Class 10 Science · Chapter 13

Our Environment

298 Class 10 Science questions on Our Environment from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 7 of 15

1. Q121 CBSE 2025
   Study the ​food web given below :

   1. (a)​Identify ​the food chain(s) in which the eagle receives the ​highest energy from the producers.
   2. (b)Identify the organism ​in ​which a non-biodegradable pesticide will be found in maximum concentration. Name the term ​used for this phenomenon.

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   (a) Grass to mouse to eagle and grass to mongoose to eagle (shortest chains); (b) the eagle, by biological magnification.

   (a) Grass → Mouse → Eagle and Grass → Mongoose → Eagle. These are the shortest chains in the web, with three trophic levels. Since only about 10% of the energy passes from one level to the next, the eagle receives the most energy through them: about 1% of the energy stored in the grass, against about 0.1% through the four-level chains and about 0.01% through Grass → Grass Hopper → Frog → Snake → Eagle. (b) The eagle. It is the top consumer, at the end of every chain in the web, so a non-biodegradable pesticide reaches its highest concentration in it. The phenomenon is called biological magnification (biomagnification).

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2. Q122 CBSE 2025
   “Excessive use of ​chemicals and ​pesticides ​in agriculture ​adversely effect ​the environment.” Justify ​this ​statement.
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   Chemicals wash into soil and water, magnify along food chains to harm humans, and kill soil micro-organisms, so the statement is justified.

   Pesticides and chemical fertilisers sprayed on a field are washed by rain into the soil and into nearby water bodies. From the soil they are absorbed by plants along with water and minerals, and in water they are taken up by aquatic plants and animals, so they enter the food chain. These substances are non-biodegradable: they are neither broken down nor excreted. An animal keeps all of the chemical contained in the food it eats while building only a small part of that food into its own body, so the concentration of the chemical increases at every successive trophic level. This progressive increase is called biological magnification, and because human beings occupy the top level of the food chain, the maximum concentration accumulates in our bodies and causes serious health hazards. The same chemicals also kill useful soil micro-organisms and earthworms, so decomposition stops and soil fertility falls, and the run-off pollutes ponds and rivers and kills fish and other non-target organisms such as pollinators. The excessive use of these chemicals therefore damages the environment, which justifies the statement.

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3. Q123 CBSE 2025
   1. (a)​Why ​are the organisms of first trophic level important in ​any food chain ?
   2. (b)Justify the ​following statement ​: 'The flow of energy in ​an ecosystem ​is unidirectional.'
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   Producers capture the Sun's energy for all other organisms; energy enters once, moves one way and is lost as heat: unidirectional.

   (a) The first trophic level is occupied by the producers, the green plants and other autotrophs. They are the only organisms in a food chain that can capture the sun's radiant energy and convert it by photosynthesis into the chemical energy of food, so energy enters the living world only through them. Every other organism depends on them directly or indirectly for food and energy, and without them no food chain could exist. (b) Energy enters an ecosystem only as sunlight and only at the producers; it then passes from producers to herbivores and from herbivores to carnivores, and never in the reverse direction. At each transfer only about $10\%$ of the energy reaches the next trophic level, while about $90\%$ is used up in life processes and lost to the surroundings as heat, which no organism can capture and turn back into food. Since energy enters at one point, travels one way and is steadily lost as unusable heat, its flow in an ecosystem is unidirectional.

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4. Q124 CBSE 2025
   We do ​not ​clean ​natural ponds or lakes ​whereas an aquarium ​or a swimming pool ​needs to be ​cleaned regularly. Why ?
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   A pond has decomposers and cleans itself; an aquarium is an artificial, incomplete ecosystem, so wastes collect and it needs cleaning.

   A pond or a lake is a natural, complete and self-sustaining ecosystem. Along with its producers and consumers it contains decomposers, the bacteria and fungi, which break down the dead remains and the wastes of the organisms in it and return the nutrients to the water for the producers to use again. Nothing accumulates, so the pond cleans itself. An aquarium or a swimming pool is an artificial and incomplete ecosystem with few or no decomposers and no natural cycling of materials, so the wastes and dead matter simply collect in it and have to be removed by us. That is why they must be cleaned regularly.

