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 11 of 15

1. Q201 CBSE 2023
   1. (i)Why does a kitchen garden called an artificial ecosystem while a forest is considered to be a natural ecosystem ​?
   2. (ii)While designing an artificial ​ecosystem at home, write any two ​things ​to be kept in mind to convert it into a self-sustaining system. Give ​reason ​to justify your ​answer.
   Show answer

   A kitchen garden needs human care, whereas a forest sustains itself; a home ecosystem needs producers, consumers and decomposers, plus balanced abiotic conditions.

   (i) A kitchen garden is called an artificial ecosystem because it is created and maintained by human beings: people choose and sow the plants, water them, add manure, remove the weeds and keep out the pests, and it would not continue without that care. A forest is a natural ecosystem because it comes into being and keeps itself going entirely by natural processes, without any human interference; its plants, animals and decomposers are present in balanced numbers, so it is self-regulating and self-sustaining. (ii) Two things to keep in mind: 1. Include all three kinds of organism - producers (green plants), consumers, and especially decomposers such as bacteria and fungi. Reason: the decomposers break down dead remains and waste and return the nutrients to the soil, so that the plants can use them again; without them the nutrients stay locked up and the system runs down. 2. Provide the abiotic components in the right measure - enough sunlight, air, water and a suitable temperature - and keep the numbers at each trophic level balanced. Reason: sunlight is the only energy input the system has, since energy flows through it and is lost as heat, and a balance of numbers stops the consumers from finishing off the producers.

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2. Q202 CBSE 2023
   1. (i)Construct a food chain of ​four trophic levels comprising ​the following : Hawk, ​snake, plants, rat.
   2. (ii)20,000 J of energy ​was transferred by the ​producers to the organism ​of second ​trophic level. Calculate the amount of energy that will be transferred by organisms of the third trophic ​level to the organisms of the fourth trophic level.
   Show answer

   The third trophic level transfers 200 J to the fourth; the chain is plants, rat, snake, hawk.

   (i) Plants $\to$ Rat $\to$ Snake $\to$ Hawk. (Plants are the producers and the first trophic level, the rat the second, the snake the third and the hawk the fourth.) (ii) By the ten per cent law, energy transferred to the third trophic level $=\frac{10}{100}\times 20,000=2,000$ J, and energy transferred by the third trophic level to the fourth $=\frac{10}{100}\times 2,000=200$ J.

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3. Q203 CBSE 2022
   Kulhads (disposable ​cups ​made of clay) and disposable paper cups both are used as an alternative for ​disposable plastic cups. Which one of these ​two can ​be ​considered ​as a ​better alternative to plastic cups and why ?
   Show answer

   Disposable paper cups are better: paper is biodegradable and recyclable, whereas large-scale kulhad making would remove fertile top-soil.

   Disposable paper cups can be considered the better alternative. The paper they are made of is biodegradable — it is broken down by micro-organisms — and paper can be recycled. Kulhads are made of clay dug from the soil, and making them on the large scale at which disposable cups are used would result in the loss of fertile top-soil.

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4. Q204 CBSE 2022
   Human beings are most ​adversely ​affected by ​the ​Biological Magnification. State the reason. Why can ordinary ​washing of ​the edibles (fruits and vegetables) not reduce ​the ​effect of biological magnification ?
   Show answer

   Humans, at the top trophic level, receive the maximum concentration of non-degradable chemicals; washing removes only surface residue, not chemicals inside the tissues.

   Reason: Pesticides and other chemicals used on crops are not degradable. They are absorbed by plants from the soil and water and get accumulated progressively at each trophic level, so their concentration rises up the food chain. Human beings are usually at the top trophic level of the food chains they belong to, so the maximum concentration of these chemicals accumulates in human bodies. Washing: The plants take up these chemicals along with water and nutrients, so the chemicals are inside the tissues of the fruits and vegetables, not just on their surface. Ordinary washing removes only surface residue, so it cannot remove the chemical inside the food.

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5. Q205 CBSE 2022
   “The ​improvement ​in our lifestyle has led to the ​generation of large amount ​of ​waste material.” List ​two reasons to justify this ​statement.
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   Disposable use-and-throw articles and packaging that is thrown away at once, much of it non-biodegradable, generate large amounts of waste.

