Class 10 Science Chapter 13 — Our Environment (International Edition)

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NCERT / RBSE Syllabus 2026–27 · Learn by doing · Includes four labelled diagrams

Prepared by: NCERTClasses Team — textbook specialists and experienced teachers.
Based on: NCERT Class 10 Science textbook, Chapter 13, and the Board of Secondary Education, Rajasthan syllabus 2026–27.
Published: 25 September 2026 · Last updated: 25 September 2026

If every bacterium and fungus on Earth vanished overnight, no predator would die, no disease would spread — yet scientists agree the entire ecosystem would collapse within a few years. Why?

And a plastic bag can sit in soil for hundreds of years, while a banana peel disappears in days — where does that difference actually come from?

How to read this — first you think or try it, then comes the explanation:
🔍 Wonder · 🧪 Try it · 📖 Understand · 🔗 Apply · ✅ Check yourself
"Check yourself" has three levels — 1, 2, 3.

Contents

1. Foundation check
2. Ecosystem — biotic and abiotic components
3. Food chains and food webs
4. Energy flow — the 10% law
5. Biological magnification
6. Ozone layer depletion
7. Waste management
8. Your ladder
9. Marking scheme
10. Common mistakes
11. Final test
12. Glossary
13. For parents and teachers
14. Beyond NCERT

Foundation check

1. What is photosynthesis? (Class 9)
2. What is the difference between autotrophs and heterotrophs?
3. Is bacteria unicellular or multicellular?

Answers

1. The process by which plants make their own food (glucose) using sunlight. 2. Autotrophs make their own food (like plants); heterotrophs depend on others (like animals). 3. Unicellular. This chapter examines exactly how these tiny organisms hold an entire ecosystem together.

1. Ecosystem — biotic and abiotic components

🔍 Wonder — A pond, a forest, and a home aquarium — what do all three have in common, and why does an aquarium need constant cleaning and balancing while a pond keeps running on its own?

📖 Understand — All the living organisms in an area (biotic components) together with their physical surroundings — soil, air, water, temperature (abiotic components) — form an ecosystem. It is a functional unit of nature, where both components continuously influence one another.

Type	Examples
Natural ecosystem	Forest, pond, ocean
Human-made ecosystem	Crop field, aquarium, garden

Biotic components are classified by how they obtain food:

Group	Identity	Example
Producers	Make their own food using sunlight	Green plants, algae
Primary consumers (herbivores)	Eat producers directly	Deer, goats, grasshoppers
Secondary consumers (carnivores)	Eat primary consumers	Frogs, snakes
Tertiary consumers	Top predators	Lions, hawks
Decomposers	Break down dead organisms and waste into simpler substances	Bacteria, fungi

🔗 Apply — An aquarium has few decomposers or other natural balancing organisms, so it needs frequent cleaning; a pond has an entire ecosystem present (from producers to decomposers), so it balances itself.

Answering the opening question: without bacteria and fungi, dead plants and animals wouldn't decompose, nutrients wouldn't return to the soil — the soil would gradually become infertile, and the whole ecosystem would suffer.

✅ Check yourself

Level 1 What are the two main components of an ecosystem? — Answer: Biotic and abiotic components.
Level 2 What do decomposers do? — Answer: Break down dead organisms and waste into simpler substances, returning nutrients to the soil.
Level 3 What would an aquarium need to become as self-sufficient as a natural pond? — Answer: Decomposers and a balanced food chain (the right ratio of producers to consumers) — so nutrient recycling happens on its own, without relying on external cleaning.

2. Food chains and food webs

🔍 Wonder — If every lion were removed from a forest, would only the deer population rise, or would the whole forest change?

📖 Understand — A sequence of organisms in which one is eaten by the next, passing energy and nutrients along, is a food chain:

Producer → Primary consumer → Secondary consumer → Tertiary consumer

Each step is called a trophic level. In real nature, organisms rarely eat just one thing — many food chains interlink into a complex food web.

