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Class 10 Our Environment notes: NCERT Science Chapter 13 revision

CMDLearn · 3 November 2026

These are short revision notes for Chapter 13 of the NCERT Class 10 Science textbook (Reprint 2026-27). It is one of the shortest chapters in the book, pages 208 to 217, and it has two halves: how an ecosystem works (food chains and the flow of energy) and two problems people have caused (the thinning ozone layer and the waste we throw away). The notes follow the book's sections, then give the key words, the diagram to practise, the mistakes students make most often and 8 quick questions of our own with answers.

Keep the textbook open next to these notes. The wording here is our own; the textbook is the authority.

How much it counts

CBSE's Class 10 Science curriculum for 2026-27 puts Our Environment in Unit V, Natural Resources, worth 5 of the 80 theory marks (curriculum, PDF page 4). The unit lists the ecosystem, environmental problems, ozone depletion, waste production and its solutions, and biodegradable and non-biodegradable substances (PDF page 6). It is a small unit, but these are some of the easiest marks in the paper to secure. More on the units: CBSE Class 10 Science syllabus 2026-27.

1. What an ecosystem is (section 13.1, pages 208-209)

• An ecosystem = all the living things of an area that act on one another, together with the non-living parts of their surroundings.
• Biotic components: the living organisms (plants, animals, microorganisms).
• Abiotic components: physical factors such as temperature, rainfall, wind, soil and minerals. They affect how the living things grow and reproduce.
• Natural ecosystems: forests, ponds and lakes. Human-made (artificial) ecosystems: gardens, crop fields and an aquarium.

2. Producers, consumers and decomposers (page 209)

3. Food chains, trophic levels and food webs (section 13.1.1, pages 210-211)

• A food chain is a series of organisms, each feeding on the one before it (Fig. 13.1 shows one in a forest, one in a grassland and one in a pond).
• Each step of the chain is a trophic level (Fig. 13.2):

• Most organisms are eaten by two or more kinds of organisms, so real feeding links branch. The branching picture is a food web (Fig. 13.3).

4. How energy flows (pages 210-211)

• Producers capture energy from sunlight and store it as chemical energy. From them it passes to consumers and decomposers.
• Every time energy changes form, some of it is lost to the surroundings in a form that cannot be used again.
• The numbers to remember: - Green plants on land turn about 1% of the sunlight falling on their leaves into food energy. - On average only about 10% of the food eaten becomes the eater's body and is passed on to the next level. A lot is lost as heat; some goes into digestion and work.
• Because so little reaches each new level, food chains usually have only three or four steps; after four levels very little usable energy is left.
• The lower trophic levels usually have the most individuals, and producers are the most numerous.
• Fig. 13.4 shows the flow as a stack: sunlight → producers → herbivores → carnivores → top carnivores. Two things follow from it: - energy flow is one-way (unidirectional): energy that reaches the herbivores never goes back to the plants, and energy the plants caught never goes back to the Sun; - the energy available shrinks at every level.

5. Harmful chemicals in the food chain (pages 211-212)

• Pesticides and other chemicals sprayed on crops wash into the soil and into water bodies.
• Plants take them up from the soil with water and minerals; water plants and animals take them up from lakes and rivers. That is how they enter the food chain.
• These chemicals are not degradable, so they build up at every trophic level.
• People sit at the top of almost any food chain, so the highest concentration ends up in our bodies. This build-up is called biological magnification.
• That is why grains, vegetables, fruits and even meat can carry pesticide residues, which washing cannot always remove.

6. The ozone layer (section 13.2.1, pages 212-213)

• Ozone (O₃) is a molecule of three oxygen atoms. Ordinary oxygen (O₂) is needed by all aerobic life; ozone itself is a deadly poison.
• High up in the atmosphere, though, ozone does vital work: it shields the Earth's surface from the Sun's ultraviolet (UV) radiation, which harms living things and can cause skin cancer in people.
• How ozone forms up there: 1. High-energy UV radiation splits some O₂ molecules into single oxygen atoms: O₂ → O + O. 2. Each free atom joins an O₂ molecule: O + O₂ → O₃ (ozone).
• The amount of ozone began to fall sharply in the 1980s. The fall has been linked to man-made chlorofluorocarbons (CFCs), which were used in refrigerators and fire extinguishers.
• 1987: countries agreed, through the United Nations Environment Programme (UNEP), that CFC production would stay frozen at its 1986 level. Manufacturers everywhere must now make CFC-free refrigerators.