   Two reasons: 1. Change in attitude: more and more of the things we use are disposable (use-and-throw), such as paper and plastic cups and plates, disposable pens and syringes. They are thrown away after a single use instead of being used again, so every use adds to the waste. 2. Change in packaging: many of the things we buy now come wrapped or packed in paper, plastic, polythene or cardboard, and this packaging is usually thrown away as soon as the product is opened; much of it is also non-biodegradable. (Greater purchasing power, which leads people to buy more and to replace things such as gadgets while they are still usable, is another reason that justifies the statement.)

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6. Q206 CBSE 2022
   “The ​change in packaging has ​resulted in waste ​becoming non-biodegradable.” Giving two examples ​from ​daily life, justify ​this statement.
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   Biodegradable cloth, jute, leaf and paper packaging was replaced by polythene and plastic, which decomposers cannot break down, so waste became non-biodegradable.

   Biodegradable substances are broken down by decomposers such as bacteria and fungi; non-biodegradable substances, such as plastics, are not, and they persist in the environment for a long time. 1. Carry bags: earlier, shopping was carried home in cloth or jute bags, or wrapped in paper - all biodegradable, and the bags were reused. Now polythene carry bags are used once and thrown away, and they are non-biodegradable. 2. Serving and packing food: earlier, snacks and sweets were served on leaf plates and in leaf bowls or wrapped in paper, which decompose. Now they come on thermocol plates, in plastic cups and in plastic packets, and milk comes in polythene pouches - all non-biodegradable. In both examples biodegradable packaging has been replaced by plastic packaging that decomposers cannot break down, so the change in packaging has made our waste non-biodegradable.

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7. Q207 CBSE 2022
   Why do we ​say that the flow ​of ​energy in an ecosystem is unidirectional ? Explain ​with the ​help of a food ​chain ​having ​three trophic levels.
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   Energy flows only from the Sun to grass, deer and lion, with about $10\%$ passed on each step, and never flows back.

   The flow of energy in an ecosystem is said to be unidirectional because energy moves in one direction only - from the Sun to the producers, and then from one trophic level to the next - and never flows back. The energy captured by the autotrophs (producers) 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. At each level a large part of the energy is also used in life processes or lost as heat, which cannot be used again by the producers. Food chain with three trophic levels: grass $\to$ deer $\to$ lion. The grass (producer, first trophic level) captures about $1\%$ of the sunlight falling on its leaves; in this chain the deer (herbivore, second trophic level) gets energy only by eating grass, and the lion (carnivore, third trophic level) only by eating deer. Only about $10\%$ of the energy of each level passes to the next - for example $10,000$ J in the grass, about $1,000$ J in the deer and about $100$ J in the lion - and the rest is used up or lost as heat. None of this energy flows back from the lion to the deer, from the deer to the grass, or from the grass to the Sun.

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8. Q208 CBSE 2022
   What ​is ozone ? How is ​it ​formed in ​the upper layers of the Earth’s ​atmosphere ? ​How does ozone affect our ecosystem ​?
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   Ozone is ${\text{O}}_3$, formed from oxygen by UV radiation; the ozone layer shields organisms from the Sun's harmful UV.

   Ozone is a molecule made of three atoms of oxygen, ${\text{O}}_3$. Formation: at the higher levels of the atmosphere, high-energy ultraviolet (UV) radiation from the Sun splits some oxygen molecules into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$; each free oxygen atom then combines with a molecule of oxygen to form ozone, $\text{O}+{\text{O}}_2\to {\text{O}}_3$. Effect on our ecosystem: the ozone layer shields the surface of the Earth from the Sun's ultraviolet radiation, which is highly damaging to organisms (it causes skin cancer in human beings, for example). By absorbing most of this radiation, ozone in the upper atmosphere protects the plants, animals and human beings of the ecosystem; where the layer has been depleted by chlorofluorocarbons (CFCs), more ultraviolet radiation reaches them. Near the ground, however, ozone is a deadly poison.

