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Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Science — all chaptersOur Environment
Start here: why this chapter matters
In simple wordsThink of nature as a household. Plants are the earners who bring in energy from the Sun; animals spend it; bacteria and fungi are the cleaners who recycle the leftovers. This chapter shows how that household runs — who eats whom and how little energy reaches the top — and then two ways we are damaging it: thinning the Earth's sunscreen (the ozone layer) and piling up rubbish that the cleaners cannot handle.
This is Chapter 13, the last chapter of NCERT's Class 10 Science textbook (pages 208–217 of the 2026-27 reprint; the contents page and the PDF agree). It has two parts: 13.1 Eco-system — what are its components? with 13.1.1 Food chains and webs, and 13.2 How do our activities affect the environment? with 13.2.1 the ozone layer and 13.2.2 managing garbage. It has 9 activities, Figures 13.1–13.4, a ‘Think it over’ box on disposable cups, three ‘Questions’ boxes (7 questions) and 9 exercises. It is short, so this page has nine short concept sections rather than long ones.
- Neighbouring chapters: Ecosystem (Class 12 Biology, Chapter 12) takes food chains, energy flow, the 10 per cent law and ecological pyramids much further — read it next rather than expecting pyramids here. Life Processes (Chapter 5 of this book) explains photosynthesis and the enzymes this chapter uses to explain why plastics persist. Geography's Composition and Structure of Atmosphere (Class 11) places the ozone layer in the atmosphere's layers; Environment and Sustainable Development (Class 11 economics) and Environment and Natural Resources (Class 12 political science) cover the global environmental agreements.
- Prelims — environment and science: biological magnification of DDT and chlorinated pesticides (1997, 2010), why DDT was banned (1987), persistent ‘forever chemicals’ (2024), ozone and UV (1987), CFCs and ozone depletion (1992), the Montreal Protocol (2015), food chains and decomposers (2013, 2014), a top carnivore's fragility (1992), losing phytoplankton (2012), what an ecosystem is (2015) and e-waste metals (2013).
- GS3 — Environment: solid waste disposal and toxic waste (Mains 2018); pollution, pesticide residues, ozone protection and waste rules. GS2 touches it through local bodies (panchayats, municipalities) that collect and treat waste — the subject of Activity 13.7.
| Word | What it means here |
|---|---|
| Ecosystem | all the interacting organisms in an area together with the non-living parts of their environment |
| Biotic / abiotic | living components / physical factors such as temperature, rainfall, wind, soil and minerals |
| Producer | an organism that makes food (sugar, starch) from inorganic substances using sunlight and chlorophyll — green plants, certain bacteria |
| Consumer | an organism that eats producers directly or other consumers — herbivore, carnivore, omnivore, parasite |
| Decomposer | a microorganism (bacterium, fungus) that breaks dead remains and wastes into simple inorganic substances |
| Trophic level | each step or level of a food chain |
| Biological magnification | progressive build-up of a non-degradable chemical at each higher trophic level |
| Ozone (O₃) | a molecule of three oxygen atoms; poisonous at ground level, a UV shield high in the atmosphere |
| Biodegradable | broken down by biological processes (bacteria, other saprophytes); non-biodegradable substances are not |
Reading NCERT's chapter opening (page 208)A QR code labelled 1064CH15 (from the older numbering, when this was Chapter 15) and a small picture of the Sun shining over the curve of the green-and-blue Earth, beside the title. The opening paragraph notes how often ‘environment’ is heard — in the news, from elders who say it is not what it was, and at global summits of developed and developing countries — and promises to study how its parts interact and how we affect it. The point: the chapter moves from how nature works to what we do to it.
Common confusions
- This reprint teaches no ecological pyramids, productivity or nutrient cycles — those are in the Class 12 Ecosystem chapter. Here only the trophic-level diagram (Figure 13.2) and the 10 per cent rule appear.
- Environment is the broad surroundings; an ecosystem is a defined unit of organisms plus their non-living environment.
- The ozone problem (UV getting through) and global warming (greenhouse gases trapping heat) are different problems; the chapter discusses only the first.
At a glance — the six ideas to carry away
- Ecosystem = all interacting organisms of an area (biotic) + non-living factors (abiotic: temperature, rainfall, wind, soil, minerals). Natural: forest, pond, lake. Human-made: garden, crop field, aquarium.
- Producers: green plants and certain bacteria make food by photosynthesis. Consumers: herbivores, carnivores, omnivores, parasites. Decomposers: bacteria and fungi break dead remains into simple inorganic substances that plants reuse.
- Food chain: each step a trophic level — producers (I), herbivores (II), small carnivores (III), large carnivores (IV). Food web: chains branch because each organism is eaten by two or more kinds.
- Energy: green plants store about 1% of the sunlight on their leaves; about 10% passes to each next level; so chains have 3–4 steps. Flow is unidirectional and shrinks at each level; producers are the most numerous.
- Biological magnification: non-degradable pesticides absorbed from soil and water accumulate progressively up the chain; humans at the top get the most; residues in grain, fruit, meat can't always be washed off.
- Ozone (O₃) high up absorbs Sun's UV (skin cancer); O₂ —UV→ O + O, O + O₂ → O₃. Fell sharply in the 1980s — CFCs; 1987 UNEP agreement froze CFC production at 1986 levels. Garbage: biodegradable (broken down by microbes) vs non-biodegradable (plastics — enzymes are specific).
1. What an ecosystem is made of
In simple wordsLook at a school garden. The grass, roses, frogs, insects and birds are the living cast; the sunshine, rain, wind, soil and minerals are the stage and the weather. The cast and the stage keep affecting each other — a dry month changes which insects appear, and the plants change the soil. Cast plus stage, working together, is an ecosystem.
All organisms — plants, animals, microorganisms and human beings — and their physical surroundings interact and keep a balance in nature. All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem. It therefore has biotic components (living organisms) and abiotic components — physical factors like temperature, rainfall, wind, soil and minerals. A garden is an ecosystem: grasses and trees, flowering plants such as rose, jasmine and sunflower, frogs, insects and birds all interact, and their growth and reproduction are shaped by the abiotic factors. Forests, ponds and lakes are natural ecosystems; gardens and crop fields are human-made (artificial) ones.
