Our Environment

Why this chapter matters for UPSC: This is Chapter 13, "Our Environment", the last chapter of NCERT's Class X Science (Reprint 2026-27, pp. 208-217). It defines an ecosystem, sorts living things into producers, consumers and decomposers, follows energy along food chains and food webs, and shows how chemicals that cannot be broken down climb the chain until they reach us. It then takes two problems caused by human activity in detail: the depletion of the ozone layer and the disposal of the waste we produce. For Prelims the chapter supplies the basic vocabulary of ecology (trophic level, food web, biological magnification, biodegradable) and two numbers that recur everywhere, 1 per cent and 10 per cent. GS3 meets the same ideas in solid and hazardous waste, the pollution of rivers and soil, and the treaties that protect the ozone layer.

Contemporary hook: On 3 October 2026 the United Nations treaty record listed 198 parties to the Montreal Protocol on Substances that Deplete the Ozone Layer. It is the 1987 agreement that NCERT describes on p. 213 without naming it. Every southern spring, from August to October, NASA's Ozone Watch maps the Antarctic ozone hole, which it defines as the region over Antarctica with total ozone of 220 Dobson Units or lower. Values that low were not observed before 1979.

🧠 First Principles — Read This First

1. An ecosystem is living things plus their physical surroundings. Biotic components (plants, animals, microorganisms) interact with abiotic components, the physical factors such as temperature, rainfall, wind, soil and minerals. Forests, ponds and lakes are natural ecosystems; gardens and crop-fields are human-made, and so is an aquarium (pp. 208-209).
2. Every ecosystem has three working roles. Producers (all green plants and certain bacteria) make food by photosynthesis. Consumers eat that food, directly or by eating other consumers. Decomposers (bacteria and fungi) break dead remains into simple inorganic substances that go back into the soil for plants to use again (p. 209).
3. Energy flows one way and thins out at every step. Green plants on land capture about 1% of the sunlight falling on their leaves. On average only 10% of the food eaten at one level is turned into body mass for the next. So food chains generally have only three or four steps (pp. 210-211).
4. What cannot be broken down builds up. Pesticides that are not degradable pass from soil and water into plants and animals and accumulate at each trophic level. Humans, at the top, carry the highest concentration. This is biological magnification (pp. 211-212).
5. Ozone is a shield high up. Ultraviolet radiation splits some oxygen molecules into atoms, which join other oxygen molecules to form ozone. This upper ozone keeps UV radiation away from the surface. Ozone fell sharply in the 1980s, and the fall was linked to CFCs used as refrigerants and in fire extinguishers (pp. 212-213).
6. Enzymes decide what rots. Enzymes are specific, so the bacteria and other saprophytes that break down food and leaves cannot break down materials such as plastics. Substances broken down by biological processes are biodegradable; the rest persist or harm the ecosystem (p. 214).
7. Waste is a product of how we live. Better life-styles, disposable goods and new packaging have increased waste and made much of it non-biodegradable. Even a clay substitute for a plastic cup carries a cost, the loss of fertile top-soil (p. 215).

PART 1 — Quick Reference

Table 1: The ecosystem and its components (13.1)

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, section 13.1 and Activity 13.1 (pp. 208-209).

Table 2: Food chains, trophic levels and the flow of energy (13.1.1)

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, section 13.1.1 and Figs 13.1-13.4 (pp. 210-211).

Table 3: The 10 per cent average worked along a chain (arithmetic)

Source: arithmetic applying NCERT's 10% average (Science Class X, ch. 13, Reprint 2026-27, p. 210) to the grazing food chain grass → goat → man (producer, primary consumer, secondary consumer) in NCERT, Biology Class XII, ch. 12, Reprint 2025-26 (p. 209). The figures are illustrative, not measurements.

Table 4: How pesticides reach our food (biological magnification)

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, section 13.1.1 (pp. 211-212).

