ScienceCBSE Class 10 · Chapter 13

Our Environment Class 10: Notes, Concepts and Board Questions

Updated 14 October 2026 · Fact-checked

Our Environment is the Class 10 Biology chapter on ecosystems, food chains and webs, energy flow between trophic levels, biological magnification, ozone depletion by CFCs and biodegradable versus non-biodegradable waste. The board asks MCQs and assertion-reason items, 2 to 3 mark reasoning answers, energy flow numericals and case-based questions on food chains and waste.

What Our Environment is about

This chapter shows you how living things and their non-living surroundings work together as one system, the ecosystem. You follow food and energy as they pass from plants to animals along food chains and food webs, and you see why so little energy reaches the top. Then the chapter turns to us: how our activities harm the environment through ozone depletion and the growing pile of garbage we throw away.

It links closely to Life Processes. Producers are the autotrophs that make food by photosynthesis, consumers are heterotrophs, and the idea that enzymes are specific explains why plastics are not broken down. It also builds on what you learnt about pollution in Class IX. In the board paper this chapter sits in Section A (Biology).

Board questions come in a few clear shapes: MCQs asking which group of items is biodegradable or which trophic level an organism occupies, assertion-reason items on energy transfer or decomposers, short answers on biological magnification, ozone and waste disposal, simple energy calculations using the 10 per cent law, and case-based questions built around a food chain, an energy pyramid or a waste problem.

Weightage: This chapter is Unit V, Natural Resources, which carries 5 of the 80 theory marks.Source: CBSE Science syllabus 2026-27, Unit V Natural Resources

In real board papers: asked in 21 of the 21 board papers we hold (2023 to 2026), on average 4.9 marks per paper. See the questions

Key concepts

Ecosystem and its components

NCERT section 13.1 Eco-system — What are its Components?

An ecosystem is all the living organisms of an area interacting with each other and with the non-living parts of their surroundings. The living part is the biotic component: plants, animals, microorganisms and humans. The non-living part is the abiotic component: physical factors such as temperature, rainfall, wind, soil and minerals.

Why treat them as one system? Because neither part works alone. A garden's plants, frogs, insects and birds feed on and depend on each other, and their growth and reproduction are controlled by abiotic factors like rain and soil. Together they keep a balance in nature.

Ecosystems are of two kinds:

Natural: forests, ponds and lakes.
Human-made (artificial): gardens, crop-fields and an aquarium.

An aquarium becomes close to self-sustaining when you add aquatic plants and animals, which shows how the parts support one another.

Worked example

[Board, 3 marks] Classify the following parts of a pond into biotic and abiotic components: frog, water temperature, algae, soil at the bottom, minerals dissolved in the water, fungi. Is a pond a natural or a human-made ecosystem? Give a reason.

Try it first, then show the solutionSolution

Biotic components (living): frog, algae, fungi. (1)
Abiotic components (non-living physical factors): water temperature, soil at the bottom, dissolved minerals. (1)
A pond is a natural ecosystem because it forms and runs on its own in nature, unlike a garden, crop-field or aquarium, which humans set up. (1)

Answer: Biotic: frog, algae, fungi. Abiotic: water temperature, soil, minerals. A pond is a natural ecosystem.

Producers, consumers and decomposers

NCERT section 13.1 Eco-system — What are its Components?

Organisms are grouped by how they get their food.

Producers: all green plants and certain bacteria. Using sunlight and chlorophyll, they make organic compounds such as sugar and starch from inorganic substances by photosynthesis.
Consumers: organisms that eat food made by producers, either directly or by eating other consumers. They are herbivores, carnivores, omnivores and parasites.
Decomposers: microorganisms, that is bacteria and fungi, that break down dead remains and waste products of organisms.

Decomposers matter because they turn complex organic substances into simple inorganic substances. These go back into the soil and plants use them again. Without decomposers, garbage and dead plants and animals would pile up, and the soil would not be naturally replenished.

So every group depends on the others. Producers feed consumers, and decomposers return raw materials to producers.

Worked example

[Board, 3 marks] (a) State the role of decomposers in an ecosystem. (b) Predict two things that would happen in a forest if all its bacteria and fungi were suddenly removed.

Try it first, then show the solutionSolution

(a) Decomposers (bacteria and fungi) break down complex organic substances in dead remains and wastes into simple inorganic substances that return to the soil for plants to use again. (1)
(b) Dead plants, dead animals and waste would pile up because nothing would break them down. (1)
(b) The soil would not be naturally replenished with inorganic substances, so plants, and in turn all the consumers that depend on them, would suffer. (1)

Answer: Decomposers recycle nutrients from dead matter into the soil. Without them, dead matter piles up and soil is not replenished, harming plants and every organism that depends on them.

Food chains and trophic levels

NCERT section 13.1.1 Food Chains and Webs

A food chain is a series of organisms in which each one feeds on the one before it. Each step of the chain is a trophic level.

First trophic level: producers (autotrophs). They fix solar energy and make it available to consumers.
Second: herbivores, the primary consumers.
Third: small carnivores, the secondary consumers.
Fourth: larger carnivores, the tertiary consumers.

The arrow in a food chain always points from the organism eaten to the organism that eats it, because that is the direction in which food and energy move. For example: grass → goat → human.

To find an organism's trophic level, count from the producer. The producer is level 1, and each step adds one. An organism's trophic level depends on the chain it is in, not on its name. For example, a fish that eats insect larvae which eat algae is a secondary consumer.

Worked example

[Board, 1 mark] In a grassland, grasshoppers eat grass, frogs eat grasshoppers and snakes eat frogs. Which statement is correct?
(a) The frog is a primary consumer at the second trophic level.
(b) The frog is a secondary consumer at the third trophic level.
(c) The snake is a tertiary consumer at the third trophic level.
(d) The grasshopper is a producer at the first trophic level.

Try it first, then show the solutionSolution

Write the chain: grass → grasshopper → frog → snake.
Count from the producer: grass is level 1, grasshopper level 2 (primary consumer), frog level 3 (secondary consumer), snake level 4 (tertiary consumer).
Only option (b) matches. (1)

Answer: (b) The frog is a secondary consumer at the third trophic level.

Flow of energy and the 10 per cent law

NCERT section 13.1.1 Food Chains and Webs

All life runs on energy from the Sun. Producers capture sunlight and change it into chemical energy stored in food. From them, energy passes to consumers and decomposers.

Key facts from NCERT:

Green plants in a land ecosystem capture only about 1% of the sunlight falling on their leaves as food energy.
When an animal eats, much of the energy is lost as heat, and some is used in digestion and doing work. Only about 10% of the food eaten becomes its own body and is available to the next level. This is the 10 per cent law.

Two results follow. First, energy flow is unidirectional: it never goes back from herbivores to plants or from plants to the Sun. Second, energy decreases at each level. That is why food chains usually have only three or four steps: after four levels very little usable energy is left. It also explains why the producers have the greatest number of individuals.

Worked example

[Advanced, 4 marks] In a field, 200000 J of sunlight falls on the leaves of plants. Using the NCERT values, calculate the energy available as food (a) in the plants, (b) to the herbivores, (c) to the secondary consumers. (d) Use your answers to explain why food chains are short.

Try it first, then show the solutionSolution

(a) Plants capture about 1% of sunlight: 1% of 200000 J = 2000 J. (1)
(b) About 10% passes to the next level: 10% of 2000 J = 200 J for herbivores. (1)
(c) 10% of 200 J = 20 J for secondary consumers. (1)
(d) Energy falls to one-tenth at each step, because most is lost as heat or used in digestion and work. After three or four levels too little usable energy remains to support another level, so food chains are short. (1)

Answer: (a) 2000 J (b) 200 J (c) 20 J (d) Large energy loss at each level limits food chains to three or four steps.

Food webs

NCERT section 13.1.1 Food Chains and Webs

In nature, feeding is rarely a single straight line. Each organism is usually eaten by two or more kinds of organisms, and those are eaten by several others. So the real feeding relationships form a network of branching, interlinked food chains called a food web.

How does this connect with food chains? A food web is simply many food chains joined together. A single food chain shows only one path that energy can take. A food web shows all the paths.

The lengths and complexity of food chains vary a great deal from one ecosystem to another. Even inside a web, the energy rules still apply: energy flows in one direction and decreases at each trophic level. When you are asked to trace a chain inside a web, start at a producer and follow the arrows to the eater.

Worked example

[Board, 3 marks] Distinguish between a food chain and a food web. Why are feeding relationships in nature better shown as a food web?

Try it first, then show the solutionSolution

A food chain is a single series of organisms, each feeding on the one before it, shown as one straight line. (1)
A food web is a network of many interlinked food chains shown as branching lines. (1)
In nature, each organism is generally eaten by two or more kinds of organisms, which in turn are eaten by several others, so a single straight chain cannot show all the relationships. (1)

Answer: A food chain is one straight feeding series; a food web is many interconnected chains. A web is more realistic because each organism has several eaters.

Biological magnification

NCERT section 13.1.1 Food Chains and Webs

Farmers spray pesticides and other chemicals to protect their crops from pests and diseases. Water carries these chemicals down into the soil or into ponds, lakes and rivers. Crops pull them up from the soil along with water and minerals, and water plants and animals soak them up from the water. That is one route by which the chemicals get into the food chain.

The key point is that these chemicals are not degradable. Nothing breaks them down, so each animal keeps what it takes in from everything it eats. Because an animal eats many organisms from the level below, its body ends up holding more of the chemical than its food did, and the amount keeps rising as you go up the chain.

Humans sit at the top of food chains, so we end up with the highest concentration. This rise in the concentration of harmful chemicals at higher trophic levels is called biological magnification.

So grains like wheat and rice, vegetables, fruit and meat can all carry small traces of pesticide, and washing may not get rid of them. Notice the link with energy: energy gets less as you go up the chain, but the concentration of a non-degradable chemical gets more.