Building an aquarium (Activity 13.1). Fish need free space to swim (a large jar will do), water, oxygen and food; an aerator supplies oxygen and food can be bought. Add a few aquatic plants and animals and the tank can become nearly self-sustaining — the plants make food and oxygen, the animals eat, and microbes break down waste. Yet a tank still has to be cleaned, while a pond or lake does not: the tank is tiny and closed, so wastes build up faster than its few decomposers can clear them, whereas a large natural water body has enough decomposers, inflow and space to recycle its own waste. An aquarium is a human-made ecosystem.
Reading NCERT's Activity 13.1 (pages 208–209)Design an aquarium: what do fish need (space, water, oxygen, food); supply oxygen with an aerator and food from the market; add aquatic plants and animals to make it self-sustaining; then ask why it must still be cleaned now and then, and whether ponds and lakes need the same. The point: a human-made ecosystem works on the same parts as a natural one, but a small, closed one cannot fully recycle its own waste.
| Natural ecosystem | Human-made (artificial) ecosystem | |
|---|---|---|
| NCERT's examples | forest, pond, lake | garden, crop field, aquarium |
| Who chooses the species | nature | people (what to plant, which fish to keep) |
| Needs human input? | no — self-sustaining | usually: water, fertiliser, aeration, cleaning, weeding |
| Variety of species | usually many | usually few (a crop field may be one crop) |
Common confusions
- An ecosystem is the organisms together with their physical environment — not the community of organisms alone, and not ‘the inhabited part of the Earth’ (UPSC 2015).
- Human-made ecosystems are still ecosystems: a crop field and an aquarium both count.
- NCERT's abiotic list is temperature, rainfall, wind, soil, minerals; light, water and air are abiotic too.
- Human beings are part of the biotic component, not outside it — ‘we are an integral part of the environment’ (page 212).
Beyond NCERT
- The word ‘ecosystem’ was introduced by the British ecologist A. G. Tansley in 1935.
- Ecosystems range from a drop of pond water to the whole biosphere; many ecologists treat the biosphere as one global ecosystem (Class 12 Ecosystem chapter).
2. Producers, consumers and decomposers
In simple wordsA kitchen, a dining hall and a recycling bin. Plants are the kitchen: they cook food from air, water and sunlight. Animals are the diners, some eating the plants, some eating the diners. Bacteria and fungi are the recycling bin that turns scraps back into raw ingredients for the kitchen. Shut any one down and the whole house stops working.
Organisms can be grouped by how they get their food. Producers make organic compounds such as sugar and starch from inorganic substances, using the radiant energy of the Sun in the presence of chlorophyll: all green plants and certain bacteria that photosynthesise. Consumers depend on producers directly or indirectly — eating producers, or eating other consumers. They are classed as herbivores (cow, deer, grasshopper), carnivores (tiger, frog, snake), omnivores (humans, crows, bears) and parasites (tapeworm, lice, the dodder plant Cuscuta).
When an organism dies, decomposers — microorganisms, bacteria and fungi — break down its dead remains and the waste products of organisms. They turn complex organic substances into simple inorganic substances that go into the soil and are used again by plants. Without them, garbage and dead plants and animals would pile up and the soil would not be replenished naturally. Activity 13.2 asks whether any one group is of ‘primary importance’: producers bring all the energy in, but the groups depend on one another — consumers keep plant and animal numbers in check, and decomposers keep the nutrients moving.
⤵ From a dead leaf back to a living plant
Death and wastea leaf falls, an animal dies, wastes are passed
Decomposers actbacteria and fungi break down the complex organic substances
Simple substancesinorganic substances (minerals) are released into the soil or water
Uptakeplant roots absorb them with water
New foodthe plant uses them, with sunlight, to build new organic matter
Reading NCERT's Activity 13.2 (page 209)In the aquarium: did you avoid adding an animal that would eat the others, and what would have happened if you had? Discuss in groups how producers, consumers and decomposers depend on each other; write the aquatic organisms in order of who eats whom as a chain of at least three steps (the box prints two empty arrows to fill in); and ask whether any one group is of primary importance. The point: feeding relationships make food chains, and the three groups are interdependent — the first item of the chapter's summary.
Common confusions
- Decomposers are bacteria and fungi, not viruses (UPSC 2013). Earthworms help break down dead matter but are usually called detritivores, not decomposers (Class 12 Ecosystem chapter).
- Producers include certain bacteria, not only green plants.
- Parasites are consumers — they take food from a living host.
- Decomposers turn organic into inorganic; producers turn inorganic into organic.
- Every group matters: answer ‘is any group of primary importance?’ with interdependence, not a single winner.
Beyond NCERT
- Some bacteria at deep-sea hydrothermal vents make food by chemosynthesis (chemical energy, not sunlight) — the only ecosystems not run on the Sun.
- Decomposers are also called saprotrophs (or saprophytes, the word NCERT uses on page 214): they digest dead matter outside their bodies and absorb the products.
3. Food chains and trophic levels
In simple wordsA food chain is a relay race of meals: grass hands its energy to a grasshopper, the grasshopper to a frog, the frog to a snake, the snake to an eagle. Each runner's spot in the race is its trophic level. The race always starts with a plant, because only plants (and some bacteria) can take energy from sunlight.
A series of organisms feeding on one another, each at a different biotic level, is a food chain. Each step or level is a trophic level: autotrophs (producers) at the first, fixing solar energy for the heterotrophs; herbivores (primary consumers) at the second; small carnivores (secondary consumers) at the third; and larger carnivores (tertiary consumers) at the fourth. NCERT's Figure 13.1 shows chains in a forest, a grassland and a pond, and Figure 13.2 stacks the four trophic levels with producers at the broad base.
Reading NCERT's Figure 13.1 (page 210)‘Food chain in nature’: three columns of small drawings linked by black arrows — (a) in a forest, trees lead to a deer and then a tiger; (b) in a grassland, grass leads to a grasshopper, a frog, a snake and an eagle; (c) in a pond, a water plant leads through a small aquatic insect and a fish to a heron. The point: chains differ in length — three, five and four organisms — but every one starts with a producer, and the arrow points from the eaten to the eater.
