil is plowed and tilled to aerate and loosen it up. This allows water to percolate down the layers of soil and reach to the roots of plants. Aeration of soil helps roots to grow and breathe. It also aids in the growth of worms and microbes in the soil. Worms and microbes make the soil fertile.

Question 2. What are the different methods of sowing seeds?

Answer Seeds can be sown into the soil using broadcasting, seed drills, or by transplanting. In broadcasting, seeds are broadcasted or scattered over the plowed soil by hand. A seed drill is a small device that sows seeds at equal distances. In transplantation, seeds are sown in nursery beds and once they grow into young plants, they are uprooted and planted in the field.

Question 3. What are the basic requirements for growing any crop?

Answer
The basic requirements for growing a crop are:

1. A proper type of soil as per the need of the crop
2. Sowing good quality, healthy seeds at the right time
3. Use of manures or fertilizers
4. Regular irrigation
5. Proper weeding
6. Protection from pests
7. Harvesting at the right time

Food from Animals

• Animals provide us with many useful products such as milk, eggs, meat, fibers, hide, honey, etc. These animals need to be provided with food, shelter, and care. Rearing of animals on a large scale for food, clothes or other products is known as animal husbandry. Besides providing these products, animals help us in various tasks. Such animals are called draught animals. Examples of draught animals are horses, ox, etc.

Crop Production And Management Activity 3 Objective Type Questions

State whether the following statements are true or false.

1. Rearing of honeybees on a large scale is known as apiculture.
2. We get hide and meat from poultry birds.
3. Animals such as goats and sheep provide us with fibers.
4. Cows and buffaloes provide us with milk.
5. Honey is obtained from cows.

Answers:

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

Multiple Choice Questions.

Question 1. ‘What is rearing of silkworms for producing silk called?

1. Pisciculture
2. Sericulture
3. Apiculture
4. None of these

Question 2. Which of the following products is provided by cows and buffaloes?

1. Milk
2. Hide
3. Fuel
4. All of these

Question 3. ‘What is the name of the process in which seeds or grains are separated from the plant bulk Dy beating it on a hard floor?

1. Winnowing
2. Broadcasting
3. Threshing
4. Weeding

Question 4. ‘Which of the following is used for removing weeds?

1. Weedicides
2. Pesticides
3. Fertilizers
4. Manures

Question 5. ‘Which of the following is a traditional method of irrigation?

1. Canal irrigation
2. Furrow irrigation
3. Chain pumps
4. All of these

Question 6. Which of the following is provided to us by animals?

1. Wood
2. Rubber
3. Wheat
4. Wool

Question 7. ‘What is the name given to crops which are mainly grown for money?

1. Food crops
2. Cash crops
3. Vegetables
4. Oilseeds and nuts

Question 8. What do animals get from crops?

1. Fodder
2. Oilcake
3. Grains
4. All of these

Answers:

1. (2) Sericulture
2. (4) All of these
3. (3) Threshing
4. (1) Weedicides
5. (4) All of these
6. (4) Wool
7. (2) Cash crops
8. (4) All of these

Fill In the blanks.

1. _____ and ______are milk yielding animals.
2. ____ are provided by a hen.

Answers:

1. Buffalo, cow
2. Eggs

Crop Production and Management Short Answer Type Questions

Question 1. Explain how soil gets affected by the continuous plantation of the same plant. How can this be prevented?
Answer
By repeated growth of the same plant, specific nutrients get depleted from the soil. To prevent this, different types of crops are grown in succession and this practice is known as crop rotation.

Exercises

Question 1. Select the correct word from the following list and fill in the blanks:

float, water, crops, nutrients, preparation

1. The same kind of plants grown and cultivated on a large scale at a place are called
2. The first step before growing crops is the of soil.
3. Damaged seeds would on top of water.
4. For growing of crops, sufficient sunlight from the soil are essential.

Answers:

1. Crop
2. Preparation
3. Float
4. Water, nutrients

Question 2. Give two examples of each.

1. Kharif crop
2. Rabi crop

Answers:

1. Paddy and Maize
2. Wheat and Gram

Question 3. Write a paragraph in your own words on each of the following.

Answer

• Preparation of soil
•  Sowing
• Weeding
• Threshing
• Preparation of soil: Soil is loosened and overturned to make it better ventilated and suitable for the growth of tiny organisms living in it. The ent reprocess is called ’tillage and plowing’. During dry season, plowing turns the soil into big mud pieces or crumbs, which are then broken down with a plank. The plowed soil is liable to be removed by wind and water. Therefore, a wooden leveler is used for pressing the soil.
• Sowing: The process of planting seeds into the soil is called sowing. It is the most important part of crop production. Before sowing, good-quality seeds must be selected. Seeds can be sown in fields by hand (broadcasting) or with the help of i seed drill.
• Weeding: It is a process of the removal of unwanted plants, i.e., weeds from fields. It is necessary to remove weeds as they compete with the main crop for water, nutrients, sunlight, etc. Weeds may be removed manually by uprooting or cutting or by using chemicals called weedicides. Examples of weedicides are linazine, dilation, etc. The best time to remove weeds is before they produce flowers and seeds. Hand removal of weeds can be done with burps or a tractor-driven harrow.
• Threshing: Separation of grains from the bulk of harvested plants is called threshing. Animals are used on a large scale for threshing. In large ”arms, a machine, a combined harvester, is used for both harvesting and threshing.

Question 4. Explain how fertilisers are different from manure.

Answer
Fertiliser Manure
a) These are chemical compounds rich in nitrogen, phosphorus, and potassium. a) These are organic substances prepared from the decomposition of plants and animal wastes.
b) They are man-made and manufactured on a large scale in factories. b) They are made by natural processes either on a small or large scale.
c) Chemical fertilisers are nutrient specific, i.e., nitrogenous, phosphatic, etc. c) They contain a mixture of various nutrients recycled from biomass wastes.

Question 5. What is irrigation? Describe two methods of irrigation which conserve water.

Answer
Applying water to the soil at regular intervals of time and in regular quantity is called irrigation. The time and frequency of irrigation vary from crop to crop, soil to soil, and season to season. The latest irrigation methods that help in conserving water are:

• Sprinkler system: This system is very useful on uneven land or on sandy soil. The pipes of the sprinkler system have rotating nozzles and are joined to the main pipeline at regular intervals. Water comes out from the rotating nozzles and is sprinkled on the crops as if it raining.
• Drip system: In this system, water is added drop by drop to the soil just at the position of the roots. Water wastage is minimised and that is why the drip system is a boon for regions where availability of water is limited.

Question 6. If wheat is sown in the Kharif season, what would happen? Discuss.

Answer
Wheat is a rabi crop, i.e., it is grown in the winter season If it is sown in the Kharif season, it will get more water, which is harmful to the crop. The wheat crop will neither be healthy nor will it provide a good yield.

Question 7. Explain how soil gets affected by the continuous plantation of crops in a field.

Answer
For better growth of crops, fertilisers are added regularly to a field. Fertilisers are nutrients like nitrogen, phosphorus, potassium, etc. Using them year after may change the nature of the soil. The soil may become too alkaline or acidic and less fertile.

Question 8. What are weeds? How can we control them?

Answer
Weeds are undesirable plants that grow naturally along with the crops. Removal of weeds is called weeding. The following methods can be used to control weeds:

• Tilling: The process of loosening and turning of the soil is called till ng or plowing. This is done by using a plow. Tilling uproots and kills weeds.
• Manual removal: This method includes physical removal of weeds by uprooting or cutting them close to the ground. This is done with the help of a khurpi or harrow.
• Chemical method: In this method, weeds are controlled or killed by using chemicals called weedicides. They are sprayed in fields with a sprayer.

Question 9. Arrange the following boxes in the proper order to make a flow chart of sugarcane crop production.
Answer:

Question 10. Complete the following word puzzle with the help of the clues given below.
Answer:

Down
1. Providing water to the crops
2. Keeping crop grains for a long time under proper
5. Certain plants of the same kind that are grown on a large scale Across

Across
3. A machine used for cutting matured crop
4. A rabi crop that is also one of the pulses
6. A process of separating grains from chaff

Answers

Down
1. Irrigation
2. Storage
5. Crop

Across
3. Harvestor
4. Grams
6. WINNOWING

Crop Production and Management Hots Corner

Question 1. Which irrigation method would you recommend for a region in Rajasthan? Why?
Answer: Rajasthan is a dry region and hardly gets any rain. The soil found there is sandy and has very poor water-holding capacity. Hence, modem methods of irrigation like drip irrigation and sprinkler irrigation should be used to water crops in such regions.

Question 2. The following table shows the yield of wheat crops grown by Ram Singh between the years 2005 and 2010. Look at the table below and answer the questions that follow.
Answer:

1. What could be the reason for a decrease in the yield of wheat crops in the year 2007?

Answer
Growing the same crop in the same field year after year decreases the fertility of soil. The soil gets depleted of nutrients. Hence, the yield of wheat crops in the year 2007 is low.

2. What could be the reason for all increase in die yield of wheat crops in the year 2009?

Answer
Peas are leguminous plants. Rhizobium bacteria present in the root nodules of leguminous plants fix atmospheric nitrogen into the soil. This probably increased the fertility of the soil and resulted in an increase in the yield of wheat crops in the year 2009.

Question 3. A farmer grew seeds in his field but only a small number of plants germinated. The field was plowed, tilled, leveled, and manured properly. He also irrigated the soil regularly and took care of weeds, pests, and insects. What do you think is the reason behind the poor germination of seeds?

Answer
The reason behind it might be unhealthy seeds. Everything else has been taken care of and hence, this is the only possible reason for poor germination of seeds.

Question 4. In a musk melon field, it was observed that a few fruits would sometimes crack. What do you think is the reason behind it?

Answer
Fruits in a field would sometimes crack due to excess absorption of water. This may happen due to improper drainage or leveling of the field.

Crop Production and Management Practice Exercise

Objective Type Questions

A. Circle the odd one out.

1. Wheat, rice, maize, mango, millet
2. Sugarcane, jute, cotton, flax, coconut
3. ‘Wheat, mustard, gram, soybeans, linseed
4.  Plowing, tilling, harvesting, leveling, manuring
5. Eggs, silk, paper, wool, honey

Answers:

1. Mango
2. Sugarcane
3. Soybeans
4. Harvesting
5. Paper

B. Give one word for the following.

1. Nitrogen-deficient soil can be made nitrogen-rich by applying this farming practice
2. Chemicals that are used for killing pests
3. The process by which seeds are scattered in the field by hand
4. Nitrogen-fixing bacteria, Rhizobium, lives in the root nodules of these plants
5. Rearing of fish on a scale

Answers:

1. Crop rotation
2. Pesticides
3. Broadcasting
4. Legumes
5. Pisciculture

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

1. The process of cutting and gathering crops is called harvesting.
2. Paddy is a rabi crop.
3. Tea is a leguminous crop.
4. The microorganisms found in the roots of leguminous plants are Rhizobium bacteria.

Answers:

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

Crop Production and Management Short Answer Type Questions

Question 1. What are weeds?

Answer
Weeds are unwanted plants that grow alongside crops and compete with crops for food, water, and sunlight. By taking up nutrients, space, water, and sunlight; weeds adversely affect crop yield and hence, should be removed as soon as possible.

Question 2. What is poultry farming?

Answer
The rearing and breeding of chickens and hens on a large scale is called poultry farming. It is done to produce eggs and meat

Question 3. What is crop rotation? What are its advantages?

Answer

Growing the same type of crop on the same land, again and again, depletes the soil of nutrients, deteriorates soil quality, and leads to poor crop yield. Growing crops of different kinds in succession is termed crop rotation. It helps maintain the quality of the soil.
Let us consider the case of nitrogen-fixing bacteria. These are found in the roots of leguminous plants. Nitrogen-fixing bacteria help in fixing atmospheric nitrogen in the soil. Planting legumes and Plants in the soil will result in recovering nitrogen in the soil and increase its fertility. A sample four-year crop rotation plan may be as follows:

Leaf crop —> Fruit crop —> Root crop —> Leguminous crop

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 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Force And Pressure Concepts

• Force
• Pressure

Force And Pressure Objectives

• A push or pull applied on a body is called force.
• The strength of a force is expressed by its magnitude.
• Force is denoted by the letter ‘P and its unit is Newton (N).
• When two forces act at the same point in opposing directions, the resultant force is equal to the difference between the applied forces and is in the direction of the greater force.
• When two forces act at the same point in the same direction, the resultant force is equal to the sum of the two applied forces.
• The effect of force changes when its direction and magnitude change.
• Force applied brings a change in the state of motion of an object.
• The speed of an object increases if the force applied to the object is in the direction of the motion of the object.
• The speed of an object decreases if the force applied to the object is in the direction opposite to that of the motion of the object
• Force can be of two types: contact force and non-contact force.
• Contact force acts when two or more objects are in direct physical contact with each other and bring about necessary changes.
• Muscular force, frictional force, mechanical force, etc., are examples of contact forces.
• The force exerted by the muscles of a body is called muscular force.
• A force that opposes the motion of an object and arises due to contact between two surfaces is called frictional force.
• The force applied by a machine is called mechanical force.
• Non-contact forces act from a distance without coming into direct contact with the body. Magnetic forces, electrostatic forces, gravitational forces, etc., are a few examples of non-contact forces.
• The force exerted by a magnet is called magnetic force.
• The force exerted by a charged body on another body is called electrostatic force.
• The force with which the Earth attracts all objects towards itself is called gravitational force.
• The amount of offeree acting per unit area is called pressure. The SI unit of pressure is Pascal (Pa). Pressure = Force/Area
• Pressure increases if the area over which the force is applied decreases and vice-versa.
• Liquids do not have a definite shape and hence, the pressure exerted by them depends upon the depth of the liquid column.
• Liquids exert pressure not only on the base of the container but also on its sides.
• The pressure exerted by the atmosphere on an object is called atmospheric pressure.
• Atmospheric pressure decreases with an increase in altitude.

Read and Learn More UP Board Notes for Class 8 Science

Force And Pressure Important Terms And Definitions

Pull: It is the force that is applied while picking up an object
Push: It is the force that is applied while kicking or hitting an object.
Weight: It is the force with which the Earth pulls a body towards itself.

Force

Applying force can move a stationary object or stop a moving object Force can also change the direction j of the motion of an object If the force applied is in the direction of the motion of the object, it increases the: speed of the object.

Any object thrown up in the air falls back to the Earth due to the Earth’s gravitational force. The gravitational force exerted by the Earth on an object is called its weight

Force And Pressure Activity 1

Aim: To find the effect offeree on a stationary object
Procedure:

1. Take a heavy box and try- to push it by yourself.
2. Ask one of your friends to help you push the box in the same direction.
3. Next, push the box and ask your friend to push it from the opposite direction.

Observation: In the first case, it was difficult to push the box alone. However, with the help of a friend, it was easier to move the box in the same direction.
It was difficult to move the box when your friend applied force in the opposite direction.
Conclusion: In step 2, the forces were applied in the same direction and the resultant force was the sum of the two forces. Hence, the box moved easily. In step 3, the forces applied were in opposite directions and the resultant force was the difference of the two forces. The box moved in the direction of the greater force.

Force And Pressure Activity 2

Aim: To observe the effect of force on the motion of a moving object
Procedure:

1. Take a rubber ball and place it on a table.
2. Push the ball gently and observe if the ball begins to move.
3. Push the moving ball again.
4. Place your palm in front of the moving ball and remove it as soon as the ball touches your palm.

Observation: The Dali starts moving when it is pushed gently. When it is pushed again, the speed of the moving ball increases. When pain is placed in its path, the speed of the moving ball decreases.
Conclusion: Force can change the motion of an object and can even bring it to rest.

Force And Pressure Activity 3

Aim: To observe the effect of an applied force on the direction of motion of an object Procedure:

1. Take a ball and push it gently.
2. Place a ruler in front of the moving ball.
3. Observe if the moving ball changes its direction on coming in contact with the ruler.

Observation: The direction of the moving ball changes on coming in contact with the ruler. Conclusion: Force can change the direction of motion of an object.

Force And Pressure Activity 4

Aim: To study the effect offeree on the shape of different objects
Observation table:

Conclusion: Force can change the shape of objects.

Force And Pressure Activity 5

Aim: To observe attraction and repulsion between two magnets Which are not in contact.
Procedure:

1. Take a pair of bar magnets.
2. Place one of the magnets over three round-shaped pencils or wooden rollers.
3. Bring one end of the other magnet near the end of the magnet placed on the rollers. Ensure that the two magnets do not touch each other.
4. Now, bring the other end of the magnet near the same end of the magnet placed on the rollers and note down the observation.

