CBSE Class 10 · Biology
CBSE Class 10 Biology Our Environment Previous Year Questions
Help your child confidently tackle CBSE Class 10 Biology Our Environment Previous Year Questions with this curated collection sourced from real board papers spanning 2015–2024. Each question comes with a detailed step-by-step answer, covering food chains, energy flow, ozone depletion, biodegradable waste, and biomagnification — topics that consistently carry marks in the board exam.
CBSE Class 10 Biology Our Environment — Questions with Solutions
Which group of organisms are not constituents of a food chain?
(i) Grass, lion, rabbit, wolf
(ii) Plankton, man, fish, grasshopper
(iii) Wolf, grass, snake, tiger
(iv) Frog, snake, eagle, grass, grasshopper
(i) Grass, lion, rabbit, wolf
(ii) Plankton, man, fish, grasshopper
(iii) Wolf, grass, snake, tiger
(iv) Frog, snake, eagle, grass, grasshopper
Solution
Option (C) is Correct.
Explanation:
• Food chain (ii): It is an aquatic food chain, so the grasshopper cannot be a part of it.
• Food chain (iii): Wolf, snake, and tiger are all carnivores. There are no herbivores to eat the grass, therefore grass cannot be a part of this food chain.
Explanation:
• Food chain (ii): It is an aquatic food chain, so the grasshopper cannot be a part of it.
• Food chain (iii): Wolf, snake, and tiger are all carnivores. There are no herbivores to eat the grass, therefore grass cannot be a part of this food chain.
In 1987, an agreement was formulated by the United Nations Environment Programme (UNEP) to freeze the production of ‘X’ to prevent depletion of ‘Y’.
‘X’ and ‘Y’ respectively referred here are:
‘X’ and ‘Y’ respectively referred here are:
Solution
Option (C) is Correct.
Explanation: The Montreal Protocol, finalised in 1987, is a global agreement formulated by the United Nations Environment Programme (UNEP) to freeze the production of CFCs to prevent depletion of the ozone layer.
Explanation: The Montreal Protocol, finalised in 1987, is a global agreement formulated by the United Nations Environment Programme (UNEP) to freeze the production of CFCs to prevent depletion of the ozone layer.
Choose the option that correctly matches the items given in Column I and Column II.
| Column I | Column II |
|---|---|
| (a) Physical environment | (iii) Abiotic components |
| (b) Exposure to UV radiation | (iv) Skin cancer |
| (c) Chlorofluorocarbon compounds | (i) Ozone depletion |
| (d) Decomposers | (ii) Bacteria and fungi |
Solution
Option (B) is Correct.
Explanation: The correct matching is: Physical environment → Abiotic components; Exposure to UV radiation → Skin cancer; Chlorofluorocarbon compounds → Ozone depletion; Decomposers → Bacteria and fungi.
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Explanation: The correct matching is: Physical environment → Abiotic components; Exposure to UV radiation → Skin cancer; Chlorofluorocarbon compounds → Ozone depletion; Decomposers → Bacteria and fungi.
Which one of the following greenhouse gases is a contributor due to incomplete combustion of coal and petroleum?
Solution
Carbon monoxide (CO) — Option (C) is Correct.
Explanation:
• Carbon monoxide (CO) is produced due to the incomplete combustion of carbon-containing fuels such as coal, petroleum, and natural gas.
• During incomplete combustion, there is insufficient oxygen for carbon to convert completely into carbon dioxide (CO₂).
• As a result, carbon monoxide is formed instead.
• Though CO is not as potent a greenhouse gas as CO₂ or methane, it indirectly contributes to the greenhouse effect by reacting with other atmospheric gases.
Explanation:
• Carbon monoxide (CO) is produced due to the incomplete combustion of carbon-containing fuels such as coal, petroleum, and natural gas.
• During incomplete combustion, there is insufficient oxygen for carbon to convert completely into carbon dioxide (CO₂).
• As a result, carbon monoxide is formed instead.
• Though CO is not as potent a greenhouse gas as CO₂ or methane, it indirectly contributes to the greenhouse effect by reacting with other atmospheric gases.
Depletion of ozone is mainly due to _________.
Solution
Option (A) is Correct.
Explanation: Depletion of the ozone layer occurs due to chlorofluorocarbons (CFCs). Other chemicals such as carbon monoxide, methane, and pesticides do not cause depletion of the ozone layer.
Explanation: Depletion of the ozone layer occurs due to chlorofluorocarbons (CFCs). Other chemicals such as carbon monoxide, methane, and pesticides do not cause depletion of the ozone layer.
Which of the following features relates to biodegradable substances?
Solution
Option (A) is Correct.
Biodegradable materials are substances that can be decomposed or broken down by microorganisms or decomposers.
Biodegradable materials are substances that can be decomposed or broken down by microorganisms or decomposers.
Which of these statements is incorrect about a balanced ecosystem?
Solution
Option (C) is Correct.