Worked example

[Advanced, 3 marks] A non-degradable pesticide sprayed on fields near a lake is washed into the water. In the lake, water plants are eaten by small fish, and small fish are eaten by fish-eating birds. (a) How does the pesticide enter this food chain? (b) In which organism will its concentration be highest, and why? (c) Name the phenomenon and state whether washing food can always remove such residues.

Try it first, then show the solutionSolution

(a) The pesticide washes into the lake and is taken up by the aquatic plants, the first trophic level. (½)
(b) Being non-degradable, it is not broken down, so it builds up more and more at each higher trophic level. (1)
(b) So the fish-eating bird, at the top of this chain, has the highest concentration. (½)
(c) The phenomenon is biological magnification. (½)
(c) No, washing does not always get rid of such residues. (½)

Answer: It enters through aquatic plants, builds up at each level and is highest in the fish-eating birds. This is biological magnification; washing cannot always remove the residues.

Ozone layer and its depletion

NCERT section 13.2.1 Ozone Layer and How it is Getting Depleted

**Ozone ( $O_{3}$ )** is a molecule of three oxygen atoms. The oxygen we breathe ( $O_{2}$ ) is essential for aerobic life, but ozone itself is a deadly poison. Yet high up in the atmosphere ozone does a vital job: it shields the Earth's surface from ultraviolet (UV) radiation from the Sun. UV radiation is highly damaging to organisms; for example, it causes skin cancer in humans.

How does ozone form up there? High-energy UV radiation splits some $O_{2}$ molecules into free oxygen atoms: $O_{2} UV O + O$ . Each free atom then joins an $O_{2}$ molecule: $O + O_{2} \to O_{3}$ .

Ozone levels started falling steeply in the 1980s. This fall has been linked to man-made chemicals called chlorofluorocarbons (CFCs), which were used as cooling chemicals in refrigerators and in fire-fighting equipment. In 1987, the United Nations Environment Programme (UNEP) brought countries to an agreement to hold CFC production at its 1986 level. Today every manufacturer has to make refrigerators without CFCs.

Worked example

[Advanced, 5 marks] (a) Explain with equations how ozone is formed in the upper atmosphere. (b) Ozone is called a deadly poison, yet the ozone layer is essential. Explain. (c) What has caused the depletion of the ozone layer, and what steps have been taken to limit it?

Try it first, then show the solutionSolution

(a) High-energy UV radiation splits molecular oxygen into free oxygen atoms: $O_{2} UV O + O$ . (1)
(a) The free oxygen atoms combine with molecular oxygen to form ozone: $O + O_{2} \to O_{3}$ . (1)
(b) Ozone is poisonous to living things, but at higher levels of the atmosphere it shields the Earth from the Sun's UV radiation, which damages organisms and causes skin cancer in humans. (1)
(c) Ozone fell steeply from the 1980s; this is linked to synthetic CFCs, which were used as cooling chemicals in refrigerators and in fire-fighting equipment. (1)
(c) In 1987 UNEP got countries to agree to hold CFC production at the 1986 level, and refrigerators must now be made without CFCs. (1)

Answer: UV splits $O_{2}$ into O atoms, which join $O_{2}$ to give $O_{3}$ . Ozone is poisonous but shields life from UV high up. CFCs are linked to the depletion; the 1987 UNEP agreement held CFC production at the 1986 level and CFC-free refrigerators are now compulsory.

Biodegradable and non-biodegradable substances

NCERT section 13.2.2 Managing the Garbage we Produce

Biodegradable substances are broken down by biological processes, that is, by bacteria and other saprophytes. Vegetable peels, spoilt food, used tea leaves, paper, wood and grass are examples. Non-biodegradable substances are not broken down this way. Plastics are the main example.

Why the difference? Breaking down a substance needs enzymes, and enzymes are specific: each works only on a particular substance. That is why you would get no energy by eating coal. In the same way, microbes do not have enzymes to break down many human-made materials like plastics.

Physical processes such as heat and pressure can change non-biodegradable materials. But in the everyday conditions around us, very little happens to them, so they last for a very long time. Some just sit there without reacting with anything; others can do harm to the plants and animals of the ecosystem.

A tip: something that breaks down only with heat or pressure is still non-biodegradable. The test is breakdown by living organisms.

Worked example

[Board, 1 mark]
Assertion (A): Plastic articles remain in the environment for a very long time.
Reason (R): Plastics can be acted upon by physical processes such as heat and pressure.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Try it first, then show the solutionSolution

A is true: under ordinary conditions plastics are not broken down biologically, so they persist for a long time.
R is true: heat and pressure can act on such materials.
But R does not explain A. Plastics persist because enzymes are specific and microbes have none that can break plastics down, not because heat and pressure act on them. So the answer is (b). (1)

Answer: (b) Both A and R are true but R is not the correct explanation of A.

Waste production and its solutions

NCERT section 13.2.2 Managing the Garbage we Produce

We now throw away more than people did earlier. NCERT links this to three changes in how we live:

Better living standards: as life-style improves, we buy and use more things, so more ends up as waste.
A use-and-throw habit: many everyday items are now meant to be used once and then discarded.
New kinds of packaging: much of what we wrap things in is now material that microbes cannot break down.

The result is heaps of garbage in towns and litter at tourist places. Waste from homes, sewage and industries can pollute soil and water bodies if it is not treated. Electronic items contain hazardous materials that must be handled carefully when they are disposed of.

Solutions you can write in an answer:

Segregate biodegradable and non-biodegradable waste so each can be treated separately.
Let biodegradable waste decompose, for example by burying it in a pit under soil.
Recycle plastics and similar materials instead of throwing them away.
Reduce waste: carry cloth bags when shopping and avoid disposable items.

Other environment-friendly habits include switching off unnecessary lights and fans and walking short distances.

Worked example

[HOTS, 3 marks] A school canteen throws away vegetable peels, used tea leaves, plastic milk packets and plastic disposable cups every day, all in one bin. (a) Sort this waste into two groups. (b) Suggest one suitable way to deal with each group. (c) Why should the two groups be kept separate?

Try it first, then show the solutionSolution

(a) Biodegradable: vegetable peels, used tea leaves. Non-biodegradable: plastic milk packets, plastic disposable cups. (1)
(b) Biodegradable waste can be buried in a pit covered with soil and kept moist so microbes break it down. (1)
(b) Plastic waste should be collected separately and sent for recycling, and the canteen can replace disposable plastic cups with reusable ones. (½)
(c) The two groups need different treatment. Mixed together, the plastic would persist in soil while the biodegradable part could not be used separately. (½)

Answer: Peels and tea leaves are biodegradable (bury in a pit to decompose); milk packets and cups are non-biodegradable (recycle and reduce). Segregation lets each be treated the right way.

Science in real life: Our Environment

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Ecosystem and its components

Imagine you set up an aquarium at home in a large glass jar. You add water, gravel, an aerator, a few small fish and some aquatic plants. After some weeks, a few plant leaves die and fish waste settles at the bottom.

(a) Sort the living things in your aquarium into producers, consumers and decomposers. (b) Explain how these three groups together help the aquarium become close to self-sustaining. (c) Your friend wants to add a large fish that eats small fish. Predict what would happen, and explain why your aquarium still needs cleaning now and then while a natural pond does not.

Try it first, then show the solutionSolution

(a) Producers: the green aquatic plants, which make food by photosynthesis. Consumers: the small fish. Decomposers: the bacteria and fungi in the water and gravel.
(b) The plants make food, the fish depend on that food, and the decomposers break dead leaves and fish waste into simple inorganic substances.
(b) The plants reuse those simple substances, so the materials keep going round and the system partly supports itself.
(c) The large fish would eat the small fish, so the small fish would decline and the balance of this small system would be destroyed.
(c) In a small aquarium, dead matter and waste build up faster than they are broken down, so it must be cleaned. A pond is a natural ecosystem whose decomposers and balance deal with dead matter on their own.

Answer: Plants are producers, fish are consumers, bacteria and fungi are decomposers; together they recycle materials. A predatory fish would eat the small fish and upset the balance. The aquarium needs cleaning because waste builds up faster than it is broken down, unlike in a natural pond.

Memory hook: A jar of fish is a tiny human-made ecosystem.

Flow of energy and the 10 per cent law

Suppose 1000000 J of sunlight falls on the grass in your school garden in a day. Grasshoppers eat the grass, frogs eat the grasshoppers and a snake eats the frogs.

Calculate the food energy made by the grass and the energy reaching each level of consumers. Then explain why this chain cannot support another level.

Try it first, then show the solutionSolution

The grass converts about 1% of the sunlight on its leaves into food: 1% of 1000000 J = 10000 J.
Grasshoppers get 10% of 10000 J = 1000 J.
Frogs get 10% of 1000 J = 100 J.
The snake gets 10% of 100 J = 10 J.
At every step most of the energy is lost as heat or used in digestion and work, so only 10 J is left after four levels.
That is too little usable energy to support a further level, which is why food chains generally stop at three or four steps.

Answer: Grass 10000 J, grasshoppers 1000 J, frogs 100 J, snake 10 J. So little usable energy remains after four trophic levels that no further level can be supported.

Memory hook: Every bite up the chain keeps only one-tenth.

Biological magnification

Imagine your family eats rice, vegetables and fish from your area, and the farms nearby spray non-degradable pesticides. Your mother carefully washes the vegetables before cooking.

Explain how the pesticide can still reach your body, why humans may carry more of it than the plants do, and whether washing guarantees safety.

Try it first, then show the solutionSolution

The pesticide is washed into the soil and nearby water bodies.
Crops absorb it from the soil with water and minerals; aquatic plants and animals take it up from water, so it enters the food chain.
It is not degradable, so nothing breaks it down and the amount rises at each higher trophic level.
Humans are at the top of the food chain, so the highest concentration builds up in our bodies: biological magnification.
Washing may not get rid of such pesticide residues.