Observation: In the first case, the magnets repelled each other. In the second case, they attracted each other.
Conclusion: Two magnets can attract or repel each other without touching each other. Thus, a magnetic force is a non-contact force.

Force And Pressure Activity 6

Aim: To observe the properties of electrostatic force
Procedure:

1. Take a plastic straw and cut it into two equal pieces.
2. Suspend one of the pieces from the edge of a table with the help of a thread.
3. Hold the other piece of the straw in your hand and rub its free end with a sheet of paper.
4. Bring the rubbed end of the straw close to the suspended straw. Make sure that these two do not touch each other. Observe what happens and note it down.
5. Now, rub the free end of the suspended piece of the straw’ with a sheet of paper.
6. Bring the piece of the straw that was rubbed earlier near the free end of the suspended straw again.

Observation: In the first case, both the straws attract each other due to unlike changes present on them. While in the second case, they repel each other due to the same type of change present in them.
Conclusion: Electrostatic force shows similar properties to a magnet. Like charges repel each other, while unlike charges attract each other. Thus, an electrostatic force is a non-contact force.

Objective Type Questions

A. Multiple Choice Questions.

1. Which of the following is an example of contact force?

1. Magnetic force
2. Electrostatic force
3. Gravitational force
4. Frictional force

2. ‘What is the SI unit of force?

1. meter/second
2. Newton
3. meter/hour
4. Pascal

3. Which force is exerted by the Earth on all objects?

1. Gravitational force
2. Frictional force
3. Electrostatic force
4. Mechanical force

Answers:

1. (4) Frictional force
2. (2) Newton
3. (1) Gravitational force

B. Fill in the blanks.

1. The strength of a force is expressed by its______________.
2. The force exerted by a charged body or another body)’ is__________.
3. Muscular force is also known as__________ force.

Answers:

1. Magnitude
2. Electrostatic force
3. Contact

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

1. Two objects must interact with each other for any force to act upon them.
2. A charged body attracts another charged body due to gravitational force.
3. A push or pull on a body is called force.

Answers:

1. False
2. False
3. True

Short Answer Type Questions

Question 1. Why force of gravity is termed a non-contact force? Explain.

Answer
The force of gravity can act on distant objects that are not in direct contact with the Earth, for example, falling leaves. Hence, it is termed a non-contact force.

Question 2. Why does an object moving on a smooth surface stop by itself?

Answer
An object moving on a smooth surface stops by itself due to the frictional force, which acts between it and the surface on which it is moving. Friction acts in a direction opposite to that of the motion of the object. The surface that looks smooth has many irregularities on it These irregularities lock with the irregularities on the surface of the moving object and stops it.

Question 3. How does the force apply to change the speed of an object?

Answer
If the force is applied in the direction of motion of the object, the speed of the object increases. However, if the force is applied in the direction opposite to that of the motion of the object, the speed of the object decreases.

Question 4. When is the net force applied equally to zero? Give an example.

Answer
When two forces are applied from opposite directions and are equal in magnitude then the net force applied is equal to zero. A game of tug of war is an example of this.

I 1.2 Pressure

• Force per unit area is called pressure. Just like solids, liquids and gases also exert pressure. The air; around us exerts pressure on us. It is called atmospheric pressure. Astronauts wear a special ‘space suit’! to maintain normal atmospheric pressure for their bodies as there is no atmosphere in space.

Force And Pressure Activity 7

Aim: To determine if the pressure exerted by a liquid at the bottom of the container in which it is kept depends on the amount of liquid present in the container
Procedure:

1. Take a transparent glass tube or a Plastic pipe of about 15 cm in length and 5-5.75 cm in diameter.
2. Take a piece of a thin sheet of a good-quality rubber balloon.
3. Stretch and fasten the rubber sheet tightly over one end of the pipe.
4. Hold the pipe in the middle, keeping it in a vertical position, and pour some amount of water into the pipe through its open end.
5. Note down your observation.
6. Pour some more amount of water into the pipe and note down your observation again.

Observation: When some amount of water is poured in the pipe, the rubber balloon bulges out The bulging increases upon pouring more water in the pipe.
Conclusion: The pressure exerted by a liquid at the bottom of the container in which it is kept depends on the amount of liquid present in the container.

Force And Pressure Activity 8

Aim: To determine that liquid exerts pressure on the walls of the container in which they are kept
Procedure:

1. Take a discarded plastic bottle.
2. Fix a cylindrical glass tube, a few centimeters long near the bottom of the bottle by using molten wax.
3. Cover the mouth of the glass tube with a thin rubber sheet and fill the bottle half with water.
4. Note down your observation.
5. Pour some more water into the bottle and observe the rubber sheet.

Observation: The rubber sheet bulges out. On adding more water in the bottle, the bulging of the rubber sheet increases.
Conclusion: The bulging of the rubber sheet shows that liquids exert pressure on the walls of the container in which they are kept.

Force And Pressure Activity 9

Aim: To find if liquids exert equal pressure at the same depth
Procedure:

1. Take an empty plastic bottle.
2. Drill four holes close to the base of the bottle. Make sure that the holes are at the same height as the tie base.
3. Fill the bottle with water and observe.

Observation: Four different streams of water coming out of the holes fall at the same distance from the bottle.
Conclusion: Water falling at the same distances proves that liquids exert equal pressure at the same depth.

Force And Pressure Activity 10

Aim: To determine the effect of atmospheric pressure on an object
Procedure:

1. Take a good-quality rubber sucker and press it hard on a smooth plane surface.
2. Now, try to pull the rubber sucker off the surface.

Observation: The rubber sucker sticks to the plane surface when pressed hard. It is very difficult to pull the rubber sucker from the plane surface.
Conclusion: The rubber sucker sticks to the plane surface because atmospheric pressure acts on it The rubber sucker can be pulled off from the surface only when the applied force is more than the atmospheric pressure.

Objective Type Questions

A. Multiple Choice Questions.

1. ‘Which of the following statements is correct?
(a) atmospheric pressure increases as height increases
(b) Atmospheric pressure remains the same at all heights
(c) Atmospheric pressure decreases as height increases
(d) atmospheric pressure decreases as height decreases

2. How does pressure exerted by a liquid change with depth?
(a) It decreases with depth
(b) It increases with depth
(c) It does not change with depth
(d) It cannot be determined

3. What is pressure?
(a) Volume/Area
(b) Force/Area
(c) Mass/Area
(d) Density/Area

Answers:

1. (c) Atmospheric pressure decreases as height increases
2. (b) ll increases will deeply
3. (b) Force/Area

B. Fill in the blanks.

1. The SI unit of pressure is_____.
2. The envelope of air all around us is known as__________.

Answers:

1. Pascal (Pa)
2. Atmosphere

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

1. Pressure depends on the area of contact between two surfaces.
2. ‘When a liquid is put in a vessel, it exerts pressure at its bottom only.

Answers:

1. True
2. False

Short Answer Type Questions

Question 1. Why are nail tips made pointed?

Answer
Pressure is force per unit area. The lesser the area the more is pressure. The tips of the nails are made pointed so that maximum pressure can Degenerate per unit force applied and hence, the nails can penetrate in an object easily.

Question 2. Why do divers wear special suits?

Answer
The pressure deep in the sea is very high and can cause damage to the human body. That is why divers wear specially designed suits to protect themselves from high pressu^.

Question 3. Why do cutting instruments have a sharper edge?

Answer
As pressure is force per unit area, all instruments used for cutting, like knives, blades, etc., have a sharp cutting edge. This decreases the area of contact between the surface of the instrument and the object and the pressure exerted by a given force increases.

Textbook Exercises

Question 1. Give two examples of each of the situations in which you push or pull to change the state of motion of objects.

Answer

1. A batsman hitting a ball (Push)
2. Drawing water from a well (Pull)

Question 2. Give two examples of situations in which applied force causes a change in the shape of an object.

Answer

1. Pressing a lump of dough on a plate.
2. A spring fixed in the seat of a bicycle pressed down due to the weight of the rider.

Question 3. Fill in the blanks in the following statements.

1. To draw water from a well we have to___________ the rope.
2. A charged body__________ an uncharged body towards it
3. To move a loaded trolley we have to___________ it.
4. The north pole of a magnet____________the north pole of another magnet.

Answer:

1. Pull
2. Attracts
3. Push or pull
4. Repels

Question 4. An archer stretches her bow while aiming at the target. She then releases the arrow, which begins to move toward the target. Based on this information, fill up the blanks in the following statements.

1. To stretch the bow, the archer applies a force that causes a change in its.__________
2. The force applied by the archer to stretch the bow is an example of___________ force.
3. The type of force responsible for a change in the state of motion of the arrow is an example of a__________ force.
4. While the arrow-‘ moves toward the target, the forces acting on it are due to
   that due to__________ of air.

Answer:

1. Shape
2. Muscular
3. Contact
4. Gravity, friction

Question 5. In the following situations, identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
Answer:

Question 6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Answer
The force applied by the hammer flattens the piece of iron, i.e., it changes its shape and size.

Question 7. An inflated balloon was pressed against a wall after it had been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the sticking balloon and the wall?

Answer
The electrostatic force is responsible for the attraction between the balloon and the wall.

Question 8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer
Forces acting on the plastic bucket are:
(i) Force of gravity acting downwards
(ii) The muscular force of arms acting upwards
The two forces do not bring any change in the state of motion of the bucket because both these forces are equal in magnitude and opposite in direction. Thus, they cancel each other and the net force on the bucket is zero.

Question 9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Answer
When the rocket leaves the launching pad, the ‘following forces act on it:
(i) Gravitational force of the Earth which pulls rocket towards the ground.
(ii) Frictional force due to the Earth’s atmosphere which opposes its mot on.

Question 10. When we press the bulb of a dropper with its nozzle in water, the air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to:
a) Pressure of water
b) Gravity of the Earth
c) Shape of a rubber bulb
d) Atmospheric pressure

Answer:
(d) Atmospheric pressure

Force and Pressure Hots Corner

A. Why do your ears ‘pop’ as you go higher up on mountains?
As we go higher up on mountains, our ears ‘pop’ because the air pressure around the ears decreases, causing an imbalance between the pressure inside and outside the ears.

Practice Exercise

Objective Type Questions

A. Give one word for each of the following.

1. Pressure exerted by the air around us
2. Force acting on a surface per unit area
3. A substance used for reducing friction
4. The force applied by the muscles of a human being or animal
5. A push or pull acting on a body, which changes its state o* rest or motion
6. The force exerted by a charged body on another charged or uncharged body

Answers:

1. Atmospheric pressure
2. Pressure
3. Lubricants
4. Muscular force
5. Force
6. Electrostatic force

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

1. If two forces act on an object from opposite directions, the resultant force is the sum of both forces.
2. Pressure exerted by a liquid at the base of the tie container in which it is kept depends on the depth of the liquid column.
3. The lesser the area of contact, the lesser is the effect of the force acting on an object
4. Mountaineers often suffer from nose bleeds at high altitudes.

Answers:

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

Force and Pressure Short Answer Type Questions

Question 1. Why school bags are provided with broad straps?

Answer
We know that the higher is the area of contact, the lesser is the pressure applied on an object School bags with broad straps occupy larger area and do not exert too much pressure on the shoulders. Hence, children do not feel strain on their shoulders.

Question 2. State where pressure is the greatest and the least inside a bottle filled with water.

Answer
In a bottle filled with water, pressure is the greatest at the bottom of the bottle and it is the least at the top. This is because the pressure exerted by liquids depends upon the depth o{the liquid column.

Question 3. If atmospheric pressure is so high, why are we not crushed by it?

Answer
The body of living organisms is made up of cells, which have fluids that exert pressure from within. The pressure exerted from inside the cell is equal to the atmospheric pressure. Thus, we are prevented from being crushed.

Question 4. Mountaineers always carry oxygen cylinders while climbing a mountain. What will happen if they do not carry it and why?

Answer
Mountaineers carry oxygen cylinders while climbing high mountains to avoid suffocation. As we go to a higher altitude, the layer of air that surrounds us becomes thinner. Due to lesser air, there is a decrease in the number of oxygen molecules per breath and this makes a person suffocate at high altitudes.

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 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Friction Concepts

• Force of Friction and factors affecting friction
• Friction: A Necessary Evil
• Increasing and reducing friction
• Fluid Friction

 Friction Objectives

• The force that opposes the relative motion between two objects in contact is called friction.
• Friction always acts in the direction opposite to the direction of the applied force.
• Friction is caused due to the irregularities on both the surfaces which are in contact
• The force of friction depends on the nature of the surfaces in contact and the mass of the objects in contact
• The force of friction is greater on rough surfaces as compared to smooth surfaces.
• Friction causes wear and tear, and wastage of energy.
• Friction can be considered as a ‘necessary evil’ because our normal life would not function properly without friction.
• Friction can be reduced by either polishing the surfaces or using lubricant wheels call bearings. Friction can be increased by increasing the roughness of the surface and mass of the object Friction is of three types static friction, sliding friction, and rolling friction.
• The frictional force exerted by fluids is called fluid friction.
• Fluid friction depends on the shape of the object and the nature of the fluid.
• The shape of the bodies of birds and fishes is such that they overcome fluid friction easily using less energy. Such shapes are called streamlined.

Read and Learn More UP Board Notes for Class 8 Science

Friction Important terms and Definitions

Ball bearing: Ball Bearings are rolling elements that use balls to reduce friction between the moving parts of the bearing.
Drag: The force of friction exerted by fluids is called drag.
Static friction: The force required to overcome friction when an object starts moving from rest is called static friction.
Sliding friction: The force required to keep an object moving with a uniform speed is called sliding friction.
Rolling friction: The force required to keep an object moving when it rolls over the other is called rolling friction.

12.1 Force of Friction and factors affecting friction

• The force that opposes the relative motion of two objects, when their surfaces are in contact with each other is called friction. If we push a ball forward, the force of friction acts in the backward direction and tends to stop the ball. Friction is caused by the irregularities present on the surfaces which are in contact Thus, rough surfaces exert greater frictional force as compared to smooth surfaces. The force of friction acting on a body depends upon the mass of the body and the nature of the surface. The force of friction increases if the two surfaces are pressed harder on each other.
• The force required to move an object from rest is called static friction. The force required to keep an object in motion is called sliding friction. The force exerted when two objects roll over each other is called rolling friction. The value of static friction is more than sliding and rolling friction. The value of rolling friction is less than sliding friction.

Friction Activity 1

Aim: To demonstrate the presence of friction
Procedure:

1. Gently push a book on the table.
2. After the book stops, repeat the activity by using the book from the opposite side.

Observation: When pushed the book slides for some time and then stops.
Conclusion: The motion of the book on the table is opposed by the frictional force. The force which opposes the motion of one body over the other is called friction.

Friction Activity 2

Aim: To demonstrate that rough surfaces exert greater frictional force as compared to smooth surfaces
Procedure:

1. Take a string around a brick and pull the brick by a spring balance.
2. Note down the reading on the spring balance when the brick begins to move.
3. Now, wrap a polythene sheet around the brick and repeat the activity.

Observation: The fading on the spring balance for the uncovered brick was more than that for the brick covered with polythene.
Conclusion: When polythene was not wrapped on the brick the surface of the brick was rough. So, the brick experienced greater frictional force. When polythene was wrapped around the brick the surface of the brick became smooth. So, the brick experienced lesser frictional force. Thus, rough surfaces exert greater frictional force as compared to smooth surfaces.

Friction Activity 3

Aim: To determine that friction depends on the nature of the surfaces in contact
Procedure:

1. Make an inclined plane on a smooth floor, or on a table as shown in the diagram below.
2. Put a mark with a pen at any point on the inclined plane.
3. Let a pencil cell move down from this point
4. Note down the distance the pencil cell moves on the table before coming to rest.
5. Spread a piece of cloth over the table but make sure that there are no wrinkles on the cloth.
6. Repeat the activity.

Observation:
The pencil cell rolls to a greater distance on the smooth floor or table than the floor or table covered with cloth.
Conclusion:
The resistance offered by the smooth table or floor is less as compared to the resistance offered by the cloth. Thus, the pencil cell covers a shorter distance on the cloth. The distance covered by the pencil cell depends on the nature of the surface on which it moves. Thus, friction depends on the nature of the surfaces in contact.

Objective Type Questions

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

1. Rolling friction is less than sliding friction.
2. Rolling friction exists between the surfaces when the body rolls over another.
3. Friction depends on gravity.

Answers:

1. True
2. True
3. False

B. Fill in the blanks.

1. The frictional force__________ with an increase in the force applied.
2. The force required to move an object from rest is called____________ friction.
3. The value of rolling friction is___________ then static friction.