Reading NCERT's Figure 13.2 (page 210)‘Trophic levels’: a triangle cut into four horizontal bands — producers at the wide base, then primary consumers, secondary consumers, and tertiary consumers at the narrow top. The point: the levels are numbered from the producers up, and each level is smaller than the one below it (NCERT adds that producers are the most numerous).
| Trophic level | Also called | Feeds on | Examples |
|---|---|---|---|
| I | producers (autotrophs) | makes its own food from sunlight | grass, trees, algae, phytoplankton |
| II | primary consumers | producers | grasshopper, deer, rabbit, zooplankton |
| III | secondary consumers (small carnivores) | herbivores | frog, small fish, insect-eating birds |
| IV | tertiary consumers (larger carnivores) | small carnivores | snake, large fish, hawk |
Common confusions
- A food chain shows the order in which organisms feed on each other — not how many are eaten, and not relations inside one species' population (UPSC 2013).
- The chain always starts with the producer: diatoms → crustaceans → herrings (UPSC 2014).
- Primary consumer = herbivore; secondary consumer = small carnivore — the consumer numbering is one less than the trophic level number.
- ‘Three or four steps’ and Figure 13.1(b)'s five organisms are not in conflict — chains usually stop at three or four levels, but longer ones exist.
- NCERT's text says the chain was made ‘in Activity 13.4’; it was Activity 13.2 (13.4 is the ozone activity) — a leftover from renumbering.
Beyond NCERT
- Chains that start with living plants are grazing food chains; chains that start with dead matter are detritus food chains — on land more energy flows through the detritus chain (Class 12 Ecosystem chapter).
- One species can sit at more than one trophic level: humans eating rice are primary consumers; eating fish that ate smaller fish, they are at the fourth level.
4. Energy flow and the 10 per cent rule
In simple wordsThink of energy as money passed down a line where each person spends nine-tenths on their own living and passes on only a tenth. Start with ₹1,000 and the fourth person gets ₹1. That is why a forest has millions of plants, thousands of deer and only a handful of tigers — there simply is not enough energy left at the top.
Food is fuel. Autotrophs capture the energy in sunlight and convert it into chemical energy, which passes to heterotrophs and decomposers. Every time energy changes form, some is lost to the environment in forms that cannot be used again. Studies of ecosystems show that: green plants in a terrestrial ecosystem capture about 1% of the energy of the sunlight falling on their leaves and turn it into food energy; when plants are eaten, a great deal of energy is lost as heat, some goes into digestion and work, and the rest into growth and reproduction — on average 10% of the food eaten becomes the eater's own body, available to the next level. Since so little is passed on, food chains generally have only three or four steps; there are generally more individuals at the lower levels, the most being producers.
From the energy flow diagram two things follow. First, the flow of energy is unidirectional: energy captured by autotrophs does not go back to the solar input, and energy passed to herbivores does not come back to the autotrophs. Second, the energy available at each trophic level diminishes progressively. Worked example (our numbers, NCERT's percentages): of 100,000 J of sunlight on the leaves, plants store 1,000 J; herbivores get 100 J, small carnivores 10 J and large carnivores 1 J — just 0.001% of the sunlight; a fifth level would get a tenth of a joule (all asserted in the source file).
Reading NCERT's Figure 13.4 (page 211)‘Diagram showing flow of energy in an ecosystem’: five coloured bars stacked with upward arrows — Sunlight (widest, orange) at the bottom, then Producers, Herbivores, Carnivores and Top Carnivores, each narrower than the one below. The point: energy moves one way, upward, and less is available at each level. The figure shows no heat-loss arrows; the text supplies the losses (heat, digestion, work).
| Step | Energy (J) | Share of sunlight on leaves | Lost before the next level |
|---|---|---|---|
| Sunlight on leaves | 100,000 | 100% | 99,000 not stored as food |
| Producers (I) | 1,000 | 1% | 900 |
| Herbivores (II) | 100 | 0.1% | 90 |
| Small carnivores (III) | 10 | 0.01% | 9 |
| Large carnivores (IV) | 1 | 0.001% | — |
⤵ Following sunlight through a food chain
Capturegreen plants store about 1% of the sunlight on their leaves as food energy
First transferherbivores keep about 10% of what they eat as body mass; the rest is heat, digestion, work
Second transfersmall carnivores keep about 10% of the herbivores' energy
Third transferlarge carnivores keep about 10% again — 0.001% of the original sunlight
Stoptoo little usable energy remains for a further level, so chains have three or four steps
Common confusions
- 1% is what plants capture from the sunlight falling on their leaves; 10% is what passes between consumer levels. Don't swap them.
- Energy flow is one way; nutrients are recycled by decomposers.
- The 10% is an average, not a fixed law of physics.
- Lost energy is not destroyed — it leaves as heat that living things cannot use again.
- NCERT points back to a ‘previous Chapter on Sources of Energy’; that chapter is no longer in this book (the contents end with Chapter 12, Magnetic Effects, then this one) — treat the idea of energy loss on conversion as self-standing.
Beyond NCERT
- The ten per cent rule is usually traced to Raymond Lindeman's 1942 study of trophic dynamics; real transfer efficiencies vary widely between ecosystems.
- The Class 12 Ecosystem chapter puts the capture figure differently: plants capture 2–10% of the photosynthetically active radiation (PAR), which is under half the incident sunlight — consistent with NCERT's ‘about 1%’ here as a round average.
5. Food webs and removing a trophic level
In simple wordsA single food chain is like a one-lane road: block it and nothing moves. A food web is a city's road network: if one road closes, traffic finds another route. Because most animals eat several kinds of food and are eaten by several enemies, nature is a web, and a web survives losses better than a chain.
The length and complexity of food chains vary greatly. Each organism is generally eaten by two or more kinds of organisms, which in turn are eaten by several others, so instead of a straight line the relationships form branching lines — a food web (Figure 13.3). Exercises 4 and 5 ask what happens if all organisms of one trophic level are killed: the level above loses its food and the level below loses its check. Removing producers collapses everything; removing herbivores lets plants grow unchecked and starves carnivores; removing top carnivores lets herbivores multiply and overgraze; removing decomposers stops nutrient return. No level can be removed without some damage, though the effects differ.