Explanation: The components of an ecosystem depend on each other to maintain the ecological balance. Plants are also dependent on animals for carbon dioxide and other processes like pollination and dispersal of seeds. Therefore, Option (C) is the incorrect statement.
Explanation: The components of an ecosystem depend on each other to maintain the ecological balance. Plants are also dependent on animals for carbon dioxide and other processes like pollination and dispersal of seeds. Therefore, Option (C) is the incorrect statement.
Pyramid of energy is the:
Solution
Option (D) is Correct.
Explanation: When the energy level is represented in the form of a pyramid, it is known as a pyramid of energy — a graphic representation of the amount of energy at each trophic level.
Explanation: When the energy level is represented in the form of a pyramid, it is known as a pyramid of energy — a graphic representation of the amount of energy at each trophic level.
Excessive exposure of humans to UV rays results in:
(i) Damage to immune system
(ii) Damage to lungs
(iii) Skin cancer
(iv) Peptic ulcers
(i) Damage to immune system
(ii) Damage to lungs
(iii) Skin cancer
(iv) Peptic ulcers
Solution
Option (C) is Correct.
Explanation: Excessive exposure of humans to ultraviolet (UV) rays results in:
(i) Skin cancer
(ii) Damage to the immune system of the body
Explanation: Excessive exposure of humans to ultraviolet (UV) rays results in:
(i) Skin cancer
(ii) Damage to the immune system of the body
Give reason why a food chain cannot have more than four trophic levels.
Answer
The loss of energy at each step is so great that very little usable energy remains after four trophic levels.
According to the 10% law of energy transfer, only 10% of the energy available at one trophic level passes to the next. By the fourth or fifth trophic level, the remaining energy is too little to sustain a new level of organisms.
According to the 10% law of energy transfer, only 10% of the energy available at one trophic level passes to the next. By the fourth or fifth trophic level, the remaining energy is too little to sustain a new level of organisms.
In the following food chain, only 2 J of energy was available to the peacocks. How much energy would have been present in grass? Justify your answer.
Food Chain: Grass → Grasshopper → Frog → Snake → Peacock
Food Chain: Grass → Grasshopper → Frog → Snake → Peacock
Answer
According to the 10% Law: Only 10% of energy is transferred from one trophic level to the next.
Calculations (working backwards from Peacock):
Energy at Snake level: $\dfrac{2 \times 100}{10} = 20 \text{ J}$
Energy at Frog level: $\dfrac{20 \times 100}{10} = 200 \text{ J}$
Energy at Grasshopper level: $\dfrac{200 \times 100}{10} = 2000 \text{ J}$
Energy at Grass (producer) level: $\dfrac{2000 \times 100}{10} = \mathbf{20{,}000 \text{ J}}$
Food Chain Energy Representation:
Grass (20,000 J) → Grasshopper (2,000 J) → Frog (200 J) → Snake (20 J) → Peacock (2 J)
Calculations (working backwards from Peacock):
Energy at Snake level: $\dfrac{2 \times 100}{10} = 20 \text{ J}$
Energy at Frog level: $\dfrac{20 \times 100}{10} = 200 \text{ J}$
Energy at Grasshopper level: $\dfrac{200 \times 100}{10} = 2000 \text{ J}$
Energy at Grass (producer) level: $\dfrac{2000 \times 100}{10} = \mathbf{20{,}000 \text{ J}}$
Food Chain Energy Representation:
Grass (20,000 J) → Grasshopper (2,000 J) → Frog (200 J) → Snake (20 J) → Peacock (2 J)
Insects, Hawk, Grass, Snake, Frog — the following organisms form a food chain.
Which of these will have the highest concentration of non-biodegradable chemicals?
Name the phenomenon associated with it.
Which of these will have the highest concentration of non-biodegradable chemicals?
Name the phenomenon associated with it.
Answer
Organism with highest concentration: Hawk
Phenomenon: Biomagnification (also called Biological Magnification)
Non-biodegradable chemicals accumulate progressively at each trophic level. Since Hawk is at the top of the food chain (Grass → Insects → Frog → Snake → Hawk), it receives and retains the highest concentration of these substances.
Phenomenon: Biomagnification (also called Biological Magnification)
Non-biodegradable chemicals accumulate progressively at each trophic level. Since Hawk is at the top of the food chain (Grass → Insects → Frog → Snake → Hawk), it receives and retains the highest concentration of these substances.
DDT was sprayed in a lake to regulate the breeding of mosquitoes.
How would it affect the trophic levels in the following food chain associated with the lake? Justify your answer.
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How would it affect the trophic levels in the following food chain associated with the lake? Justify your answer.
Answer
Explanation:
DDT, being a non-biodegradable pesticide, will enter the food chain from the first trophic level, i.e., plankton.
Non-biodegradable pesticides accumulate progressively at each trophic level. This phenomenon is known as biological magnification.
The hawk (or top carnivore) will have the highest level of pesticide, as DDT concentration increases at every successive trophic level and cannot be broken down by body processes.
DDT, being a non-biodegradable pesticide, will enter the food chain from the first trophic level, i.e., plankton.