Answers:

1. Increases
2. Static
3. Less

Short Answer Type Questions

12.2 Friction: A Necessary Evil

• Friction plays an important role in our daily life. It would be impossible to walk f there was no friction between the soles of ou^ shoes and the ground. Matchsticks are lit due to the force of friction between the match-head and the rough side of the matchbox. Cars and buses are able to move on the road because of friction between the tires and the road. Braking a vehicle is possible due to friction between the brake lining and the wheels of the vehicle.
• However, the tires of vehicles and the soles of shoes wear out due to friction. Friction causes a waste of energy; this is because anything that moves has to overcome the force of friction. This energy is converted into heat energy that, in turn, increases the temperature. The heat produced in the moving parts of machinery due to friction results in wear and tear of the parts. Friction reduces the speed of moving vehicles to a great extent.
  Thus, friction is considered a necessary evil.

Objective Type Questions

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

1. Friction causes a waste of energy.
2. Meteors get burnt due to the friction of air.
3. A rough surface experiences less wear and tear.

Answers:

1. True
2. True
3. False

B. Fill in the blanks.

1.  A bicycle comes to rest when we stop pedaling due to_________.
2.  ‘When we rub both of our hands with each other, they become warm due to__________
   produced by __________.

Answers:

1. Friction
2. Heat, Friction

C. Multiple Choice Questions.

1. Which of the following devices was used in ancient times to keep track of time?
(a) Digital Watch
(b) Stop Watch
(c) Wall Clock
(d) Sundial

2. Which of the following is true for two pendulums A and B of lengths 5 cm and 10 cm respectively?
(a) Both have the same time period
(b) A has a greater time period than B
(c) B has a greater time period than A
(d) Time period of A and B cannot be determined

Answer:

1. (d) Sundial
2. (c) B has a greater time period than A

Short Answer Type Questions

Question 1. Why are spaceships provided with heat shields?

Answer
When spaceships enter the Earth’s atmosphere they experience friction. The heat shields protect; them from the heat generated due to friction during their entry into the earth’s atmosphere.

12.3 Increasing and reducing friction

• Friction is caused by the roughness of the surfaces. friction can be reduced by: polishing the surfaces to make them smooth, using suitable lubricants or grease, using wheels or ball bearings, and giving an object a streamlined shape.
• Friction can be increased by increasing the roughness of the surface or the mass of the object. Spikes and grooves are provided in the soles of the shoes of athletes to increase friction.

Friction Activity 4

Aim: To demonstrate that rolling friction is smaller than sliding friction
Procedure:

1. Place a book on the table and push it from one side.
2. Place three or four cylindrical pencils on the table parallel to each other.
3. Now, place the books over these pencils as shown in the diagram below.
4. Push the book and observe the pencils rolling as the book moves.

Observation:
It was easier to move the books when placed on cylindrical pencils.
Conclusion:
The value of rolling friction is less than sliding friction.

Objective Type Questions

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

1. Spikes in shoes are provided to decrease friction.
2. Lubricants are oil-like substances used to reduce friction.

Answers:

1. False
2. True

B. Multiple-Choice Questions

1. Which of the following methods is used to increase friction?
(a) Polishing
(b) Roughening the surface
(c) Sfr’eamlining
(d) Lubrication

2. Use of bail bearings between hubs and the axles of ceiling fans is an example of______.
(a) Rolling Friction
(b) Static Friction
(c) Sliding Friction
(d) Kinetic Friction

Answers:

1. (b) Roughening the surface
2. (c) Sliding Friction

Short Answer Type Questions

Question 1. How are ball bearings useful to reduce friction?

Answer
The value of rolling friction is less than si ding friction. Ball bearings convert sliding friction into rolling friction and thus reduce friction. They are used in most of industrial machinery that has moving parts, car axles, etc.

12.4 Fluid Friction

• Fluids exert friction on the objects moving through them. This friction is called fluid friction or drag. The fluid friction exerted on a body depends on the shape of the moving body, the velocity of the moving body, and the nature of the fluid. Objects with streamlined shape experience less fluid friction. It is for this reason that boats and ships are streamlined. Automobiles are narrow from the front, so as to reduce drag in the air. The streamlined shape of fish enables them to move easily in water by minimizing fluid friction. Airplanes, rockets, and missiles have streamlined shapes to move in the air easily.

 Friction Objective Type Questions

A. State whether the statements are true or false.

1. Fluids exert force on the bodies moving through them. This force is called surface tension.
2. Fluid friction is also called drag.

Answer:

1. False
2. True

B. Multiple Choice Questions.

1. Which of the following factor has no effect on fluid friction?
(a) Mass of the object
(b) Shape of the object
(c) Velocity of the Object
(d) Nature of the fluid

2. Airplanes and rockets have___________ shape to reduce friction.
(a) Circular
(b) Triangular
(c) Streamlined
(d) None of the above

Answer:

1. (a) Mass of the object
2. (c) Streamlined

 Friction Short Answer Type Questions

Question 1. What is the reason behind the shape of airplanes?
Answer
The objects with streamlined shape experience less fluid friction. Airplanes have to streamline shape so that they can overcome the frictional force of the air.

Textbook Exercises

Question 1. Fill in the blanks.

1. Friction opposes the__________ between the surfaces in contact with each other.
2. Friction depends on________of the surfaces.
3. Friction produces___________ .
4. Sprinkling of powder on the carom board__________ friction.
5. Sliding friction is___________ than the static friction.

Answers:

1. relative motion
2. nature
3. heat
4. decreases
5. less

Question 2. Four children were asked to arrange forces due to rolling, static, and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.

1. Rolling, static, sliding
2. Rolling, sliding, static
3. Static, sliding, rolling
4. Sliding, static, rolling

Answer

3. Static, sliding, rolling

Question 3. Alida runs her car on a dry marble floor, wet marble floor, newspaper, and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be

1. Wet marble f our, dry marble floor, newspaper, and towel.
2. Newspaper, towel, dry marble floor, wet marble floor.
3. Towel, newspaper, dry marble floor, wet marble floor.
4. Wet marble floor, dry marble floor, towel, newspaper.

Answer:

4.  Wet marble floor, dry marble floor, newspaper, and towel.

Question 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of the frictional force acting on it.
Answer:

The frictional force acting on it is in the upward direction.

Question 5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer:

Soapy water will make walking more difficult Soapy floor acts as a smooth surface as it has no irregularities in it and soap is also very slippery in nature.

Question 6. Explain why sportsmen use shoes with spikes.
Answer:

Sportsmen use shoes with spikes to get a better grip on the ground. Spikes act as irregularities and help in increasing friction.

Question 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer:

Seema will experience more frictional force since the heavier box applies greater pressure on the floor as compared to the lighter box

Question 8. Explain why sliding friction is less than static friction.
Answer:

When a body is at rest, then the force acting between the two surfaces in contact is more due to irregularities on the two surfaces locked into one another. However, when a body is moving or sliding, the contact points do not get enough time to lock into one another. So sliding friction is always less than static friction.

Question 9. Give examples to show that friction is both a friend and a foe.
Answer: Friction is considered a friend. It is because of friction that:

1. We can write with a pen or pencil.
2. A teacher can write with chalk on the blackboard.
3. We can walk on the floor.
4. We can fix a nail on the wall.

Friction is considered a foe. It is because of friction that

1. Things experience wear and tear.
2. Heat is produced in machines that damage their parts.
3. The speed of the vehicle is reduced due to friction.

Question 10. Explain why objects moving in fluids must have special shapes.
Answer: When objects move through fluids, they have to overcome friction acting on them. In this process, they lose energy. Efforts are made to minimize friction by giving special shapes to the objects.

Friction Hots Corner

Question 1. Why do tires in the vehicles have designs and patterns?

Answer
Tires are the vehicles have designs and patterns (treads) with grooves on the surface to increase
friction and channel away water that, act as a lubricant on the road.

Question 2. Why it takes much longer time and distance to stop a moving ship in water than a moving car on the road?

Answer
The friction between the surface of water and the ship is very less due to the streamlined shape of the ship and hence the ship takes more time to stop and covers a large distance. On the contrary, the friction between the tires and the surface of the road is quite large. Hence, a car stops immediately; when brakes are applied.

Practice Exercises

 Friction Objective Type Questions

A. Fill in the blanks.

1. Grooves are made in the tyres of vehicles to__________ friction.
2. Polishing a rough surface________ friction.
3. Friction always acts in the direction_________ to the motion of the object
4. Liquids and gases are called__________.
5. The___________ of both the surfaces with each other causes friction.

Answers:

1. Reduce
2. Reduces
3. Opposite
4. Fluids
5. Interlocking

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

1. Friction exerted on an object does not depend on its mass.
2. It is easier to roll an object than to slide it.
3. Friction produces heat.
4. Friction can be increased by drying the surfaces in contact.
5. If two surfaces are pressed harder, the force of friction increases.

Answer:

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

C. Give reasons for each of the following.

1. A ball rolling on the ground stops after some time.
2. The shoes of athletes have grooves on their soles.
3. Machine parts are oiled occasionally.
4. It is difficult to write on glazed paper.
5. Airplanes and cars are given streamlined shapes.

Answer:

1. A ball rolling on the ground stops after some time because of the friction between the ball and the surface of the ground.
2. Spikes and grooves are provided in the soles of the shoes of athletes to increase friction.
3. Machine parts are oiled occasionally to reduce friction. This in turn reduces wear and tear of the machine parts.
4. It is difficult to write on glaze paper because the smooth surface of the glazed paper reduces friction.
5. Airplanes and cars have streamlined shapes to reduce frictional force.

 Friction Short Answer Questions

Question 1. Why are the handles of the bikes provided with rough surfaces?

Answer
The handles of bikes are provided with rough surfaces so as to provide a better grip due to friction.

Question 2. Why does a horse require more force to start a tonga than to keep it moving?

Answer
In the first few steps when the horse starts the tonga, it works against state friction. After that tonga begins to move, the horse has to work only against rolling friction. The value of static friction is more than rolling friction. Thus, the horse applies more force to start a tonga than to keep it moving.

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 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Chemical Effects Of Electric Current Concepts

• Electric Current
• Chemical Ejects of Electric Current
• Electroplating

Chemical Effects Of Electric Current Abstract

• Based on electrical conductivity, materials can be classified as conductors or insulators.
• Substances that allow an electric current to pass through them are called conductors, for example, copper, iron, etc.
• Substances that do not allow electric current to pass through them are called insulators, for example, wood, rubber, etc.
• Under certain circumstances, most of the substances can conduct electricity. Hence, substances through which electric current can flow easily are good conductors, while the substances through which electric current can pass negligibly or in a very small quantity are poor conductors.
• The passage of electric current through conducting liquids causes chemical changes in the conducting liquid.
• This phenomenon is called the chemical effect of electric current.
• The process of depositing a layer of desired metal on another material with the help of electricity is called electroplating.

Read and Learn More UP Board Notes for Class 8 Science

Important Terms And Definitions

Anode: The electrode which is connected to the positive terminal of a batter/ is known as the anode. Cathode: The electrode which is connected to the negative terminal of a battery is known as the cathode. Electrodes: Both anode and cathode are collectively called electrodes.
Electrolysis: The process of decomposition of chemicals in a solution or a liquid by passing electric current through it is called electrolysis.
Electrolytes: The substances which give ions in a solution are called electrolytes.
Light Emitting Diode (LED): It glows even when a small amount of current passes through the electric circuit

Electric Current

• Electric current or electricity can flow through different types of substances including liquids. Most of the liquids that conduct electricity are solutions of acids, bases and salts.
  

Chemical Effects of Electric Current Activity 1

Aim: To test whether lemon juice or vinegar is a good or bad conductor of electricity
Procedure:
1. Collect a few small plastics or rubber caps from the discarded bottles and clean them.
2. Pour one teaspoon of lemon juice or vinegar into one cap. Then, bring your tester over this cap and let the ends of the tester dip into lemon juice or vinegar as shown below.
3. Take care that the ends are not more than I cm apart but at the same time, they do not touch each other.
4. Check to see if the bulb of the tester glows or not

Observation: The bulb glows when dipped in the image here lemon juice or vinegar.
Conclusion: Lemon juice or vinegar is a good conductor of electricity.

Chemical Effects of Electric Current Activity 2

Aim: To prepare an electromagnetic tester based on the magnetic effect of current Precaution: Wash and wipe dry the ends of the test- after testing each liquid.
Procedure:
1. Take the tray of a discarded matchbox
2. Wrap an electric wire a few times around the tray and place a small compass needle inside it
3. Connect one free end of the wire to the terminal of a battery.
4. Leave the other end free. Take another piece of the wire and connect it to the other terminal of the batter.

5. Join the free ends of two wires momentarily. The compass needle should show deflection. Your tester with two free ends of the wire is ready.
6. Repeat ‘Activity 2’ above using this tester. You will notice that there is a deflection in the compass needle the moment the free ends of the tester are dipped in the lemon juice.
7. Take out the ends of the tester from the lemon juice. Dip them in water and then wipe them dry.
8. Repeat the activity with other liquids such as milk, honey, etc.
9. In each case, observe whether the magnetic needle shows deflection or not.

Observation:

Conclusion: Lemon juice, vinegar, tap water, milk, curd water and soda are all good conductors of electricity. However, vegetable oil, honey, kerosene oil and distilled water are all bad conductors of electricity.

Chemical Effects of Electric Current Activity 3

Aim: To show that distilled or pure water is a poor conductor of electricity
Procedure:

1. Take about two teaspoons full of distilled water in a clean, dry plastic or rubber cap of a bottle.
2. Use the tester to test if distilled water conducts electricity.
3. Dissolve a pinch of common salt in the distilled water and test again.

Observation:

1. The bulb of the tester does not glow when the tester is put into distilled water.
2. When a pinch of salt is dissolved in the distilled water and the tester is immersed in it, the bulb glows.

Conclusion: Distilled or pure water is a poor conductor of electricity.

Chemical Effects of Electric Current Activity 4

Aim: To show that water containing any salt, acid or alkali is a good conductor of electricity
Procedure:

1. Take three clean plastic or rubber caps of bottles.
2. Pour about two teaspoonfuls of distilled water into each of them.
3. Add a few drops of lemon juice or dilute hydrochloric acid to the distilled water in the first cap.
4. In the second cap, add a few drops of a base such as caustic soda or potassium iodide.
5. Add a small amount of sugar to the distilled water in the third cap and dissolve it.
6. Test to see which of these solutions conduct electricity.

Observation: Solutions in the first and second cap conduct electricity, while the one in the third does not conduct electricity.
Conclusion: Water containing any salt, acid or alkali is a good conductor of electricity.

Objective Type Questions

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

1. All liquids allow electric current to flow through them.
2. The deflection of the needle of a magnetic compass kept near an electric circuit indicates the flow of electric current.
3. An LED glows even when a small amount of electric current is passed through it.
4. Adding sugar to distilled water makes it a good conductor of electricity.

Answers:

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

B. Multiple Choice Questions.

1. Which of the following does not allow the passage of electric current through them?
(a) Tap water
(b) Vinegar
(c) Distilled water
(d) Seawater

2. ‘Which of these substances is a poor conductor of electricity?
(a) Wood
(b) Iron
(c) Copper
(d) Graphite

Answers:

1. (c) Distilled water
2. (a) Wood

C. Fill in the blanks.

1. Materials that do not allow electric current to easily pass through them are called__________
conductors of electricity.
2. Tap water is a__________ conductor of electricity.
3. An LED consists of two wires called_________.

Answers:

1. Poor
2. Good
3. Leads

Chemical Effects of Electric Current Activity-4 Short Answer Type Questions

Question 1. What are electrodes?

Answer
Electrodes are electrical conductors that are used to make contact with the non-metallic part of a circuit such as a conducting liquid, air or a semiconductor.

Question 2. Explain how an LED should be connected to a circuit

Answer
LED or light emitting diode comprises a bulb with two wires (called leads), where one lead is longer than the other. The longer lead is connected to the positive terminal of the battery and the shorter lead is connected to the negative terminal of the battery.

Chemical Effects of Electric Current

• The passage of electric current through conducting liquids causes chemical changes in the liquids. This phenomenon is called the chemical effect of electric current. The chemical reactions or changes vary depending on the type of solution and electrodes used.

Chemical Effects Of Electric Current Activity 5

Aim: To show the production of gases during electrolysis of water
Procedure:
1. Carefully take out carbon rods from two discarded batteries.
2. Clean their metal caps with sandpaper.
3. Wrap copper wires around the metal caps of the carbon rods and connect them to a battery.