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9. Q209 CBSE 2022
   In the following ​food chain, only 2J of ​energy was available to the peacocks. How much energy ​would have been present in ​Grass ? Justify your answer. GRASS → GRASS HOPPER ​→ FROG → ​SNAKE → ​PEACOCK
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   The grass would have had $2\times 10^4$ J, that is 20,000 J.

   About $20,000$ J ($2\times 10^4$ J, that is $20$ kJ) would have been present in the grass. Justification: by the ten per cent law, only about $10\%$ of the energy at one trophic level is transferred to the next. The chain has five trophic levels - grass (1), grasshopper (2), frog (3), snake (4), peacock (5) - so going back from the peacock to the grass the energy is multiplied by $10$ four times: $2\to 20\to 200\to 2000\to 20,000$ J.

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10. Q210 CBSE 2022
    1. (i)Why ​are ​crop fields considered as artificial ecosystems ?
    2. (ii)Write ​a common ​food ​chain ​of ​four steps operating ​in a terrestrial ecosystem.
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    A crop field is artificial as humans create and maintain it; a four-step terrestrial food chain is grass, grasshopper, frog, snake.

    (i) Crop fields are considered artificial ecosystems because they are created and maintained by human beings. People decide which crop is grown and control its conditions: they prepare the soil, sow the seeds, irrigate, add manure and fertilisers, remove weeds, control pests and harvest the crop. Without this human care the field would not continue as a crop field, unlike a natural ecosystem such as a forest or a pond, which maintains itself. (ii) Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake (producer, herbivore, carnivore and top carnivore of the chain: four trophic levels).

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11. Q211 CBSE 2022
    What are ​Decomposers ? ​List two important ​roles of ​decomposers in ​an ​ecosystem.
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    Decomposers are bacteria and fungi that break dead remains and wastes into simple substances; they clean the environment and recycle nutrients.

    Decomposers are micro-organisms, mainly bacteria and fungi, that break down the dead remains and waste products of plants and animals, converting the complex organic substances in them into simple inorganic substances. Two important roles in an ecosystem: 1. They break down the dead bodies and wastes of organisms, and so clean the environment; without them, dead remains and wastes would pile up. 2. They return nutrients to the soil: the simple inorganic substances they release go into the soil and are used up once more by plants, so nutrients are recycled and the soil is replenished.

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12. Q212 CBSE 2022
    1. (i)List two human-made ecosystems.
    2. (ii)“We ​do ​not ​clean a pond ​in the same manner ​as we do in ​an aquarium.” Give ​reason to justify this ​statement.
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    Garden and crop field are human-made ecosystems; a pond's decomposers recycle its waste, an aquarium has too few and needs cleaning.

    (i) Two human-made (artificial) ecosystems: a garden and a crop field. (An aquarium is another.) (ii) A pond is a natural, self-sustaining ecosystem. Besides producers (algae and water plants) and consumers (fish and other animals), it has plenty of decomposers, bacteria and fungi, which break down dead remains and waste products into simple substances that go back into the water and are used again by the producers. Its waste is recycled and does not pile up, so the pond does not need to be cleaned by us as an aquarium does. An aquarium is a human-made ecosystem: its fish are fed by us from outside, and it has too few decomposers to break down the leftover food, excreta and dead matter as fast as they are produced, so the waste collects and fouls the water. That is why an aquarium has to be cleaned by us regularly, while a pond is not cleaned in the same manner.

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13. Q213 CBSE 2022
    1. (a)What is ​meant by ​garbage ? List two classes into which garbage ​is classified.
    2. (b)​What do ​we actually mean ​when we say that the ​“enzymes ​are specific in their action” ?
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    Garbage is waste material, either biodegradable or non-biodegradable; each enzyme acts only on a particular substance.

    (a) Garbage is the waste material - things we no longer need and throw away - produced by our daily activities in homes, schools, offices, markets and industries, for example vegetable peels, leftover food, waste paper, packaging, plastic bags and broken glass. It is classified into two classes: biodegradable wastes, which are broken down by biological processes (by bacteria and other saprophytes), such as vegetable peels and paper; and non-biodegradable wastes, which are not broken down in this way and persist for a long time, such as plastics and glass. (b) It means that each enzyme acts on a particular substance, so a specific enzyme is needed to break down each substance and no single enzyme can break down everything. For example, salivary amylase breaks down starch but not proteins. That is why we get no energy by eating coal, and why many human-made materials, such as plastics, are not broken down by decomposers - no enzyme of theirs acts on them - so they are non-biodegradable.