Answer: The pesticide passes along the food chain and builds up at each level, reaching its highest concentration in humans (biological magnification). Washing does not always remove it.

Memory hook: Energy shrinks up the chain; poison grows.

Ozone layer and its depletion

Suppose your family is replacing an old refrigerator, and the new one is labelled CFC-free. Your younger cousin asks why that label matters, and adds that he thinks UV rays from the Sun are what destroy the ozone layer.

Explain to your cousin, step by step: (a) how ozone forms high in the atmosphere, (b) how CFCs end up harming living things, and (c) what step has been taken worldwide. Also correct his idea about UV rays.

Try it first, then show the solutionSolution

(a) High-energy UV radiation splits some oxygen molecules into free atoms: $O_{2} UV O + O$ .
(a) Each free atom joins an oxygen molecule to give ozone: $O + O_{2} \to O_{3}$ .
(b) The steep fall in ozone from the 1980s has been linked to CFCs, man-made chemicals that were used for cooling in refrigerators and in fire-fighting equipment.
(b) With less ozone high up, less of the Sun's UV radiation is blocked. UV is highly damaging to organisms and causes skin cancer in humans.
(c) In 1987 UNEP brought countries to an agreement to hold CFC production at the 1986 level, and refrigerators must now be made without CFCs.
Correction: UV radiation actually forms ozone; ozone blocks UV; the depletion is linked to CFCs, not to UV.

Answer: UV splits $O_{2}$ and the atoms join $O_{2}$ to form $O_{3}$ . CFCs are linked to ozone loss, which lets more harmful UV reach living things. The 1987 UNEP agreement held CFC production at the 1986 level, leading to CFC-free refrigerators. UV makes ozone; it does not destroy it.

Memory hook: UV makes ozone, CFCs thin it, ozone blocks UV.

Biodegradable and non-biodegradable substances

Imagine you collect one day's kitchen waste: vegetable peels, used tea leaves, spoilt food, an empty milk packet, a used medicine strip and a broken plastic toy. You bury it all in a pit in the school garden, cover it with soil and keep it moist.

Predict which items will change over a few weeks and which will stay the same, and explain why using the idea of enzymes.

Try it first, then show the solutionSolution

Vegetable peels, tea leaves and spoilt food will change form and break down: they are biodegradable.
The milk packet, medicine strip and plastic toy will stay largely unchanged: they are non-biodegradable.
Microbes break substances down with enzymes, and enzymes are specific to particular substances.
Microbes lack enzymes to break down plastics, so these persist for a long time.

Answer: Peels, tea leaves and food decompose (biodegradable); the plastic items persist (non-biodegradable) because microbial enzymes cannot act on them.

Memory hook: No matching enzyme, no breakdown.

Waste production and its solutions

Suppose your family brings home 5 plastic carry bags a day from the market. Every day it also throws 1.5 kg of kitchen waste (peels, spoilt food, tea leaves) and 0.5 kg of plastic packets and wrappers into one bin, which is emptied onto a heap at the end of the street.

(a) How many plastic bags would carrying a cloth bag save in a 30-day month? (b) How much kitchen waste and how much plastic packet waste reach the heap in 30 days? (c) Make a reasoned plan to deal with each kind of waste.

Try it first, then show the solutionSolution

(a) Bags saved = 5 bags per day × 30 days = 150 bags.
(b) Kitchen waste = 1.5 kg × 30 = 45 kg. Plastic packets = 0.5 kg × 30 = 15 kg.
(c) Use two bins so biodegradable and non-biodegradable waste are kept separate and can be treated differently.
(c) Bury the 45 kg of kitchen waste in a pit under soil and keep it moist, so decomposers break it into simple substances that enrich the soil.
(c) Send the 15 kg of plastic for recycling, because microbes cannot break it down and it would otherwise persist in the heap.
(c) Reduce the plastic itself: carry cloth bags and avoid disposable items.

Answer: (a) 150 bags. (b) 45 kg kitchen waste and 15 kg plastic. (c) Segregate; compost or bury kitchen waste so decomposers return it to the soil; recycle the plastic; reduce plastic by using cloth bags and avoiding disposables.

Memory hook: Separate, rot the peels, recycle the plastic, refuse the bag.

Topics in this chapter

Ecosystem and Its Components
Food Chains and Food Webs
Ozone Layer Depletion
Biodegradable and Non-biodegradable WasteComing soon
Waste Production and Garbage ManagementComing soon

Our Environment: formulas and key facts

Ecosystem: $E cosy s t e m = bi o t i cco m p o n e n t s (l i v in g or g ani s m s) + abi o t i cco m p o n e n t s (t e m p er a t u r e, r ain f a l l, w in d, so i l, min er a l s) o f ana r e a, in t er a c t in g t o g e t h er$; Use for definitions and for classifying parts of a pond, garden or aquarium.
Order of trophic levels: 1.Producers (1st)
2.herbivores / primary consumers (2nd)
3.small carnivores / secondary consumers (3rd)
4.larger carnivores / tertiary consumers (4th); Count from the producer to name any organism's level in a given chain.
Energy captured by plants: Food energy made by green plants in a terrestrial ecosystem ≈ 1% of the sunlight falling on their leaves; NCERT gives this value for green plants in a terrestrial (land) ecosystem. Use it only for the Sun-to-producer step in a numerical, never between animals.
10 per cent law: $E_{n + 1} = 0.1 \times E_{n}$; Energy at the next trophic level is about one-tenth of the energy at the level below. Use for every step from producers upwards.
Energy at any trophic level: $E_{n} = E_{1} \times (0.1)^{n - 1}$; Here $E_{1}$ is the food energy in producers. Quick check for multi-step questions: divide by 10 at each step.
Direction of energy flow: Energy flow is unidirectional and decreases at each trophic level; Use in reasoning and assertion-reason questions; energy never returns to a lower level or to the Sun.
Length of food chains: Food chains generally have only 3 or 4 steps; Reason: very little usable energy is left after four trophic levels.
Numbers at each level: The greatest number of individuals is at the lowest level, the producers; Use when interpreting a pyramid or comparing trophic levels.
Biological magnification: Concentration of non-degradable chemicals increases at each higher trophic level; maximum at the top (humans); Opposite trend to energy. Use for pesticide and food-chain questions.
Ozone molecule: Ozone is $O_{3}$ (three oxygen atoms); the oxygen we breathe is $O_{2}$; Ozone is a deadly poison, but in the upper atmosphere it shields Earth from UV radiation.
Ozone formation, step 1: $O_{2} UV O + O$; High-energy UV radiation splits molecular oxygen into free oxygen atoms.
Ozone formation, step 2: $O + O_{2} \to O_{3}$; Write both steps for full marks; never write $O_{2} \to O_{3}$ alone.
Cause of ozone depletion: Cause chlorofluorocarbons (CFCs), man-made chemicals
Uses cooling in refrigerators, fire-fighting equipment
Effect linked to the steep fall in ozone from the 1980s; Name CFCs and at least one use.
Step taken against depletion: 1987: UNEP brought countries to agree to hold CFC production at the 1986 level; refrigerators must now be made CFC-free; Use for 'what steps are being taken' questions.
Biodegradable: Broken down by biological processes (bacteria and other saprophytes); Examples: vegetable peels, spoilt food, tea leaves, paper, wood, grass.
Non-biodegradable: Not broken down by biological processes; persists for a long time (e.g. plastics); Reason: enzymes are specific. Heat and pressure can act on them, but that is physical, not biological.

Cause	chlorofluorocarbons (CFCs), man-made chemicals
Uses	cooling in refrigerators, fire-fighting equipment
Effect	linked to the steep fall in ozone from the 1980s

Our Environment: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Pyramid of trophic levels

"Draw a labelled diagram showing the trophic levels in an ecosystem", or a case-based question that gives a pyramid and asks you to name the levels.

Draw

Draw a triangle split by three horizontal lines into four layers, widest at the bottom.
Bottom layer: first trophic level (producers).
Second layer: second trophic level (herbivores, primary consumers).
Third layer: third trophic level (small carnivores, secondary consumers).
Top layer: fourth trophic level (large carnivores, tertiary consumers).
Optionally add an upward arrow beside the pyramid marked 'energy decreases at each level'.

Label

Producers / autotrophs (first trophic level)
Herbivores / primary consumers (second trophic level)
Small carnivores / secondary consumers (third trophic level)
Large carnivores / tertiary consumers (fourth trophic level)

Watch out: Keep the producers at the wide base. Write both names for each level, the feeding type and the trophic level number. Do not put decomposers as a fifth layer on top.

Food chain

"Give an example of a food chain and label its trophic levels" (short answer).

Draw

Write the organisms in a line, starting with a green plant.
Join them with arrows pointing from the organism eaten to the organism that eats it, e.g. grass → goat → human.
Under each organism, write its trophic level and its role.

Label

Grass: producer, first trophic level
Goat: herbivore, primary consumer, second trophic level
Human: carnivore here, secondary consumer, third trophic level

Watch out: The arrow shows the direction food and energy move, so it points towards the eater. A backward arrow loses the mark. Always begin with a producer.

Food web

"Distinguish a food chain from a food web" with a diagram, or questions that show branching feeding links.

Draw

Place one or two producers at the bottom.
Place several herbivores above them, with arrows from each plant to every herbivore that eats it.
Place carnivores at the top, with arrows from each prey to each predator.
Make sure at least one organism is eaten by two different organisms, so the lines branch and cross.

Label

Producers
Herbivores (primary consumers)
Carnivores (secondary and tertiary consumers)
Arrows showing who is eaten by whom

Watch out: A food web is many food chains linked together. If your drawing is just one straight line, it is a food chain, not a web.