Note: Instead of carbon rods, you may take bio-iron nails about 6 cm long.
4. Pour a cupful of water into a glass or a plastic bowl.
5. Add a teaspoonful of salt or a few drops of lemon juice to the water to make it more conducting.
6. Immerse the electrodes in this solution. Ensure that the metal caps of the carbon rods are outside the water.
7. Wait for 3-4 minutes.
8. Observe the electrodes.
Observation: Gas bubbles are seen at the ends of the electrodes.
Conclusion:
1. Due to the passage of electric current, water is broken down into its constituent gases, i.e., hydrogen and oxygen. This process of breaking up a*i electrolyte by passing an electric current through it is known as electrolysis.
2. Oxygen bubbles are formed on the positively charged electrode (anode) and hydrogen bubbles are formed on the negatively charged electrode (cathode).

Objective Type Questions

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

1. When electric current passes through copper sulphate solution, copper gets deposited at the anode.
2. Electric current can bring a chemical change in a liquid solution.

Answers:

1. False
2. True

B. Multiple Choice Questions.

1. ‘What does a positively charged electrode know as?
(a) Cathode
(b) Anode
(c) Electrolyte
(d) Electrolysis

2. When electrodes are immersed in water and electric current is passed through them, on which terminal of the battery oxygen bubbles are formed?
(a) Positive terminal
(b) Negative terminal
(c) Both the terminals
(d) Neither of the terminal

Answers:

1. (b) Anode
2. (a) Positive terminal

C. Fill in the blanks.

1.  When electrodes are immersed in water and an electric current is passed through them, bubbles of
   __________and_________are produced.
2. The closed path through which electric current ‘lows is called a________.
3. The negative terminal of a batter)’ or negatively charged electrode is also known as the_______.

Answers:

1. Oxygen, hydrogen
2. Circuit
3. Cathode

Chemical Effects Of Electric Current Activity 5 Short Answer Type Questions

Question 1. During electroplating, the copper deposited on the plate connected to the negative terminal comes from the solution. How is this loss of copper from the solution compensated?

Answer
The loss of copper from the solution is compensated by the other plate. An equal amount of copper gets dissolved in the solution and the process keeps on going. This means that copper gets transferred from one plate to another.

Question 2. Solid NaCI does not conduct electricity but when dissolved in water, it conducts electricity. Why?

Answer
Solid NaCI consists of ions that cannot move. However, when solid NaCI is dissolved in water, the ions separate and can move easily. Thus, solid NaCI cannot conduct electricity, whereas NaCI solution can conduct electricity.

14.3 Electroplating

• Electroplating is the process of depositing a layer of a desired metal on another material with the help of electricity. Artificial jewellery is made with coatings of less expensive metals such as iron and aluminium with costlier metals, such as silver and gold to give them a rich look.

Chemical Effects Of Electric Current Activity 6

Aim: To demonstrate the process of electroplating
Materials Required: Two copperplates approximately 10 cm 4 cm in size, copper sulphate, distilled water, a beaker, dilute sulphuric acid, sandpaper and batter)’
Procedure:
1. Take 250 ml of distilled water in a clean and dry beaker.
2. Dissolve two teaspoonfuls of copper sulphate in rt. Also, add a few drops of dilute sulphuric acid to the copper sulphate solution to make it more conducting.
3. Clean copper plates with a sandpaper.
4. Rinse the plates with water and let them dry/.
5. Connect the copper plates to the terminals of a batter)- and immerse them into the copper sulphate solution.
6. Allow the electric current to pass through the circuit for abo it 15 minutes. Now, remove the electrodes from the solution and observe them.

Observation: Copper metal gets deposited on the plate at the negative terminal of the battery.

Objective Type Questions

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

1. During electroplating, the object to be electroplated is connected to the positive terminal of the batter).
2. Copper and iron are the only metals that can be used for electroplating purposes.
3. A coating of zinc is deposited on iron to protect it from corrosion.

Answers:

1. False
2. False
3. True

B. Multiple Choice Questions.

1. Electroplating is based on which of these effects of electricity?
(a) Magnetic effect
(b) Physical effect
(c) Heating effect
(d) Chemical effect

2. Which of these is true about electroplating?
(a) It is a method of making metal plates.
(b) It is the process of coating a layer of metal over another metal by using an electric current.
(c) It is the technique of making plates that do not conduct electricity,
(d) None of these

Answers:

1. (d) Chemical effect
2. (b) ft is the process of coating a layer of metal over another metal by using electric current

C. Fill in the blanks.

1.__________is used to coat bath taps, bicycle handlebars, etc., and give them a shiny look.
2.________is one of the most common applications of chemical effects of electric current.

Answers:

1. Chromium
2. Electroplating

Chemical Effects Of Electric Current Activity 6 Textbook Exercises

Question 1. Fill in the blanks.

1. Most liquids that conduct electricity are solutions of_________,__________ and_________
2. The passage of an electric current through a solution causes__________effects.
3. If the current passes through a copper sulphate solution, copper gets deposited on the plate connected
   to the__________terminal of the battery.
4. The process of depositing a layer of any desired metal on another material using electricity
   is called____________.

Answers:

1. Acids, bases, salts
2. Chemical
3. Negative
4. Electroplating

Question 2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Answer
The needle shows deflection because the solution is a good conductor of electricity and electric current has a magnetic effect

Question 3. Name three liquids, which when tested in the manner given below, may cause the magnetic needle to deflect.
Answer: Tap water, lime water and vinegar cause the magnetic needle to deflect.

Question 4. The bulb does not glow in the setup shown in the figure given below. List the possible reasons. Explain your answer.
Answer:

In the given setup, if the bulb does not glow, it may be because the bulb may be fused or the connection of the wires may be loose. Replace the bulb and check once again. If the bulb still does not glow, check the connection of the wires. However, after tightening the connections and testing the bulb, if it still does not glow, then the only possible reason can be that the solution is a poor conductor of electricity.

Question 5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightfor liquid A, while it glows very dimly for liquid B. You would conclude that

1. liquid A is a better conductor than Liquid B
2. liquid B is a better conductor than Liquid A
3. both liquids are equally conducting
4. conducting properties of liquids cannot be compared in this manner

Answer:

1. liquid A is a better conductor than liquid B.

Question 6. Does pure water conduct electricity? If not, what can we do to make it conduct?

Answer
No, pure water does not conduct electricity. However, adding some salt, acid or alkali can make pure water a good conductor of electricity.

Question 7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Answer
Tap water is a good conductor of electricity. Hence, to avoid electric shock, firemen shut off the main electrical supply before using water hoses.

Question 8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Answer
Drinking water is generally free of salts and minerals, whereas seawater is rich in salts and minerals. Thus, seawater is a better conductor of electricity than drinking water. Hence, the compass needle deflects more in the case of seawater as compared to drinking water

Question 9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpours?

Answer
Explain. No. it is not at all safe to perform outdoor electrical repairs when it is raining. Rather, it is dangerous because the electrician may get an electric shock as rainwater is a good conductor of electricity.

Question 10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester To her surprise she found that the compass needle showed deflection. What could be the reason?

Answer
Though rainwater is as pure as distilled water it may be contaminated by the impurities suspended in the atmosphere. These impurities make rainwater a better conductor of electricity. This is the reason that the compass needle showed deflection.

Question 11. Prepare a list of objects around you that are e electroplated.

Answer
Bicycle rims, door handles, taps, metallic pens, artificial jewellery, utensils, belts, and buckles.

Question 12. The process that you saw in Activity 7 is used for the purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod j is sought to be transferred to the thin copper plate. Which electrode should be attached to the • positive terminal of the battery and why?

Answer
The impure copper rod should be attached to the positive terminal because copper ions will get drawn towards the negative terminal and will be deposited there. So, the copper from the impure rod will get deposited on the pure copper plate.

Chemical Effects of Electric Current Hots Corner

Question 1. We are advised not to touch electrical appliances with wet hands. Why?

Answer
Small quantities of mineral salts are naturally present in water. The salts make water a good conductor of electricity. Therefore, it may be dangerous to touch any electrical appliance with wet hands because of the potential risk of being electrocuted.

Practice Exercise Objective Type Questions

A. Give one word for the following.

1. Substances that do not allow an electric current to pass through them
2. The metal rods dipped in liquids to which batteries are attached in a circuit
3. Substances that conduct electricity in a liquid state or when dissolved in water and break up chemically during the process

Answers:

1. Insulators
2. Electrodes
3. Electrolytes

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

1. Electric bulbs glow due to the chemical effect of electricity.
2. Some liquids are good conductors of electricity and some are poor conductors.
3. Electrolysis is a chemical change.
4. Bath taps are electroplated to prevent them from corrosion.
5. A solution in which very few mobile ions are present is a strong electrolyte.

Answers:

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

C. Circle the odd one out.

1. Plastic, copper, aluminium, graphite
2. Lemon juice, tap water, vegetable oil, and common salt solution

Answers:

1. Plastic
2. Vegetable oil

D. Fill in the blanks.

1. Electroplating __________rusting of objects.
2. is an electrolyte and____________ is a non-electrolyte.
3. Distilled water is a____________ conductor of electricity.
4. Carbonic acid is a____________ electrolyte.

Answers:

1. Prevents
2. Sulphuric acid, ethyl alcohol
3. Poor
4. Weak

Chemical Effects Of Electric Current Short Answer Type Questions

Question 1. Name four liquids that can conduct electricity.

Answer
Sea water, vinegar, soda and milk

Question 2. A pencil sharpened at both ends can be used as a conductor in laboratories. Why?

Answer
A pencil sharpened at both ends can be used as a conductor because pencil lead is made up of graphite and graphite is a good conductor of electricity.

Question 3. Why high voltage electric current is dangerous for our body?

Answer
The brain and heart send tiny electrical impulses generated within the body. If high voltage electric current passes through our body, the brain or heart could stop working. Thus, high-voltage electric current is dangerous for our bodies.

• 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 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Sound Concepts

•  Sound and its Production
•  Propagation of Sound
• Characteristics of Sound

Sound Abstract

• Sound is produced by a vibrating object.
• The to-and-fro motion of an object is termed a vibration. Vibration is an example of oscillatory motion.
• In human beings, the sound is produced by the voice box, which is also called the larynx.
• Two cords called the vocal cords, are stretched across the larynx in such a way that there is a narrow slit between them for the passage of air. When air is forced through this slit vocal cords vibrate and sound is produced.
• Sound needs a medium to propagate. It cannot travel in a vacuum.
• Sound is received by our outer ear from where it travels down a canal.
• At the end of the ear canal is a tightly stretched, thin membrane called the eardrum.
• The eardrum is just like a stretched rubber sheet. When sound strikes the eardrum, it vibrates. The eardrum then sends vibrations to the inner ear.
• From the inner ear, the signals are further transmitted to the brain and we can hear.
• The number of oscillations per second is called the frequency. It is measured in hertz (Hz).
• The maximum displacement of a vibrating body from its mean position is called amplitude.
• The time taken to complete one oscillation is called the period.
• Amplitude and frequency are the two important properties of any sound.
• The amplitude of the vibrating body producing the sound determines the loudness of the sound. The larger the amplitude of the vibration, the louder the sound is produced.
• Frequency determines the shrillness or pitch of a sound.
• The higher the frequency of vibration, the higher the pitch.
• Sound can also be classified as audible and inaudible.
• Sound of frequencies less than about 20 vibrations per second (i.e. 20 Hz) and more than about
• 20,0 vibrations per second (i.e. 20 kHz) are not audible to the human ear. Such sounds are called inaudible sounds.
• Thus, for a normal human ear, the audible frequency range is roughly between 20 and 20,000 Hz. A sound that is pleasing to the ear is called a musical sound, while an unpleasant sound is called noise.
• Noise pollution may cause many health-related problems such as lack of sleep, hypertension {high blood pressure), anxiety, etc.
• A person who is continuously exposed to a loud sound may suffer from temporary or even permanent hearing impairment.
• Planting trees along the roadside and in the surroundings can reduce noise pollution.

Read and Learn More UP Board Notes for Class 8 Science

Sound and its Production

• A sound is a form of energy which is produced by a vibrating body. In human beings, the larynx helps in; the production of sound. It is situated in the neck at the upper end of the windpipe. Sound is controlled; by the vocal cords. The quality of sound produced from loose and thick vocal cords is different from the ’ sound produced by tight and thin vocal cords.

Sound Activity 1

Aim: To create and feel vibrations
Procedure:
1. Take a metal plate (or a shallow frying pan) and hang it at a convenient place in such a way that it does not touch any walls.
2. Now strike it with a stick and touch the plate or pan gently with your finger. Note down your observation.
3. Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Note down your observation again.
4. Touch the plate after it stops producing sound. Note down your observation.

Observations: When the plate was struck, sound and vibrations produced were felt on touching the plate gently. The sound produced vanished on holding the plate tightly. When the plate stopped producing the sound, no vibrations were felt.
Conclusions: When a metal object is struck vibrations are produced. Sound is also produced by a vibrating object.

Sound Activity 2

Aim: To produce sound Dv creating vibrations
Procedure:
1. Take a rubber band and put it around the longer side of a pencil box
2. Insert two pencils between the box and the stretched rubber and then pluck the rubber band somewhere in the middle. Note down your observations.

Observations: On plucking, the band vibrates and a sound is produced.
Conclusion: Sound is produced by a vibrating object.

Sound Activity 3

Aim: To create waves by producing vibrations
Procedure:
1. Take a metal dish and pour water into it. Strike it at its edge with a spoon
2. Note down your observations.

2. Strike the plate again and then touch it. Note down your observations.
3. Strike the dish again and look at the surface of the water.
4. Then hold the dish and note down your observations.
Observations: In steps I and 2, both sound and vibrations were produced. In step 3, waves were seen on the water’s surface. But on holding the dish, vibrations stopped and there were no waves.
Conclusion: Vibrations produce sound as well as waves.

Sound Activity 4

Aim: To make an Ektara and identify its vibrating part
Procedure:
1. Take a hollow coconut shell or an earthen pot and make a musical instrument Ektara.
2. Play this instrument and identify its vibrating part.

Observations: The vibrating part is its strings.
Conclusion: The vibrating part of an Ektara is its string, which produces sound vibrating.

Sound Activity 5

Aim: To make a Jaitarang
Procedure:
1. Take 6-8 metal bowls or tumblers and fill them with water up to different levels, increasing gradually from one end to the other.
2. Then take a pencil and strike the bowls gently, in succession. Note down your observation.

Observations: Different types of sounds are produced on vibrating bowls filled with water up to different levels.
Conclusion: Vibrations produce sound. The sound produced varies with the amount of water present in a bowl.

Sound Activity 6

Aim: To find out how vocal cords produce sound
Procedure:

1. Take two rubber strips of the same size. Place them one above the other and stretch them tight.
2. Now blow air through the gap between them
3. Record your observation.

4. You can also take a piece of paper with a narrow slit and hold it between your fingers
5. Now’ blow’ through the slit and listen to the sound.
Observation: As the air blows through the stretched rubber strips or the piece of paper, a sound is produced.
Conclusion: In human beings, sound is produced by the voice box or the larynx. Two cords called the vocal

Objective Type Questions

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

1. The to-and-fro motion of an object is called vibration.
2. In human beings, the sound is produced by vocal cords.
3. ‘When we pluck the string of an instrument, like the sitar, not only its string but the whole instrument vibrates.
4. Women have longer vocal cords than men.

Answer

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

B. Multiple choice questions.

1. What does a vibrating body transfer to the adjacent molecules in a medium?
(a) Energy
(b) Force
(c) Speed
(d) Pressure

2. What is the length of the vocal cord in men?
(a) 10 mm
(b) 15 mm
(c) 20 mm
(d) 25 mm

Answers:

1. (a) Energy
2. (c) 20mm

C. Fill in the blanks.

1. Sound is a form of_________
2. Sound is produced when objects__________
3. The voice box is also called the_________
4. The voice box is situated in the neck at the upper end of the________

Answers:

1. Energy
2. Vibrate
3. Larynx
4. Windpipe

Sound Short Answer Type Questions

Question 1. How do human beings produce voices?

Answer
In human beings, the sound is produced by the voice box, which is also called the larynx. Two cords called the vocal cords, are stretched across the larynx in such a way that there is a narrow slit between them for the passage of the air. When air is forced through this slit, the vocal cords vibrate and sound is produced.

 Propagation of Sound

• Sound needs a medium for its propagation. Sound can travel through solids, liquids, and gases.  Sound cannot travel through a vacuum. The outer part of the ear, which is shaped like a funnel, – receives the sound. From here, the sound travels through the middle ear and reaches the inner ear; which transmits signals to the brain. That is how we hear.

Sound Activity 7

Aim: To observe that sound needs a medium to travel
Procedure:
1. Take a dry metal tumbler and place a cell phone inside it
2. Ask your friend to call you on that cell phone from another cell phone. Listen to the ring carefully.
3. surround the nim of the tumbler with your hands and put your mouth on the opening between your hands.