7. Managing waste (section 13.2.2, pages 213-215)

• Enzymes are specific: each one breaks down only particular substances. That is why eating coal would give us no energy, and why bacteria and other saprophytes cannot break down many human-made materials such as plastics.
• Biodegradable: broken down by biological processes (by living things).
• Non-biodegradable: not broken down that way. Such substances may lie unchanged for a long time, or harm members of the ecosystem. Heat and pressure can act on them, but not at ordinary conditions.
• More comfortable lifestyles, more disposable things and new kinds of packaging mean we now produce more waste, and more of it is non-biodegradable.
• The book also asks you to look at how your town collects and separates waste, how sewage and factory waste are treated, and what happens to old electronics and recycled plastic (pages 215-216).

Key words at a glance

• Ecosystem: living things of an area plus the non-living parts of their surroundings.
• Biotic / abiotic: living / non-living components.
• Producer: an organism that makes its own food from inorganic substances using sunlight.
• Decomposer: a bacterium or fungus that breaks dead matter into simple inorganic substances.
• Trophic level: one step of a food chain.
• Food web: many food chains linked together.
• 10% law: about a tenth of the food energy at one level reaches the next.
• Biological magnification: the build-up of a non-degradable chemical at each step of a food chain.
• Ozone layer: ozone high in the atmosphere that blocks the Sun's UV rays.
• CFCs: chlorofluorocarbons, the chemicals linked to the loss of ozone.
• Biodegradable / non-biodegradable: can / cannot be broken down by biological processes.

The diagram to know: trophic levels and energy flow

Board papers often ask you to show trophic levels or the flow of energy. Practise both:

1. The pyramid (Fig. 13.2): draw a triangle cut into four bands. Label from the bottom: producers, primary consumers, secondary consumers, tertiary consumers. The widest band (producers) is at the bottom.
2. The energy-flow stack (Fig. 13.4): draw five boxes one above the other: sunlight at the bottom, then producers, herbivores, carnivores and top carnivores. Join them with arrows pointing upwards only.
3. Write "energy lost as heat" beside each step if you have room, and add "1%" beside the arrow from sunlight to producers and "about 10%" beside the arrows above it.

The upward-only arrows earn the mark for "unidirectional"; an arrow pointing back down is a common slip.

Mistakes that cost marks

• Saying energy cycles in an ecosystem. Nutrients go round through decomposers; energy only flows one way.
• Saying 10% of the energy is lost. It is the other way round: about 10% is passed on, and most is lost.
• Putting decomposers at one trophic level. They break down dead matter from every level.
• Saying ozone is good everywhere. It is poisonous to breathe; it is useful high in the atmosphere because it blocks UV rays.
• Writing that the ozone layer stops global warming. The chapter's point is UV protection.
• Calling glass or metal biodegradable because they are "natural". The test is whether living things can break them down.
• Saying washing removes pesticide residues. The book says it cannot always do so.

Quick check: 8 questions of our own

These are our own questions, written for these notes and checked against the textbook pages named; no practising teacher has reviewed them yet.

Question 1. On her building's terrace, Sana keeps a vegetable patch. She lists: tomato plants, earthworms, the soil, ladybirds, sunlight, rainwater, a sparrow that visits. (a) Sort the list into biotic and abiotic components. (b) Is the patch a natural or a human-made ecosystem?

Answer: (a) biotic: tomato plants, earthworms, ladybirds and the sparrow; abiotic: soil, sunlight and rainwater; (b) human-made. Living organisms are the biotic components, and physical factors such as soil, rainfall and temperature are abiotic. The book counts gardens and crop fields as human-made ecosystems; forests, ponds and lakes are natural ones (page 208). Sunlight counts as non-living: it is the energy source, not an organism.

Question 2. In a paddy field, rats eat the rice plants, snakes eat the rats, and hawks eat the snakes. (a) Give each organism its trophic level and its name as a consumer (if it is one). (b) When a hawk dies, which organisms break its body down, and where do the products go?

Answer: (a) rice plants: first level, producers; rats: second level, primary consumers (herbivores); snakes: third level, secondary consumers; hawks: fourth level, tertiary consumers; (b) bacteria and fungi, and the simple inorganic substances they produce go into the soil for plants to use again. The levels follow the book's order (page 210). Decomposers break dead remains into simple inorganic substances that return to the soil (page 209).

Question 3 (assertion and reason). Assertion (A): The energy a herbivore takes in from grass never passes back to the grass. Reason (R): In an ecosystem, energy flows in one direction only. Choose: (A) both true and R explains A; (B) both true but R does not explain A; (C) A true, R false; (D) A false, R true.

Answer: (A). The book's energy-flow diagram shows that energy passed to herbivores does not come back to the autotrophs, because the flow of energy is unidirectional (page 211). R is the reason A is true.