Table 5: Ozone and its depletion (13.2.1)

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, section 13.2.1 (pp. 212-213).

Table 6: Biodegradable and non-biodegradable substances (13.2.2)

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, section 13.2.2 and Exercise 1 (pp. 214, 216) and Answers (p. 219).

Table 7: The activities and the box

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, Activities 13.1-13.9 and the box "Disposable cups in trains" (pp. 208-216).

Table 8: In-text questions, with answers from the chapter

NCERT's answer key covers only the exercise MCQs, so these answers are drawn from the chapter's own text.

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, in-text questions (pp. 212, 214, 216) and the sections cited.

Table 9: Exercises and NCERT's answers

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27, Exercises (pp. 216-217) and Answers (p. 219). NCERT's key gives answers only for Exercises 1-3; the answers to 4-9 are drawn from the pages cited.

Table 10: NCERT lines to read with care

Source: NCERT, Science Class X, ch. 13, Reprint 2026-27 (pp. 209-216) and Answers (p. 219); 2020-21 edition, ch. 15 (p. 258); NCERT, Biology Class XII, ch. 12, Reprint 2025-26 (pp. 207, 209, 211-214); UNEP Ozone Secretariat, "The Montreal Protocol"; PIB releases of 16 September 2023 (MoEFCC) and 18 August 2021 (Cabinet); NASA Ozone Watch, "Ozone hole facts".

PART 2 — Concepts & Narrative

What an ecosystem is (13.1)

The chapter opens with a word we hear everywhere. Our elders say the environment is not what it used to be, others say we should work in a healthy environment, and global summits involving developed and developing countries are regularly held to discuss environmental issues. NCERT's aim is narrower and more useful: to study how the components of the environment interact with each other and how we affect them (p. 208).

Plants, animals, microorganisms and human beings interact with each other and with their physical surroundings, and together they maintain a balance in nature. All the interacting organisms in an area, together with the non-living constituents of the environment, form an ecosystem. Its biotic components are the living organisms; its abiotic components are physical factors like temperature, rainfall, wind, soil and minerals (p. 208).

A garden is NCERT's first example. It holds grasses and trees, flowering plants like rose, jasmine and sunflower, and animals like frogs, insects and birds. All of them interact, and their growth, reproduction and other activities are affected by the abiotic components. Forests, ponds and lakes are natural ecosystems; gardens and crop-fields are human-made, or artificial, ecosystems (p. 208).

An aquarium as a small ecosystem (Activity 13.1)

Activity 13.1 asks the student to design an aquarium. The fish need free space for swimming (a large jar will do), water, oxygen and food. Oxygen can come from an oxygen pump (an aerator) and food from the market. If a few aquatic plants and animals are added, the aquarium can become a self-sustaining system. NCERT calls the aquarium an example of a human-made ecosystem (pp. 208-209).

The activity ends with questions rather than answers. Why must an aquarium be cleaned once in a while, and do we clean ponds or lakes in the same way? The chapter returns to this a page later, when it asks what happens if the aquarium is left uncleaned and some fish and plants die. That question leads straight to the decomposers (p. 209).

Producers, consumers and decomposers

NCERT groups organisms by how they obtain their sustenance from the environment (p. 209).

• Producers make organic compounds like sugar and starch from inorganic substances, using the radiant energy of the Sun in the presence of chlorophyll. All green plants and certain bacteria which can produce food by photosynthesis belong here.
• Consumers depend on the producers, directly or indirectly. They eat food either directly from producers or by feeding on other consumers, and they can be classed as herbivores, carnivores, omnivores and parasites.
• Decomposers are microorganisms, bacteria and fungi, which break down the dead remains and waste products of organisms. They turn complex organic substances into simple inorganic substances that go into the soil and are used up once more by the plants.

NCERT then asks what would happen to garbage, dead animals and dead plants if there were no decomposers, and whether the natural replenishment of the soil would take place without them (p. 209). The answer the chapter is steering towards is no: decomposers close the loop between death and new growth.