Flow of energy in an ecosystem

"Show with a diagram that energy flow is unidirectional", or a 5-mark answer on energy flow and the 10% law.

Draw

Draw the Sun on the left with an arrow to a box labelled Producers.
Draw arrows in one direction only: Producers → Herbivores → Carnivores (small) → Carnivores (large).
From every box, draw a side arrow marked 'heat lost'.
Draw arrows from every level to a box labelled Decomposers, since energy from all levels also reaches them.
Make each box smaller than the one before, to show less energy at each level.

Label

Sun / solar energy
Producers (about 1% of sunlight on leaves captured)
Herbivores
Small carnivores
Large carnivores
Heat lost at each level
Decomposers

Watch out: Never draw an arrow going back to an earlier level or to the Sun. Energy flow is one-way, and showing that is the point of the diagram.

How the board asks Our Environment

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Identify the biodegradable or non-biodegradable group (MCQ)	Four groups of three items; you pick the group that is entirely biodegradable, or the one that is not biodegradable.	Check every item in a group. One plastic, polythene or metal item makes the whole group non-biodegradable. Things from living sources such as peels, wood, grass, cake and paper break down biologically.
Name the trophic level (MCQ or short answer)	A food chain or a description of who eats whom is given, and you name an organism's trophic level or consumer type.	First write the chain with arrows from eaten to eater. Number the producer 1 and count up. Level 2 is a primary consumer, level 3 secondary, level 4 tertiary.
Energy flow numerical	Energy at one level, or the sunlight falling on leaves, is given; you find energy at another level and explain why chains are short.	Use 1% only for sunlight to plants. Then take one-tenth at each step. Show each step with the unit, then give the reason: most energy is lost as heat, digestion and work.
Assertion-reason on ecosystem facts	Statements on the 1% and 10% values, numbers at lower trophic levels, decomposers and soil replenishment, or ozone.	Judge A and R separately as true or false. Only then ask whether R actually explains why A happens. Watch for swapped 1% and 10% values.
Biological magnification reasoning	Pesticides used in fields or paddy reach water bodies; you explain how they reach humans, which level has most, and name the phenomenon.	Write the path: soil or water, then plants, then each consumer. Say the chemical is non-degradable so it accumulates at each level, give the highest at the top, and name biological magnification.
Ozone formation and depletion	Explain ozone formation with equations, why depletion is a concern, its cause and the steps taken; sometimes a 5-mark long answer.	Give both equations, the UV shield role with skin cancer as the harm, CFCs with their uses, and the 1987 UNEP agreement plus CFC-free refrigerators.
Waste disposal and environment-friendly practices	Suggest ways to deal with kitchen, plastic or mixed waste, or pick the environment-friendly practice from options.	Segregate first, then match each type: biodegradable to decompose in a pit, plastics to recycling, and reduction through cloth bags and avoiding disposables. Give one clear method per item.
Case-based on an ecosystem or pyramid	A passage about a pond, garden, aquarium or energy pyramid with 3 to 4 linked sub-parts and an internal choice.	Read the data carefully, label trophic levels, apply the 10 per cent law or the magnification idea, and answer each sub-part in one or two precise sentences.

Tricks to remember Our Environment

Short memory aids. Say each one aloud once, then use it on a question the same day.

One, then ten
Sun to plant: 1%. Every step after that: 10%.
Use it when: Stops you mixing the two values in numericals and assertion-reason questions.
Plants Help Small Tigers
Producers, Herbivores, Small carnivores, Tertiary (larger) carnivores: levels 1 to 4.
Use it when: Recall the order of trophic levels and their names.
Arrow points to the mouth
In a food chain, the arrow goes to whoever does the eating.
Use it when: Draw food chains in the correct direction of energy flow.
Energy down, poison up
Energy decreases at each higher level; non-degradable chemicals increase.
Use it when: Answer biological magnification and energy questions without confusing the two trends.
Ozone: a poison that shields
$O_{3}$ is itself a deadly poison, yet high in the atmosphere it protects us from UV.
Use it when: Explain why ozone is harmful yet the ozone layer is essential.
One bad apple check
In a 'which group is biodegradable' MCQ, one plastic, polythene or metal item spoils the whole group.
Use it when: Solve biodegradable-group MCQs fast and safely.

Our Environment: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Spot the biodegradable group · 1 mark · MCQBoard level

Which group contains only biodegradable items?
(a) Used tea leaves, vegetable peels, spoilt food
(b) Vegetable peels, plastic milk packet, paper
(c) Wood, grass, polythene bag
(d) Medicine bubble pack, cake, fruit peel

Try it first, then show the solutionSolution

Check each item in each option.
(b), (c) and (d) each contain a plastic item (milk packet, polythene bag, bubble pack), which is non-biodegradable.
Every item in (a) is broken down by microbes.

Answer: (a) Used tea leaves, vegetable peels, spoilt food.

Example 2: 1% or 10%? · 1 mark · Assertion-reasonBoard level

Assertion (A): Green plants in a terrestrial ecosystem convert about 10% of the sunlight falling on their leaves into food energy.
Reason (R): On average, about 10% of the food eaten by an organism becomes its own body and is available to the next trophic level.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Try it first, then show the solutionSolution

A is false: green plants in a terrestrial ecosystem capture only about 1% of the sunlight falling on their leaves.
R is true: about 10% of the food eaten is turned into the organism's body and made available to the next level.
A is false and R is true, so the answer is (d). (1)

Answer: (d) A is false but R is true.

Example 3: Ozone: formation and depletion · 4 marks · Short answerHarder board level

(a) Write the two steps by which ozone forms in the upper atmosphere. (b) Name the chemicals linked to ozone depletion and state where they were used. (c) What international step was taken in 1987 to limit them? (d) A classmate writes, 'UV rays from the Sun are destroying the ozone layer.' Correct this statement. (4 marks)

Try it first, then show the solutionSolution

(a) Step 1: high-energy UV radiation splits oxygen molecules into free atoms: $O_{2} UV O + O$ . (½)
(a) Step 2: each free atom joins an oxygen molecule: $O + O_{2} \to O_{3}$ . (½)
(b) Chlorofluorocarbons (CFCs), man-made chemicals that were used for cooling in refrigerators and in fire-fighting equipment. (1)
(c) In 1987 the United Nations Environment Programme (UNEP) brought countries to an agreement to hold CFC production at the 1986 level; manufacturers must now make refrigerators without CFCs. (1)
(d) The statement is wrong. UV radiation actually makes ozone from oxygen, and ozone in turn blocks UV. The fall in ozone is linked to CFCs, not to UV. (1)

Answer: $O_{2} UV O + O$ , then $O + O_{2} \to O_{3}$ . CFCs (cooling in refrigerators, fire-fighting equipment) are linked to depletion. The 1987 UNEP agreement held CFC production at the 1986 level, and refrigerators must now be CFC-free. UV forms ozone; CFCs, not UV, deplete it.

Example 4: Draw and label the trophic levels · 3 marks · Short answerBoard level

Draw a labelled diagram to show the four trophic levels of an ecosystem. Why is the base of this diagram the widest? (3 marks)

Try it first, then show the solutionSolution

Draw and label: a triangle split into four horizontal layers, widest at the bottom.
Labels from base to top: producers (first trophic level); herbivores / primary consumers (second); small carnivores / secondary consumers (third); large carnivores / tertiary consumers (fourth).
Common slip: do not place decomposers as a fifth layer, and do not draw the pyramid upside down.
Reason: there are generally more individuals at lower trophic levels, and producers are the most numerous. Energy also decreases at each higher level, so fewer organisms can be supported there.

Answer: A four-layer pyramid with producers at the base and large carnivores at the top. The base is widest because producers are the most numerous, and less energy is available at each higher level.

Example 5: Energy numerical with the 1% and 10% values · 5 marks · Long answerHarder board level

In a grassland (a terrestrial ecosystem), the food chain is grass → grasshopper → frog → snake. The grass has made 50000 J of food energy. Using the 1% and 10% values, find: (a) the sunlight energy that fell on the grass; (b) the energy available to the grasshoppers, frogs and snake; (c) the energy reaching the snake as a percentage of the producers' energy. (d) Explain why this chain cannot usefully go on to many more levels, and (e) why energy flow is called unidirectional. (5 marks)

Try it first, then show the solutionSolution

(a) Green plants in a terrestrial ecosystem capture about 1% of the sunlight on their leaves, so sunlight = 50000 J ÷ 0.01 = 5000000 J. (1)
(b) At each step only about 10% passes on. Grasshoppers: 10% of 50000 = 5000 J. Frogs: 10% of 5000 = 500 J. Snake: 10% of 500 = 50 J. (1½)
(c) $\frac{50}{50000} \times 100 = 0.1%$ . (½)
(d) A great deal of energy is lost as heat and used in digestion and work at each step. After four levels so little usable energy remains that a further level cannot be supported. (1)
(e) Energy moves only forward. It does not return to the Sun from the grass, or to the grass from the grasshoppers. (1)

Answer: (a) 5000000 J. (b) 5000 J, 500 J, 50 J. (c) 0.1%. (d) Large losses at each step leave too little energy after four levels. (e) Energy never returns to an earlier level.

Example 6: Biodegradable vs non-biodegradable waste in depth · 5 marks · Long answerHarder board level

(a) Define biodegradable and non-biodegradable substances, with two examples each. (b) Explain why plastics are non-biodegradable. (c) State two harmful effects of non-biodegradable substances. (d) Suggest two ways you can reduce problems of waste disposal at home. (5 marks)

Try it first, then show the solutionSolution

(a) Biodegradable: broken down by biological processes, e.g. fruit peels and wood.
Non-biodegradable: not broken down biologically, e.g. plastic bags and plastic milk packets.
(b) Enzymes are specific, and bacteria and other saprophytes have none that act on plastics. So plastics are not broken down; under ordinary conditions they persist for a long time.
(c) They stay in the environment for a long time and pile up as garbage. Some harm members of the ecosystem; for example, non-degradable pesticides build up along food chains.
(d) Keep biodegradable and non-biodegradable waste separate, and bury or compost kitchen waste. Carry cloth bags instead of plastic carry bags, and send plastics for recycling.