Reading NCERT's Figure 13.3 (page 211)‘Food web, consisting of many food chains’: drawings of producers (grass, reeds, shrubs, a tree) along the bottom, with herbivores (insects, rabbit, rodents, goat-like animals), then frog, snake, fox, birds, a peacock and a tiger above, joined by many curving black arrows; blue lines run across the picture separating the rough bands of trophic levels. The point: arrows branch and cross — one producer feeds several animals, one predator eats several prey — so real feeding links form a web.
| Level wiped out | Immediate effect | Knock-on effect |
|---|---|---|
| Producers | no food enters the system | all consumers die out — the most damaging loss |
| Herbivores | plants grow unchecked | carnivores starve and decline |
| Small carnivores | herbivores multiply | plants are overgrazed; top carnivores lose food |
| Top carnivores | their prey multiply | overgrazing; usually the least damaging, but the balance shifts |
| Decomposers | dead matter and waste pile up | soil is not replenished; producers suffer |
Common confusions
- A food web is many interlinked chains, not a longer chain.
- ‘Can any trophic level be removed without damage?’ — no; the damage differs, it is never zero.
- Top carnivores are the most vulnerable level (they depend on everything below) — UPSC 1992 called them the ‘most delicately balanced’ — while removing producers does the most damage to the web.
- If phytoplankton in an ocean were destroyed, food chains would be hit (UPSC 2012) — the producer level is the base of the web.
Beyond NCERT
- The return of wolves to Yellowstone National Park (USA) in 1995 is a famous example of how removing or restoring a top predator changes the numbers of herbivores and plants below — a trophic cascade.
- Many-linked webs are generally more stable than simple ones; that is one reason species-rich ecosystems recover better.
6. Biological magnification
In simple wordsImagine a rice grain with one invisible speck of poison. A rat eats thousands of grains in its life and keeps every speck; a snake eats dozens of rats; a hawk eats many snakes. Energy is burnt away at each step, but the poison is not — it piles up. The animal at the top ends up carrying the most, and in many of our food chains, the animal at the top is us.
Some harmful chemicals enter our bodies through the food chain without our knowing. Pesticides and other chemicals used to protect crops from diseases and pests wash into the soil or into water bodies. From the soil, plants absorb them with water and minerals; from water bodies, aquatic plants and animals take them up — and so they enter the food chain. As these chemicals are not degradable, they accumulate progressively at each trophic level. Human beings, eating at the top of many food chains, accumulate the highest concentrations — biological magnification. That is why food grains such as wheat and rice, vegetables, fruits and even meat contain varying amounts of pesticide residues, which cannot always be removed by washing.
⤵ How a pesticide reaches your plate
Sprayingpesticides are sprayed on crops against pests and diseases
Run-offrain washes them into the soil and into ponds, lakes and rivers
Entryplants absorb them with water and minerals; aquatic plants and animals take them up
Accumulationthey are not broken down, so each eater stores what was in all its food
Top of the chainhumans and birds of prey carry the highest concentrations; washing food does not always remove residues
Reading NCERT's Activity 13.3 (page 212)Newspapers report pesticide levels in ready-made food, and some states have banned such products: debate in groups whether such bans are needed; work out where the pesticides come from and whether other foods carry them too; and discuss how to reduce our intake. The point: biological magnification is a public-health and policy issue, not only a textbook idea — see the positions table below.
| Position on banning pesticide-laden food products (attributed) | What it argues |
|---|---|
| Consumer and public-health advocates | residues above safe limits should keep a product off the shelves; bans and testing protect children and frequent consumers who cannot detect residues themselves |
| Food manufacturers and traders | residues often come from raw materials grown by others; clear, uniform limits, testing and time to comply are fairer than sudden bans on whole products |
| Farmers and agricultural scientists | pesticides protect yields; the answer lies in correct doses and waiting periods before harvest, safer chemicals and integrated pest management rather than blanket bans |
| Regulators | set maximum residue limits, register which pesticides may be used on which crops, test samples, and ban specific chemicals when evidence warrants |
Common confusions
- Energy decreases up the chain; the concentration of non-degradable chemicals increases — opposite directions.
- The organism with the highest concentration is the highest trophic level: hawk in crop → rat → snake → hawk (UPSC 2010); snake among grasshopper, toad, snake, cattle (UPSC 1997).
- Only non-degradable (persistent) chemicals magnify; biodegradable ones are broken down.
- DDT was banned in many western countries because it persists in the environment for a long time (UPSC 1987).
- NCERT says humans ‘occupy the top level in any food chain’ — loosely put: humans are omnivores and are at the top only of the chains that end with them. Safe line: ‘humans, feeding at the top of many food chains, accumulate the most’.
Beyond NCERT
- Bioaccumulation is the build-up of a chemical inside one organism over its life; biomagnification is the rise in concentration from one trophic level to the next.
- Rachel Carson's Silent Spring (1962) drew world attention to DDT's harm to birds; in birds of prey DDT thinned eggshells.
- The Stockholm Convention on Persistent Organic Pollutants (2001) restricts chemicals such as DDT; mercury and some industrial ‘forever chemicals’ (PFAS) also bioaccumulate (UPSC 2024).
7. The ozone layer and its depletion
In simple wordsThe Earth wears sunscreen. Far above our heads, a thin layer of ozone soaks up most of the Sun's harmful ultraviolet rays. Down at ground level the same gas is a poison, but up there it protects every living thing. Certain man-made chemicals used in old fridges drift up and eat holes in that sunscreen — and the world agreed in 1987 to stop making them.
Ozone (O₃) is a molecule of three oxygen atoms. Ordinary oxygen (O₂) is essential for all aerobic life, but ozone is a deadly poison. At the higher levels of the atmosphere, though, it does an essential job: it shields the Earth's surface from ultraviolet (UV) radiation from the Sun, which is highly damaging to organisms — it is known to cause skin cancer in human beings. High up, ozone is made by UV acting on oxygen: higher-energy UV splits some oxygen molecules into free oxygen atoms (O₂ → O + O), and these atoms combine with oxygen molecules to form ozone (O + O₂ → O₃).
