Chapter Overview & Weightage
Our Environment is Chapter 15 in CBSE Class 10 Science (NCERT). It is the final chapter in the science curriculum and deals with ecosystems, food chains, waste management, and environmental issues — tying together concepts from biology, chemistry, and geography.
This chapter carries 8–10 marks in CBSE Class 10 Science board exams. Food chains/webs, ozone depletion, and biodegradable vs non-biodegradable waste are the most frequently tested topics. A 5-mark question on ecosystem or human impact on environment appears in most board papers.
What this chapter covers:
- Ecosystem components (biotic and abiotic)
- Food chains and food webs
- Trophic levels and energy flow (10% energy law)
- Biodegradable and non-biodegradable substances
- Ozone layer and its depletion
- Managing household and industrial waste
- Harmful effects of chemicals in the food chain (biomagnification)
Key Concepts You Must Know
Ecosystem and Its Components
An ecosystem is the unit of nature where living organisms interact with each other and with their non-living environment.
Biotic components — living: producers, consumers, decomposers
Abiotic components — non-living: temperature, water, sunlight, soil, wind, minerals
Producers (autotrophs): Plants, algae, phytoplankton — make their own food using photosynthesis.
Consumers (heterotrophs): Primary consumers (herbivores), secondary consumers (carnivores eating herbivores), tertiary consumers.
Decomposers: Bacteria and fungi that break down dead organic matter, returning nutrients to the soil.
Food Chains and Food Webs
A food chain shows a linear sequence of energy transfer:
Example: Grass → Grasshopper → Frog → Snake → Eagle
A food web is an interconnected network of food chains — more realistic representation of nature.
Trophic Levels and the 10% Energy Law
Each step in a food chain is a trophic level.
Only about 10% of energy transfers from one trophic level to the next. The rest (~90%) is lost as heat (respiration and metabolic processes).
This is why food chains rarely exceed 4–5 trophic levels — there isn’t enough energy left after 4–5 transfers.
Biomagnification
Biomagnification (biological magnification) — the progressive increase in concentration of a substance (like DDT, mercury) as it moves up the food chain.
The substance is not metabolised or excreted; it accumulates in fatty tissues. Top predators (eagles, humans) accumulate the highest concentrations.
Classic example: DDT sprayed on fields → accumulates in phytoplankton → zooplankton → fish → fish-eating birds → birds show reproductive failure (thin eggshells).
Important Concepts
Energy available at trophic level = 10% of energy at trophic level
If producers fix 10,000 J → primary consumers get 1,000 J → secondary consumers get 100 J → tertiary consumers get 10 J
UV radiation + CFC → chlorine radicals
(regenerates Cl, which destroys more ozone)
One Cl radical can destroy thousands of ozone molecules.
Solved Previous Year Questions
PYQ 1 — Food Chain and Energy
Q: In a food chain: plants → deer → tiger. If plants have 1,000 J of energy, how much energy is available to the tiger? (CBSE 2023)
Solution: Applying 10% law at each step:
Plants: 1,000 J → Deer: 10% of 1,000 = 100 J → Tiger: 10% of 100 = 10 J
The tiger receives 10 J of energy.
PYQ 2 — Biodegradable vs Non-biodegradable
Q: Define biodegradable substances. Give two examples each of biodegradable and non-biodegradable waste. (CBSE — 2 marks)
Solution:
Biodegradable: Substances that are broken down by microorganisms (bacteria, fungi) into simpler compounds.
Examples: Kitchen waste (vegetable peels, food scraps), paper, cotton, wood.
Non-biodegradable: Substances that cannot be broken down by natural processes (or take very long — hundreds of years).
Examples: Plastic, polythene bags, glass, synthetic fibres, DDT, heavy metals.
PYQ 3 — Ozone Depletion
Q: What is the importance of the ozone layer? Name the chemicals responsible for its depletion. (CBSE)
Solution:
Importance: The ozone layer (in the stratosphere, 15–35 km altitude) absorbs harmful ultraviolet (UV) radiation (especially UV-B and UV-C) from the Sun. Without it, UV radiation would reach Earth’s surface, causing: skin cancer, cataracts, immune suppression, DNA damage in organisms, harm to phytoplankton.
Chemicals responsible for depletion: Chlorofluorocarbons (CFCs) — used in refrigerants, air conditioners, aerosol spray cans. Also: methyl bromide, N₂O from fertilisers, methyl chloroform.
The Montreal Protocol (1987) is the international agreement to phase out CFCs.
Difficulty Distribution
| Difficulty | Topic | Marks |
|---|---|---|
| Easy (35%) | Definitions; classify biodegradable/non-biodegradable; 1-2 trophic levels | 1–2 marks |
| Medium (40%) | Energy calculation in food chain; effects of ozone depletion; biomagnification | 2–3 marks |
| Hard (25%) | Long answer on ecosystem components; effects of human activity; draw food web | 4–5 marks |
Expert Strategy
For energy flow questions: practise applying the 10% rule repeatedly. If asked “how many kg of grain to support 1 kg of human via cattle?”— that’s 2 trophic levels above plants: Grain → Cattle → Human. Human gets 10% of cattle’s energy = 10% of 10% of grain = 1% of grain. So 100 kg of grain supports 1 kg of human via cattle. These calculations appear in 3-mark questions.
For the ozone question — always mention: UV radiation, skin cancer, CFCs, Montreal Protocol. These are the keywords examiners look for.
Common Traps
Trap 1 — Confusing greenhouse effect with ozone depletion: These are different problems. Ozone depletion = UV radiation problem (CFCs → ozone destruction). Greenhouse effect = global warming (CO₂, CH₄ → heat trapping). CFCs contribute to BOTH problems, which causes confusion. Examiners exploit this.
Trap 2 — Thinking decomposers are consumers: Decomposers (bacteria, fungi) are not consumers in the traditional sense — they don’t “eat” prey. They absorb nutrients from dead organic matter. In food chain diagrams, they are often shown separately, not as a link in the main chain.
Trap 3 — 10% rule is an average, not exact: The 10% energy transfer is an approximation. The actual value varies (5–15%) depending on the organism and ecosystem. In board answers, use “approximately 10%” not “exactly 10%.”
Trap 4 — Biomagnification vs bioaccumulation: Bioaccumulation is the build-up of a substance within a single organism. Biomagnification is the increase in concentration ACROSS trophic levels (from one organism to another higher in the food chain). Know both terms for the board exam.