🔗 Apply — This is exactly why removing one species ripples through the whole web — removing lions wouldn't just increase deer numbers; heavier grazing would reduce grass, affecting every other herbivore that depends on it too — the whole balance can shift.

The same principle applies to aquatic ecosystems like a pond — algae (producer) → small fish (primary consumer) → larger fish (secondary consumer) → a fish-eating bird like a heron (tertiary consumer). Whether terrestrial or aquatic, the basic structure of a food chain stays the same — only the organisms change.

Decomposers matter directly to farmers too — manure and crop residue added to soil are broken down by decomposers, enriching it naturally; this same nutrient cycle is what reduces dependence on chemical fertilisers.

✅ Check yourself

Level 1 What is each step of a food chain called? — Answer: A trophic level.
Level 2 How does a food web differ from a food chain? — Answer: A food chain is a single linear sequence; a food web is a complex network of many interconnected chains, much closer to how nature actually works.

3. Energy flow — the 10% law

🔍 Wonder — Why does a food chain like "grass → deer → lion → hawk" almost always stop after 4-5 steps — why don't we ever see a 20-step chain in nature?

📖 Understand — When one organism eats another, not all the energy moves forward. According to ecologist Raymond Lindeman's 10% law, only about 10% of the energy at any trophic level passes to the next — the remaining 90% is lost as heat through respiration, movement, and other biological processes.

Worked example 1: If producers in a grassland hold 100,000 J of energy in total, how much energy reaches the third trophic level (secondary consumers)?
Solution: Producers (level 1) = 100,000 J. Primary consumers (level 2) = 10% of 100,000 = 10,000 J. Secondary consumers (level 3) = 10% of 10,000 = 1,000 J — only 1% of the original energy reaches the third level.

🔗 Apply — This is exactly why food chains rarely exceed 4-5 steps — too little energy remains to practically support another level. It also explains why a vegetarian diet is more energy-efficient — eating directly from producers (trophic level 1) loses far less energy than eating meat, which has already lost energy through an extra trophic step.

✅ Check yourself

Level 1 Who proposed the 10% law? — Answer: Raymond Lindeman.
Level 2 What happens to the remaining 90% of energy? — Answer: It's lost as heat through respiration, movement, and other biological activities.
Level 3 If producers hold 50,000 J of energy, how much reaches the fourth trophic level (tertiary consumers)? — Answer: Level 1=50,000, Level 2=5,000, Level 3=500, Level 4=50 J — just 0.1% of the original energy.

4. Biological magnification

🔍 Wonder — Farmers spray pesticides on their fields to kill insects — so how does that same chemical end up in the highest concentration inside a hawk or a human, far from the field?

📖 Understand — Some harmful chemicals, like pesticides, aren't easily excreted from the body — they accumulate in fatty tissue instead. When an organism eats many smaller organisms, all their stored chemical accumulates in its own body. This repeats at every trophic level, so the concentration keeps increasing. This is called biological magnification.

A real-world example — DDT concentrations measured after spraying a pond:

Trophic level	Organism	DDT concentration (approx.)
Producers (plankton)	Algae	0.04 ppm
Primary consumers	Small fish, clams	0.23 ppm
Secondary consumers	Sea gulls (birds)	Even higher
Tertiary consumers (top)	Predators like hawks	Highest

🔗 Apply — This is exactly the same food chain studied earlier, except here it's the concentration of a chemical that rises, rather than energy that falls — and it's precisely why humans, who often eat from near the top of a food chain, face the greatest risk from these harmful chemicals.

✅ Check yourself

Level 1 In which direction does concentration increase in biological magnification? — Answer: From lower to higher trophic levels.
Level 2 Why do these chemicals keep accumulating in the body? — Answer: Because they aren't easily excreted — they build up in fatty tissue.
Level 3 Why are humans especially affected by this problem? — Answer: Because humans often eat from the top trophic levels of a food chain (large fish, meat), where biological magnification has already concentrated harmful chemicals to their highest levels.