Non-biodegradable pesticides accumulate progressively at each trophic level. This phenomenon is known as biological magnification.
The hawk (or top carnivore) will have the highest level of pesticide, as DDT concentration increases at every successive trophic level and cannot be broken down by body processes.
In the following food chain, vertical arrows indicate the energy lost to the environment and horizontal arrows indicate energy transferred to the next trophic level.
Which one of the three vertical arrows (A, C, and E) and which one of the two horizontal arrows (B and D) will represent more energy transfer? Give reason for your answer.
Which one of the three vertical arrows (A, C, and E) and which one of the two horizontal arrows (B and D) will represent more energy transfer? Give reason for your answer.
Answer
• Arrow A will represent more energy transfer (lost to environment) as compared to C and E.
• Arrow B will represent more energy transfer (to next trophic level) as compared to D.
Explanation:
When green plants are eaten by primary consumers, a great amount of energy is lost as heat to the environment. Some goes into digestion and doing work, while the rest is used for growth and reproduction.
In accordance with the 10% law of energy transfer, only 10% of the energy available at one trophic level is transferred to the next. This loss of energy takes place at every trophic level, meaning the highest absolute amounts of energy are present and transferred at the producer and primary consumer levels (A and B).
• Arrow B will represent more energy transfer (to next trophic level) as compared to D.
Explanation:
When green plants are eaten by primary consumers, a great amount of energy is lost as heat to the environment. Some goes into digestion and doing work, while the rest is used for growth and reproduction.
In accordance with the 10% law of energy transfer, only 10% of the energy available at one trophic level is transferred to the next. This loss of energy takes place at every trophic level, meaning the highest absolute amounts of energy are present and transferred at the producer and primary consumer levels (A and B).
Study the food chain given below and answer the questions that follow:
(a) If the amount of energy available at the third trophic level is 100 joules, then how much energy will be available at the producer level? Justify your answer.
(b) Is it possible to have two more trophic levels in this food chain just before the fourth trophic level? Justify your answer.
(a) If the amount of energy available at the third trophic level is 100 joules, then how much energy will be available at the producer level? Justify your answer.
(b) Is it possible to have two more trophic levels in this food chain just before the fourth trophic level? Justify your answer.
Answer
(a) Energy at the producer level = 10,000 J
Because only 10% of energy is available for the next trophic level:
• Third trophic level = 100 J
• Second trophic level = 100 × 10 = 1,000 J
• Producer level = 1,000 × 10 = 10,000 J
(b) No, it is not possible to have two more trophic levels just before the fourth trophic level. The loss of energy at each step is so great that very little usable energy remains after four trophic levels. Adding more levels would leave insufficient energy to sustain life at those levels.
Because only 10% of energy is available for the next trophic level:
• Third trophic level = 100 J
• Second trophic level = 100 × 10 = 1,000 J
• Producer level = 1,000 × 10 = 10,000 J
(b) No, it is not possible to have two more trophic levels just before the fourth trophic level. The loss of energy at each step is so great that very little usable energy remains after four trophic levels. Adding more levels would leave insufficient energy to sustain life at those levels.
What is trophic level?
Why are autotrophs considered to be at the first trophic level of all food chains?
State the reason for the limited number of trophic levels in nature.
Why are autotrophs considered to be at the first trophic level of all food chains?
State the reason for the limited number of trophic levels in nature.
Answer
A specific place of organisms in the food chain is known as their trophic level.
Producers (green plants) constitute the first trophic level in a food chain because they take nutrients from the soil or water, and manufacture their own food by photosynthesis using energy from the sun.
In a food chain, the number of trophic levels is limited to 4–5. This is because, according to the 10% law of energy transfer, only 10% of energy passes from one trophic level to the next. Thus, the amount of energy decreases with successive trophic levels and becomes too little to sustain organisms beyond the fourth or fifth level.
Producers (green plants) constitute the first trophic level in a food chain because they take nutrients from the soil or water, and manufacture their own food by photosynthesis using energy from the sun.
In a food chain, the number of trophic levels is limited to 4–5. This is because, according to the 10% law of energy transfer, only 10% of energy passes from one trophic level to the next. Thus, the amount of energy decreases with successive trophic levels and becomes too little to sustain organisms beyond the fourth or fifth level.
Define a food chain.
Design a terrestrial food chain of four trophic levels.
If a pollutant enters at the producer level, the organisms of which trophic level will have the maximum concentration of the pollutant in their bodies?
What is this phenomenon called?
Design a terrestrial food chain of four trophic levels.
If a pollutant enters at the producer level, the organisms of which trophic level will have the maximum concentration of the pollutant in their bodies?
What is this phenomenon called?
Answer
A chain of organisms formed as a result of eating or being eaten by other organisms is called a food chain. It represents the transfer of energy and nutrients from one organism to the next.
Example of a Terrestrial Food Chain (4 trophic levels):
Grass → Insect → Frog → Snake
(Producer → Herbivore → Carnivore → Top Carnivore)
Trophic Level with Maximum Pollutant Concentration:
The organisms at the fourth trophic level (Top Carnivore / Snake) will have the maximum concentration.