4. Indicate to your friend to call on that cell phone again. Listen to the ring while sucking air from the tumbler.
5. Remove the tumbler from your mouth. Note down your observation.

Observations: The sound became fainter as the air was sucked from the tumbler. The sound became loud again after removing the mouth.
Conclusion: As the air was sucked, a partial vacuum was created inside the tumbler and the sound became fainter. Once air entered the tumbler again, the sound became louder. This proves that sound needs a medium to travel.

Sound Activity 8

Aim: To determine if sound can travel through liquids
Procedure:
1. Take a bucket or a bathtub and fill it with clean water.
2. Take a small bell in one hand. Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub.
3. Place your ear gently on the water surface and note down your observations.

Observations: The sound of the bell is heard.
Conclusion: Sound can travel through liquids.

Sound Activity 9

Aim: To determine if sound can travel through solids
Procedure:
1. Take a meter scale or a long metal rod and hold its one end to your ear.
2. Ask your friend to gently scratch or tap the other end of the scale.

Observations: The sound of the scratch is heard.
Conclusion: Sound can travel through solids.

Sound Activity 10

Aim: To observe how an eardrum works
Procedure:
1. Take a tin can and cut its ends.
2. Stretch a piece of rubber balloon across one end of the can and fasten it with a rubber band.
3. Put four or five grains of dry cereal on the stretched rubber and ask your friend to speak “Hurray, Hurray’1 from the open end.

Observations: On speaking, grains jump up and down.
Conclusion: Grains jump up and down due to the vibrations. The vibrations produce movements. The eardrum is just like a stretched rubber sheet. Sound vibrations make the eardrum vibrate and thus, we can hear sound.

Objective Type Questions

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

1. Sound needs a medium for its propagation.
2. The eardrum sends vibrations directly to the brain.

Answers:

1. True
2. False

B. Multiple-Choice Questions

1. What is the name of the membrane present in the ear?
(a) Ear foil
(b) Eardrum
(c) Ear sheet
(d) Hammer

2. In which of the following mediums does sound travel the fastest?
(a) Water
(b) Air
(c) Iron
(d)Vacuum

Answers:

1. (b) Eardrum
2. (c) Iron

C. Fill in the blanks.

1. The_______sends vibrations to the inner ear.
2. Sound travels the_________in solids and_______in gases.

Answers:

1. Eardrum
2. Fastest, Slowest

Sound Short Answer Type Question

Question 1. How do we hear?
‘When sound enters into our ear, it travels through a canal at the end of which there is a tightly stretched, thin membrane called the eardrum. When sound strikes the eardrum, it vibrates. The eardrum then sends vibrations to the inner ear. From there, the signal is further transmitted to the brain. That is how we hear.

 Characteristics of Sound

• Amplitude and frequency are two important properties of sound. Frequency determines the shrillness or pitch of a sound. The male voice has a low pitch, whereas the female voice has a high pitch. Scientifically, a musical sound is produced by regular vibrations, and noise is produced by irregular vibrations. Too much noise in our surroundings is harmful to us and is known as noise pollution.

Sound Activity 11

Aim: To find the relation between the loudness and amplitude of the sound
Procedure:
1. Take a metallic tumbler and a tablespoon.
2. Strike the tablespoon gently at the brim of the tumbler.
3. Hear the sound produced.
4. Now’ Dang the spoon on the tumbler and hear the sound produced again.
5. Next suspend a small thermocol ball touching the rim of the tumbler.
6. vibrate the tumbler by striking it and check how far the ball is displaced.
7. Strike the tumbler gently and then with some force.
8. Compare the displacements (or amplitudes} of vibrations in both cases.

Observation: In steps, I and 2, the sound produced was loud when the tumbler was struck hard. In steps 3 and 4, the vibrations produced were more when force was applied.
Conclusion: The loudness of the sound was more when the amplitude of the vibration of the tumbler was greater. Thus, the loudness of sound is proportional to its amplitude.

Objective Type Questions

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

1. The number of oscillations per second is called the amplitude of oscillation.
2. Amplitude and frequency are two important: properties of any sound.
3. The larger the amplitude of vibrations, the louder the sound produced.
4. For a normal human ear, the audible frequency range is roughly between 200 and 20,000 Hz.

Answers:

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

B. Multiple-Choice Questions

1. Which of the following determines the shrillness or pitch of a sound?
(a) Frequency
(b) Amplitude
(c) Period
(d) None of these

2. ‘Which characteristic of sound distinguishes a male and a female voice?
(a) Loudness
(b) Period
(c) Pitch
(d) Amplitude

Answers:

1. (a) Frequency
2. (c) Pitch

C. Fill in the blanks.

1. The higher the frequency of vibration, the__________is the pitch.
2. The maximum displacement of a vibrating body from its mean position is called___________.
3. Sounds of frequencies less than about___________ GHz are not audible to the human ear.
4. A complete to-and-fro motion of a vibrating body about its mean position is called a /an_________.

Answers:

1. Higher
2. Amplitude
3. 20
4. Oscillation

Sound Textbook Exercises

Question 1. Choose the correct answer.
Sound can travel through

(a) Gases only
(b) Solids only
(c) Liquids only
(d) Solids, liquids, and gases

Answer

(d) Solids, liquids, and gases

Question 2. Voice of which of the following is likely to have a minimum frequency?

(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman

Answer

(c) A man.

Question 3. In the following statements, tick (T) against those which are true, and (F) against those which are false.

1. Sound cannot travel in a vacuum.
2. The number of oscillations per second of a vibrating object is called its period.
3. If the amplitude of the vibration is large, the sound is feeble.
4. For human ears, the audible range is 20 Hz to 20,000 Hz.
5. The lower the frequency of vibration, the higher the pitch.
6. An unwanted or unpleasant sound is termed as music.
7. Noise pollution may cause partial hearing impairment

Answers:

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

Question 4. Fill in the blanks with suitable words.

1. The time taken by an object to complete one oscillation is called.
2. Loudness is determined by the vibration.
3. The unit of frequency is.
4. Unwanted sound is called
5. The Shrillness of a sound is determined by the vibration.

Answer:

1. Period
2. Amplitude
3. Hertz (Hz)
4. Noise
5. Frequency.

Question 5. A pendulum oscillates 40 times in 4 seconds. Find its period and frequency.

Answer

Frequency=Number of oscillations/Time in seconds

Frequency = 40/4

Frequency = 10 Hz

Time period =1/Frequency

Time period = 1/10

Period — 0.1 s

Question 6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the period of the vibration?

Answer

Frequency = 500 vibrations per second – 500 Hz
We know, that time period =1/Frequency
Time period = 1/500
Time period = 2 x 10 3 s

Question 7. Identify the part that vibrates to produce sound in the following instruments:
(a) Dholak (b) Sitar (c) Flute

Answer

(a) Stretched membrane
(b) String
(c) Air column

Question 8. What is the difference between noise and music? Can music become noise sometimes?
Answer:

Differences between noise and music are:

Yes, music can become noise. When music is too loud or the sounds produced by the instruments are not in sync with each other then music becomes noise.

Question 9. List sources of noise pollution in your surroundings.

Answer
Sources of noise pollution are:

1. Noise of vehicles
2. Loudspeakers
3. Working machines
4. Bursting of crackers
5. Air coolers
6. Radios and televisions at high volumes
7. Kitchen Appliances
8. Hawkers etc.

Question 10. Explain in what ways noise pollution is harmful to humans.

Answer
Too much noise in our surroundings is known as noise pollution. It is very harmful to us and can cause irritability, loss of concentration, high blood pressure, headache, stress, sleep disturbances, etc. It can even damage hearing permanently.

Question 11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Answer
My parents should buy the house, which is located three lanes away from the roadside because a roadside house has many disadvantages such as:

1. A lot of noise due to passing vehicles.
2. Smoke and dust are produced by running vehicles.
3. Sound of loud horns of vehicles at the time of traffic jams.

Question 12. Sketch the larynx and explain its function in your own words.
Answer:

Functions of the larynx: When air passes through the vocal cords, they vibrate to produce sound. Vocal cords may become loose/thick or tight/thin on vibration, thus producing different Types of sounds.

Question 13. Lightning and thunder take place in the sky at the same time and the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?

Answer
The speed of light is 3 * 10® m/s, while that of sound is only 330 m/s. So, although lightning and thunder take place simultaneously in the sky and at the same distance from the Earth, lightning is seen first.

Sound Hots Corner

1. Can two people talk on the Moon in the same way as they can on the Earth?

Answer
No, there is no air or atmosphere on the Moon and sound needs a medium to travel. Therefore, the sound from one person will not travel to the ear of the other person. Hence, we cannot talk on the Moon in the same way as we do on the Earth.

Practice Exercise

A. Give one word for the following.

1. The position where the bob of a pendulum stops
2. Number of oscillations per second
3. The sound produced by regular vibrations
4. The factor on which the loudness of a sound depends
5. Unit in which the loudness of a sound is measured

Answers:

1. Mean position
2. Frequency
3. Music
4. Amplitude
5. Decibel

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

1. Sound travels at different speeds in different substances.
2. Scientifically, a musical sound is produced by regular vibrations.
3. The male voice has a high pitch.
4. The time period is denoted by the letter T.
5. Loudness of a sound is inversely proportional to the amplitude of the vibration.

Answers:

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

Sound Short Answer Type Questions

Question 1. Sound cannot travel through a vacuum. Why?

Answer
Sound is a form of energy and is produced by a vibrating body. The vibrating body transfers its energy to the neighboring molecules in the medium, which in turn pass on their energy to the other; molecules. Since no molecules are present in a vacuum, there is no transfer of energy. Thus, sound \ cannot travel in a vacuum.

Question 2. The bob of a pendulum oscillates twice per second. Find its period and frequency.

Answer
Frequency = Number of oscillations/Time (in seconds)

Frequency = 2/1 = 2 Hz.

Time period = 1/Frequency

Time period = — = 0.5 s

Question 3. Write any three methods to reduce noise pollution.

Answer

Three methods to reduce noise pollution are:

1. The use of loudspeakers in public places and residential areas should be banned.
2. Vehicles should be fitted with effective silencers and should not be allowed to play loud music.
3. Planting more trees on the roadsides reduces noise pollution as they absorb the sound.

Question 4. What are ultrasounds?

Answer
Sound frequencies more than 20,000 Hz (20 k Hz) are called ultrasounds. Human beings cannot perceive ultrasonic sounds. Hence, ultrasounds are also called ‘inaudible’ sounds.

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 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Absorption by Roots Learning Objectives

After completing this chapter, you will be able to:

• Explain how roots are adapted for absorbing water;
• Describe the adaptation in roots for absorption of water and minerals;
• Explain diffusion, osmosis, imbibition, active transport, turgidity and plasmolysis and their importance for plants;
• Differentiate between diffusion, osmosis and active transport;
• Differentiate between turgidity and flaccidity, plasmolysis and Deplasmolysis;
• Explain the process of absorption of water and minerals by roots;
• Explain the ascent of sap in plants;
• Discuss the causative forces in the ascent of sap, namely cohesive, adhesive forces and transpiration pull.

Like animals, plants also need water and nutrients to survive. Plants absorb water and mineral nutrients from the soil through roots. The roots conduct these into the stem for supplying to upper parts like leaves, flowers, fruits, etc. How do water and minerals absorb from the soil move from one part to another part of the plant body? In this chapter, we will study about some fundamental processes that help in the absorption of water and minerals by the roots in plants.

Absorption by Roots Why Do Plants Need Water And Minerals?

Plants need water and minerals for many purposes as mentioned subsequently.

Need for water

• For photosynthesis: Water is used as a raw material for the synthesis of glucose during the process of photosynthesis.
• For transpiration: It is used for cooling the tree in warm weather and for generating a pull/suction force for the movement of sap by transpiration.
• For transportation: It helps in the upward transport of minerals from the roots into the shoot system and for the transport of food manufactured in leaves to other parts.
• For mechanical strength: It is required for providing turgidity, which makes plant tissues stiff and gives them strength.

Need for mineral nutrients

• For movement of substances through the cell membrane (calcium)
• For respiration (iron) and cell division (phosphorus)
• For activating the enzymes (potassium, magnesium)
• For making chlorophyll (magnesium)
• For being part of nucleic acids, chlorophyll, proteins (nitrogen)
• For serving as building blocks of many compounds synthesized by plants such as a new protoplasm, etc.

Absorption by Roots How Are Roots Adapted For Absorption Of Water?

• Roots contain root hair that provide large surface area: A plant may contain millions of root hair, which together provide large surface area for the absorption of water.
• The epidermis of root hair is permeable to water: The root hair has very thin walls that are freely permeable to water. The thin walls help in easy movement of water and minerals in and out of cells.
• Most of the absorption of water and minerals occurs near root tips. The soil particles, which are usually coated with water and dissolved minerals, adhere tightly to the root hair. The soil solution flows in and out of the root hair cells.
• Root hair contains cell sap, which is at a higher concentration of minerals than the surrounding water: Root hair grows from the outer layer of the cortex. Between the cells of the cortex are large air spaces (vacuoles) that allow diffusion of gases and movement of water. The spaces allow water to get into the root by capillary action. The large vacuoles in the plant cells contain a solution called cell sap, which contains dissolved salts and is therefore of a higher concentration than the surrounding water. This facilitates osmosis, as a result of which water from outside is drawn inside the root hair.
  

How Do Absorption And Conduction Of Water And Minerals Occur?

The absorption of water and minerals from the soil by the root hair, their movement through the thickness of the root and then upward conduction through the stem to the leaves of the plant takes place through the following processes.

• Diffusion
• Osmosis
•  Imbibition
• Active transport
• Turgidity and flaccidity

Diffusion

The movement of molecules of a substance from a region of their higher concentration to a region of their lower concentration is called diffusion.
Large molecules move much more strongly than small molecules. If you add a small drop of dye (like ink) to one end of a tub of water without disturbing it, the dye starts dissolving. It would take a long time for the ink molecules to diffuse throughout the tub and reach a state of equilibrium.

A-barrier-separates-two-kinds-of-molecules

When the barrier is removed, random movement of molecules results in both kinds of molecules moving from a region of higher concentration to a region of lower concentration.

Eventually, an equilibrium (even distribution) is reached. The diffusion gradually slows down as equilibrium is approached.

Absorption by Roots Activity 1

To study the diffusion of a soluble dye in water. Take a beaker containing water. Put a small crystal of potassium permanganate in one corner of the beaker. Observe for some time. You will observe that the potassium permanganate crystal slowly starts dissolving. After some time, the molecules of the dye distribute uniformly throughout the water.

• The molecules of dye are more concentrated in crystal form. When added to water they begin to dissolve.
• The molecules move away from the region where they were added (region of higher concentration) to a region where they are less in number (region of lower concentration).
• Finally, the molecules of dye have been uniformly distributed (state of equilibrium).

Importance of diffusion

• Diffusion of water keeps the wall of the internal plant tissue moist.
• It helps in the distribution of ions and molecules throughout the protoplast.
• Loss of water vapour from the stomata during transpiration is through diffusion.
• Aroma (smell) of flowers is due to diffusion of aromatic compounds from flowers to attract pollinators.

Osmosis

• The movement of water molecules from a region of their higher concentration (more dilute solution) to their lower concentration (less diluted solution) through a semi-permeable membrane is called osmosis.
• In other words, osmosis is the movement of only water from its pure state or solvent from a dilute solution into a stronger or concentrated solution through a semi-permeable membrane.

Endosmosis and Exosmosis

• The inward movement of water molecules through a semi-permeable membrane when the surrounding solution is less concentrated is called endosmosis (endo: inward). Endosmosis leads to swelling up of cells.
• The outward movement of water molecules through a semi-permeable membrane when the surrounding solution is more concentrated is called exosmosis (Exo: out.vard). Exosmosis leads to the shrinking of the cells.

Absorption by Roots Activity- 2

To demonstrate osmosis through a thistle funnel.

• Take a thistle funnel and fill it with concentrated sugar solution. Cover the mouth of the thistle funnel with cellophane paper. Tie the cellophane paper as shown in the Figure.
• Now take a beaker filled with water. Invert the thistle funnel in the beaker and suspend it with a stand as shown in Figure 4.7. Mark the level of sugar solution on the stem end of funnel and level of water in the beaker. Leave the set-up for about two hours.
• Set the same experiment but without taking sugar solution in the thistle funnel. Instead, take water in the thistle funnel and mark the reading. This serves as a control set-up.
• After about two hours you will observe that the level of sugar solution in the thistle rises only in the experimental set-up and not in the control.

• The level of water in the beaker in the experimental set-up drops slightly while it remained unchanged in the control.