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5. Q125 CBSE 2025
   Indicate the flow ​of ​energy in a food chain operating in an ​ecosystem. ​Why is it ​uni-directional ​? Give ​reason.
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   Energy flows from the Sun to producers, herbivores and carnivores, about $10\%$ passing on each step, and never returns.

   Flow of energy: Sun → producers (green plants) → herbivores (primary consumers) → carnivores (secondary consumers) → top carnivores (tertiary consumers). Green plants capture about 1% of the sunlight falling on their leaves, and on average only about 10% of the energy at one level is passed on to the next; much of the rest is used in life processes or lost to the surroundings as heat. It is uni-directional because the energy captured by the autotrophs does not revert to the solar input, and the energy that passes to the herbivores does not come back to the autotrophs: as it moves progressively through the trophic levels it is no longer available to the previous level.

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6. Q126 CBSE 2025
   Write the percentage ​of the energy of sunlight captured by green plants, to convert it into food energy. ​Explain the fate ​of this energy ​when ​green plants are eaten ​by primary ​consumers.
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   Plants capture about 1% of sunlight's energy; when eaten, much is lost as heat and only about 10% passes to the next level.

   Green plants capture about 1% of the energy of the sunlight falling on their leaves and convert it into food energy. When primary consumers eat the plants, a great deal of this energy is lost as heat to the environment, some is used in digestion and in doing work, and the rest goes into growth and reproduction. On average only about 10% of the food eaten is turned into the consumer’s own body and made available to the next level of consumers.

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7. Q127 CBSE 2025
   Define the term ​'biodegradable waste'. Select biodegradable ​waste(s) ​from the ​following ​: Insecticides, agricultural ​waste, plastics, sewage
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   Biodegradable waste is broken down by micro-organisms into simpler harmless substances; here, agricultural waste and sewage.

   Biodegradable waste is waste that is broken down into simpler, harmless substances by the action of micro-organisms such as bacteria and fungi. From the given list, the biodegradable wastes are agricultural waste and sewage; insecticides and plastics are non-biodegradable.

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8. Q128 CBSE 2025
   Why ​are ​bacteria and fungi called decomposers ? ​List ​any ​two ​advantages ​of decomposers.
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   Bacteria and fungi break dead remains and wastes into simple substances; they clean the environment and return nutrients to the soil.

   Bacteria and fungi are called decomposers because they feed on the dead remains and waste products of plants and animals and break this complex organic matter down into simple inorganic substances such as carbon dioxide, water and mineral salts, which are returned to the soil and the air. Two advantages: (i) they clean up the environment by disposing of dead bodies and organic waste, which would otherwise accumulate; (ii) they return nutrients to the soil, keeping it fertile and keeping the cycling of materials in the ecosystem going.

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9. Q129 CBSE 2025
   Write the essential function performed by ozone at the ​higher levels of the atmosphere. How is ​it ​formed ​in the ​upper atmosphere ? ​Write ​the name of the group of chemicals mainly responsible for the depletion of ozone layer.
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   Ozone absorbs harmful UV radiation; it forms when UV splits ${\text{O}}_2$ and the atoms combine with ${\text{O}}_2$; CFCs deplete it.

   Function: the ozone at the higher levels of the atmosphere absorbs the harmful ultraviolet radiation of the Sun and shields the surface of the Earth from it, so preventing skin cancer, cataract and damage to crops. Formation: high in the atmosphere the intense ultraviolet radiation splits oxygen molecules into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$, and these atoms combine with oxygen molecules to form ozone, $\text{O}+{\text{O}}_2⟶{\text{O}}_3$. Chemicals responsible for depletion: chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers.

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10. Q130 CBSE 2025
    1. (a)“In ​a ​food chain energy ​flow is unidirectional.” Give two reasons for ​the given statement.
    2. (b)If 10,000 J energy is ​available ​at the producer level, how ​much energy will be available to the secondary consumers ? Give reason to justify your answer.
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    Energy enters only as sunlight and about 90% is lost as heat at each level, so flow is one-way; secondary consumers get 100 J.