The amount of ozone in the atmosphere began to drop sharply in the 1980s. The fall has been linked to synthetic chemicals, chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. In 1987, the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at 1986 levels, and it is now mandatory for manufacturers worldwide to make CFC-free refrigerators. NCERT does not name the agreement: it is the Montreal Protocol on Substances that Deplete the Ozone Layer (1987).
⤵ How ozone is made — and unmade
Splittinghigh-energy UV splits an oxygen molecule into two free oxygen atoms
Joininga free oxygen atom joins an oxygen molecule to form ozone (O3)
Shieldingthe ozone layer absorbs most of the Sun's UV before it reaches the ground
AttackCFCs drift up; UV frees chlorine from them, and chlorine breaks ozone apart (Beyond NCERT)
Responsethe 1987 UNEP agreement froze CFC production at 1986 levels; CFC-free refrigerators became mandatory
Reading NCERT's ozone equations (page 213)Two short equations set in the text: an oxygen molecule with ‘UV’ over the arrow giving O + O, and O + O₂ giving O₃ with ‘(Ozone)’ beneath. The point: ozone high up is made by the very UV it later absorbs, from ordinary oxygen. (The text's ‘moleculer’ is a typo for ‘molecular’.)
Reading NCERT's Activity 13.4 (page 213)Find out from the library, internet or newspapers which chemicals deplete the ozone layer, whether the regulations to control them have reduced the damage, and whether the size of the ozone hole has changed in recent years. The point: the ozone story is a test of whether international rules work — see Beyond NCERT below for the answer scientists give.
Common confusions
- Ozone absorbs ultraviolet rays (UPSC 1987) — not infrared, which greenhouse gases such as CO₂ absorb.
- CFCs deplete ozone (UPSC 1992); sulphur dioxide and nitrogen oxides cause acid rain; CO₂ causes warming.
- The Montreal Protocol (1987) deals with ozone-depleting substances (UPSC 2015); the Kyoto Protocol deals with greenhouse gases.
- Ozone high up protects; ozone at ground level is a pollutant (part of smog).
- NCERT says CFCs were used ‘in fire extinguishers’: the ozone-depleting fire-extinguisher gases were mainly halons, close bromine-containing relatives of CFCs, controlled by the same protocol. Safe line: ‘CFCs (refrigerants, aerosols, foams) and halons (fire extinguishers)’.
- The 1987 agreement first froze production at 1986 levels; later amendments moved to phase-outs.
Why this matters for Prelims and GS3The ozone story is UPSC's favourite example of a successful environmental treaty: a clear scientific cause (CFCs), substitutes available, and binding production controls. In GS3 answers on climate agreements, it is the usual comparison — and its Kigali Amendment (below) links ozone protection to climate change.
Beyond NCERT
- The ozone layer lies mainly in the stratosphere. Scientists of the British Antarctic Survey reported a seasonal ‘ozone hole’ over Antarctica in 1985; very cold polar stratospheric clouds there speed up chlorine's attack (UPSC 2011).
- Molina, Rowland and Crutzen shared the 1995 Nobel Prize in Chemistry for work on how ozone is formed and destroyed.
- The Vienna Convention (1985) was the framework; the Montreal Protocol (1987) set the controls. Its Kigali Amendment (2016) adds a phase-down of HFCs — CFC substitutes that do not harm ozone but are powerful greenhouse gases.
- 16 September, the day the Montreal Protocol was signed in 1987, is observed as the International Day for the Preservation of the Ozone Layer.
8. Biodegradable and non-biodegradable waste
In simple wordsBacteria and fungi are like locksmiths who each carry only a few keys. A banana peel has a lock they can open, so it rots away in weeks. A plastic bag has a lock none of them can open, so it just lies there for years. The keys are enzymes — and each enzyme fits only certain substances.
Every day we throw away materials. Bury them (Activity 13.5) and some — kitchen waste, peels, tea leaves — change form within weeks, while others stay unchanged for a long time. The reason lies in enzymes: they are specific in their action, so a particular enzyme is needed to break down a particular substance — which is why we get no energy from eating coal. In the same way, many human-made materials such as plastics are not broken down by bacteria or other saprophytes. Heat and pressure act on them, but under ordinary environmental conditions they persist for a long time.
Substances broken down by biological processes are biodegradable; substances not broken down this way are non-biodegradable. Non-biodegradable substances may be inert and simply persist, or may harm various members of the ecosystem. Activity 13.6 asks how long such substances last and whether newer plastics sold as ‘biodegradable’ actually avoid harm.
⤵ Activity 13.5 step by step
Collecta day's household waste: kitchen waste, paper, medicine strips and bubble packs, old clothes, broken footwear
Buryin a pit in the school garden, or in an old bucket or flower pot, under at least 15 cm of soil
Keep moistmoisture helps bacteria and fungi work
Observe every 15 daysnote what has changed form and what has not
Classifyfast-changing and slow-changing materials are biodegradable; unchanged ones are non-biodegradable
Reading NCERT's Activity 13.5 (page 213)Collect a day's household waste — spoilt food, peels, used tea leaves, milk packets, empty cartons, waste paper, empty medicine bottles, strips and bubble packs, old and torn clothes, broken footwear; bury it in a pit or a bucket under at least 15 cm of soil; keep it moist and look every 15 days; which materials stay unchanged, which change, which change fastest? The point: a home-made test that sorts waste into biodegradable and non-biodegradable.
Reading NCERT's Activity 13.6 (page 214)Use a library or the internet to learn more about biodegradable and non-biodegradable substances, how long various non-biodegradable ones last, and whether the new plastics sold as ‘biodegradable’ do or do not harm the environment. The point: ‘biodegradable’ on a label is a claim to check, not a guarantee.
| Biodegradable | Non-biodegradable | |
|---|---|---|
| Broken down by | bacteria and other saprophytes (biological processes) | not by biological processes; only physical processes like heat and pressure, slowly |
| Examples | vegetable peels, food, paper, cotton, wood, leather, grass | plastics, polythene, glass, many synthetic materials, some pesticides |
| Effect if dumped | rots: smell, flies, disease, methane in heaps, oxygen loss in water | persists: chokes drains and animals, enters soil and food chains |
| Best handled by | composting, biogas | reduce, reuse, recycle; safe disposal |
Common confusions
- ‘Biodegradable’ means broken down by living organisms, not by sunlight, heat or pressure.