Answer: Biodegradable waste is broken down by microbes; non-biodegradable waste is not, because microbes lack enzymes for it. Non-biodegradable waste persists and can harm organisms. Reduce the problem by segregating waste, composting kitchen waste, using cloth bags and recycling plastics.

Example 7: Reading a waste-burial activity · 3 marks · Short answerHOTS / competency

A class buried some household waste in the school garden, kept it moist, and recorded what they found after 30 days:

Vegetable peels: gone, mixed with the soil
Used tea leaves: mostly gone
Waste paper: soft and broken into bits
Plastic milk packet: unchanged
Medicine bubble pack: unchanged

(a) Which items are biodegradable? (b) Which item changed fastest, and which changed slowest among the biodegradable items? (c) Explain why the last two items were unchanged, and what this means for littering parks with such wrappers. (3 marks)

Try it first, then show the solutionSolution

(a) Vegetable peels, used tea leaves and waste paper, because all of them changed form.
(b) Fastest: vegetable peels. Slowest: waste paper, which was only softened.
(c) Microbes have no specific enzymes to break down plastic, so these items persist.
Wrappers of this kind littered in parks will stay for a long time and build up as garbage. They should be collected separately, not left in soil.

Answer: Peels, tea leaves and paper are biodegradable. Peels changed fastest and paper slowest. Plastic packets stay unchanged because microbes cannot break them down, so littered wrappers persist.

Example 8: Interpreting pesticide data · 3 marks · Short answerHOTS / competency

Suppose a survey of a lake polluted by a non-degradable pesticide gives these hypothetical concentrations (in parts per million): water 0.01, aquatic plants 0.4, small fish 2, fish-eating birds 20. (a) Name the phenomenon shown. (b) How many times higher is the concentration in the birds than in the water? (c) Explain why the concentration rises at each level, and why people who eat fish from this lake are at risk. (3 marks)

Try it first, then show the solutionSolution

(a) Biological magnification.
(b) 20 ÷ 0.01 = 2000 times.
(c) The pesticide is not degradable, so it is not broken down in the body. Each organism eats many organisms of the level below, so the amount rises at every step up the chain.
Humans who eat these fish occupy the top level, so the chemical can reach its maximum concentration in their bodies.

Answer: Biological magnification. The birds have 2000 times the concentration in the water. The non-degradable chemical builds up at each step, so humans at the top of the chain collect the most.

Example 9: Removing a link from a food web · 4 marks · Short answerHarder board level

Suppose in a grassland food web, grass is eaten by grasshoppers and by rabbits, frogs eat grasshoppers, and snakes eat both frogs and rabbits. (a) Write the two food chains in this web that end in the snake. (b) State the trophic level of the snake in each chain. (c) A disease kills all the frogs. Predict the effects on the grasshoppers, the grass and the snakes, and explain why the snakes are less badly hit than they would be in a single straight food chain. (4 marks)

Try it first, then show the solutionSolution

(a) Chain 1: grass → grasshopper → frog → snake. Chain 2: grass → rabbit → snake. (1)
(b) The snake is at the fourth trophic level (tertiary consumer) in chain 1 and at the third trophic level (secondary consumer) in chain 2. (1)
(c) Without frogs, grasshoppers are no longer eaten, so their numbers rise; they eat more grass, so the grass decreases. (1)
(c) The snakes lose one food source but can still eat rabbits, so they may survive in smaller numbers. In a single straight chain the frog would be their only food, so they would starve. (1)

Answer: (a) grass → grasshopper → frog → snake and grass → rabbit → snake. (b) Fourth level in the first, third level in the second. (c) Grasshoppers increase, grass decreases, snakes decline but survive on rabbits, because a food web gives them more than one food path.

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 1 + 1 + 2 + 2 marks.

A family's daily waste

Suppose a student weighs her family's waste for one day and records:

vegetable peels and spoilt food: 1.2 kg
used tea leaves: 0.1 kg
waste paper: 0.2 kg
plastic milk packets and wrappers: 0.4 kg
an empty medicine bubble pack: 0.1 kg

At present, all of it goes into a single bin and is dumped at a heap at the end of the street. She wants to suggest changes to her family, using what she learnt in her Science class about how waste breaks down. Answer (A), (B) and either (C) or (D).

(i) · 1 mark
(A) What percentage of this waste is biodegradable?
Answer
Answer: 75%
Biodegradable = 1.2 + 0.1 + 0.2 = 1.5 kg. Total = 1.5 + 0.4 + 0.1 = 2.0 kg. So 1.5 ÷ 2.0 × 100 = 75%.
(ii) · 1 mark
(B) Which items on her list will remain in the heap for a very long time, and why?
Answer
Answer: The plastic milk packets, wrappers and bubble pack.
These are non-biodegradable. Enzymes are specific, and microbes have none that can break these materials down, so they persist.
(iii) · 2 marks
(C) At this rate, how much plastic waste (packets, wrappers and bubble pack) will the family add to the heap in 30 days? Suggest a plan for each kind of waste and explain why separating the waste helps.
Answer
Answer: 0.5 kg × 30 = 15 kg of plastic. Use two bins; bury or compost the kitchen waste and paper, and send the plastics for recycling.
Plastic per day = 0.4 + 0.1 = 0.5 kg, so 15 kg in 30 days (½). Plan: separate bins, kitchen waste buried in a pit under soil, plastics recycled (½). Separation lets decomposers break the biodegradable part into simple substances for the soil, while plastics are recycled instead of lying in the heap for years (1).
(iv) · 2 marks
(D) OR (instead of C): Her brother says, 'If all our waste were biodegradable, dumping it on the street heap would cause no problem at all.' Do you agree? Give two reasons.
Answer
Answer: No, it would still cause problems.
Decomposition takes time, so large amounts of waste dumped every day would still pile up as a garbage heap (1). Dumped in a heap rather than managed, for example buried in a pit, it is not put to use for the soil and can foul the surroundings, so it still needs proper handling (1).

Energy at each level of a grassland

Suppose ecologists measure the energy in a hypothetical grassland, a terrestrial ecosystem, over a fixed period. The food chain is grass → grasshoppers → frogs → snakes. Their table reads:

sunlight falling on the grass leaves: 1000000 J
grass (producers): 10000 J
grasshoppers (primary consumers): 1000 J
frogs (secondary consumers): value missing
snakes (tertiary consumers): 10 J

When drawn as a pyramid, each layer is narrower than the one below it. Answer (A), (B) and either (C) or (D).

(i) · 1 mark
(A) What percentage of the sunlight falling on the leaves did the grass convert into food energy? Does this match the NCERT value?
Answer
Answer: 1%; yes, it matches.
10000 ÷ 1000000 × 100 = 1%, which matches the NCERT value for green plants in a terrestrial ecosystem.
(ii) · 1 mark
(B) Find the missing energy value for the frogs and state the rule you used.
Answer
Answer: 100 J.
By the 10 per cent law, about 10% of the energy at one level reaches the next: 10% of 1000 J = 100 J. Check: 10% of 100 J = 10 J for snakes, which matches the table.
(iii) · 2 marks
(C) Give two reasons why the layers of this pyramid get narrower towards the top, and explain why no arrow of energy flow can point downwards.
Answer
Answer: Energy is lost at every step, and fewer organisms can be supported higher up; energy flow is unidirectional.
At each level much energy is lost as heat and used in digestion and work, so only about 10% passes on (½). Less energy can support fewer individuals, so the producers are the most numerous and each higher level is smaller (½). Energy captured at one level is no longer available to the level before it; it never returns from frogs to grasshoppers or from grass to the Sun, so the flow is one-way (1).
(iv) · 2 marks
(D) OR (instead of C): Suppose a disease kills most of the grasshoppers. Predict the effect on the grass and on the frogs and snakes, using the idea of energy flow.
Answer
Answer: Grass increases; frogs and snakes decline.
With few grasshoppers eating it, the grass is not consumed and may increase, upsetting the balance (1). Energy from the grass can no longer pass up the chain, so the frogs lose their food and decline, and the snakes, which depend on frogs, decline too (1).

Pesticide in a pond food web

Suppose a pond lies next to paddy fields that are sprayed with a non-degradable pesticide. In the pond, water plants are eaten by insect larvae, insect larvae are eaten by small fish, and small fish are eaten both by large fish and by fish-eating birds. A hypothetical survey records the pesticide (in parts per million):

pond water: 0.02
water plants: 0.5
insect larvae: 2
small fish: 8
large fish: 40

Answer (A), (B) and either (C) or (D).

(i) · 1 mark
(A) Write the longest food chain in this web and name the trophic level of the large fish.
Answer
Answer: Water plants → insect larvae → small fish → large fish; the large fish is at the fourth trophic level (tertiary consumer).
Start at the producer and follow each eater: water plants (1) → insect larvae (2) → small fish (3) → large fish (4).
(ii) · 1 mark
(B) How many times greater is the pesticide concentration in the large fish than in the water plants?
Answer
Answer: 80 times.
40 ÷ 0.5 = 80.
(iii) · 2 marks
(C) Name the phenomenon shown in the table and explain why the concentration rises at each level. The birds were not sampled; predict whether their concentration would be higher or lower than in the small fish, with a reason.
Answer
Answer: Biological magnification; the birds would have a higher concentration than the small fish.
The phenomenon is biological magnification (½). The pesticide is not degradable, so each organism keeps what it takes in from the many organisms it eats, and the amount rises at each step (½). The birds eat many small fish and sit one level above them, so their concentration would be higher than 8 ppm (1).
(iv) · 2 marks
(D) OR (instead of C): A farmer says, 'We wash our rice and the fish before cooking, so the pesticide cannot harm our family.' Evaluate this claim using the table.
Answer
Answer: The claim is not correct.
The table shows the pesticide building up inside living organisms at each level, so it is within the food, not just on its surface (1). Humans eat crops and fish and are at the top of food chains, so they can collect the highest concentration, and washing cannot always remove such residues (1).