Food chains, trophic levels and food webs (13.1.1)

Activity 13.2 asks the student to write the aquatic organisms in order of who eats whom, forming a chain of at least three steps (p. 209). Such a series of organisms feeding on one another is a food chain. Each step or level of the chain is a trophic level (p. 210).

• The autotrophs, or producers, are at the first trophic level. They fix up the solar energy and make it available for heterotrophs, the consumers.
• The herbivores, or primary consumers, come at the second level.
• Small carnivores, or secondary consumers, are at the third.
• Larger carnivores, or tertiary consumers, form the fourth trophic level (Fig. 13.2).

Real feeding relations are rarely a single line. Each organism is generally eaten by two or more other kinds of organisms, which in turn are eaten by several others. So instead of a straight-line food chain, the relationship is better shown as a series of branching lines called a food web (Fig. 13.3). The length and complexity of food chains vary greatly (p. 210).

Activity 13.2 also asks whether any one group of organisms is of primary importance. The chapter's own summary answers the spirit of the question: the components of an ecosystem are interdependent, and it is the producers that make the energy from sunlight available to the rest of the ecosystem (p. 216).

Why energy runs out along a chain

The food we eat acts as a fuel that gives us energy to do work, so the interactions among the components of an ecosystem involve a flow of energy. Autotrophs capture the energy in sunlight and convert it into chemical energy. From them it goes to the heterotrophs and the decomposers. At every conversion some energy is lost to the environment in forms which cannot be used again (p. 210).

NCERT then gives the findings that define this topic for the exam (p. 210):

• Green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy.
• When green plants are eaten by primary consumers, a great deal of energy is lost as heat to the environment, some goes into digestion and doing work, and the rest goes towards growth and reproduction. An average of 10% of the food eaten is turned into the consumer's own body and made available for the next level.
• So 10% can be taken as the average amount of organic matter present at each step that reaches the next level of consumers.
• Since so little energy reaches the next level, food chains generally consist of only three or four steps. Very little usable energy remains after four trophic levels.
• There are generally more individuals at the lower trophic levels, and the greatest number is of the producers.

The energy flow diagram (Fig. 13.4) adds two conclusions (p. 211). First, the flow of energy is unidirectional. The energy captured by autotrophs does not go back to the solar input, and the energy that passes to the herbivores does not come back to the autotrophs; once it moves up a level it is no longer available to the level below. Second, the energy available at each trophic level diminishes progressively because energy is lost at each level.

Biological magnification (pp. 211-212)

The food chain also explains how harmful chemicals reach us without our knowing. One reason water gets polluted is the use of pesticides and other chemicals to protect crops from diseases and pests. These chemicals are washed down into the soil or into water bodies. From the soil they are absorbed by plants along with water and minerals; from water bodies they are taken up by aquatic plants and animals. This is one of the ways in which they enter the food chain (pp. 211-212).

Because these chemicals are not degradable, they accumulate progressively at each trophic level. Human beings occupy the top level in any food chain, so the maximum concentration of these chemicals accumulates in our bodies. This phenomenon is known as biological magnification. It is why food grains such as wheat and rice, vegetables and fruits, and even meat contain varying amounts of pesticide residues, which cannot always be removed by washing or other means (p. 212).

Activity 13.3 turns this into a civic question. Newspapers report pesticide levels in ready-made food items, and some states have banned these products. Students are asked to debate the need for such bans, to trace where the pesticides in these foods come from and whether they could enter our bodies through other foods too, and to discuss methods to reduce our intake (p. 212).

How our activities affect the environment (13.2)

We are an integral part of the environment: changes in it affect us, and our activities change it. NCERT notes that Class IX showed how our activities pollute the environment, and here takes up two problems in detail, the depletion of the ozone layer and waste disposal (p. 212).