- Leather and wood are biodegradable, though slow; a material that changes slowly is not ‘non-biodegradable’.
- NCERT's exercise 1 asks ‘which group contains only biodegradable items’, but its answer page gives three options — (a), (c) and (d); only (b), with plastic, is wrong (asserted in the source file).
- Biodegradable waste is not harmless in excess — rotting heaps smell, breed flies and pollute water (exercise 8).
- Plastics persist because no microbe has the specific enzyme to digest them — not because they are ‘too hard’.
Beyond NCERT
- Many ‘biodegradable’ or ‘compostable’ plastics break down only in industrial composting conditions; ordinary plastics break into microplastics rather than degrading.
- Rotting organic waste in landfills without air produces methane, a greenhouse gas.
9. Managing the garbage we produce
In simple wordsYour grandparents' tea came in a glass returned to the vendor. Then came throw-away plastic cups — clean, but millions a day into the bin. Clay cups looked like the answer, until someone worked out how much fertile topsoil they would eat. Every waste solution has a cost somewhere; managing garbage means choosing the least harmful option and producing less in the first place.
Heaps of garbage in towns and litter at tourist places show the scale of the problem. NCERT gives three reasons for more waste: improvements in lifestyle mean more material is used and thrown away; changes in attitude make more and more things disposable; and changes in packaging have made much of our waste non-biodegradable. Activities 13.7–13.9 send students to find out how their household and local body (panchayat, municipal corporation, resident welfare association) collect waste and whether they treat biodegradable and non-biodegradable waste separately; how much waste a home and a classroom generate; how local sewage and industrial waste are treated so that soil and water are not polluted; what hazardous materials electronic items contain; and how plastics are recycled and whether recycling itself affects the environment.
⤵ What can happen to a day's garbage
Generate lesscarry cloth bags, avoid disposables, buy less packaging
Segregate at homekeep biodegradable and non-biodegradable waste apart
Biodegradablekitchen and garden waste
Compost or biogasmicrobes return nutrients to the soil
Non-biodegradableplastic, glass, metal, paper for recycling
Reuse or recyclewhat cannot be recycled goes to safe disposal
Special careelectronic and hazardous waste, sewage and industrial effluents need treatment before release
Reading NCERT's Activities 13.7 and 13.8 (page 215)13.7: find out what happens to household waste, how the local body deals with it and whether biodegradable and non-biodegradable wastes are treated separately; calculate the waste a home and a classroom produce in a day and how much of it is biodegradable; suggest ways to handle it. 13.8: find out how sewage in your locality is treated and whether local water bodies are protected from untreated sewage; and how local industries treat their wastes so that soil and water are not polluted. The point: waste management is a local-government job that starts with measuring and separating.
Reading NCERT's ‘Think it over’ box — Disposable cups in trains (page 215)Tea on trains was once served in plastic glasses returned to the vendor; disposable cups were welcomed for hygiene, without thought for millions thrown away daily; clay kulhads were suggested, but making them on a large scale would cost fertile topsoil; now disposable paper cups are used — what are their advantages over plastic ones? The point: every alternative has an environmental cost, so options must be compared, not assumed green.
| Tea cup on a train | For | Against |
|---|---|---|
| Returnable plastic glass | reused many times; little waste | hygiene worries if not washed well |
| Disposable plastic cup | hygienic, cheap, light | non-biodegradable; millions discarded daily |
| Clay kulhad | biodegradable; local livelihood | large-scale making uses up fertile topsoil |
| Disposable paper cup | biodegradable paper, hygienic | uses wood pulp, water and energy; a plastic lining makes some cups hard to recycle (Beyond NCERT) |
Reading NCERT's Activity 13.9 (page 216)Search the internet or library for the hazardous materials that must be handled when disposing of electronic items and how they affect the environment, and for how plastics are recycled and whether recycling has its own environmental impact. The point: e-waste is a special, toxic waste stream, and recycling is not cost-free.
Common confusions
- Segregation (separating at source) comes before treatment; mixed waste is hard to compost or recycle.
- Reduce comes before reuse and recycle — the waste never made needs no handling.
- The kulhad is biodegradable but not impact-free — the topsoil cost is the book's point.
- E-waste hazards are heavy metals such as lead, mercury, cadmium, chromium and beryllium — not pesticides such as heptachlor, and not plutonium (UPSC 2013).
- Environment-friendly practices in exercise 3 — cloth bags, switching off lights and fans, walking to school — are all correct: answer (d).
Why this matters for GS3 (and GS2)Mains 2018 asked about the impediments to disposing of huge quantities of solid waste and how to remove toxic wastes safely. Build the answer on this chapter's ideas — biodegradable vs non-biodegradable, segregation at source, composting, recycling, the cost of each alternative — then add the institutional layer: urban and rural local bodies, waste rules, extended producer responsibility for e-waste and plastics, and citizens' role.
Beyond NCERT
- India's Solid Waste Management Rules, 2016 require waste generators to segregate waste at source; the Plastic Waste Management Rules, 2016 and the E-Waste (Management) Rules place extended producer responsibility (EPR) on producers. EPR first appeared in the e-waste rules of 2011 (UPSC 2019).
- India banned a list of identified single-use plastic items from 1 July 2022.
- The Swachh Bharat Mission (launched 2 October 2014) includes solid waste management in cities and villages.
NCERT's in-text questions — short answers
The chapter has three ‘Questions’ boxes (pages 212, 214 and 216; 7 questions) and asks questions inside Activities 13.1–13.3. Short answers:
Activity 13.1 (page 209): Can we leave the aquarium as such after we set it up? Why does it have to be cleaned once in a while? Do we have to clean ponds or lakes in the same manner?