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Distinguish between biodegradable and non-biodegradable substances. Give two examples of each. (2 marks) (2 marks)

Step	Marks
Biodegradable substances are broken down by biological processes, that is, by bacteria and other saprophytes.	0.5
Non-biodegradable substances are not broken down in this way. They persist in the environment for a long time or may harm members of the ecosystem.	0.5
Two biodegradable examples, e.g. vegetable peels, used tea leaves, cake, wood or grass.	0.5
Two non-biodegradable examples, e.g. plastic milk packets or plastic bags.	0.5

(a) Why do food chains generally have only three or four steps? (b) Why is a food web a more realistic picture of feeding relationships than a food chain? (3 marks) (3 marks)

Step	Marks
At each step a great deal of energy is lost as heat, or used in digestion and work. Only about 10% of the food eaten reaches the next trophic level.	1
So very little usable energy is left after four trophic levels, which cannot support a further level. This keeps chains short.	1
Each organism is usually eaten by two or more kinds of organisms, which are themselves eaten by several others. So the links branch into a web rather than a single straight line.	1

(a) Define an ecosystem and name its two types of components, with two examples of each. (b) Name the three groups of organisms in an ecosystem by the way they get their food. Give one example and the role of each. (c) Would the soil be replenished naturally if decomposers were absent? Explain. (5 marks) (5 marks)

Step	Marks
Ecosystem: all the interacting organisms of an area together with the non-living parts of their environment.	1
Biotic components are living organisms, e.g. plants and animals. Abiotic components are physical factors, e.g. temperature and soil (also rainfall, wind, minerals).	1
Producers, e.g. green plants: they make sugar and starch from inorganic substances using sunlight in the presence of chlorophyll.	0.5
Consumers, e.g. a goat (herbivore): they depend on producers directly, or indirectly by eating other consumers.	0.5
Decomposers, e.g. bacteria and fungi: they break down dead remains and wastes of organisms.	0.5
No. Decomposers convert complex organic substances into simple inorganic substances.	0.5
These substances return to the soil and are used again by plants. Without decomposers, dead matter and garbage would pile up and the soil would not be replenished.	1

Where students lose marks in Our Environment

The slips examiners see most often, and what to write instead.

Mistake: Drawing food chain arrows the wrong way
Students think 'the snake eats the frog' and point the arrow from eater to food.
Wrong working
Snake → Frog → Grasshopper → Grass
Right working
Grass → Grasshopper → Frog → Snake (arrows show the direction of energy flow, from the eaten to the eater)
Do this: Always start with the producer on the left and point each arrow towards the eater.
Usually costs: ½ to 1 mark
Mistake: Saying only 10% of energy is lost at each level
The 10 per cent law is half-remembered and turned upside down.
Wrong working
At each trophic level, 10% of the energy is lost.
Right working
Only about 10% of the food eaten is passed on to the next level; most of the rest is lost as heat or used in digestion and work.
Do this: Say '10% passes on', never '10% is lost'.
Usually costs: 1 mark
Mistake: Using 10% for the Sun-to-plant step
Students apply one number to every step.
Wrong working
Sunlight on leaves = 50000 J, so plants get 10% = 5000 J.
Right working
Plants capture about 1% of sunlight: 1% of 50000 J = 500 J. Then herbivores get 10% of 500 J = 50 J.
Do this: 1% for sunlight to producers; 10% for every level after that.
Usually costs: 1 to 2 marks in a numerical
Mistake: Writing ozone formation as a single step
Students shortcut to the product and also leave the equation unbalanced.
Wrong working
$O_{2} \to O_{3}$
Right working
$O_{2} UV O + O$ , then $O + O_{2} \to O_{3}$
Do this: Write both steps, put UV over the first arrow and count oxygen atoms on each side.
Usually costs: 1 mark
Mistake: Saying UV rays destroy the ozone layer
UV and depletion are mentioned together, so the roles get mixed up.
Wrong working
The ozone layer is being depleted by UV rays from the Sun.
Right working
UV radiation actually forms ozone from oxygen. The fall in ozone is linked to CFCs, man-made chemicals that were used for cooling in refrigerators and in fire-fighting equipment.
Do this: Remember: UV makes ozone, CFCs deplete it, ozone blocks UV.
Usually costs: 1 mark
Mistake: Putting the highest pesticide concentration in producers
Students reason that plants absorb the pesticide first, so they must hold the most.
Wrong working
Plants absorb pesticides directly, so they have the highest concentration.
Right working
Plants take in the pesticide first, but because it is non-degradable it builds up at each level; the top level (humans) has the maximum.
Do this: Link 'non-degradable' to 'accumulates' to 'highest at the top'.
Usually costs: 1 to 2 marks
Mistake: Calling plastic biodegradable because heat changes it
Any breakdown is mistaken for biodegradation.
Wrong working
Plastic melts and breaks down in heat, so it is biodegradable.
Right working
Heat and pressure are physical processes. Plastic is not broken down by bacteria or other saprophytes, so it is non-biodegradable.
Do this: Biodegradable means broken down by living organisms, nothing else.
Usually costs: 1 mark

NCERT exercise guide: Our Environment

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

In-text questions after section 13.1.1 (Food Chains and Webs)

What it tests: The meaning of trophic levels, writing a food chain with its levels named, and the role of decomposers.

How to approach it: Define the term first, then give a correct food chain that starts with a producer and has arrows towards the eater. Label every level with its number and its role. For decomposers, say what they break down, what they turn it into and where it goes.

Mirrors the in-text question on trophic levels

What is meant by a trophic level? Write a pond food chain of four steps and name the trophic level of each organism.

Show the solutionSolution

A trophic level is each step or level of a food chain at which organisms get their food.
Pond food chain: aquatic plants → small insects → small fish → large fish.
Aquatic plants: first trophic level (producers).
Small insects: second trophic level (herbivores, primary consumers).
Small fish: third trophic level (secondary consumers). Large fish: fourth trophic level (tertiary consumers).

Answer: A trophic level is a step in a food chain. In aquatic plants → small insects → small fish → large fish, the levels are first (producer), second (primary consumer), third (secondary consumer) and fourth (tertiary consumer).

In-text questions on biodegradable and non-biodegradable substances (section 13.2.2)

What it tests: Why some substances break down and others do not (enzyme specificity), and the effects of both kinds of waste on the environment.

How to approach it: Link your reason to enzymes. Each enzyme breaks down only a particular substance, so microbes have no enzymes for many human-made materials such as plastics. For effects, give two separate, clear points for each kind of waste.

Mirrors the in-text question on why substances differ

Vegetable peels buried in soil disappear in a few weeks, but a plastic packet buried alongside stays unchanged. Explain why.

Show the solutionSolution

Bacteria and other saprophytes break down substances using enzymes.
Enzymes are specific: each one acts only on a particular substance.
These microbes have enzymes that act on vegetable peels, so the peels are biodegradable.
They have no enzymes that act on plastic, so it is not broken down biologically. Under normal conditions it persists for a long time.

Answer: Peels are broken down by the specific enzymes of microbes. Plastic is not, because microbes lack enzymes for it, so it is non-biodegradable.

In-text questions at the end of section 13.2 (ozone and waste disposal)

What it tests: What ozone is, its effect on ecosystems, and practical ways you can reduce waste.

How to approach it: For ozone, first give its formula and its protective role, then explain what loss of that protection would mean. For waste, give actions you can actually do, each with a one-line reason.

Mirrors the in-text question on ozone

What is ozone? How would a thinner ozone layer affect living things?

Show the solutionSolution

Ozone, $O_{3}$ , is a molecule made of three oxygen atoms.
Ozone is a deadly poison to living things, yet in the higher atmosphere it shields the earth's surface from the Sun's UV radiation.
UV radiation is highly damaging to organisms. For example, it causes skin cancer in humans.
A thinner ozone layer lets more UV reach the surface, harming organisms and so the ecosystem.

Answer: Ozone ( $O_{3}$ ) in the upper atmosphere protects the earth from UV radiation. If the layer thins, more UV reaches living things and damages them, e.g. by causing skin cancer in humans.

End-of-chapter exercises

What it tests: Choosing biodegradable groups and correct food chains (MCQs), environment-friendly habits, the effect of removing a trophic level, biological magnification, problems of non-biodegradable waste, whether biodegradable waste has impact, and the ozone layer.

How to approach it: For MCQs, check every item in an option. One non-biodegradable item or one wrong link rules out that option. For reasoning questions, write cause → effect → consequence for the whole ecosystem. Answer every part of a two-part question separately.

Mirrors the exercise on removing a trophic level

In a grassland, all the herbivores die from a disease. Predict what happens to the producers and to the carnivores.

Show the solutionSolution

Herbivores are the second trophic level. They link the plants to the carnivores.
Without herbivores, plants are not eaten, so plant numbers may increase greatly and the natural balance is upset.
Carnivores lose their food source, so their numbers fall. They may starve or die out.
The flow of energy from producers to higher levels is cut, so the ecosystem becomes unbalanced.

Answer: Plants increase unchecked, carnivores lose their food and decline, and energy flow beyond the first level is broken, so the ecosystem's balance is lost.