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14. Q214 CBSE 2022
    1. (i)Why ​should ​biodegradable and non-biodegradable wastes be discarded separately ?
    2. (ii)What destructive effect do chlorofluorocarbons ​bring about ​in ​the atmosphere and ​to ​the mankind ?
    Show answer

    Separate disposal lets each kind be composted or recycled; CFCs deplete the ozone layer, so more UV reaches Earth and causes skin cancer.

    (i) Biodegradable wastes (vegetable peels, leftover food, paper) are broken down by decomposers such as bacteria and fungi into simple substances and can be turned into compost (manure) or biogas, whereas non-biodegradable wastes (plastics, polythene, glass, metals) are not broken down by biological processes and remain in the environment for a long time, harming soil, water and animals; many of them can be reused or recycled. They should be discarded separately so that each can be dealt with properly - the biodegradable part by composting or biogas, the non-biodegradable part by recycling or safe disposal. When they are mixed, the plastic contaminates the compost and the wet waste spoils the recyclable materials, so neither can be processed and more waste ends up in dumps. (ii) Chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers, rise into the upper atmosphere and break down ozone, so they deplete (thin) the ozone layer. The ozone layer shields the Earth from the harmful ultraviolet (UV) radiation of the Sun. With the layer thinned, more UV radiation reaches the Earth, and this can cause skin cancer in human beings, damage the eyes (cataract) and weaken the immune system, and it also harms plants and animals.

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15. Q215 CBSE 2022
    Distinguish ​between Biodegradable ​and Non-biodegradable ​substances giving ​one ​example for ​each.
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    Decomposers break down biodegradable substances (vegetable peels) but not non-biodegradable ones (polythene bags), which persist and may harm the ecosystem.

    1. What breaks them down: Biodegradable substances are broken down by biological processes, that is, by the action of decomposers such as bacteria and fungi. Non-biodegradable substances are not broken down by biological processes. 2. How long they last: Biodegradable substances are broken down into simpler substances and do not persist in the environment. Non-biodegradable substances persist in the environment for a long time. 3. Effect on the ecosystem: The materials of biodegradable substances return to the soil and are reused in natural cycles. Non-biodegradable substances may remain as inert waste or may harm members of the ecosystem; some, such as DDT, build up in food chains. Examples: biodegradable - vegetable peels (or paper, cotton cloth); non-biodegradable - polythene bags and other plastics (or glass, DDT).

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16. Q216 CBSE 2022
    What ​are human-made ecosystems ? Give an ​example. Can ​a human-made ecosystem become ​a self-sustaining ​ecosystem ? Give ​reason to justify your ​answer.
    Show answer

    Human-made ecosystems (e.g. a crop field) are created by humans; one can become self-sustaining if producers, consumers and decomposers are in balance.

    Human-made (artificial) ecosystems are ecosystems that are created and maintained by human beings, as distinct from natural ecosystems such as forests, ponds and lakes. Example: a crop field (gardens, parks and aquaria are others). Yes, a human-made ecosystem can become self-sustaining, provided it contains producers, consumers and decomposers in balance. For example, when a few aquatic plants and animals are added to an aquarium, the plants make food and oxygen by photosynthesis using sunlight, the animals feed on them and release carbon dioxide that the plants use, and decomposers break down the wastes and dead remains into minerals that the plants take up again. Materials such as carbon (in food and carbon dioxide), oxygen and minerals are then recycled within the tank, so it can keep going without being fed or cleaned, with sunlight as its source of energy. Without that balance, as in a crop field that has to be sown, watered, fertilised and weeded, or a tank holding only fish, the ecosystem depends on people and is not self-sustaining.