The ozone layer and how it is getting depleted (13.2.1)

Ozone (O₃) is a molecule of three oxygen atoms. Ordinary oxygen, O₂, is essential for all aerobic forms of life, but ozone itself is a deadly poison. At the higher levels of the atmosphere, however, ozone performs an essential function. It shields the surface of the earth from ultraviolet (UV) radiation from the Sun, which is highly damaging to organisms and is known to cause skin cancer in human beings (pp. 212-213).

Upper-atmosphere ozone is a product of UV radiation acting on oxygen. The higher energy UV radiation splits some molecular oxygen into free oxygen atoms, and these atoms combine with molecular oxygen to form ozone (p. 213):

• O₂ → O + O (in the presence of UV)
• O + O₂ → O₃ (ozone)

The amount of ozone in the atmosphere began to drop sharply in the 1980s. The decrease has been linked to synthetic chemicals like chlorofluorocarbons (CFCs), which are used as refrigerants and in fire extinguishers. In 1987 UNEP succeeded in forging an agreement to freeze CFC production at 1986 levels, and NCERT adds that it is now mandatory for manufacturing companies throughout the world to make CFC-free refrigerators (p. 213).

Activity 13.4 sends the student to the library, the internet or newspapers to find which chemicals deplete the ozone layer, whether the regulations controlling their emission have reduced the damage, and whether the size of the ozone hole has changed in recent years (p. 213). The chapter does not answer these questions; the ozone-treaty box below does, from the treaty record and NASA.

Managing the garbage we produce (13.2.2)

We throw away a great deal every day. Activity 13.5 asks students to collect a day's waste from home, bury it in a pit in the school garden or under at least 15 cm of soil in an old bucket or flower pot, keep it moist, and look at it every 15 days. Which materials stay unchanged for a long time, which change their form and structure, and which change fastest (p. 213)?

The answer comes from a chapter studied earlier, "Life Processes": the food we eat is digested by various enzymes. The same enzyme does not break down everything, because enzymes are specific in their action; a particular substance needs a specific enzyme. That is why, as NCERT puts it, we will not get any energy if we try to eat coal. For the same reason many human-made materials like plastics are not broken down by bacteria or other saprophytes. They are acted upon by physical processes like heat and pressure, but under the ambient conditions of our environment they persist for a long time (p. 214).

Substances broken down by biological processes are biodegradable. Substances not broken down in this manner are non-biodegradable; they may be inert and simply persist in the environment for a long time, or they may harm the various members of the ecosystem (p. 214). Activity 13.6 asks how long various non-biodegradable substances last, and whether the new plastics said to be biodegradable do or do not harm the environment (p. 214).

Garbage, life-style and the kulhad (p. 215)

Any town or city has heaps of garbage, and places of tourist interest are littered with empty food wrappers (p. 214). Activity 13.7 asks students to find out what happens to household waste, whether there is a system to collect it, how the local body (a panchayat, a municipal corporation or a resident welfare association) deals with it, and whether biodegradable and non-biodegradable wastes are treated separately. Students then measure the waste made at home and in the classroom in a day, work out how much of it is biodegradable, and suggest ways of dealing with it (p. 215).

NCERT names three causes for the growth of waste. Improvements in our life-style have produced greater amounts of waste. Changes in attitude have made more and more of the things we use disposable. Changes in packaging have made much of our waste non-biodegradable (p. 215). Activity 13.8 extends the inquiry to sewage and to the wastes of local industries, asking whether there are mechanisms to keep water bodies and soil from being polluted (p. 215).

The box "Disposable cups in trains" (p. 215) is a short lesson in trade-offs. Tea in trains was once served in plastic glasses that had to be returned to the vendor. Disposable cups were then hailed as a step forward for hygiene, without thought for the disposal of millions of cups every day. Kulhads, disposable clay cups, were suggested as an alternative, but making them on a large scale would result in the loss of the fertile top-soil. Disposable paper cups are used now, and the box asks what their advantages are over disposable plastic cups. The point is that a substitute has to be judged by its whole cost, not only by whether it rots.