Phenomenon: Biological magnification / Biomagnification
Example of a Terrestrial Food Chain (4 trophic levels):
Grass → Insect → Frog → Snake
(Producer → Herbivore → Carnivore → Top Carnivore)
Trophic Level with Maximum Pollutant Concentration:
The organisms at the fourth trophic level (Top Carnivore / Snake) will have the maximum concentration.
Phenomenon: Biological magnification / Biomagnification
Your mother always thought that fruit juices are very healthy for everyone. One day, she reads in the newspaper that some brands of fruit juices in the market have been found to contain certain levels of pesticides in them. She got worried as pesticides are injurious to our health.
(a) How would you explain to your mother about fruit juices getting contaminated with pesticides?
(b) It is said that when these harmful pesticides enter our body as well as in the bodies of other organisms, they get accumulated, and beyond a limit may cause harm and damage our organs. Name the phenomenon and write about it.
(a) How would you explain to your mother about fruit juices getting contaminated with pesticides?
(b) It is said that when these harmful pesticides enter our body as well as in the bodies of other organisms, they get accumulated, and beyond a limit may cause harm and damage our organs. Name the phenomenon and write about it.
Answer
(a)
(i) Farmers generally use pesticides on fruit crops to protect them from plant diseases. However, these pesticides may contaminate the fruits, and therefore fruit juices also become contaminated.
(ii) Using contaminated groundwater for irrigation also makes the fruits infected with contaminants.
(b)
Biological magnification (or bio-magnification) is the accumulation of chemicals in individuals of higher trophic levels. These chemicals are non-biodegradable, and their concentration increases at each trophic level.
Humans, being at the top of the food chain, receive the highest concentration of these harmful chemicals, resulting in various health problems.
(i) Farmers generally use pesticides on fruit crops to protect them from plant diseases. However, these pesticides may contaminate the fruits, and therefore fruit juices also become contaminated.
(ii) Using contaminated groundwater for irrigation also makes the fruits infected with contaminants.
(b)
Biological magnification (or bio-magnification) is the accumulation of chemicals in individuals of higher trophic levels. These chemicals are non-biodegradable, and their concentration increases at each trophic level.
Humans, being at the top of the food chain, receive the highest concentration of these harmful chemicals, resulting in various health problems.
Explain how ozone being a deadly poison can still perform an essential function for our environment.
Answer
Although ozone (O₃) is a poisonous gas at ground level, in the stratosphere it forms the ozone layer which acts as a shield. The ozone layer protects us from the harmful effects of UV radiation emitted by the sun, preventing serious damage to living organisms on Earth.
A lot of waste is generated in the neighbourhood. However, almost all of it is biodegradable.
What impact will it have on the environment or human health?
What impact will it have on the environment or human health?
Answer
Excess generation of biodegradable wastes can be harmful as:
(i) Their decomposition is a slow process, leading to the production of foul smell and harmful gases during decomposition.
(ii) They can become a breeding ground for germs, creating unhygienic conditions and spreading diseases in the neighbourhood.
(i) Their decomposition is a slow process, leading to the production of foul smell and harmful gases during decomposition.
(ii) They can become a breeding ground for germs, creating unhygienic conditions and spreading diseases in the neighbourhood.
What is meant by garbage? List two classes into which garbage is classified.
Answer
Garbage is a waste substance that is no longer useful for humans. It includes household waste, kitchen waste, animal waste, plant waste, etc.
There are two types of garbage:
(i) Biodegradable Wastes:
Substances which can be decomposed by the action of microorganisms are called biodegradable substances.
Examples: Fruit and vegetable peels, cotton, jute, dung, paper, etc.
(ii) Non-biodegradable Wastes:
Substances which cannot be decomposed by the action of microorganisms are called non-biodegradable wastes.
Examples: Plastic, polythene, pesticides, etc. Most of these result from human activities.
There are two types of garbage:
(i) Biodegradable Wastes:
Substances which can be decomposed by the action of microorganisms are called biodegradable substances.
Examples: Fruit and vegetable peels, cotton, jute, dung, paper, etc.
(ii) Non-biodegradable Wastes:
Substances which cannot be decomposed by the action of microorganisms are called non-biodegradable wastes.
Examples: Plastic, polythene, pesticides, etc. Most of these result from human activities.
The improvement in our lifestyle has led to the generation of large amount of waste material. List two reasons to justify this statement.
Answer
The day-to-day improvement in our lifestyle has resulted in a greater amount of waste generation.
Evidences to support this statement:
(i) The use of disposable items like paper plates, plastic items, polythene, etc., is increasing day by day. These are usually made up of non-biodegradable substances such as plastic and styrofoam, which eventually result in polluting our environment.
(ii) The use of aluminium foil and plastic wraps for packaging, both on a small scale and large scale, has resulted in a greater amount of non-biodegradable waste.