No. A small, closed tank collects fish waste, leftover food and dead matter faster than its few decomposers can break them down, so the water fouls and must be cleaned. Ponds and lakes are large, have many decomposers and fresh inflow, and recycle their own waste — so they do not need cleaning, unless we overload them with sewage or garbage.
Page 209: Can you give examples for each of these categories of consumers — herbivores, carnivores, omnivores and parasites?
Herbivores: cow, goat, deer, grasshopper. Carnivores: tiger, lion, frog, snake. Omnivores: human, crow, bear, cockroach. Parasites: tapeworm, lice, mosquito (feeding on blood), the dodder plant Cuscuta.
Page 209: What will happen to the garbage, and dead animals and plants, in the absence of decomposers? Will natural replenishment of the soil take place?
They would pile up and not rot, and the nutrients locked in them would never return to the soil. The soil would not be replenished, and plants would eventually run short of minerals.
Activity 13.2 (page 209): Would you consider any one group of organisms to be of primary importance? Why or why not?
Producers are the entry point for energy, so nothing lives without them; but consumers keep populations in check and decomposers return nutrients, so the groups are interdependent — no single group can sustain the ecosystem alone.
Page 212, Q1: What are trophic levels? Give an example of a food chain and state the different trophic levels in it.
Each step or level of a food chain is a trophic level. Example: grass (I, producer) → grasshopper (II, primary consumer, herbivore) → frog (III, secondary consumer) → snake (IV, tertiary consumer) → eagle (V).
Page 212, Q2: What is the role of decomposers in the ecosystem?
Bacteria and fungi break down dead remains and wastes into simple inorganic substances that return to the soil and are reused by plants. They recycle nutrients, replenish the soil and clear away dead matter.
Page 214, Q1: Why are some substances biodegradable and some non-biodegradable?
Breakdown by bacteria and other saprophytes needs specific enzymes. Natural materials (food, paper, cotton) have enzymes that can digest them; many human-made materials such as plastics have none, so they are not broken down biologically.
Page 214, Q2: Give any two ways in which biodegradable substances would affect the environment.
In large heaps they (1) rot and smell, breeding flies and disease germs; (2) in water, their decay uses up dissolved oxygen, harming fish. Handled properly, they (3) return nutrients to the soil as compost or give biogas.
Page 214, Q3: Give any two ways in which non-biodegradable substances would affect the environment.
(1) They persist for long periods, piling up in land and water, choking drains and harming animals that swallow them; (2) persistent chemicals among them can enter food chains and be biologically magnified; they also reduce soil fertility.
Page 216, Q1: What is ozone and how does it affect any ecosystem?
Ozone (O₃) is a molecule of three oxygen atoms. In the higher atmosphere it absorbs the Sun's UV radiation, protecting organisms; if it thins, more UV reaches the surface, causing skin cancer in humans and damaging plants and other organisms — harming the producers on which the ecosystem depends. At ground level ozone itself is poisonous.
Page 216, Q2: How can you help in reducing the problem of waste disposal? Give any two methods.
(1) Reduce and reuse: carry cloth bags, avoid disposables and excess packaging; (2) segregate waste at home — compost kitchen waste and send plastic, paper, glass and metal for recycling.
NCERT exercise — answer outlines
Reading NCERT's ‘What you have learnt’ and Exercises (pages 216–217)Seven summary points: the components of an ecosystem are interdependent; producers make the Sun's energy available to the rest; energy is lost from one trophic level to the next, which limits the number of levels; human activities affect the environment; CFCs have endangered the ozone layer, which protects against UV; waste may be biodegradable or non-biodegradable; and its disposal is causing serious problems. Nine exercises follow (three multiple choice, six descriptive); the book's answer page gives (a), (c), (d) for exercise 1, (b) for 2 and (d) for 3.
1. Which of the following groups contain only biodegradable items? (a) Grass, flowers and leather (b) Grass, wood and plastic (c) Fruit-peels, cake and lime-juice (d) Cake, wood and grass
(a), (c) and (d) — NCERT's own answer page gives all three. Only (b) contains a non-biodegradable item, plastic. Leather is biodegradable, though slowly.
2. Which of the following constitute a food-chain? (a) Grass, wheat and mango (b) Grass, goat and human (c) Goat, cow and elephant (d) Grass, fish and goat
(b) Grass → goat → human: producer, herbivore, consumer of the goat. (a) is three producers, (c) three herbivores, and in (d) fish do not eat grass and goats do not eat fish.
3. Which of the following are environment-friendly practices? (a) Carrying cloth-bags to put purchases in while shopping (b) Switching off unnecessary lights and fans (c) Walking to school instead of getting your mother to drop you on her scooter (d) All of the above
(d) All of the above — each cuts waste or energy use (and the emissions that come with it).
4. What will happen if we kill all the organisms in one trophic level?
The level above loses its food and declines; the level below loses its check and multiplies, so the whole web is unbalanced. If producers are killed, all consumers eventually die; if herbivores, plants grow unchecked and carnivores starve; if top carnivores, herbivores multiply and overgraze.
5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?
Yes, the impact differs: removing producers is catastrophic (no energy enters); removing top carnivores is least damaging but still lets prey multiply; removing decomposers stops nutrient recycling. No level can be removed without damage, because the levels are interdependent and every level is linked through the food web.
6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?
The progressive accumulation of non-biodegradable chemicals (such as some pesticides) at successive trophic levels. Yes — the concentration is lowest in producers and highest at the top (birds of prey, humans), because each consumer eats many organisms from the level below and keeps the chemical. Example: crop → rat → snake → hawk, the hawk carrying most.
7. What are the problems caused by the non-biodegradable wastes that we generate?
They persist for long periods; pile up as litter and in landfills; block drains and cause waterlogging; harm or kill animals that swallow plastic; reduce soil fertility; and toxic ones enter food chains and are biologically magnified. Some, like e-waste, release heavy metals.
8. If all the waste we generate is biodegradable, will this have no impact on the environment?
It will still have an impact. Large amounts take time to decompose; heaps smell, breed flies and germs; in water, decay depletes oxygen and kills fish; unmanaged rotting waste releases gases, including methane (Beyond NCERT). Its impact is smaller and shorter-lived than non-biodegradable waste, and composting turns it into a resource.