Activity 13.9 closes the chapter's activities with two research tasks: the hazardous materials that must be dealt with when electronic items are discarded, and how plastics are recycled and whether recycling itself affects the environment (p. 216).

Did the 2020-21 edition differ?

Not in the text. In the 2020-21 edition this was Chapter 15 (pp. 256-265), and the wording is the same apart from the numbering of sections, activities and figures: 15.1-15.2 in place of 13.1-13.2, Activities 15.1-15.9 in place of 13.1-13.9. The exercises and NCERT's answers to Exercises 1-3 are also unchanged (2020-21 Answers, p. 282). The slip on p. 210 is old too: the 2020-21 text read "In Activity 15.4". What changed is the chapter's place in the book. Chapter 14, "Sources of Energy", which this chapter still calls "the previous Chapter", and Chapter 16, "Sustainable Management of Natural Resources", which followed it, were both dropped in NCERT's 2022 rationalisation, so "Our Environment" is now the last chapter of the book.

Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.

PART 3 — UPSC Integration

Frames for Mains Answers

1. Why ecosystems support so few top consumers. Start from NCERT's numbers: plants capture about 1% of the sunlight falling on their leaves, and on average 10% of the food eaten at one level reaches the next, so chains stop at three or four steps (Class X, p. 210). Add Class XII's pyramid of energy, which is always upright. Use this to explain why the energy captured by producers limits how many consumers an area can support, which is the ecological core of carrying capacity.
2. Pesticides, food safety and biological magnification. Trace the path: crop chemicals, run-off into soil and water, uptake by plants and aquatic life, accumulation at each level, the highest concentration in humans, and residues that washing cannot always remove (pp. 211-212). NCERT's own Activity 13.3 frames the policy question: bans on contaminated ready-made foods and ways to reduce intake.
3. The ozone regime as a model agreement. NCERT's 1987 freeze at 1986 levels grew into a phase-out of the production and consumption of ozone-depleting substances; the Protocol has 198 parties in the UN treaty record, India phased out CFCs for controlled uses by 1 January 2010 and HCFCs are due to go by 1 January 2030. The Kigali Amendment then tackled HFCs, which spare the ozone layer but warm the climate. UNEP says the ozone layer is well on its way to recovery. Contrast this with ground-level ozone, a constituent of photochemical smog.
4. Waste: the limits of biology and the choices of people. Enzyme specificity explains why plastics persist (p. 214); life-style, disposability and packaging explain why waste has grown (p. 215). The kulhad box shows that substitutes have costs of their own. Bring in what the activities point to (segregation by local bodies, sewage and industrial treatment, e-waste, the impact of recycling) and India's 2022 ban on listed single-use plastic items.
5. Decomposition as a natural service. For questions on how organic matter such as cellulose breaks down, use Class XII's five steps (fragmentation, leaching, catabolism, humification, mineralisation), the role of humus as a nutrient reservoir, and the factors that speed or slow decay (oxygen, lignin and chitin, nitrogen and sugars, warmth and moisture). Link it to Class X's decomposers, which return simple substances to the soil.

Exam Strategy

Prelims fact-traps:

• Class X's "about 1%" is the share of sunlight falling on leaves that land plants turn into food energy. Class XII's "2-10 per cent" is the share of PAR captured, and PAR itself is less than 50 per cent of incident radiation.
• Class X calls 10% an average; Class XII calls it the 10 per cent law. Food chains generally have three or four steps.
• The flow of energy is unidirectional; nutrients are returned to the soil by decomposers.
• The pyramid of energy is always upright; the pyramid of biomass in the sea is generally inverted.
• Decomposers are bacteria and fungi; the earthworm is a detritivore, which does fragmentation.
• Ozone forms in two steps: O₂ → O + O under UV, then O + O₂ → O₃.
• The ozone hole is a region of depleted ozone, not an empty hole; NASA's line is 220 Dobson Units, and one Dobson Unit is 0.001 atm-cm.
• The Montreal Protocol was signed in 1987 and entered into force in 1989; the Kigali Amendment is dated 15 October 2016 and entered into force on 1 January 2019. HFCs do not deplete ozone.
• India: CFCs out for controlled uses as on 1 January 2010; HCFCs completely by 1 January 2030; HFCs down by 85% in 2047.
• Single-use plastics: listed items banned from 1 July 2022; carry bags under 75 microns from 30 September 2021 and under 120 microns from 31 December 2022; banners under 100 micron.
• NCERT's key to Exercise 1 accepts three options and counts leather as biodegradable.

Mains: Use the chapter as the spine of an answer: energy flow explains limits, biological magnification explains toxicity, enzyme specificity explains persistence, and the ozone story shows that a global rule can reverse damage. Add one dated fact from the treaty record or a government release for each claim about today.

Practice Questions

Questions 1-3 are the NCERT exercise MCQs, with answers from NCERT's key. Practice (UPSC-pattern, not past papers): questions 4-13.

1. Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather
(b) Grass, wood and plastic
(c) Fruit-peels, cake and lime-juice
(d) Cake, wood and grass

Answer: (a), (c) and (d) (NCERT Answers, p. 219). Only (b), which contains plastic, is wrong.

2. Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat

Answer: (b) (NCERT Answers, p. 219).

3. Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping
(b) Switching off unnecessary lights and fans
(c) Walking to school instead of getting your mother to drop you on her scooter
(d) All of the above

Answer: (d) (NCERT Answers, p. 219).

4. Consider the following statements about energy in an ecosystem, as NCERT's Class X book gives them:
1. Green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves.
2. On average, 10% of the food eaten at one trophic level is made available to the next level.
3. Part of the energy that passes to herbivores returns to the autotrophs.
Which of the statements given above are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (b). The flow of energy is unidirectional: energy passed to herbivores does not come back to autotrophs (Class X, pp. 210-211).

5. With reference to ecological pyramids, consider the following statements:
1. The pyramid of energy in the sea is generally inverted.
2. The pyramid of biomass in the sea is generally inverted.
3. Many insects feeding on a single big tree are an exception to the upright pyramid of numbers.
Which of the statements given above are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (c). The pyramid of energy is always upright and can never be inverted, because some energy is lost as heat at each step (NCERT Biology Class XII, ch. 12, p. 213).

6. Why do human beings carry the highest concentration of certain pesticides?
(a) Humans eat more food by weight than any other animal
(b) The pesticides are not degradable, accumulate at each trophic level, and humans occupy the top level
(c) The pesticides are formed inside the human body from food
(d) Washing removes the residues from plant food, so they come only from meat

Answer: (b). This is biological magnification; NCERT adds that residues in grains, vegetables, fruits and meat cannot always be removed by washing (Class X, p. 212).

7. Consider the following statements:
1. Higher energy UV radiation splits some molecular oxygen into free oxygen atoms, which combine with molecular oxygen to form ozone.
2. NASA treats the region over Antarctica with total ozone of 220 Dobson Units or lower as the ozone hole.
3. One Dobson Unit is 0.01 atm-cm.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). One Dobson Unit is 0.001 atm-cm, so a 0.3 atm-cm column is 300 Dobson Units (Class X, p. 213; NASA Ozone Watch).

8. With reference to the Montreal Protocol, consider the following statements:
1. It was signed in 1987 and entered into force in 1989.
2. India became a party to it by accession in 1992.
3. Its Kigali Amendment controls hydrofluorocarbons because they deplete the ozone layer faster than CFCs.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). HFCs do not deplete the ozone layer; they are controlled because of their high global warming potential (UN Treaty Collection; PIB, 16 September 2023).