Conclusion

This shows that:

1. the water molecules are able to move from a dilute solution (water in a beaker) into the concentrated sugar solution (in the thistle funnel) through the cellophane paper.
2. Sugar solution from the funnel did not pass into the beaker.
3. Cellophane paper acts as a semi-permeable membrane. Only water molecules could pass through it.

What will happen if you use a rubber sheet instead of cellophane as a barrier?

There will be no change in the level of sugar solution as the rubber sheet is an impermeable membrane and would not allow the water molecules from the beaker to cross over to the other side.

What will happen if you use muslin cloth instead of cellophane as a barrier?

Since the pores in the muslin cloth are very large, they will not hold back even the sugar molecules and all the sugar solution will flow out to a common level due to gravity.

Absorption by Roots Activity-3

To demonstrate osmosis through a Viking bag.

Repeat the earlier mentioned activity (no. 2) by using a Viking bag. The Viking bag acts as a semi-permeable membrane.

• Take a Viking bag and tie a knot at one end. Fill this bag with sugar solution from the other end. Insert a long glass capillary tube into it. You will find that the sugar solution rises in the capillary.
  
• Now immerse the visking bag in a beaker containing water and clamp the capillary tube vertically. Leave the experimental set-up for about one hour.
• After about 1 hour you will find that the level of sugar solution in the capillary tube rises. This happens because the water from the beaker diffuses inside through the walls of the visking bag into the more concentrated sugar solution.

Osmotic pressure

• The results of the above activities show that the pressure builds up in the sugar solution and forces the solution up the capillary tube. The pressure results from the rapid diffusion of water molecules from the dilute to the more concentrated solution.
• The osmotic pressure is the minimum pressure that needs to be applied to a solution to prevent the inward flow of pure solvent (water) into the solution when separated by a semi-permeable membrane.
• Osmotic pressure can also be defined as the pressure required to completely stop the entry of water into a solution across a semi-permeable membrane. It can also be defined as the measure of the tendency of a solution to take in water by osmosis.

Osmotic potential

• The osmotic potential of a solution is a measure of the tendency of water molecules to diffuse out of it. A concentrated solution that has relatively few water molecules has a low osmotic potential. On the other hand, a dilute solution with a larger proportion of water molecules has a high osmotic potential.
• Pure water has the highest possible osmotic potential.
• Movement of water in a plant occurs from a dilute solution of a high osmotic potential to a concentrated solution of a low osmotic potential.

Importance of osmosis

• Water absorbed by roots moves to the upper parts of a plant from cell to cell through osmosis.
• Osmosis plays a key role in the growth of radicles and plumules during seed germination.
• Cell-to-cell movement of water occurs through osmosis.
• Living cells remain turgid or distended due to osmosis.
• The stomata are open and close in response to increase or decrease in osmotic pressure of the guard cell.
• The differences between diffusion and osmosis are given in the Table.

Tonicity: Isotonic, Hypotonic And Hypertonic Solutions

• The relative concentration of a solution which determines the direction and extent of diffusion is called tonicity. Based on tonicity, solutions can be classified into three types – isotonic, hypotonic and hypertonic.
• If you take a plant cell and place it in solutions that are of different concentrations you will find that the cell shrinks in a hypertonic solution, swells in a hypotonic solution and remains unchanged in an isotonic solution.

Imbibition

• Osmosis is not the only force involved in the absorption of water by plants. Substances such as cellulose and starch are hydrophilic, i.e. they have a strong affinity for water. They can absorb water and swell up.
• Imbibition is the process by which plant cells (living or dead) absorb water by surface attraction. Imbibition results in swelling of tissues and rupturing of seed coat for germination of seeds. Swelling of wooden doors during rainy season is also due to imbibition.

Passive transport

• Diffusion, osmosis and imbibition are examples of passive transport in plants. Passive transport always takes place along the concentration gradient and requires no energy from the cell.

Active Transport

The passage of salt or ions of a substance from its lower concentration to a higher concentration utilizing the energy from the cell through a living membrane is known as active transport. Active transport is just opposite to diffusion. The ions of certain elements such as nitrates, sulphates, manganese, etc., cannot easily pass through the cell membranes of root cells. This is because their concentration is higher inside the root cells than in the surrounding soil water. Thus, the ions of such elements move inside root cells (the region. of their higher concentration) from outside (the region of their lower concentration) using the energy (ATP). The differences between diffusion and active transport are given in Table.

Turgidity And Flaccidity

Turgidity

The root hair of plants are permeable to water. The cell sap inside the vacuole contains salts and sugars and is highly concentrated. If this cell is surrounded by water, osmosis will cause water to enter the cell sap. As a result, the vacuole would expand, pushing the cell cytoplasm against the cell wall. Eventually, a condition will arise when no more water can enter the cell. Such a cell cannot accommodate any more water and is called turgid and the condition is called turgidity.

Turgor pressure

The pressure of the cell contents against the cell wall due to movement of water into the cell during osmosis is called turgor pressure. The pressure exerted by the cell wall on the cell contents is called wall pressure.

Turgor pressure and wall pressure counterbalance each other. As a result, even if the concentration of solute inside the cell is greater than that outside of a cell, further absorption of water does not take place.

1. Salts and sugars in cell sap make it concentrate inside
2. water enters the cell by osmosis
3. The cell sap volume increases, pushing the cell wall outward making it turgid.

Uses of turgor pressure

• Turgor pressure keeps the cells and their organelles stretched, which is essential for the proper functioning of a cell.
• It is necessary for the enlargement of cells.
• It provides support to living tissues like parenchyma.
• It keeps the leaves fully expanded and oriented towards the light. In case of loss of turgidity, the shoots droop down and leave wilt. Rapid drooping of the leaves of the sensitive plant, Mimosa pudica in response to touch is due to turgor movement.
• Flowers, soft stems and other soft parts of a plant are able to maintain their shape due to turgidity or turgor pressure.
• It keeps a check on the excessive entry of water into the cells.
• A plant cell may burst when turgor pressure exceeds wall pressure.

Turgor pressure in root cells builds up root pressure

Root pressure is the pressure developed in the roots due to the continued inflow of water by cell to cell osmosis. This helps in ascent of sap upwards through the stem. If you cut off the shoot of a plant, the water

Absorption by Roots Activity- 4

To study turgor pressure in root cells-Root pressure

• Take a well-watered, potted plant such as balsam and cut it a few centimetres above the soil level.
• Immediately fix a glass tubing over the cut portion with the help of a rubber connection. The other end of this tubing is connected to a manometer.
• You will observe that water starts coming out of the cut end of the plant and exerts pressure and raises the level of mercury in the connected manometer. This upward movement of water is due to the heavy root pressure.
• immediately comes out from the cut portion. This is due to root pressure. To understand it better let us perform an activity (activity 4).

Turgor pressure helps in the opening and closing of stomata

• During photosynthesis, glucose is synthesized from CO₂ and H₂O. This causes an increase in the osmotic pressure of the contents of guard cells.

• As a result, guard cells absorb more water from the neighbouring cells, thus becoming turgid. The high turgor pressure causes the guard cells to bulge out and the stoma opens. At night, since no photosynthesis takes place, there is a shortage of water in the leaf and the guard cells become flaccid, their inner walls become straight and the stomata are closed.

Turgor movement

• In the sensitive plant, Mimosa pudica, the stimulus of touch leads to the loss of turgor pressure at the base of leaflets and the leaves droop down (fold) within 2-3 seconds of touching. This is an example of the turgor movement.
• The bending of certain flowers (e.g. sunflower) towards sun is due to the turgor movement.

Flaccidity

Flaccidity reverse of turgidity. If a fully-distended (turgid) cell is placed in a hypertonic solution, the
water moves out of the cell due to exosmosis and the cell loses turgidity. The cell is called flaccid and the phenomenon is known as flaccidity. Cells remain flaccid when placed in isotonic solution.

A condition in which the cell contents shrink and the cell loses its turgidity is called flaccidity.

1. Solution outside is more concentrated than the cell sap.
2. Water passes out of the vacuole by osmosis.
3. The vacuole shrinks, pulling the cytoplasm away from the cell wall and leaving the cell flaccid

Plasmolysis And Deplasmolysis

Plasmolysis

• Shrinkage or contraction of the cytoplasm (cell content) of a cell from its cell wall when placed in a hypertonic solution is called plasmolysis.
• If we place a living turgid cell in a hypertonic solution, outward movement of water (exosmosis) occurs from the central region of the cell. As a result, the size of the cytoplasm reduces, and the plasma membrane is withdrawn from the cell wall. This is called plasmolysis and the cell is called a plasmolysed cell.

a. A cell in normal turgid condition;
b.-d. Successive stages in shrinkage of cytoplasm from the cell wall after being placed in a hypertonic solution

Deplasmolysis

• If a plasmolysed cell is placed in water, its shrunk cytoplasm swells up and presses against the cell wall.

• This happens due to endosmosis. The swelling up of a plasmolysed cell under the influence
  of hypotonic solution or water is called deplasmolysis. Deplasmolysis is possible only if the cell is alive and its cytoplasm is not dead or damaged. Differences between plasmolysis and deplasmolysis are given in Table.

Absorption of Water And Minerals By The Root

Absorption of water

The absorption of water occurs through root hair. Root hair are thin-walled extensions from the cells. of the outer layer of a root. They grow out pushing between the soil particles. There is a film of water that surrounds the soil particles and in turn root hair also.

1. The root hair contains cell sap which has a higher concentration of salts than the outside soil water. This causes osmosis and the water from outside diffuses into the cells of root hair (let us take it as cell A). This is due to root pressure.
2. As water enters the vacuole of cell A, it dilutes the concentration of sugar and salts in its cell sap.
3. Another cell (assume cell B) next to cell A has a higher concentration of cell sap (salts and sugars). As a result water from cell A moves to cell B.
4. The water entering cell B makes its cell sap dilute and then moves to cell C. This way water moves from one cell to another by cell-to-cell osmosis.
5. The water ultimately passes into the xylem vessels at the centre of root and is conducted up the root and stem into the leaves.
6. The water and minerals absorbed by the roots are transported through xylem tissue through the ascent of sap.

Absorption of minerals

Absorption of mineral elements by the root from the soil takes place by active transport. The water film around the soil particles also contains a low concentration of mineral elements. These mineral elements move from soil into the root cells against the concentration gradient. Energy in the form of ATP is required by the cell for the absorption of minerals.

Ascent Of Sap

The upward movement of water and mineral salts from roots to the aerial parts (leaves, flowers, branches, etc.) of the plant, against the gravitational force, is called ascent of sap.

The elongated, lignified tracheids and xylem vessels, are placed end-to-end without any cross wall. They form the pipeline for conducting water and minerals from the roots to the leaves.

Water enters the root hair cells by imbibition and then by the process of osmosis. This water from the root hair cells passes into the xylem vessels through the cells of the cortex, endodermis and pericycle.

Ascent of sap takes place from the root, into the stem and finally the leaf veins through the xylem vessels and tracheids by means of a pull exerted by the leaf cells at the top of the sap column.

The cohesive force between the water molecules also helps to maintain the continuity of the water column.

The cohesive force is the force adhesion which keeps molecules of the same substance together, for example, water molecules.

a. Transverse section through a dicot root showing absorption of water by root hair

b. Pathway of water through the plant

Absorption by Roots Activity 5

To show that xylem is the path of ascent of sap.

• Cut two leaf shoots of the balsam plant underwater. Keep their lower ends dipped in water. In one shoot remove 2-4 cm long ring of bark (phloem) roughly from the middle region of the shoot.
• Remove the xylem from the central part of the second shoot (Fig. 4.19b). Fix the shoots with the bola of the help of a stand and leave the apparatus as such for 1-2 days.
• Result: In the first case (Set-up a), the leaves remain turgid even after 24 hours which shows that water continues to rise even if the phloem is removed. Leaves of the second shoot (Set-up b) get wilted. This shows that when xylem is removed, water cannot rise up.

The experiment proves that water rises through the xylem vessels.

In the leaf blade, water passes from the xylem into the cells of the mesophyll and epidermis by the process of cell-to-cell osmosis.

Causative Forces For Ascent Of Sap

• As the water moves upward from roots to the leaves, a lot of it evaporates through the stomata present on the leaf epidermis. This process of evaporation of water from leaves and other aerial parts of the plant is called transpiration.
• The xylem sap (water containing minerals) rises against the gravity without the help of any mechanical pump. The xylem sap is largely pulled upward by transpiration – cohesion-tension mechanism, also called transpiration pull.

1. Root pressure (pushing xylem sap): Root pressure is a pressure created due to the continuous influx of water in the xylem vessels from the root hair and root cortex. Root pressure causes guttation, the exudation of water droplets that can be seen on leaf surface. In most plants, root pressure is not the major mechanism during the ascent of sap. At the most, root pressure pushes the sap in the xylem vessels up to a certain height. Later on, the sap moves with cohesion, adhesion and transpiration pull.
2. Capillarity nature of xylem vessels: Xylem vessels are very narrow. This causes the water from a lower level to rise by capillary action in order to fill up the vacuum created at the leaves due to the loss of water by transpiration.
3. Pulling xylem sap: Transpiration – Cohesion- tension mechanism or transpiration pull: Stomata are the sites of exchange of CO₂ and O₂ between photosynthetic tissues and atmosphere. They are also the sites for transpiration. The air in these stomata is saturated with water vapour since it is in contact with most walls of the mesophyll cells (Fig. 4.20). On most of the days, the air is drier outside the leaf, i.e. it has a lower water concentration outside than inside the leaf. Thus, the gaseous water diffuses outside the leaf through the stomata and there is a loss of water during transpiration.
4. This leads to a generation of tension (negative pressure) in the leaf due to the unique physical property of water. The thin film of water vapour present in the mesophyll cells replaces the water vapour which is lost from the leaf stomata by transpiration.
5. During this process, water is pulled on by the adhesive and cohesive forces between the molecules of two different substances.
6. This tension is the pulling force or suction force which draws water from the leaf xylem through the mesophyll cells toward stomata. The water lost via transpiration is replaced by the water that is pulled out of the leaf xylem.

• Cohesion and adhesion of water: The transpiration pull on water extends from the leaves up to the root tip and even into the soil solution.
• This cohesion of water is due to hydrogen bonding between water molecules. Each water molecule is attached (adhered) to an adjacent water molecule and this pull is relayed from molecule to molecule down the entire column of water in the xylem.

Absorption by Roots Activity 6

To show that water is conducted through xylem tissues.

1. Take a young, medium-sized balsam plant. Remove this plant from soil, wash it and place it in a beaker. Half fill this beaker with water containing eosin stain solution (pink colour).
2. Ensure that the roots are completely submerged in the solution. Leave this set-up for about 3-4 hours. Now remove this plant from beaker and wash it in running tap water.
3. Cut transverse section passing through roots, stem and leaves with the help of a sharp razor or a blade. Mount the sections on the slides and observe under a microscope.
4. You will observe that in the centre xylem vessels appear red due to conduction of eosin stain dye. This shows that water is conducted through xylem tissue.

Absorption by Roots Summary

• Plants need water and minerals for many purposes such as growth, photosynthesis, transpiration, mechanical strength and transportation of nutrients.
• Roots bear root hair that provides enormous surface area. The epidermis of root hair is permeable to water.
• The movement of molecules or ions of a substance from a region of their higher concentration to a region of their lower concentration is called diffusion. Diffusion of water molecules keeps the cell wall of the internal plant tissues moist and helps in the transpiration of water vapour from stomata.
• Osmosis is the movement of water from its pure state or diluted solution into a concentrated solution through a semi-permeable membrane. The inward movement of water is called endosmosis and outward movement of water is called exosmosis.
• The osmotic pressure is the maximum pressure which can develop in an osmotically active solution when it
  Is separated by a semi-permeable membrane to stop further endosmosis of water from a region of low concentration to the region of higher concentration of solute.
• Imbibition is the process by which plant cells absorb water by surface attraction.
• The passage of salt or lons of a substance from their lower concentration to higher concentration through a living cell membrane using the energy from the cell is called active transport. The loss of elements moves into roots through active transport.
• The condition, when a cell reaches a state where it cannot accommodate any more water is called turgidity.
• The pressure of the cell contents against the cell wall is called turgor pressure. Flaccidity is reverse of turgidity.
• Shrinkage of cytoplasm of a cell from its cell wall under the influence of a hypertonic solution is called plasmolysis. The condition opposite to it is deplasmolysis.
• Absorption of water occurs through root hair by the process of osmosis. Minerals move from soil into root hair through active transport.
• The water and mineral salts enter the root by moving between the cells before entering xylem. Water also enters root hair; then passes through the cells of the cortex and endodermis to reach xylem vessels.
• The water and minerals absorbed by roots are conducted up through the xylem tissue.
• The upward movement of water and minerals from the roots to the aerial parts of plants against the gravitational force is called ascent of sap.
• Xylem sap is pulled upward by root pressure, capillary action of xylem vessels, transpiration-cohesion-tension mechanisms, and cohesion and adhesion of water.