Question 4. A cotton farm next to a lake is sprayed with a pesticide that does not break down. In the lake, tiny water plants are eaten by small fish, small fish by large fish, and large fish by fish-eating birds and by people. (a) Where in this chain will the pesticide be most concentrated? (b) What is this build-up called? (c) Will washing the fish before cooking remove the pesticide completely?

Answer: (a) in the birds and the people at the top; (b) biological magnification; (c) not always. Chemicals that are not degradable build up at each trophic level, so the top of the chain gets the most. The book adds that washing does not always get rid of pesticide residues in food (pages 211-212).

Question 5. Ravi says: "Ozone is just a kind of oxygen, so breathing it is good for us, and it helps us everywhere." (a) What is wrong with his statement? (b) Where does ozone do something useful, and what does it do? (c) Write the two steps by which it forms there.

Answer: (a) ozone is a deadly poison to breathe; (b) high in the atmosphere, where it shields the Earth from the Sun's UV radiation; (c) O₂ → O + O (split by UV), then O + O₂ → O₃. Ozone is a molecule of three oxygen atoms, unlike the O₂ that aerobic life needs. Up high, it blocks UV radiation, which damages organisms and can cause skin cancer. UV splits some O₂ into free atoms, which then join O₂ to form ozone (pages 212-213).

Question 6. After a festival, a housing society collects: a wilted marigold garland, a glass jam jar, a polythene carry bag, sawdust from the stage, a steel spoon and a coconut shell. (a) Which are biodegradable? (b) Why can bacteria break down the sawdust but not the polythene bag?

Answer: (a) the marigold garland, the sawdust and the coconut shell; the glass jar, polythene bag and steel spoon are not; (b) enzymes are specific, and decomposers have none that act on plastics. Substances broken down by biological processes are biodegradable; those that are not are non-biodegradable and may persist for a long time (page 214). The book explains that each enzyme breaks down only particular substances, which is why many human-made materials such as plastics are not broken down by bacteria and other saprophytes (page 214). The coconut shell breaks down slowly, but it does break down.

Question 7. In a field, the plants have 4,00,000 kJ of sunlight falling on their leaves in a day. Grasshoppers eat the plants and frogs eat the grasshoppers. Neha says: "The frogs get 10% of the energy that the plants made." (a) How much food energy do the plants make? (b) How much reaches the grasshoppers, and how much the frogs? (c) Is Neha right?

Answer: (a) about 4,000 kJ; (b) about 400 kJ and 40 kJ; (c) no: the frogs get about 1% of the plants' food energy, not 10%. Green plants on land turn about 1% of the sunlight on their leaves into food energy: 1% of 4,00,000 = 4,000 kJ. About 10% is passed to each next level: 4,000 × 0.1 = 400 kJ for the grasshoppers and 400 × 0.1 = 40 kJ for the frogs (page 210). 40 is 1% of 4,000, because the 10% step happens twice: 10% of 10% is 1%.

Question 8. In a grassland, hawks eat snakes, rats and small birds; snakes eat rats and frogs; rats eat grass seeds. (a) Why is this better shown as a food web than as one food chain? (b) Rats start to disappear because of a disease. Which do you expect to cope better, the hawks or the snakes? Give a reason.

Answer: (a) because the organisms are eaten by, and eat, more than one kind of organism, so the links branch; (b) the hawks, because they have more other foods (snakes and small birds) to fall back on. The book points out that most organisms are food for at least two different kinds of animals, so the relationships form a branching food web rather than a single straight chain (page 210). Here the snakes would still have frogs, but the hawks have two other foods, so losing rats leaves them less short of food.

Revise with questions

Test yourself next: Class 10 Our Environment, 8 practice questions with answers. The other notes in this set: Life Processes, Metals and Non-metals and Heredity. For timed practice, the free Class 10 revision papers include a Science paper laid out in 1, 2, 3 and 5-mark sections.

Where CMDLearn fits

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Sources

All checked on 3 November 2026. NCERT page numbers are the printed page numbers of the textbook.

• NCERT, Science, Textbook for Class X, Chapter 13 "Our Environment" (Reprint 2026-27), pages 208-217: section 13.1 (pages 208-209); 13.1.1, Figs. 13.1 to 13.4 and biological magnification (pages 210-212); 13.2.1, ozone (pages 212-213); 13.2.2, waste (pages 213-216); "What you have learnt" (page 216). ncert.nic.in/textbook/pdf/jesc113.pdf
• CBSE, Science curriculum, Class X, 2026-27: unit marks (PDF page 4) and Unit V contents (PDF page 6): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf

The facts, section numbers, figure numbers and percentages are from the documents above. The wording of these notes, the diagram steps, the list of mistakes and the 8 questions are our own.

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