5. Ozone layer depletion

📖 Understand — Ozone (O₃) gas in the upper atmosphere absorbs harmful ultraviolet (UV) radiation from the sun, keeping life on Earth safe. Chemicals such as chlorofluorocarbons (CFCs) — once widely used in refrigerators, air conditioners, and sprays — reach the atmosphere and break down ozone molecules, thinning this protective layer.

Effect: a weakened ozone layer lets more UV radiation reach the surface — raising the risk of skin cancer and cataracts. UV radiation can also damage crop yields and marine plankton (which form the base of many aquatic food chains) — the impact isn't limited to humans, it spreads through the whole ecosystem.

Interestingly, ozone depletion and global warming, though often confused, are driven by largely different mechanisms — yet some CFC substitutes turned out to be potent greenhouse gases themselves, which is why the more recent Kigali Amendment to the Montreal Protocol specifically targets phasing those substitutes down too.

🔗 Apply — This exact concern led to the 1987 Montreal Protocol, an international agreement in which countries worldwide committed to phasing out CFCs and similar chemicals.

✅ Check yourself

Level 1 What does the ozone layer protect us from? — Answer: Harmful ultraviolet radiation from the sun.
Level 2 Which chemical group is the main cause of ozone depletion? — Answer: Chlorofluorocarbons (CFCs).

6. Waste management

🧪 Try it — List everything you threw in the bin today. Now think — which of these will rot away, and which will remain unchanged for years?

📖 Understand — Waste comes in two kinds:

Type	Identity	Example
Biodegradable	Naturally broken down by microorganisms	Fruit/vegetable peels, paper, dung
Non-biodegradable	Microorganisms can't break it down; remains for years to decades	Plastic, metal, glass

The three Rs — the key to reducing waste:

• Reduce — generate less waste from the start, like avoiding unnecessary packaging
• Reuse — use an item more than once (like refilling a glass bottle)
• Recycle — process old materials (paper, plastic, metal) into new products

🔗 Apply — This answers the opening question: microorganisms can easily break down the substances in a banana peel (biodegradable), while plastic's chemical structure is such that ordinary microorganisms simply can't break it down (non-biodegradable) — which is exactly why it persists in the environment for decades.

E-waste — old mobile phones, computers, and other electronics are a newer kind of non-biodegradable problem. They contain some valuable metals (like gold, copper) alongside some harmful substances (like lead) — which is why they should go to dedicated e-waste recycling centres rather than ordinary bins.

✅ Check yourself

Level 1 Is plastic biodegradable or non-biodegradable? — Answer: Non-biodegradable.
Level 2 What do the three Rs stand for? — Answer: Reduce, Reuse, Recycle.

Your ladder

Level	Total	If you got this many right	You are
Level 1	11	9+	Familiar
Level 2	11	8+	Proficient — board-exam ready
Level 3	5	3+	Skilled

Marking scheme

(An example of a typical examiner's approach — not an official scheme.)

Question (5 marks): What is the 10% law? Based on it, explain why trophic levels in a food chain are limited. Illustrate with a calculation.

Definition of the law. 1 mark
Explanation of where the 90% goes. 1 mark
Logical reasoning for why trophic levels are limited. 1 mark
Correct numerical illustration. 2 marks

Common mistakes

Common mistake	Correct fact
"The 10% law means 10% of energy is lost"	The reverse — only 10% passes on; 90% is lost.
"Energy increases in biological magnification"	Not energy — the concentration of a harmful chemical increases; energy still decreases at every level as usual.
"All biodegradable substances vanish instantly"	No — biodegradation still takes time (days to weeks); it just happens, unlike with plastic.
"Ozone depletion and global warming are the same problem"	Different issues — ozone depletion relates to UV radiation; global warming to greenhouse gases and rising temperature.
"A food web and a food chain are the same thing"	A food chain is one linear link; a food web is a network of many interconnected chains.

Final test — mixed

9+ correct = chapter mastered.