Photosynthesis Learning Objectives

After completing this chapter, you will be able to:

• Define the term photosynthesis;
• Describe the importance of photosynthesis for living beings;
• Construct the generalized equation of photosynthesis;
• List the raw materials required for photosynthesis and describe the role of chlorophyll;
• Describe the structure of chloroplast and the functions of its various parts;
• Explain what happens in the light-dependent reactions and light-independent reactions of photosynthesis and list the end products of the two reactions;
• Describe various factors affecting photosynthesis;
• Give suitable experiments to prove the raw materials required and products formed as a result of photosynthesis;
• Describe carbon cycle occurring in nature.

You know that all green plants can synthesize their own food from simple inorganic raw materials through photosynthesis. Hence, green plants are called producers or autotrophs and the mode of nutrition in green plants is called autotrophic nutrition. In this chapter, you will study about photosynthesis and its importance for life.

What Is Photosynthesis?

Photosynthesis is a biochemical process by which living cells of plants containing chlorophyll manufacture their own food (glucose) using carbon dioxide and water as raw materials in the presence of light energy. Oxygen is released as a by-product of photosynthesis. Photosynthesis is an important activity that occurs in all green plants.

Photosynthesis is the only process by which solar energy is converted into chemical energy.

All living beings depend on photosynthesis for two reasons.

• For food prepared by plants, and
• For oxygen is released as a by-product.

Significance of photosynthesis

1. Photosynthesis provides food for all. The process of photosynthesis occurs in green plants which are the primary producers in a food chain.
2. Photosynthesis is the ultimate biological source of oxygen and energy for all living beings on earth. It is essential for sustaining life.
3. It is necessary for the synthesis of organic compounds from inorganic compounds.
4. It takes in atmospheric carbon dioxide (given out during respiration and other activities) and releases oxygen.

Raw Materials For Photosynthesis

To perform photosynthesis, plants require carbon dioxide (CO₂), water (H₂O), light energy, and chlorophyll. CO₂ and H₂O serve as raw materials and sunlight serves as a source of energy. The process of photosynthesis takes place in chloroplasts.

Carbon dioxide

The main source of CO₂ for land plants is the atmosphere, which contains 0.03-0.04 percent of CO2. Aquatic plants use CO₂ dissolved in water. Two main processes, photosynthesis, and respiration take place side by side but photosynthesis does not take place in the absence of light whereas respiration continues throughout the day and night.

CO2 Compensation Point
During the day when the light intensity is high, the rate of CO₂ consumption for photosynthesis is higher than that of CO₂ liberation by respiration in plants, hence, CO₂ is continuously absorbed from the atmosphere through stomata. During morning and evening hours, the intensity of light is usually low. At this time, a stage may come when CO₂ liberated during respiration in plants is equal to CO₂ used in photosynthesis. At this stage, no exchange of CO₂ takes place between the environment and plants. This stage is known as the compensation point.

Water
Plants absorb water from the soil by their root hair. This water is then transported up to the stem and leaves through the xylem vessels. Aquatic plants absorb water through their general body surface because they have poor root system. Water rarely serves as a limiting factor in photosynthesis because less than 1 percent of the water absorbed by a plant is used in photosynthesis.

Light
Light is very important for photosynthesis. In photosynthesis, light energy of the sun is converted into chemical energy. The sun is the main source of light energy. Artificial light is also effective in photosynthesis but this light should be of a required intensity.

The rate of photosynthesis is affected both by the quality as well as the quantity (intensity and duration) of light. Too much light intensity may destroy chlorophyll. In red-colored light, the rate of photosynthesis is maximum, whereas in green-colored light, photosynthesis does not occur.

Chlorophyll – The Photoreceptor to Trap Solar Energy

Leaves contain chlorophyll which is a photoreceptor molecule. Chlorophyll absorbs photons unit of sunlight. Chlorophyll is present in the chloroplast. Chloroplasts are green-colored plastids containing chlorophyll pigments and are mostly present in leaves  That is why leaves are called photosynthetic organs. Chloroplasts are also present in young stems and fruits. A cell may contain about 40-50 chloroplasts and a leaf may contain more than 500000 chloroplasts per sq mm of leaf surface. The green color of the plants is due to the chlorophyll.

The internal structure of a chloroplast as seen through an electron microscope

Photosynthesis Structure of chloroplasts

A Chloroplast has three parts:

A double membrane envelope covers each chloroplast. There is a minute space available between the membranes. The membranes are selectively permeable.

1. Stroma: On the inside, the lumen of chloroplast is filled with a colorless ground substance or matrix called the stroma.
2. Thylakoids: Thylakoids are flattened membranous sac-like structures present in stroma. These thylakoids are stacked over one another like a stack of coins. A pile of thylakoids is called grana (singular: granum).

The closely packed thylakoids arranged in piles are called grana.

The grana are connected to each other by interconnecting bars or lamellae called fret or stroma lamellae.
The chlorophyll pigment is contained in the membranes of the thylakoids. Chloroplasts are mainly located in the mesophyll cells between upper and lower epidermis (palisade and spongy cells) of leaves. Guard cells of stomata also contain chloroplasts.

Chlorophyll is mainly of nine types. Of these, chlorophyll a and chlorophyll b are most important as they receive solar energy to bring about splitting of water. Chlorophyll is composed of carbon, oxygen, hydrogen, nitrogen, and magnesium. Water passes into the palisade cells of mesophyll tissue by osmosis from the xylem and carbon dioxide diffuses in from the atmosphere.

Location and functions of some structures

Sunlight is absorbed by the chlorophyll. By using this energy, carbon dioxide and water are utilized in the chloroplast with the help of a number of enzymes to yield sugar which is readily converted into a storable form of food, starch. The oxygen formed in the reaction diffuses out of the cells and is released into the atmosphere through the stomata.

Opening And Closing Of Stomata

Stomata (singular: stoma) are minute pores present either on the lower or both the surfaces of the leaf to facilitate the exchange of gases (CO₂ and O₂) between the leaf and the atmosphere.

Each stoma consists of a stomatal aperture and two surrounding guard cells. The guard cells are kidney-shaped and contain chloroplasts. The inner wall of each guard cell is thick and outer wall is thin.
There are two theories for opening and closing of stomata, namely

1. sugar concentration theory and
2. K+ ion concentration theory.

Sugar concentration theory

This is an old theory. During the daytime, the cell-sap concentration becomes high due to the accumulation s increased and of sugar. As a result, osmotic pressure water is drawn inside the guard cells from adjoining cells due to endosmosis. This makes guard cells turgid so that their thin outer walls get stretched and bulge outside. This widens the stomatal pore causing the opening of stomata. The pressure developed in guard cells is turgor pressure. As the stomata open, the diffusion of gases takes place and CO₂ is let in and O₂ is let out.
During the night, when there is no photosynthesis, carbon dioxide gets accumulated in guard cells.

Stomatal apparatus opening and closing of stomatal pore.

This carbon dioxide then combines with water to form carbonic acid which has a pH of 5.0. It then converts the sugar into starch which is insoluble in water. As a result, exosmosis takes place and guard cells become flaccid or lose turgidity. Thus, the slit-like stomatal pore narrows down. The inner thick walls straighten and stomata close.

Potassium ion (K*) concentration theory
Recently a new theory of opening and closing of stomata has come into existence. According to this theory, opening and closing of stomata depend on the generation of potassium ion (K+) gradient.

Opening of stomata: During daytime, photosynthesis takes place inside chloroplasts in the guard cells as a result of which ATP is produced. This ATP helps in the pumping of potassium ions (K+) of the adjoining cells into the guard cells. As a result, the concentration of K+ions increases in the guard cells making them hypertonic. Thus, more water from the adjacent cells move into the guard cells making them turgid. The guard cells open out leading to the opening of stomata.

Closing of stomata: During night, just opposite happens. The ATP formation stops during night as no photosynthesis is taking place. Thus K ions move out of guard cells leading to hypotonic condition. The water moves out of the guard cells and they become flaccid, leading to closing of the stomatal pore.

How are the leaves adapted for photosynthesis?

1. Large surface area: Leaves have large surface area for maximum absorption of light.
2. More number of stomata: Leaves have more number of stomata to allow rapid exchange of O₂ and CO₂ gases.
3. Arrangement of leaves at right angles: Leaves are arranged at right angles to light source so as to trap maximum light.
4. Concentration of chloroplasts on upper epidermis: Chloroplasts are more concentrated on the upper epidermis of the leaf so as to obtain maximum light energy.
5. Extensive vein system: The vein system is extensively developed for rapid transport of water to and from mesophyll cells.
6. Thinning of leaves: This reduces distance between cells for faster transport of gases and water.

The Mechanism Of Photosynthesis

As already discussed in the previous section, mesophyll tissue in a leaf is the principal site of photosynthesis. Mesophyll tissue is differentiated into palisade and spongy tissue. Water is transported into palisade cells by cell-to-cell osmosis from xylem tissues and carbon dioxide diffuses in from the atmosphere. Sunlight is absorbed by the chlorophyll of the upper layers of mesophyll cells.
By using the light energy from sunlight, carbon. dioxide and water are combined in the chloroplast with the help of a number of enzymes to yield sugar (glucose). It is readily converted into a storable form, starch. The oxygen evolved in this process is given out in the atmosphere through stomata.
The overall chemical equation of photosynthesis is as follows:

6CO₂ +12H₂O ————- > C6H12O6 + 6H₂O +60₂

The six molecules of H₂O liberated at the end of the reaction are those that have been re-formed during chain of reactions in this process.

Phases of photosynthesis
There are two phases of photosynthesis:

1. the light-dependent phase and
2. the light-independent phase (dark phase).

Light-dependent phase or reaction – Hill’s reaction (Photochemical phase)

This is the photochemical phase of photosynthesis. As the name suggests, this is a light-dependent reaction, that is, light plays a key role in this reaction. This reaction takes place in the thylakoids of the chloroplasts.
A series of chemical reactions occur in quick succession, initiated by light. The main steps of this reaction are as follows.

Step 1: Excitation or activation of chlorophyll

• The photosynthetic pigments (chlorophyll) absorb light energy in the form of photons (smallest unit of light energy).
• After being exposed to photons, the chlorophyll molecule gets activated and emits electrons, which travel through electron transport chain in chloroplasts.

Step 2: Splitting of water (Photolysis)

The splitting of water, also known as photolysis (Gk. photo: light, lysis: breaking), takes place in a light reaction during which water is broken down into highly reactive hydrogen (H) ions and oxygen and electrons (e) are emitted. These electrons travel through the electron transport chain in chloroplasts.

2H₂O energy of 4 photons———–> 4H+ + O₂ + 4e¯

The free oxygen is the oxygen gas given off during photosynthesis.

Photolysis

The splitting of water (H₂O) molecules into hydrogen (HT) ions and oxygen in the presence of light energy is called photolysis. It takes place inside grana.

Step 3: Formation of ATP from ADP (Photophosphorylation)

The above-mentioned reactions in step 1 is mediated by electron acceptors, and adenosine triphosphate (ATP) is synthesized from adenosine diphosphate (ADP) and inorganic phosphate (Pi). In other words, electrons are used to convert ADP into ATP by adding one inorganic phosphate, Pi. This is known as photophosphorylation [addition of phosphate in the presence of light (photons)]. This ATP is used during dark reactions.
ADP + Pi + e + energy → ATP

End products of photolysis

1. NADP (Nicotinamide Adenine dinucleotide phosphate) is reduced to NADPH: The released H+ (hydrogen ions) are picked up by NADP molecule to form NADPH (reduced form of NADP).NADP+ + 2e + H+- enzymes —— > NADPH

2. The oxygen (O) is given out as molecular oxygen (0₂).
   20 → O2

Light-independent phase – Calvin cycle (Dark reaction or Biosynthetic phase)

This is the biosynthetic phase of photosynthesis. The dark reaction occurs in the stroma of chloroplasts. This reaction does not require light energy, but it does not mean that t occurs during dark only. This is a light-independent reaction.
The dark reaction occurs simultaneously with the light reaction and the time gap between the two is less than one-thousandth of a second. In the dark reaction, the following steps take place:

1. NADPH molecules and ATP molecules, both

Summary of light and dark reactions. The light reactions occur in thylakoids of chloroplasts where sunlight is captured, water is split and oxygen is given out, and ATP and NADPH are produced. The dark reactions occur in stroma of chloroplasts, where carbon dioxide is fixed and reduced after being incorporated into the Calvin cycle. Reduction uses the ATP and NADPH from the light reactions. produced during light reaction are utilized to produce sugar (C6H12O6) from carbon dioxide.

2. Reduction of carbon dioxide occurs in the stroma of the chloroplast by means of a series of reactions known as the Calvin cycle during which fixation and reduction of carbon dioxide take place.

The fixation of CO₂ is catalyzed by the enzyme Rubisco (Ríbulose-1, 5-bisphosphate carboxylase).

The overall process of a sequence of events taking place in a plant cell during photosynthesis are shown in.

Water is transported into palisade cells by cell-to-cell osmosis. CO₂ diffuses in from atmosphere.
Photons (light energy) from sunlight absorbed by chlorophyll in chloroplast.

Chlorophyll absorbs light energy (photons)
↓
Chlorophyll molecule gets activated
↓
Photolysis: Splitting of water molecules into highly reactive
H+ ions and O₂
2H₂O → 4H* + O₂ + 4e¯
↓
ATP formed from ADP
ADP + Pi + energy → ATP
↓
NADP+ reduced to NADPH
↓
Fixation of CO₂ by enzyme Rubisco
↓
Sugar (C6H₁2O6) produced from CO₂. NADPH and ATP catalyze the reaction. (Calvin cycle)
↓
Glucose either immediately used by cells or stored as starch.

 A simple flowchart to show the process of photosynthesis in plants.

Photosynthesis Factors Affecting Photosynthesis

There are a number of factors affecting the rate of photosynthesis. These factors are categorized as follows:

external factors:

1. light intensity
2. carbon dioxide concentration
3. temperature
4. water availability

internal factors:

1.  chlorophyll
2.  leaf structure
3. protoplasm

Photosynthesis External factors

• Light intensity: The rate of photosynthesis increases linearly with an increase in the intensity of light and then stabilizes at a point of 0.02% of CO₂. However, extremely high intensities of light do not increase the rate of photosynthesis. Optimum light intensity for photosynthesis varieswith the species of the plant. In fact, very strong light intensity may bleach chlorophyll and retard photosynthesis.
• Carbon dioxide concentration: In normal conditions, carbon dioxide is the major limiting factor in photosynthesis. The rate of photosynthesis increases with an increase in the CO₂ concentration. The concentration of CO₂ in the atmosphere varies from 0.03 to 0.04 percent. A concentration of 0.02 percent of CO₂ for a short duration is optimum for increasing the rate of photosynthesis. However, over long periods, even 0.05 percent CO₂ concentration in the atmosphere can increase the rate of photosynthesis, provided the light intensity is also increased to support it.

Effect of increasing light intensity and CO₂ concentration on rate of photosynthesis.

Stage 1: Limited light intensity and at CO₂ concentration of 0.02% (In a normal condition) the rate of photosynthesis gets stabilized.

Stage 2: Rate of photosynthesis gets further increased with an increase in light intensity and CO₂ concentration of 0.03%.

Stage 3: Rate of photosynthesis is maximum at CO₂ concentration 0.05% along with maximum light intensity. Then at this point, the rate of photosynthesis gets stabilized.

• Temperature: In general, an increase in temperature results in an increase in the rate of photosynthesis when other factors are not limiting. Photosynthesis is restricted to a temperature range in which the enzymes remain active. Further, a rise of 10 °C up to the optimum temperature (35 °C) doubles the rate of photosynthesis, for example, a rise from 20 °C to 30 °C or 25 °C to 35 °C doubles the rate of photosynthesis. The maximum suitable temperature when photosynthesis occurs best is about 35 °C above which the rate falls. The process of photosynthesis falls and stops above 40 °C as the enzymes get destroyed. Similarly, low temperature also inhibits enzymatic activity and rate of photosynthesis is reduced.

Effect of temperature on the rate of photosynthesis

•  Water: Less than 1 percent of the total water absorbed by plants is utilized as a raw material in photosynthesis. Water rarely becomes a limiting factor in photosynthesis.