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17. Q217 CBSE 2022
    How is ​ozone formed ​at ​the higher ​levels of ​the atmosphere ? State how ozone performs the ​essential ​function at this level. In the 1980’s when it was noticed that the amount of ozone in the atmosphere is ​dropping sharply, what steps were taken to save the atmosphere ?
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    UV splits ${\text{O}}_2$ and the atoms form ${\text{O}}_3$, which absorbs UV; CFC production was frozen after UNEP's 1987 agreement.

    Formation: at the higher levels of the atmosphere, high-energy ultraviolet (UV) radiation from the Sun splits some oxygen molecules into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$, and each free atom combines with an oxygen molecule to form ozone, $\text{O}+{\text{O}}_2\to {\text{O}}_3$. Function: this ozone absorbs most of the harmful UV radiation from the Sun and so shields the surface of the Earth from it. UV radiation is highly damaging to organisms; for example, it is known to cause skin cancer in human beings. Steps taken: the fall was linked to synthetic chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. In 1987 the United Nations Environment Programme (UNEP) forged an agreement to freeze CFC production at 1986 levels, and, as NCERT notes, it is now mandatory for manufacturers throughout the world to make CFC-free refrigerators.

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18. Q218 CBSE 2022
    What ​is Biological magnification ​? How ​does ​it affect ​organisms ​present ​at various trophic levels, particularly the tertiary consumers ?
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    Biological magnification is the rising concentration of non-biodegradable chemicals along a food chain, so tertiary consumers are affected most.

    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. These chemicals are washed into the soil and water, are absorbed by plants and aquatic organisms, and so enter the food chain. As they are not degradable, they are not broken down and are stored in the body, and each organism eats many organisms of the level below. The concentration therefore rises at every trophic level: the producers have the least, and each higher level of consumers has more. The tertiary consumers, at the top of a four-level chain, accumulate the highest concentration and are affected the most. In the DDT example printed in NCERT Class 12 Biology, large fish (tertiary consumers) carry $2$ ppm, $50$ times the $0.04$ ppm in zooplankton, and the fish-eating birds one level above them reach $25$ ppm; in birds, high DDT concentrations disturb calcium metabolism, making eggshells thin so that they break early and bird populations fall.

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19. Q219 CBSE 2022
    What are trophic levels ? ​Why are autotrophs ​considered to ​be at the first trophic level of all ​food chains ? State the reason for limited ​number ​of trophic levels ​in nature.
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    Autotrophs begin every chain by fixing the Sun's energy; only about 10% passes on at each level, so chains have few trophic levels.

    Trophic levels are the steps of a food chain at which food, and with it energy, is transferred; each step or level of a food chain forms one trophic level, with the producers first, herbivores second, small carnivores third and large carnivores fourth. Autotrophs are placed at the first trophic level because they make their own food from carbon dioxide and water using sunlight (photosynthesis). They fix the Sun's energy in food and make it available to all the consumers, which depend on them directly or indirectly, so a food chain begins with them. The number of trophic levels is limited because the flow of energy is one-way and only about $10\%$ of the energy at one trophic level reaches the next; the rest is used in life processes and lost as heat. After three or four steps so little usable energy is left (for example, $10,000$ J in the producers leaves only $10$ J at the fourth level) that it cannot support a further level, so food chains usually have only three or four trophic levels.

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20. Q220 CBSE 2022
    1. (a)What ​are decomposers ​? State the role of decomposers ​in the natural replenishment ​of the ​soil.
    2. (b)​Why are decomposers not helpful in ​decomposing ​the plastic waste ?
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    Decomposers break dead remains into inorganic nutrients that return to the soil; they lack enzymes for plastic, so it stays undecomposed.

    (a) Decomposers are micro-organisms, such as bacteria and fungi, that break down the dead remains and waste products of plants and animals. They break the complex organic substances in them down into simple inorganic substances, which go into the soil and are absorbed again by plants. In this way the nutrients taken up from the soil are returned to it, and the soil is replenished naturally. (b) Decomposers break substances down with enzymes, and enzymes are specific in their action: a particular enzyme is needed to break down a particular substance. Decomposers have no enzymes that can act on plastics, which are human-made materials, so they cannot decompose plastic waste. Plastic is non-biodegradable and persists in the environment for a long time.

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