9. Consider the following pairs of substances and India's commitments:
1. CFCs, halons and carbon tetrachloride: phased out for controlled uses as on 1 January 2010
2. HCFCs: complete phase-out for all controlled applications as on 1 January 2030
3. HFCs: cumulative reduction of 85% by 2037
Which of the pairs given above are correctly matched?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). India's HFC steps are 10% in 2032, 20% in 2037, 30% in 2042 and 85% in 2047 (PIB, 18 August 2021).

10. Which of the following were prohibited with effect from 1 July 2022 under the Plastic Waste Management Amendment Rules, 2021?
1. Plastic plates, cups, glasses and cutlery
2. Plastic or PVC banners less than 100 micron
3. All plastic carry bags, whatever their thickness
Select the correct answer using the code given below.
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). Carry bags are banned by thickness: under 75 microns from 30 September 2021 and under 120 microns from 31 December 2022 (PIB, 12 December 2022).

11. Which one of the following statements about decomposition is not correct?
(a) Detritivores such as earthworms break detritus into smaller particles
(b) Humus is highly resistant to microbial action and decomposes at an extremely slow rate
(c) Decomposition is quicker when detritus is rich in lignin and chitin
(d) A warm and moist environment favours decomposition

Answer: (c). Detritus rich in lignin and chitin decomposes more slowly; detritus rich in nitrogen and water-soluble substances like sugars decomposes more quickly (NCERT Biology Class XII, ch. 12, p. 208).

12. Consider the following statements:
1. In an aquatic ecosystem, the grazing food chain is the major conduit for energy flow.
2. In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
3. The detritus food chain begins with green plants.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). The detritus food chain begins with dead organic matter (NCERT Biology Class XII, ch. 12, p. 210).

13. In NCERT's box "Disposable cups in trains", why were kulhads not adopted on a large scale?
(a) Clay cups are non-biodegradable
(b) Making them on a large scale would result in the loss of the fertile top-soil
(c) They could not be kept clean
(d) They cost more than plastic cups

Answer: (b) (Class X, p. 215).

📦 Revision Capsule

Sources

• NCERT, Science, Textbook for Class X, ch. 13 "Our Environment", Reprint 2026-27 — ncert.nic.in PDF.
• NCERT, Science, Class X, Answers, Reprint 2026-27 — ncert.nic.in PDF.
• NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021, ch. 15 "Our Environment" — Wayback Machine.
• NCERT, Biology, Textbook for Class XII, ch. 12 "Ecosystem", Reprint 2025-26 — ncert.nic.in PDF.
• United Nations Treaty Collection, "Montreal Protocol on Substances that Deplete the Ozone Layer", status as at 3 October 2026 — treaties.un.org.
• United Nations Treaty Collection, "Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer" (Kigali, 15 October 2016), status as at 4 October 2026 — treaties.un.org.
• UNEP Ozone Secretariat, "The Montreal Protocol on Substances that Deplete the Ozone Layer" — ozone.unep.org (accessed October 2026).
• NASA Ozone Watch, "Ozone hole facts" — ozonewatch.gsfc.nasa.gov; "Latest status of ozone" — ozonewatch.gsfc.nasa.gov (both accessed 7 October 2026).
• PIB (Cabinet), "Cabinet approves Ratification of Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer for phase down of Hydrofluorocarbons", 18 August 2021 — pib.gov.in.
• PIB (Ministry of Environment, Forest and Climate Change), release on the International Day for the Preservation of the Ozone Layer, 16 September 2023 — pib.gov.in.
• PIB (Ministry of Environment, Forest and Climate Change), Lok Sabha reply on the ban on identified single-use plastic items, 12 December 2022 — pib.gov.in.
• World Health Organization, fact sheet "Ambient (outdoor) air pollution", 24 October 2024 — who.int.
• NOAA NESDIS, "World's Fourth Mass Coral Bleaching Event Likely Ended in 2025", 2 June 2026 — nesdis.noaa.gov.