    (a) (i) Energy enters the food chain in one place only — as sunlight captured by the green plants — and then passes from producers to herbivores to carnivores, and never back, because no consumer can return energy to the level below it. (ii) At every trophic level about $90\%$ of the energy received is used up in life processes and given off to the surroundings as heat; heat lost in this way cannot be recaptured by any organism, so the flow cannot reverse. (b) $100$ J. Producers hold $10,000$ J; the primary consumers get about $10\%$ of it, which is $1000$ J; the secondary consumers get about $10\%$ of that, which is $100$ J. The reason is the ten per cent law — only about a tenth of the energy at one trophic level passes on to the next, and the secondary consumers are two transfers away from the producers.

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11. Q131 CBSE 2025
    Consider the ​following food chain : Plants → Deer → ​Lion In this food chain, state the impact of removing all the organisms ​of the second trophic level (i.e., ​Deer) on the organisms of ​the first and ​the third trophic levels ? Will the impact be the same ​for the organisms in the third ​trophic level if they were part of the food web of the ecosystem ? Justify your answer.
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    Plants increase and become overcrowded; lions starve; in a food web lions switch to other prey, so the impact is smaller.

    Impact on the **first** trophic level (plants): with no deer to graze on them the plants are no longer checked, so their population increases rapidly and becomes overcrowded; they then compete with one another for sunlight, water, minerals and space, many are stunted or die, and the habitat becomes unbalanced and degraded. Impact on the **third** trophic level (lions): in this chain the deer are the lions' only food, so the lions lose their entire food supply — they starve, their numbers fall sharply, and they either die or migrate away in search of prey. **No**, the impact on the third trophic level would **not** be the same if the lions were part of a food web. In a food web an organism has several sources of food, so a lion feeds not only on deer but also on other herbivores such as rabbits, wild boar or antelope. If the deer were removed, only one of its several food routes would be lost; the lion would switch to the alternative prey, its food supply would be reduced but not cut off, and its population would survive with only a small fall. This is exactly why a food web makes an ecosystem far more stable than a single food chain.

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12. Q132 CBSE 2025
    1. (a)Define the ​term ​biological ​magnification.
    2. (b)In the ​given aquatic food chain, which trophic level ​will ​have highest effect ​of biological ​magnification and why ? Phytoplankton $\to$ Zooplankton $\to$ Fishes $\to$ Human
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    Biological magnification is the progressive rise of harmful chemicals along a food chain; it is highest in human beings, the last level.

    (a) Biological magnification is the progressive increase in the concentration of harmful non-biodegradable chemicals, such as pesticides, at each successive trophic level of a food chain. (b) In the given chain the highest effect of biological magnification will be at the last trophic level, that is in Human beings, the fourth trophic level. The reason is that these chemicals cannot be broken down or excreted, so they are stored in the body of every organism that takes them in. At each step an organism eats a large number of organisms of the level below, so almost all the chemical stored in them is collected into one body, while only about ten per cent of the biomass is passed on. The concentration therefore multiplies at every step and is greatest in the organism at the end of the chain.

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13. Q133 CBSE 2025
    What are ​decomposers? ​Give ​two ​examples. State how ​they ​maintain ​a balance in an ecosystem.
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    Decomposers (bacteria, fungi) break down dead matter into simple substances, clean the environment and return nutrients to the soil for plants.

    Decomposers are micro-organisms that break down the complex organic matter of dead plants and animals, and their waste products, into simple inorganic substances that return to the soil, water and air. Two examples are bacteria and fungi. They maintain the balance of an ecosystem in two ways: they clean the environment by breaking down dead remains, which would otherwise pile up, and they release the nutrients locked in that dead matter back into the soil, where green plants absorb them and use them again to make food. Since matter must be used over and over in an ecosystem, decomposers are the step that closes the nutrient cycle and keeps the supply of nutrients available to the producers.