Activity 13.1 and Activity 13.2 (designing an aquarium and forming its food chain)

What it tests: An aquarium as a human-made ecosystem, what makes it self-sustaining, why it needs cleaning, and arranging aquatic organisms in a food chain.

How to approach it: Name what is added and the role each item plays (water, oxygen from the aerator, food, plants). Explain cleaning by comparing the aquarium with a natural pond. In a natural pond, decomposers and natural balance deal with dead matter; a small artificial system has no such balance.

Mirrors the aquarium activity questions

Why does an aquarium need cleaning once in a while, while a natural pond does not? Why should you avoid putting a fish that eats other animals into a small aquarium?

Show the solutionSolution

An aquarium is a small human-made ecosystem. Dead plants and fish and their wastes collect in it faster than they are broken down, so it must be cleaned.
A pond is a natural ecosystem. Its decomposers break down dead remains and wastes into simple substances that plants use again, so it maintains its own balance.
A fish that eats other animals would eat the smaller fish, and the balance of the small system would be destroyed.

Answer: In an aquarium, wastes and dead matter build up because it lacks a natural balance, so it needs cleaning. A pond's decomposers keep it balanced. A predatory fish would eat the other animals and upset the aquarium.

The waste-burying activity in section 13.2.2 (and the waste surveys, Activities 13.7 to 13.9)

What it tests: Observing which waste changes over time when buried, sorting waste, and how homes, local bodies and industries deal with waste, sewage and electronic items.

How to approach it: Write what was done, what was seen and what it shows. Materials that change form are biodegradable; materials that stay unchanged are non-biodegradable. For survey questions, always suggest keeping biodegradable and non-biodegradable waste separate.

Mirrors the burying activity

A student buries kitchen waste in a pot, covers it with 15 cm of soil, keeps it moist and checks it every 15 days. What will she observe, and what does it show?

Show the solutionSolution

Vegetable peels, spoilt food and used tea leaves change form and gradually disappear into the soil. Soft kitchen waste changes fastest.
Plastic milk packets and medicine bubble packs remain unchanged over long periods.
The first group is broken down by microbes, so it is biodegradable.
The second group is not, so it is non-biodegradable.

Answer: Kitchen waste decomposes but plastic packets stay unchanged. This shows which wastes are biodegradable and which are non-biodegradable.

Our Environment: previous year questions

Asked in 21 of the 21 board papers we hold (2023 to 2026), on average 4.9 marks per paper; 64 questions in all, counting OR alternatives. In 4 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Our Environment

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

2026, set 31/1/1, Q7 · 1 mark · MCQ

Which of the following group is not 'biodegradable'?

(A)Vegetable peels, dead leaves, paper
(B)Cow dung, leather bag, water
(C)Polythene bag, rubber band, ball pen
(D)Paper, fruits, bones

Try it first, then show our solutionSolution

Biodegradable substances are broken down by decomposers such as bacteria and fungi. Polythene, synthetic rubber and the plastic of a ball pen are not broken down by microbes. All the other groups contain natural materials that decay. Hence the answer is option (C).
1 mark

Answer: (C) Polythene bag, rubber band, ball pen

Where the marks are

Selects the group made of plastic and synthetic materials that microbes cannot decompose (1 mark)

See it in the full 2026 board paper, set 31/1/1

2026, set 31/1/1, Q13 · 3 marks · Short answer

Given below is a pyramid showing various trophic levels in an ecosystem:

(a)
From the organisms listed below, identify which one is to be placed at which trophic level? Deer, Grass, Lion, Snake, Rabbit
0 marks
(b)
Discuss the reason why primary consumers will have more energy as compared to secondary consumers?
0 marks
(c)
Why is the base of the pyramid broad?
0 marks

Figure in the paper: Four-level pyramid. Top to bottom: (iv) Tertiary consumers, (iii) Secondary consumers, (ii) Primary consumers, (i) Producers. See it in the official paper.

Try it first, then show our solutionSolution

(a) (i) Producers: Grass; (ii) Primary consumers: Deer, Rabbit; (iii) Secondary consumers: Snake, Lion; (iv) Tertiary consumer: Lion.
1 mark
(b) Primary consumers feed directly on green plants, which hold a large amount of energy. By the 10% law, only about 10% of the energy at one level passes to the next. The rest is used in life processes or lost as heat. So secondary consumers get much less energy than primary consumers.
1 mark
(c) The base is broad because producers are the most numerous and contain the maximum energy and biomass of all trophic levels. Energy, and usually number, decreases at each higher level.
1 mark

Answer: (a) Grass – producers; Deer, Rabbit – primary; Snake, Lion – secondary; Lion – tertiary. (b) Only ~10% energy passes to the next level. (c) Producers have the highest number/energy/biomass.

Where the marks are

Correct placement of all organisms (1 mark)
10% law explanation for less energy at the higher level (1 mark)
Producers have the maximum number/energy/mass, so the base is broad (1 mark)

See it in the full 2026 board paper, set 31/1/1

2026, set 31/2/1, Q12 · 2 marks · Very short answerCompetency-based

Suggest one best way to dispose of the following wastes :

(i) Kitchen waste
(ii) Metallic waste
(iii) Plastic bottles
(iv) Sanitary waste

Try it first, then show our solutionSolution

(i) Kitchen waste: composting (converting it into manure).
½ mark
(ii) Metallic waste: recycling or reuse.
½ mark
(iii) Plastic bottles: recycling or reuse.
½ mark
(iv) Sanitary waste: incineration (burning at high temperature).
½ mark

Answer: (i) Composting; (ii) Recycle/reuse; (iii) Recycle/reuse; (iv) Incineration

Where the marks are

Kitchen waste – composting (½ mark)
Metallic waste – recycling/reuse (½ mark)
Plastic bottles – recycling/reuse (½ mark)
Sanitary waste – incineration (½ mark)

See it in the full 2026 board paper, set 31/2/1

Every Our Environment question, by year

For 2027

Sample paper 2026-27: Q7 (1 mark, solved), Q9 (1 mark, solved), Q14 (3 marks, solved)

2026

Set 31/1/1: Q7 (1 mark, solved), Q13 (3 marks, solved)

Set 31/2/1: Q6 (1 mark, solved), Q7 (1 mark, solved), Q9 (1 mark, solved), Q12 (2 marks, solved)

Set 31/3/1: Q5 (1 mark, solved), Q6 (1 mark, solved), Q8 (1 mark, solved), Q10 (2 marks, solved)

Set 31/4/1: Q3 (1 mark, solved), Q9 (1 mark, solved), Q11 (2 marks, solved)

Set 31/5/1: Q6 (1 mark, solved), Q7 (1 mark, solved), Q14 (3 marks, solved)

Sample paper 2025-26: Q6 (1 mark, solved), Q7 (1 mark, solved), Q9 (1 mark, solved), Q12 (2 marks, solved)

2025

Set 31/1/1: Q15 (1 mark, solved), Q16 (1 mark, solved), Q20 (1 mark, solved), Q26 (2 marks, solved)

Set 31/2/1: Q16 (1 mark, solved), Q33 (3 marks, solved)

Set 31/3/1: Q16 (1 mark, solved), Q20 (1 mark, solved), Q33 (3 marks, solved)

Set 31/4/1: Q15 (1 mark, solved), Q16 (1 mark, solved), Q33 (3 marks, solved)

Set 31/5/1: Q15 (1 mark, solved), Q16 (1 mark, solved), Q20 (1 mark, solved), Q26 (2 marks, solved)

Set 31/6/1: Q15 (1 mark, solved), Q16 (1 mark, solved), Q20 (1 mark, solved), Q26 (2 marks, solved)

Sample paper 2024-25: Q15 (1 mark, solved), Q16 (1 mark, solved), Q20 (1 mark, solved), Q26 (2 marks, solved)

2024

Set 31/1/1: Q15 (1 mark, solved), Q18 (1 mark, solved), Q33(A) (3 marks, solved), Q33(B) (3 marks, solved)

Set 31/2/1: Q26 (2 marks, solved), Q33 (3 marks, solved)

Set 31/3/1: Q15 (1 mark, solved), Q16 (1 mark, solved), Q20 (1 mark, solved), Q26 (2 marks, solved)

Set 31/4/1: Q16 (1 mark, solved), Q20 (1 mark, solved), Q33 (3 marks, solved)

Set 31/5/1: Q16 (1 mark, solved), Q20 (1 mark, solved), Q33 (3 marks, solved)

Sample paper 2023-24: Q15 (1 mark, solved), Q16 (1 mark, solved), Q20 (1 mark, solved), Q26 (2 marks, solved)

2023

Set 31/1/1: Q26 (2 marks, solved), Q33(a) (3 marks, solved), Q33(b) (3 marks, solved)

Set 31/2/1: Q26 (2 marks, solved), Q33 (3 marks, solved)

Set 31/4/1: Q26 (2 marks, solved), Q33 (3 marks, solved)

Set 31/5/1: Q26 (2 marks, solved), Q33 (3 marks, solved)

Set 31/6/1: Q10 (1 mark, solved), Q18 (1 mark, solved), Q33 (3 marks, solved)