1. What are the two main components of an ecosystem?
2. What do decomposers do?
3. What is each step of a food chain called?
4. Who proposed the 10% law?
5. What happens to the remaining 90% of energy?
6. In which direction does concentration increase in biological magnification?
7. Which chemical group is the main cause of ozone depletion?
8. Is plastic biodegradable or non-biodegradable?
9. What do the three Rs stand for?
10. (Chapter 12) Which device's principle is Fleming's left-hand rule the basis of?
11. (Chapter 11) State Ohm's law's formula.

Answers — 1. Biotic and abiotic components | 2. Break down dead organisms into simpler substances | 3. A trophic level | 4. Raymond Lindeman | 5. Lost as heat | 6. Lower to higher trophic levels | 7. Chlorofluorocarbons (CFCs) | 8. Non-biodegradable | 9. Reduce, Reuse, Recycle | 10. Electric motor | 11. V=IR

Glossary

English	Hindi	English	Hindi
Ecosystem	पारितंत्र	10% Law	दस प्रतिशत नियम
Biotic components	जैविक घटक	Biological magnification	जैविक आवर्धन
Abiotic components	अजैविक घटक	Ozone depletion	ओज़ोन क्षरण
Decomposers	अपघटक	Biodegradable	अपघटनीय
Food chain	आहार श्रृंखला	Non-biodegradable	अनअपघटनीय
Trophic level	पोषण स्तर	Food web	आहार जाल

For parents and teachers

Five-minute check: Explain the 10% law and work through one numerical example together. Ask the difference between biological magnification and energy loss. Have them sort today's household waste into biodegradable and non-biodegradable.

To try at home: keep biodegradable and non-biodegradable waste separated for a week and observe — a practical, completely safe activity.

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Beyond NCERT — For Curious Readers

This section goes beyond the RBSE/NCERT Class 10 syllabus and is not required for the board exam. It draws only on government and internationally reputed university sources, for curious readers who want to see where this chapter leads.

The book that shook the world

In 1962, American scientist Rachel Carson published Silent Spring, laying out in detail how the biological magnification of pesticides like DDT was devastating bird populations and ecosystems. Historians widely credit this book with helping to found the modern environmental movement, and it directly contributed to DDT's agricultural ban in several countries in the years that followed.

The Antarctic ozone hole

In 1985, scientists discovered a large "hole" — a region of severely thinned ozone — over Antarctica. This discovery was a direct trigger for the Montreal Protocol and similar international agreements. According to United Nations Environment Programme reports, this hole has been gradually shrinking since CFC use declined — clear evidence that global cooperation can genuinely reverse environmental damage.

India's Swachh Bharat Mission and plastic ban

India's Swachh Bharat Mission (Clean India Mission) is a major national campaign promoting waste management and sanitation. Alongside it, the Ministry of Environment, Forest and Climate Change has banned certain single-use plastic items — a concrete policy response to exactly the "non-biodegradable waste" problem this chapter covers.

Mercury — another real case of biological magnification

Besides DDT, mercury is a well-documented example of biological magnification in marine food chains — its concentration rises many times over from small fish up to large predatory fish like tuna and shark. This is precisely why health agencies commonly advise pregnant women to limit consumption of certain large ocean fish.

Sources for this section

• ncert.nic.in — NCERT, Government of India
• ePathshala — Ministry of Education, Government of India
• moef.gov.in — Ministry of Environment, Forest and Climate Change, Government of India

Note: nothing from these sources has been copied into this text; all statements above are written in the author's own words for general educational understanding, and are not medical or professional advice.

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Continue reading
← Previous: Chapter 12 — Magnetic Effects of Electric Current (6 marks)
→ This is the final chapter of the Class 10 Science 2026-27 syllabus.

Related material

• Chapter 12 — Magnetic Effects of Electric Current (English)
• Chapter 11 — Electricity (English)
• Class 10 Science Mega Test — 200 MCQs

Based on the Class 10 Science (Code 07) syllabus 2026–2027 of the Board of Secondary Education, Rajasthan, Ajmer — Chapter 13, marks weightage 5. Textbook: Science, NCERT. All diagrams are original.