9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?
The ozone layer absorbs harmful UV radiation; thinning lets more reach the surface, causing skin cancer in humans and harming other organisms. Steps: in 1987 UNEP forged an agreement to freeze CFC production at 1986 levels (the Montreal Protocol, later strengthened to phase-outs); manufacturers must make CFC-free refrigerators; substitutes are used in refrigeration and aerosols.
What to remember
Numbers
Chapter pages208–217 (Chapter 13, the last of the book)
Activities / figures / exercises9 / 4 / 9
Questions boxes3 boxes, 7 questions (pages 212, 214, 216)
Sunlight stored by green plantsabout 1% of the light falling on their leaves
Energy passed to the next trophic levelabout 10% of the food eaten (average)
Usual length of a food chain3–4 steps
Trophic levelsI producers · II herbivores · III small carnivores · IV large carnivores
Worked energy chain100,000 J → 1,000 → 100 → 10 → 1 J
Share of the sunlight at level IV0.001%
Ozone moleculeO₃ — three oxygen atoms
Ozone began to drop sharply1980s
UNEP agreement on CFCs1987 — production frozen at 1986 levels (the Montreal Protocol)
Activity 13.5at least 15 cm of soil; observe every 15 days
Exercise 1 key (NCERT)(a), (c) and (d) — three options
One-liners
- An ecosystem is all the interacting organisms of an area plus its non-living factors.
- Gardens, crop fields and aquaria are human-made ecosystems.
- Forests, ponds and lakes are natural ecosystems.
- Producers are green plants and certain bacteria.
- Parasites are consumers.
- Decomposers are bacteria and fungi.
- Decomposers turn complex organic substances into simple inorganic ones.
- Each step of a food chain is a trophic level.
- Herbivores are primary consumers at the second trophic level.
- Green plants capture about 1% of the sunlight falling on their leaves.
- About 10% of energy passes from one trophic level to the next.
- Food chains usually have three or four steps.
- Energy flows in one direction only.
- Producers are the most numerous level.
- Branching food chains form a food web.
- Non-degradable chemicals concentrate at higher trophic levels.
- Ozone high in the atmosphere absorbs ultraviolet radiation.
- UV radiation can cause skin cancer.
- CFCs are linked to ozone depletion.
- Enzymes are specific, so plastics are not broken down by microbes.
- Biodegradable substances are broken down by biological processes.
Traps to avoid
- 1% is plant capture of sunlight; 10% is transfer between levels.
- Energy decreases up a food chain; pesticide concentration increases.
- The highest trophic level carries the most pesticide.
- Viruses are not decomposers.
- A food chain shows feeding order, not numbers eaten.
- Ozone absorbs ultraviolet, not infrared.
- Montreal Protocol is about ozone; Kyoto Protocol is about greenhouse gases.
- Ozone protects high up but is a pollutant at ground level.
- Fire-extinguisher ozone depleters were mainly halons, not CFCs.
- Leather and wood are biodegradable, only slow.
- Exercise 1 has three correct options in NCERT's own key.
- Biodegradable waste in excess still harms the environment.
- No trophic level can be removed without damage.
- Pyramids and productivity are not in this chapter.
How to strategise
Prelims — what UPSC asks
- Food chains and biomagnification: the snake carries the most DDT among grasshopper, toad, snake and cattle (1997); the hawk in crop → rat → snake → hawk (2010); DDT banned because it persists (1987); PFAS persist and bioaccumulate (2024); a food chain shows feeding order only (2013); diatoms → crustaceans → herrings (2014); decomposers are fungi and bacteria, not viruses (2013); top carnivore most delicately balanced (1992); losing ocean phytoplankton hits food chains (2012); best description of ‘ecosystem’ (2015).
- Ozone: the ozone layer absorbs ultraviolet rays (1987); CFCs deplete it (1992); the Montreal Protocol controls ozone-depleting substances (2015). Beyond this chapter UPSC has also asked about the Antarctic ozone hole's polar stratospheric clouds (2011) and CFC uses in foams and aerosols (2012) — learn them from the Beyond NCERT notes.
- Waste: e-waste releases beryllium, cadmium, chromium, mercury and lead — not heptachlor or plutonium (2013; weakest link here, built on Activity 13.9). Waste rules (Solid Waste Management Rules 2016, EPR in the 2011 e-waste rules — UPSC 2019) are Beyond NCERT.
- Method: in any biomagnification question, find the highest trophic level among the options; in any ‘protocol’ question, match ozone → Montreal (1987), greenhouse gases → Kyoto; for ‘which is biodegradable’, ask whether a microbe's enzyme can digest it.
- Likely next: bioaccumulation vs biomagnification; microplastics; HFCs and the Kigali Amendment; ground-level ozone as a pollutant; extended producer responsibility; compostable plastics.
Mains — where it appears and how to answer
- GS3 2018 (impediments to disposing of solid waste; removing toxic waste safely): open with the biodegradable/non-biodegradable distinction and why plastics persist (enzyme specificity); then impediments — volume, mixed waste with no segregation, land for landfills, weak collection, informal recycling of e-waste — and remedies: segregation at source, composting and biogas, recycling, EPR, scientific landfills, bioremediation and safe treatment of hazardous waste; end with behaviour change (reduce first).
- GS3 environment answers on pesticides, food safety or ozone: use biological magnification for the mechanism (persistence + food chain), the 10 per cent rule for why top consumers are most exposed, and the Montreal Protocol (1987) as the model of a successful treaty.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Which of the following is a human-made ecosystem according to NCERT?
2. Decomposers in an ecosystem are mainly:
3. In the chain grass → grasshopper → frog → snake → eagle, the frog is a:
4. If producers store 1,000 J and about 10 per cent passes to each next level, the fourth trophic level receives about:
5. A non-degradable pesticide is sprayed on a crop. Crop → rat → snake → hawk. The highest concentration will be found in the:
6. Ozone in the higher atmosphere is important because it:
7. The 1987 agreement forged by UNEP on CFCs:
8. Plastics are not broken down by bacteria mainly because:
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.