Evidences to support this statement:
(i) The use of disposable items like paper plates, plastic items, polythene, etc., is increasing day by day. These are usually made up of non-biodegradable substances such as plastic and styrofoam, which eventually result in polluting our environment.
(ii) The use of aluminium foil and plastic wraps for packaging, both on a small scale and large scale, has resulted in a greater amount of non-biodegradable waste.
‘The change in packaging has resulted in waste becoming non-biodegradable’.
Give two examples from daily life to justify this statement.
Give two examples from daily life to justify this statement.
Answer
Increased use of disposable items like paper plates, plastic items, and polythene (Example 1), and the use of aluminium foil and plastic wraps for packaging at both small and large scale (Example 2) have resulted in a greater amount of non-biodegradable waste.
To reduce non-biodegradable waste, polythene bags and plastic containers should be reused.
To reduce non-biodegradable waste, polythene bags and plastic containers should be reused.
(a) We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly. Why?
(b) Why is the ozone layer getting depleted at the higher levels of the atmosphere? Mention one harmful effect caused by its depletion.
(b) Why is the ozone layer getting depleted at the higher levels of the atmosphere? Mention one harmful effect caused by its depletion.
Answer
(a)
We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly because it is an artificial ecosystem and does not have enough decomposers to break down waste materials produced by aquatic animals. In contrast, ponds and lakes are natural ecosystems that have plenty of producers, consumers, and decomposers.
(b)
The ozone layer is getting depleted at the higher levels of the atmosphere due to the action of chlorofluorocarbons (CFCs), which are used in refrigerators and fire extinguishers.
Harmful effect: Depletion of the ozone layer allows a greater amount of UV radiation to reach the Earth. In humans, it may cause skin cancer, cataract of the eyes, and damage to the immune system.
We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly because it is an artificial ecosystem and does not have enough decomposers to break down waste materials produced by aquatic animals. In contrast, ponds and lakes are natural ecosystems that have plenty of producers, consumers, and decomposers.
(b)
The ozone layer is getting depleted at the higher levels of the atmosphere due to the action of chlorofluorocarbons (CFCs), which are used in refrigerators and fire extinguishers.
Harmful effect: Depletion of the ozone layer allows a greater amount of UV radiation to reach the Earth. In humans, it may cause skin cancer, cataract of the eyes, and damage to the immune system.
In 1987, the United Nations Environment Programme (UNEP) forged an agreement to freeze the production of a certain chemical to the year 1986 level.
(a) Name the chemical and the manufacturing companies in which this chemical was mostly used.
(b) In what way does this chemical damage:
(i) the upper layers of the atmosphere, and ultimately
(ii) the organisms on the Earth?
(a) Name the chemical and the manufacturing companies in which this chemical was mostly used.
(b) In what way does this chemical damage:
(i) the upper layers of the atmosphere, and ultimately
(ii) the organisms on the Earth?
Answer
(a) The name of the chemical is Chlorofluorocarbon (CFCs). They are used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants.
(b)(i) Air pollutants like CFCs cause depletion of the ozone layer in the upper atmosphere. This depletion allows a greater amount of UV radiation to reach the Earth.
(b)(ii) Effects of UV radiation on organisms:
• It affects photosynthesis in plants.
• It destroys planktons and decomposers, disturbing the ecosystem.
• In humans, it may cause skin cancer.
(b)(i) Air pollutants like CFCs cause depletion of the ozone layer in the upper atmosphere. This depletion allows a greater amount of UV radiation to reach the Earth.
(b)(ii) Effects of UV radiation on organisms:
• It affects photosynthesis in plants.
• It destroys planktons and decomposers, disturbing the ecosystem.
• In humans, it may cause skin cancer.
Plastic cups were used to serve tea in trains in the early days — these could be returned to the vendors, cleaned, and reused. Later, Kulhads were used instead of plastic cups. Now, paper cups are used for serving tea.
What are the reasons for the shift from plastic to Kulhads and then finally to paper cups?
What are the reasons for the shift from plastic to Kulhads and then finally to paper cups?
Answer
Plastic cups are non-biodegradable and harm the environment. They were, therefore, replaced by Kulhads.
However, making Kulhads, which are made of clay, on a large scale resulted in the loss of top fertile soil.
Now, disposable paper cups are used because paper can be recycled, it is biodegradable, and an eco-friendly material that does not cause any environmental pollution.
However, making Kulhads, which are made of clay, on a large scale resulted in the loss of top fertile soil.
Now, disposable paper cups are used because paper can be recycled, it is biodegradable, and an eco-friendly material that does not cause any environmental pollution.
(a) Explain the role of UV radiation in producing the ozone layer.
(b) Mention the reaction involved.
(c) Why is the excessive use of CFCs a cause of concern?
(b) Mention the reaction involved.
(c) Why is the excessive use of CFCs a cause of concern?
Answer
(a) High-energy UV radiations split apart some molecular oxygen (O₂) into free oxygen atoms (O). These free atoms combine with molecular oxygen to form ozone (O₃).