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14. Q134 CBSE 2025
    A gas 'G', which ​is deadly poisonous, is found at the higher levels of atmosphere and performs an essential function. Name the gas 'G' and state the function performed by this gas in ​the atmosphere. Name the chemicals linked to the ​decrease in ​the level of this gas. Write the measures which have been taken by an International Organisation to check the depletion of the layer ​of this gas at ​the ​higher levels of earth's atmosphere.
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    Gas G is ozone, which absorbs harmful UV; CFCs deplete it; UNEP's 1987 agreement froze CFC production at 1986 levels.

    Gas G is ozone, ${\text{O}}_3$. Its function is to shield the surface of the earth from the harmful ultraviolet radiation of the Sun: the ozone layer high in the atmosphere absorbs that radiation, which would otherwise cause skin cancer, cataract and damage to crops. The chemicals linked to the decrease in its level are the chlorofluorocarbons, the CFCs, used as refrigerants and in fire extinguishers and aerosol sprays. The measures taken internationally are those of the United Nations Environment Programme, UNEP, which in 1987 forged an agreement among the manufacturing countries, the Montreal Protocol, to freeze CFC production at the 1986 levels; the requirement now is to phase out CFC production entirely, and refrigerators, air-conditioners and aerosols are manufactured CFC free.

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15. Q135 CBSE 2025
    Some harmful chemicals get accumulated in ​human bodies through the food chain. ​Name this phenomenon. Explain the reason ​of maximum ​concentration ​of these chemicals found ​in our ​bodies.
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    The phenomenon is biological magnification; non-biodegradable chemicals build up at each trophic level, so humans at the top have the most.

    The phenomenon is biological magnification. Reason: harmful chemicals such as pesticides are washed into the soil and water bodies and are taken up by plants and aquatic organisms, so they enter the food chain. Being non-biodegradable, they are not broken down or readily excreted, so they accumulate in the organisms, and their concentration increases progressively at each trophic level as each organism eats a large quantity of food from the level below. Human beings occupy the top level of the food chains they belong to, so the maximum concentration of these chemicals accumulates in our bodies.

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16. Q136 CBSE 2024
    United ​Nations Environment ​Programme forged an ​agreement ​to

    1. (a)control ${\text{CO}}_2$ emissions in the environment
    2. (b)conserve biodiversity
    3. (c)control water pollution
    4. (d)reduce CFC production
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    (d) reduce CFC production

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17. Q137 CBSE 2024
    Identify the food chain ​in which ​the ​organisms ​of the second ​trophic ​level are ​missing :

    1. (a)Grass, goat, lion
    2. (b)Zooplankton, Phytoplankton, small fish, large fish
    3. (c)Tiger, grass, snake, frog
    4. (d)Grasshopper, grass, snake, frog, eagle
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    (c) Tiger, grass, snake, frog

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18. Q138 CBSE 2024
    A food ​chain will ​be ​more advantageous ​in terms of energy ​if it has

    1. (a)2 trophic levels
    2. (b)3 trophic levels
    3. (c)4 trophic levels
    4. (d)5 trophic levels
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    (a) 2 trophic levels

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19. Q139 CBSE 2024
    Consider the following statements ​: I. Every step in a food chain is called a trophic level. II. Decomposers convert complex inorganic matter in dead remains of organisms into simple substances ​that go into the ​soil. III. In ​a food chain, energy ​level increases ​from lower trophic level ​to higher trophic level. IV. Linkages between various food chains constitute a food web. The correct statements is/are :

    1. (a)I only
    2. (b)I and IV
    3. (c)I, II and IV
    4. (d)II and IV
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    (b) I and IV

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20. Q140 CBSE 2024
    Amongst the following groups ​of ​materials the ​group ​which contains ​only ​biodegradable materials is :

    1. (a)Polythene, Soap, Wool
    2. (b)Plastic, Leather, Grass
    3. (c)Glass, Aluminium, Neem leaves
    4. (d)Wood, Leather, Paper
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    (d) Wood, Leather, Paper

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