Internal factors

1. Chlorophyll: Inadequate amount of nutrients like minerals causes loss of chlorophyll in leaves thereby reducing the trapping of solar energy. As a result, the rate of photosynthesis is reduced.

2. Structure of leaf: The size and thickness of the leaf, and distribution of stomata influence the amount of CO₂ and light entering the leaf.

3. Protoplasm: Dehydration of protoplasm and accumulation of sugar and starch in the leaves reduce the rate of photosynthesis.

Rate Of Photosynthesis

(Blackman’s principles of limiting factors)
The three main conditions affecting the rate of photosynthesis, also known as Blackman’s principles limiting factors, are as follows:

• Optimum condition: CO₂ conc. 0.03% -0.05%, more light intensity and optimum temperature (35 °C) – More photosynthesis
•  More light intensity, optimum CO₂ but low temperature -Less photosynthesis
• More light intensity, less CO₂ and optimum temperature – Less photosynthesis
• Less light intensity, optimum CO₂, and optimum temperature – Less photosynthesis

Thus it is necessary that all the conditions must be optimum to achieve the maximum rate of photosynthesis.

Photosynthesis Carbon Cycle

The cyclic process in which carbon element is circulated continuously through the living and non-living components of the biosphere is called carbon cycle.
In fact, carbon is the most essential constituent of all the organic compounds present in the living organisms. Carbon dioxide present in the atmosphere is the main reservoir of carbon.
The main steps of the carbon cycle in nature are:

1. Photosynthesis: Carbon is present as carbon dioxide gas in the atmosphere. Green plants use this carbon dioxide and prepare their food (as carbohydrates) by the process of photosynthesis. When animals eat the plant, plant carbohydrate is converted into animal carbohydrate.

2. Respiration: When plants and animals respire by oxidizing carbohydrates in their cells to release energy, they give out carbon dioxide, which is returned to the atmosphere.

 Carbon cycle in nature

When animals and plants die, their bodies are decomposed by decomposers and carbon dioxide is returned to the atmosphere.

4. Burning of fossil fuels: Some of the dead plants and animals get buried deep under the earth. They changed into fossil fuels like coal and petroleum through slow chemical changes. Petroleum gives us fuel like kerosene, petrol, diesel, petroleum gas, etc. When these fuels burn, they give out carbon dioxide which goes into the atmosphere.
5. Weathering of rocks: Some carbon dioxide is present in the dissolved state in water. This gets converted into calcium carbonate (CaCO3) in limestone and other carbonate rocks. Weathering of carbonate-containing rocks or treatment of their minerals gives out carbon dioxide. When acid rain falls on these rocks, then carbon dioxide is released.
6. Volcanic eruptions: Volcanic eruptions and hot springs also release carbon dioxide into the atmosphere. Thus, there is a continuous exchange of carbon dioxide between atmosphere, water bodies, and living beings.

The different processes that help in the removal of carbon dioxide from the atmosphere are:

•  Photosynthesis by green plants
• Formation f fossil fuels (like coal, petroleum, natural gas,
• Formation of carbonate rocks
• Formation of carbonate shells, skeleton, etc.

The different processes that help in the addition of dioxide to the atmosphere are:

• Respiration of plants and animals
•  Decay of dead plants and animals
•  Burning of fossil fuels
• Action of acid rain on carbonate rocks, shells, etc.
• Volcanic eruptions carbon

Photosynthesis Experiments On Photosynthesis

Destarching (removing starch)

Since the presence of starch is regarded as an evidence of photosynthesis, the experimental plant should not have starch in its leaves, before the experiment is started.
The leaves of a potted plant may be detached (devoid of starch) by leaving them in a dark place for 2-3 days (48 to 72 hours).
To conduct experiments on plants in open, the selected leaves on a plant must be detached by wrapped in aluminum foil for 2 days and then experimented upon.

Photosynthesis Experiment 1

To test a leaf for the presence of starch (iodine test)

•  Detach a fresh green leaf and dip it in boiling water for 1 minute. Boiling will kill protoplasm and enzymes In it so that no further chemical change takes place. Boiling will also make the cell more permeable to water.
• Now boil the leaf in a test tube containing methylated spirit in a water bath till it becomes colorless or pale white due to the removal of chlorophyll.
• The leaf now becomes brittle and hard. Place it again in boiling water to make it soft again. Spread the leaf flat on a white surface such as a glazed tile or a Petri dish and pour a few drops of iodine solution (iodine 0.3 g: potassium iodide = 1.5 g and water = 100 mL) on the leaf surface. What do you observe?
• You will observe that the part of the leaf having starch becomes blue-black, while, the part of leaf without starch becomes brown in color.
  
  Experimental set-up to test a leaf for the presence of starch

Photosynthesis Experiment 2

To show that chlorophyll is necessary for photosynthesis.

• Take a plant with variegated leaves (which have chlorophyll only in patches). Such leaves can be found in plants like Coleus, Tradescantia, Croton, etc.
• Distarch the leaf by placing the plant in dark for 2-3 days.
• Place the plant again in daylight for a few hours. Detach the leaf and test this leaf for presence of starch.
• Only the parts which were green previously, turn blue-black with iodine solution. This shows that chlorophyll is necessary for photosynthesis.
  

Experimental set-up to show that chlorophyll is necessary for photosynthesis

Photosynthesis Experiment 3

To show that sunlight is necessary for photosynthesis.

• Take a potted plant and destarch its leaves by keeping it in dark for 2 days.
• Take black paper and cut simple ‘L’ shape in it making a stencil. Cover one leaf on either side with one such paper by clipping it. Leave the set up in daylight for 4-6 hours.
• Detach the leaf and test it for the presence of starch. You will observe that only the part of the leaf that could get light through the cut-out design and the other exposed parts of the leaf turn blue-black with iodine solution, showing the presence of starch in it. This proves that sunlight Is necessary for photosynthesis.

Experimental set-up to show that sunlight is necessary for photosynthesis

Photosynthesis Experiment 4

To show that oxygen is given out during photosynthesis.

• Take some water plants such as Elodea or Hydrilla and place them in a beaker containing pond water.
• Cover these plants with a short-stemmed inverted funnel. Slightly raise the level of funnel above the bottom of the beaker to allow free circulation of water. Insert a test tube full of water over the stem of the funnel.
• Place the apparatus in sunlight or bright light and observe.
• You will observe that bubbles of gas appear from the cut ends of stems, rise and collect in the test tube. Test the gas in the test tube by introducing a glowing splinter. The splinter bursts into flames showing oxygen is present in the test tube. Alkaline pyrogallol can be also used for testing the presence of oxygen. Thus, it is proved that oxygen is produced during photosynthesis.

Experimental set-up to show that oxygen is produced during photosynthesis

Photosynthesis Experiment 5

To show that carbon dioxide (CO₂) is necessary for photosynthesis (Moll’s half leaf experiment).

• Take a potted plant with detached leaves. Take a conical flask and put some potassium hydroxide into it. Potassium hydroxide absorbs the carbon dioxide. Insert one leaf into this conical flask through a split cork.
• Leave the set-up in sunlight for few hours.
• After few hours take out this leaf from conical flask. Take one more leaf from the same plant. Test both these leaves for presence of starch.
• You will observe that leaf in the conical flask does not turn blue-black while the one that was exposed to atmospheric air turns blue-black.

This shows that carbon dioxide is necessary for photosynthesis.

Experimental set-up to show that carbon dioxide is necessary for photosynthesis

Photosynthesis Summary

• All green plants prepare their own food, hence, the Hydrogen ions produced due to splitting of water are called producers or autotrophs. combine with NADP to produce NADPH.
• The process by which living cells of plants containing chlorophyll manufacture their own food (carbohydrate) using CO₂ and water as raw materials in the presence of sunlight is called photosynthesis. Oxygen is released as a by-product during photosynthesis.
• Photosynthesis provides food for living beings directly or indirectly and produces oxygen.
• Chlorophyll o and chlorophyll b are most important as they receive light energy from the sun to bring about splitting of water molecules during photosynthesis.
• Chlorophyll pigment is present in chloroplast. Chloroplasts contain two main parts-stroma and grana.
• Photosynthesis occurs in two phases – light-dependent reaction and light-independent (dark) reaction.
  
• Light-dependent reaction (also called photochemical phase) occurs in thylakoids of grana of chloroplasts.
  
• Light reaction includes trapping of light energy by chlorophyll, splitting of water, and formation of ATP from ADP.
• Light-independent (dark) reaction is a biosynthetic phase which occurs in the stroma of chloroplasts. This reaction does not require light energy.
• During the dark reaction, NADPH molecules and ATP molecules both produced during light reaction, are utilized to produce sugar (C6H₁206) from carbon dioxide.
• Dark reaction occurs simultaneously with light reaction. and the time gap between the two is less than 1/1000 of a second.
• As a result of photosynthesis, glucose, water and oxygen are produced. Glucose is either immediately used up by cells or is stored in the form of starch, sucrose, and cellulose. Oxygen diffuses out into atmosphere which is used by living beings for respiration.
• Light intensity, CO₂ concentration, and temperature affect the rate of photosynthesis.
• Carbon cycle involves a series of chemical reactions through which carbon dioxide is circulated in nature.

HumanChromosomes Learning Objectives

After completing this chapter, you will be able to:

• Recognize chromosomes as carriers of heredity,
• Explain the structure of a human chromosome;
• Differentiate between sex chromosomes and autosomes;
• Describe the functions of chromosomes.

You have already studied in the previous chapter about the behaviour of chromosomes during cell division. You have also studied that chromosomes are hereditary vehicles on which genes are located. In this chapter, you will study about the structure and functions of human chromosomes.

Chromosomes – The Carriers Of Heredity

• A chromosome (Gk. chroma: colour; soma: body) is a strand of deoxyribonucleic acid (DNA) molecule associated with proteins found in the nucleus of a cell. The chromosomes contain genes, thus, they serve as the carriers of heredity.

• The chromosomes are hereditary vehicles. The characteristics move from parents to the offspring in the form of genes located on the chromosomes.
• The genes are composed of DNA and proteins. DNA functions as the genetic material and forms the chemical basis of heredity.
• A gene is a segment of DNA that codes for the synthesis of a specific protein, which controls the expression of a particular characteristic in an individual. A gene is the basic unit of heredity found on a chromosome.

Number of chromosomes in an individual

• The number of chromosomes is constant for all individuals in a species and an individual has a fixed and equal number of chromosomes. The number of chromosomes in the somatic cells (body cells) of higher plants and animals is in diploid number represented by 2n (both sets of homologous chromosomes present).
• In gametes (sperm and egg), it is in haploid number and is represented by n.

The chromosome number of some plants and animals is given in Table.

• A human body cell has 46 chromosomes. These 46 chromosomes are arranged into 23 pairs of homologous chromosomes.
• We inherit half our chromosomes from our mother and a half from our father. Thus, we inherit 23 unpaired chromosomes from each parent.

Structure of chromosomes

• A chromosome is a diffuse, thread-like structure within the nucleus of a cell.
• At the time of cell division, chromosomes condense and become visible under the microscope.
• At the start of cell division, a chromosome consists of two chromatids joined at some point along the length. At the point of joining, a constriction is formed, called a centromere.
•  The spindle fibres are also attached to the centromere at the time of cell division. These spindle fibres contract and help in the separation of two sister chromatids towards the opposite poles in a dividing cell.
• On the completion of cell division, these chromatids become chromosomes. These chromatids decondense and form very thin thread-like chromatin fibres. Each chromatin fibre is made up of DNA and histone proteins with a small amount of RNA.

Chromatin fibre

• Chromatin is the complex of DNA and proteins found in the nucleus. The chromatin material largely consists of two strands of DNA and proteins (mainly histones). DNA forms about 40% while histones form about 60% of the overall part of the chromosome.
• DNA has negative charges along its length while histones are positively-charged basic protein molecules which are bound to it.
• This DNA-histone (protein) complex is called chromatin. It has been shown that the DNA helix combines with groups of eight histone molecules to form a structure known as a nucleosome. It has the appearance of beads on a string. In a human cell, there are about two million nucleosomes among 46 chromosomes.

Structure of DNA molecule

• Rosalind Franklin for the first time in 1953 studied the shape of the DNA molecule. Then, Watson and Crick in 1953 worked out the structure of DNA. Watson and Crick showed that DNA is a large molecule (macro-molecule) consisting of two polynucleotide strands, complementary in nature, wound around each other in a double helix.

• The strands run in opposite directions, i.e. they are antiparallel. Each single DNA strand is composed of repeating nucleotides. Nucleotides are made of three components, a phosphate, a sugar (pentose) arranged lengthwise and a nitrogenous base attached to the sugar inwards.
• The sugar-phosphate backbone has nitrogenous bases arranged at right angles giving a ladder-like arrangement. There are two purine bases and two pyrimidine bases.
• The bases are guanine (G), thymine (T), adenine (A) and cytosine (C). Among them, adenine and guanine are purine bases and thymine and cytosine are pyrimidine bases. The guanine is complementary to cytosine and thymine to adenine.

Chromosome

• A thread-like strand of DNA and protein present in the nucleus of a cell. They carry genes, the carrier of heredity.

Chromatid

•  One of the two thread-like strands of a chromosome formed as a result of cell division. Each chromatid contains a double helix of DNA,

Chromatin

• A complex of DNA and protein found in the nucleus of a cell. A chromosome is packaged and organised into chromatin.

How are new DNA strands formed?

• Formation of a new DNA molecule is called DNA replication. During replication, DNA double helix opens at one end, freeing the strands.
• For each of these free strands, new complementary strands begin to form in the opposite direction. This process takes place for the whole length of DNA as given in.
• DNA double helix
• Two strands of DNA double helix open at one day. A new strand is produced against each simultaneously
• Each new DNA contains one original (old) strand and one new strand

Types of chromosomes

• The chromosomes may differ in the position of the centromere. Centromere is the point on the chromosome, marked by a constriction where nitrogenous sister chromatids are attached during cell division.
• base If the centromere is near the middle, the chromosome is metacentric. If the centromere is towards one end (away from the centre), the chromosome is acrocentric. If the centromere is located at the end, the chromosome is telocentric.

• In addition to a centromere, there may also be a secondary constriction on a particular chromosome. The part of the chromosome located distal to (far from) a secondary constriction is called a chromosome satellite.

Sex chromosomes and autosomes

• In human beings, out of the 23 pairs of chromosomes, a specific pair, i.e. the 23rd pair of chromosomes, determine the sex of the individual. These are called sex chromosomes or allosomes. All other 22 pairs of chromosomes are termed autosomal chromosomes or autosomes.
• The autosomes carry genes that control somatic traits and play no role in sex determination. The two members of each pair of homologous autosomes are similar in size and shape, but this may not be true for the sex chromosomes.
• In human males, one sex chromosome is smaller than the other. The larger one is known as X chromosome and the smaller one as the Y chromosome. Thus, the condition in the male may be briefly expressed as XY and in female as XX.
• The sex chromosomes of human females are described as homomorphic and that of human males as heteromorphic. The human females produce only one type of gametes (all with X) and are said to be homogametic. The human males produce two types of gametes (X type and Y type) and are described as heterogametic.
• Human karyotype – Human somatic cells have 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY); Chromosomes are paired by matching their banding pattern and are arranged by size and shape.

Functions of chromosomes

• Chromosomes are hereditary vehicles that contain genes. All the hereditary information is located on the genes.
• Chromosomes control the synthesis of structural proteins and thus help in cell division, cell repair and cell growth.
• By directing the synthesis of enzymatic proteins, chromosomes control cell metabolism.
• Chromosomes guide development and control cell differentiation.
• Sex chromosomes (XX and XY chromosomes) determine the sex of individuals.

What is a gene?

• A gene is a basic unit of heredity or inheritance of a character passed from parents to offspring via chromosomes.
• A gene is a segment of DNA on a chromosome that encodes a particular protein which is expressed in the form of a particular characteristic of the body.
• Genes are located on specific positions (loci) on the chromosomes.
• Genes are transmitted from parents to offspring through the gametes.

HumanChromosomes Summary

• A chromosome is a thread-like strand of DNA molecule associated with proteins found in nucleus. Chromosomes carry genes and are referred to as hereditary vehicles.
• Genes are located on chromosomes.
• The number of chromosomes is constant for a species. Human beings have 46 chromosomes.
• Depending upon the location of centromere, chromosomes can be metacentric, acrocentric and telocentric.
• DNA-histone complex in chromosomes is called chromatin.
• DNA molecule is a double-helical structure. Each single DNA strand is composed of repeating nucleotides.
• Formation of a new DNA molecule is called DNA replication.
• There are two kinds of chromosomes in human beings – sex chromosomes (chromosomes that decide the sex of an individual) and autosomes (rest of the chromosomes, somatic chromosomes are same in males and females).