(b) Reactions involved:
$$\text{O}_2 \xrightarrow{\text{UV}} 2\text{O}$$ $$\text{O} + \text{O}_2 \rightarrow \text{O}_3$$
(c) Excessive use of CFCs is a cause of concern because:
• It leads to depletion of the ozone layer.
• If UV radiations reach the Earth unfiltered, they may cause skin cancer in human beings and disturb ecosystems.
(b) Reactions involved:
$$\text{O}_2 \xrightarrow{\text{UV}} 2\text{O}$$ $$\text{O} + \text{O}_2 \rightarrow \text{O}_3$$
(c) Excessive use of CFCs is a cause of concern because:
• It leads to depletion of the ozone layer.
• If UV radiations reach the Earth unfiltered, they may cause skin cancer in human beings and disturb ecosystems.
The stratosphere is very dry and rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form. These clouds are called Polar Stratospheric Clouds (PSCs).
Scientists recently discovered that polar stratospheric clouds, long known to play an important role in Antarctic ozone destruction, are now occurring with increasing frequency in the Arctic.
These high-altitude clouds form only at very low temperatures and help destroy ozone in two ways:
1. They provide a surface that converts benign forms of chlorine into reactive, ozone-destroying forms.
2. They remove nitrogen compounds that normally moderate the destructive impact of chlorine.
(Information credit: NASA)
(a) How is ozone formed in the outer atmosphere?
(b) Ozone is being continuously destroyed due to extremely low temperatures. However, ozone formation is also a continuous process. Why is there still a depletion in the ozone layer?
(c) What can be a positive effect of global warming on the depletion of the ozone layer?
(d) How does ozone layer depletion impact human health?
Scientists recently discovered that polar stratospheric clouds, long known to play an important role in Antarctic ozone destruction, are now occurring with increasing frequency in the Arctic.
These high-altitude clouds form only at very low temperatures and help destroy ozone in two ways:
1. They provide a surface that converts benign forms of chlorine into reactive, ozone-destroying forms.
2. They remove nitrogen compounds that normally moderate the destructive impact of chlorine.
(Information credit: NASA)
(a) How is ozone formed in the outer atmosphere?
(b) Ozone is being continuously destroyed due to extremely low temperatures. However, ozone formation is also a continuous process. Why is there still a depletion in the ozone layer?
(c) What can be a positive effect of global warming on the depletion of the ozone layer?
(d) How does ozone layer depletion impact human health?
Answer
(a) Ultraviolet (UV) radiations split the oxygen molecules (O₂) present in the stratosphere into free oxygen atoms (O). These free oxygen atoms then combine with molecular oxygen to form ozone (O₃).
$$\text{O}_2 \xrightarrow{\text{UV}} \text{O} + \text{O}$$ $$\text{O}_2 + \text{O} \rightarrow \text{O}_3$$
(b) There is still depletion in the ozone layer because the rate of destruction is higher than the rate of formation — primarily due to the action of CFCs and PSC-related chlorine activation.
(c) A positive effect of global warming on ozone depletion is that a rise in polar temperature might restrict the formation of Polar Stratospheric Clouds (PSCs) and therefore reduce ozone depletion.
(d) Removal of the ozone layer allows harmful UV radiations to reach the Earth, causing diseases such as skin cancer, cataract, and weakening of the immune system in humans.
$$\text{O}_2 \xrightarrow{\text{UV}} \text{O} + \text{O}$$ $$\text{O}_2 + \text{O} \rightarrow \text{O}_3$$
(b) There is still depletion in the ozone layer because the rate of destruction is higher than the rate of formation — primarily due to the action of CFCs and PSC-related chlorine activation.
(c) A positive effect of global warming on ozone depletion is that a rise in polar temperature might restrict the formation of Polar Stratospheric Clouds (PSCs) and therefore reduce ozone depletion.
(d) Removal of the ozone layer allows harmful UV radiations to reach the Earth, causing diseases such as skin cancer, cataract, and weakening of the immune system in humans.
Assertion (A): Food chain is responsible for the entry of harmful chemicals into our bodies.
Reason (R): The length and complexity of food chains vary greatly.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Reason (R): The length and complexity of food chains vary greatly.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Answer
Option (B) is Correct.
Explanation: Through bio-magnification, harmful chemicals that are not metabolised by our body pass into the food chain, irrespective of its length and complexity, which may vary in nature. Both statements are true but Reason (R) is not the correct explanation of Assertion (A).
Explanation: Through bio-magnification, harmful chemicals that are not metabolised by our body pass into the food chain, irrespective of its length and complexity, which may vary in nature. Both statements are true but Reason (R) is not the correct explanation of Assertion (A).
Assertion (A): A greater number of individuals are present in lower trophic levels.
Reason (R): The flow of energy is unidirectional.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Reason (R): The flow of energy is unidirectional.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Answer
Option (B) is Correct.
Explanation: There are generally a greater number of individuals at the lower trophic levels of an ecosystem, with the greatest number being the producers.