Practice questions on Our Environment

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In the food chain grass → goat → human, the human is a:
(a) producer
(b) primary consumer
(c) secondary consumer
(d) decomposer
Hint
Count the steps from the producer.
Answer
(c) secondary consumer. The human eats the herbivore (goat), so it is at the third trophic level.
Q2 · 2 marks · Very short answerBoard level
Distinguish between natural and human-made ecosystems, giving two examples of each.
Hint
Ask who set it up.
Answer
Natural ecosystems exist on their own in nature, e.g. forests, ponds and lakes. Human-made (artificial) ecosystems are created and maintained by people, e.g. gardens, crop-fields and aquariums.
Q3 · 1 mark · Assertion-reasonBoard level
Assertion (A): Ozone in the higher levels of the atmosphere is essential for life on earth.
Reason (R): Ozone shields the surface of the earth from the ultraviolet radiation of the Sun.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
What would happen to organisms without the shield?
Answer
(a) Both are true. The ozone layer is essential precisely because it blocks damaging UV radiation, which causes skin cancer in humans.
Q4 · 3 marks · Short answerHarder board level
Imagine all the frogs in a grassland food chain (grass → grasshopper → frog → snake) are killed. Explain three effects on the ecosystem.
Hint
Look one level below and one level above the frog.
Answer
Grasshoppers are no longer eaten, so their numbers rise. They then eat more grass, so the grass decreases. Snakes lose their food and decline. The balance of the whole ecosystem is upset.
Q5 · 3 marks · Short answerHarder board level
Will removing all organisms of a trophic level have the same impact whichever level is removed? Can any level be removed without damage? Justify.
Hint
Compare removing producers with removing top carnivores.
Answer
No. Removing the producers is the most damaging, because all consumers depend on them directly or indirectly for energy, so the whole ecosystem collapses. Removing a higher level mainly affects the levels next to it, e.g. prey numbers increase unchecked. No level can be removed without harm, because all the components are interdependent.
Q6 · 3 marks · Short answerHarder board level
In a terrestrial ecosystem, a food chain of four trophic levels has tertiary consumers that receive 30 J of energy. Using the 10% value, find the energy at the secondary consumers, primary consumers and producers. Then use the 1% value to find the sunlight that fell on the producers' leaves.
Hint
Work backwards: each lower level has 10 times more.
Answer
Secondary = 30 × 10 = 300 J. Primary = 3000 J. Producers = 30000 J. Sunlight = 30000 ÷ 0.01 = 3000000 J.
Q7 · 5 marks · Long answerHarder board level
(a) What is biological magnification? (b) Explain how a pesticide sprayed on crops reaches the human body. (c) Will its concentration be the same at all trophic levels? Justify. (d) Why can washing vegetables not always protect us?
Hint
Trace the path: soil and water → plants → animals → humans.
Answer
(a) The progressive build-up of a non-degradable chemical at each higher trophic level. (b) Pesticides wash into the soil and water bodies. Plants absorb them with water and minerals, aquatic plants and animals take them up, and they pass along the food chain to humans through grains, vegetables, fruits and meat. (c) No. The chemical is not degraded, so it builds up more at each level, and is highest in humans at the top. (d) The residues are inside the food, so washing may not get rid of them.
Q8 · 2 marks · Very short answerHarder board level
If all the waste we produce were biodegradable, would it have no impact on the environment? Give two reasons.
Hint
Think about quantity and how it is disposed of.
Answer
It would still have an impact. Large amounts of waste, especially with growing disposable use, still pile up as garbage heaps if not managed, because decomposition takes time. Untreated waste such as sewage can still pollute local water bodies.
Q9 · 3 marks · Short answerHOTS / competency
A student's notes contain three errors. Find and correct each one:
(i) Energy passed to herbivores flows back to the producers.
(ii) The greatest number of individuals is found among the top carnivores.
(iii) Decomposers convert simple inorganic substances into complex organic ones.
Hint
Recall which way energy flows and what decomposers do.
Answer
(i) Energy flow is unidirectional; it never returns to an earlier level. (ii) The greatest number is of producers; numbers are larger at lower levels. (iii) Decomposers break complex organic substances down into simple inorganic ones.
Q10 · 3 marks · Short answerHOTS / competency
Suppose your school decides to make its canteen produce as little garbage as possible. Suggest three steps and justify each using what you know about biodegradable and non-biodegradable waste.
Hint
Reduce, separate, then deal with each kind of waste.
Answer
1. Replace disposable plastic cups and packets with reusable items, since disposables add non-biodegradable waste that persists. 2. Keep separate bins for biodegradable and non-biodegradable waste, so each can be treated properly. 3. Compost food waste in a pit in the school garden, so decomposers return nutrients to the soil, and send plastics for recycling.
Q11 · 1 mark · Assertion-reasonBoard level
Assertion (A): If there were no decomposers, the soil would not be naturally replenished.
Reason (R): Decomposers convert simple inorganic substances into complex organic substances.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Check carefully which way round decomposers work.
Answer
(c) A is true but R is false. Without decomposers, dead matter would not be broken down and the soil would not be replenished, so A is true. But decomposers break complex organic substances down into simple inorganic ones, the opposite of what R says, so R is false.
Q12 · 4 marks · Short answerHarder board level
Suppose a crop-field has the food chain crop plants → grasshoppers → frogs → snakes. A farmer sprays a non-degradable pesticide that kills most of the grasshoppers. (a) Predict the effect on the frogs and the snakes. (b) A few grasshoppers survive with pesticide in their bodies. Which organism in this chain will end up with the highest concentration, and why? (c) Explain why this pesticide use can also affect the farmer's family.
Hint
Link the food-chain effect with biological magnification.
Answer
(a) Frogs lose most of their food, so their numbers fall; snakes, which eat frogs, then decline too (1). (b) The snakes. The pesticide is not degradable, so each frog collects it from many grasshoppers and each snake from many frogs; the amount rises at every level and is highest at the top (2). (c) The pesticide also stays in the crop plants that people eat, and humans are at the top of food chains, so it can build up in their bodies, and washing cannot always remove it (1).
Q13 · 3 marks · Short answerHOTS / competency
A student's energy table for a food chain in a terrestrial ecosystem reads: sunlight on leaves 500000 J; producers 50000 J; herbivores 5000 J; small carnivores 500 J; large carnivores 50 J. (a) Find the error in the table. (b) Correct all the values. (c) The student wants to add a fifth level. Using your corrected values, explain why this is unlikely.
Hint
Which step uses 1%, not 10%?
Answer
(a) The student used 10% for the Sun-to-plant step; green plants capture only about 1% of the sunlight on their leaves (1). (b) Producers = 1% of 500000 J = 5000 J; herbivores = 500 J; small carnivores = 50 J; large carnivores = 5 J (1). (c) A fifth level would get only 10% of 5 J = 0.5 J; so much energy is lost at each step that too little usable energy remains after four levels to support another level (1).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What are the two components of an ecosystem?Show answer
Biotic (living organisms) and abiotic (physical factors such as temperature, rainfall, wind, soil and minerals).
2.Name two human-made ecosystems.Show answer
Any two of: garden, crop-field, aquarium.
3.Which organisms are decomposers, and what do they produce?Show answer
Bacteria and fungi; they turn complex organic substances into simple inorganic substances that return to the soil.
4.About what fraction of sunlight falling on leaves do plants convert into food energy?Show answer
About 1% (for green plants in a terrestrial ecosystem).
5.On average, how much of the food eaten reaches the next trophic level?Show answer
About 10%.
6.Why do food chains usually have only three or four steps?Show answer
So much energy is lost at each level that very little usable energy remains after four levels.
7.Which chemicals are linked to ozone depletion, and where were they used?Show answer
Chlorofluorocarbons (CFCs), which were used for cooling in refrigerators and in fire-fighting equipment.
8.What did the 1987 UNEP agreement do?Show answer
It held CFC production at the 1986 level.

Best order to study it

Learn the ecosystem: biotic and abiotic components, natural and human-made examples, and the roles of producers, consumers and decomposers.
Practise food chains and trophic levels: write chains with correct arrows and name each level and consumer type.
Master energy flow: the 1% and 10% values, one-way flow, why chains are short. Then solve two or three numericals.
Study food webs and biological magnification together, contrasting falling energy with rising chemical concentration.
Learn the ozone layer: formation equations, its role against UV, CFCs as the cause and the 1987 UNEP step.
Finish with biodegradable and non-biodegradable substances, the enzyme reason, waste problems and practical solutions.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you define an ecosystem and list five abiotic factors?
Can you name the four trophic levels in order with their consumer names?
Can you explain what would happen without decomposers?
Can you calculate the energy at the third level if 100000 J of sunlight falls on leaves? (1000 J in plants, 100 J, 10 J)
Can you state two features of energy flow in an ecosystem?
Can you explain biological magnification in four steps, ending with humans?
Can you write both ozone formation equations, balanced, with UV over the arrow?
Can you name the cause of ozone depletion, the harm from UV and the 1987 step?
Can you explain, using enzymes, why plastics are non-biodegradable?
Can you give three ways to reduce or manage household waste?

Our Environment: frequently asked questions

Is the chapter Sources of Energy needed for Our Environment?

No. Sources of Energy is not in the 2026-27 syllabus. The Our Environment chapter mentions it once in passing, but you do not need anything from it to answer this chapter's questions.

Is the energy transferred between trophic levels 1% or 10%?

Both numbers appear, for different steps. Plants capture about 1% of the sunlight falling on their leaves. After that, about 10% of the food eaten at one level passes to the next level.

Are decomposers a trophic level in a food chain?

NCERT lists four trophic levels, from producers to tertiary consumers. Decomposers break down dead remains and wastes from all levels, so do not write them as a numbered step in a food chain unless a question specifically asks about them.

Why is ozone called a poison if the ozone layer protects us?

Ozone itself is a deadly poison to living things. High up in the atmosphere, though, it absorbs the Sun's harmful ultraviolet radiation, which damages organisms and causes skin cancer. So the ozone layer is essential even though ozone is toxic.

Will the box on disposable cups in trains come in the board exam?

No. Information in NCERT text boxes is not assessed in the board exam. It is useful for thinking about waste, but you should build your answers on the main text.

How do I decide if an item is biodegradable in an MCQ?

Ask whether bacteria and other saprophytes can break it down. Peels, food, paper, wood and grass can, while plastics and polythene cannot. If even one item in a group is non-biodegradable, that group is not fully biodegradable.

Based on: CBSE Science (086) syllabus 2026-27; NCERT Science, Textbook for Class X (Reprint 2026-27)