The flow of energy in an ecosystem is always linear or unidirectional. The energy captured by producers does not revert to the solar input, and the energy passed to herbivores does not return to autotrophs.
Both statements are individually true, but R does not directly explain A, hence Option (B).
Explanation: There are generally a greater number of individuals at the lower trophic levels of an ecosystem, with the greatest number being the producers.
The flow of energy in an ecosystem is always linear or unidirectional. The energy captured by producers does not revert to the solar input, and the energy passed to herbivores does not return to autotrophs.
Both statements are individually true, but R does not directly explain A, hence Option (B).
Assertion (A): Biodegradable substances result in the formation of compost and natural replenishment.
Reason (R): It is due to the breakdown of complex inorganic substances into simple organic substances.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Reason (R): It is due to the breakdown of complex inorganic substances into simple organic substances.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Answer
Option (C) is Correct.
Explanation: Biodegradable substances do result in the formation of compost and natural replenishment by the action of small organisms like bacteria and fungi — so Assertion (A) is true.
However, Reason (R) is false: Decomposition is the physical and chemical breakdown of complex organic matter into simple inorganic substances (not the other way around).
Explanation: Biodegradable substances do result in the formation of compost and natural replenishment by the action of small organisms like bacteria and fungi — so Assertion (A) is true.
However, Reason (R) is false: Decomposition is the physical and chemical breakdown of complex organic matter into simple inorganic substances (not the other way around).
Assertion (A): An ecosystem consists of biotic components and abiotic components.
Reason (R): Biotic and abiotic components play important roles for the sustenance of life and work independently.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Reason (R): Biotic and abiotic components play important roles for the sustenance of life and work independently.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
Answer
Option (C) is Correct.
Explanation: An ecosystem is a unit of the biosphere in which biotic and abiotic components interact with each other — so Assertion (A) is true.
However, Reason (R) is false because biotic and abiotic components do not work independently — biotic components depend on abiotic factors for their survival.
Explanation: An ecosystem is a unit of the biosphere in which biotic and abiotic components interact with each other — so Assertion (A) is true.
However, Reason (R) is false because biotic and abiotic components do not work independently — biotic components depend on abiotic factors for their survival.
The table shows some organisms and their food sources in an ecosystem.
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Identify the primary consumer(s) in the ecosystem.
(A) Snake (B) Shrew (C) Frog and Shrew (D) Cricket and Grasshopper
Identify the primary consumer(s) in the ecosystem.
(A) Snake (B) Shrew (C) Frog and Shrew (D) Cricket and Grasshopper
Answer
Option (D) is Correct.
Explanation: Cricket and Grasshopper are the primary consumers in the ecosystem as they feed directly on plants (producers).
Food Chain:
Plants → Cricket / Grasshopper → Frog → Snake → Eagle
Explanation: Cricket and Grasshopper are the primary consumers in the ecosystem as they feed directly on plants (producers).
Food Chain:
Plants → Cricket / Grasshopper → Frog → Snake → Eagle
Food chains and the energy flow within an ecosystem provide an important understanding of the contingencies and mutual dependencies of organisms.
The flow chart below depicts the energy flow within some members of a grassland ecosystem. The grass in this ecosystem transduces 120,000 J of sunlight and fixes it into 12,000 J of energy. It is established that 90% of the energy at one trophic level is not passed to the next. The energy transferred is equally shared among organisms at that trophic level.
1. How many food chains are present in the food web depicted above?
2. Indicate the amount of energy that organisms A to F may have received from the previous trophic level.
3. Identify the most energy-efficient link for the tertiary consumer.
4. Differentiate between the food habits of organisms belonging to the first and second trophic levels.
The flow chart below depicts the energy flow within some members of a grassland ecosystem. The grass in this ecosystem transduces 120,000 J of sunlight and fixes it into 12,000 J of energy. It is established that 90% of the energy at one trophic level is not passed to the next. The energy transferred is equally shared among organisms at that trophic level.
1. How many food chains are present in the food web depicted above?
2. Indicate the amount of energy that organisms A to F may have received from the previous trophic level.
3. Identify the most energy-efficient link for the tertiary consumer.
4. Differentiate between the food habits of organisms belonging to the first and second trophic levels.
Answer
1. There are five food chains depicted in the above food web.
2. Based on the Ten Per Cent Law (only 10% of the energy entering a particular trophic level is available for transfer to the next higher trophic level):
• A = 1200 J
• B = 1200 J
• C = 1200 J
• D = 120 J
• E = 12 J (via D and F) or 120 J (via A and C)
• F = 120 J
3. The tertiary consumer here is the Eagle. For the Eagle, the most energy-efficient link is the Rabbit, as the rabbit is not being eaten in any other food chain.
4. Organisms of the first trophic level are producers — they are autotrophic, i.e., they manufacture their own food from inorganic raw materials using sunlight.
Organisms of the second trophic level are herbivores — they are animals which feed on producers to obtain food and energy.
2. Based on the Ten Per Cent Law (only 10% of the energy entering a particular trophic level is available for transfer to the next higher trophic level):
• A = 1200 J
• B = 1200 J
• C = 1200 J
• D = 120 J
• E = 12 J (via D and F) or 120 J (via A and C)
• F = 120 J
3. The tertiary consumer here is the Eagle. For the Eagle, the most energy-efficient link is the Rabbit, as the rabbit is not being eaten in any other food chain.
4. Organisms of the first trophic level are producers — they are autotrophic, i.e., they manufacture their own food from inorganic raw materials using sunlight.
Organisms of the second trophic level are herbivores — they are animals which feed on producers to obtain food and energy.
Read the given passage and answer the following questions:
The green plants are autotrophs that synthesise their own food using solar energy and various inorganic constituents (like water and carbon dioxide) of the ecosystem. The food manufactured by the green plants is utilised by them and also by the herbivores. The herbivores are consumed by some carnivorous animals. In this way, one form of life is supported by another, forming a food chain.
It is estimated that only about 10% of the potential energy available at the previous trophic level is available to the next organism. The shorter the food chain, the greater is the amount of energy available to the last trophic level.
1. In the food chain comprising Tiger, Plants, and Goats, which will —
(i) Transfer the maximum amount of energy
(ii) Receive the minimum amount of energy?
2. In the following food chain, plants provide 500 J of energy to rats. How much energy will be available to hawks from snakes?
Food Chain: Plants → Rats → Snakes → Hawks
The green plants are autotrophs that synthesise their own food using solar energy and various inorganic constituents (like water and carbon dioxide) of the ecosystem. The food manufactured by the green plants is utilised by them and also by the herbivores. The herbivores are consumed by some carnivorous animals. In this way, one form of life is supported by another, forming a food chain.
It is estimated that only about 10% of the potential energy available at the previous trophic level is available to the next organism. The shorter the food chain, the greater is the amount of energy available to the last trophic level.
1. In the food chain comprising Tiger, Plants, and Goats, which will —
(i) Transfer the maximum amount of energy
(ii) Receive the minimum amount of energy?
2. In the following food chain, plants provide 500 J of energy to rats. How much energy will be available to hawks from snakes?
Food Chain: Plants → Rats → Snakes → Hawks
Answer
1.
(i) Transfer of Maximum Energy: Plants
(ii) Receive Minimum Energy: Tiger
2. Calculations:
Energy available to snakes from rats:
$$500 \text{ J} \times \frac{10}{100} = 50 \text{ J}$$
Energy available to hawks from snakes:
$$50 \text{ J} \times \frac{10}{100} = \mathbf{5 \text{ J}}$$
Energy available to hawks = 5 J
(i) Transfer of Maximum Energy: Plants
(ii) Receive Minimum Energy: Tiger
2. Calculations:
Energy available to snakes from rats:
$$500 \text{ J} \times \frac{10}{100} = 50 \text{ J}$$
Energy available to hawks from snakes:
$$50 \text{ J} \times \frac{10}{100} = \mathbf{5 \text{ J}}$$
Energy available to hawks = 5 J
Explore More CBSE Class 10 Biology Chapters
Frequently Asked Questions
What does the Our Environment chapter cover in CBSE Class 10 Biology? ⌄
The Our Environment chapter in CBSE Class 10 covers ecosystems, food chains and food webs, the flow of energy through trophic levels (including the 10% law), biodegradable and non-biodegradable substances, biomagnification, and the depletion of the ozone layer. These topics are regularly tested in board exams across all question types.
How many marks does Our Environment carry in the CBSE Class 10 board exam? ⌄
Our Environment is part of the Life Processes and related topics unit in CBSE Class 10 Science. Questions from this chapter appear as 1-mark MCQs, 2-mark short answers, 3-mark descriptive questions, and 5-mark case study questions, making it a chapter that contributes across multiple question formats and carries significant weightage in the board paper.
What are the most important topics students should focus on in Our Environment? ⌄
The most important topics are: the 10% law of energy transfer (with numerical calculations), biomagnification and how non-biodegradable substances enter food chains, the formation and depletion of the ozone layer and the role of CFCs, differentiating between biodegradable and non-biodegradable waste, and the role of decomposers in ecosystems. Questions on energy calculations and assertion-reason formats from this chapter are very frequent in recent board papers.
What common mistakes do students make when solving Our Environment questions? ⌄
A very common error is confusing biomagnification (increase in concentration of non-biodegradable chemicals at higher trophic levels) with bioaccumulation, or incorrectly applying the 10% law when calculating energy at different trophic levels. Students also frequently confuse the role of CFCs with carbon monoxide when answering ozone depletion questions. Regular practice with previous year questions helps your child avoid these errors under exam pressure.
How does Angle Belearn help students score well in Our Environment? ⌄
Angle Belearn’s CBSE specialists curate chapter-wise question banks drawn from real board papers, each paired with clear, step-by-step solutions. Students practising on Angle Belearn develop the habit of showing structured working — something that earns full marks in board exams. Regular practice with these verified questions from Our Environment builds both conceptual clarity and exam-day confidence.
