Home › Free notes › Class 10 Science › Our Environment

Our Environment – Class 10 Science notes, important questions and MCQs

NCERT Class 10 Science, chapter 13. Notes, key terms, important questions with answers and MCQs.

Chapter overview

An ecosystem is a community of living things interacting with their non-living surroundings. Energy flows one way through food chains, while human activities create waste and affect the ozone layer.

Ecosystem and food chains

Components: biotic (producers, consumers, decomposers) and abiotic (air, water, soil, temperature).
Food chain: grass → deer → tiger; interconnected chains make a food web.
Only about 10% of energy passes to the next trophic level (10% law), so chains are short.

Harmful effects

Biological magnification: non-biodegradable chemicals (like pesticides) concentrate at higher trophic levels.
Ozone (O₃) in the upper atmosphere blocks UV rays; CFCs deplete it.
Biodegradable waste is broken down by microbes; non-biodegradable waste (plastic, glass) is not.

Key terms

Trophic level: each step of a food chain
Decomposers: microbes that break down dead matter (bacteria, fungi)
Biological magnification: increase of a harmful chemical's concentration along a food chain

Formulas

10% law of energy transfer
Ozone = O₃

Important questions with answers

Why are decomposers important in an ecosystem?2 marks

Decomposers break down dead plants and animals into simple substances that return to the soil, so nutrients are recycled and waste does not pile up.

What is biological magnification? Which organism will have the highest pesticide concentration in a food chain?3 marks

Biological magnification is the increase in concentration of non-biodegradable chemicals such as DDT at each higher trophic level. The top consumer (e.g. humans or eagles) has the highest concentration because it eats many organisms from lower levels.

Explain the 10 per cent law of energy flow and why food chains usually have only 3–4 trophic levels. Also explain how the ozone layer is being depleted.5 marks

Only about 10% of the energy at one trophic level is passed to the next; the rest is used in life processes and lost as heat. Example: 1000 J in plants gives about 100 J to herbivores and 10 J to carnivores. So very little energy is left after 3–4 levels to support more organisms. Ozone (O₃) in the upper atmosphere absorbs harmful UV radiation. Chlorofluorocarbons (CFCs) used in refrigerants and fire extinguishers release chlorine that breaks down ozone, thinning the layer; their production has been limited by international agreements.

Quick revision

Energy flow is unidirectional.
CFCs damage the ozone layer.
Decomposers recycle nutrients.

Exam tips

The highest level of pesticide is found in top consumers (e.g. humans, eagles).
Use separate bins for biodegradable and non-biodegradable waste.

Practice MCQs

Which is a producer?
1. lion
2. deer
3. grass
4. mushroom
Show answer
Answer: (c) grass. Green plants make food.
Which of these is non-biodegradable?
1. paper
2. vegetable peel
3. cotton cloth
4. plastic bag
Show answer
Answer: (d) plastic bag. Plastic is not broken down by microbes.
Ozone layer protects us from
1. infrared rays
2. visible light
3. radio waves
4. ultraviolet rays
Show answer
Answer: (d) ultraviolet rays. UV radiation.
If a plant has 10,000 J of energy, a secondary consumer gets about
1. 1000 J
2. 100 J
3. 10 J
4. 1 J
Show answer
Answer: (b) 100 J. 10% at each step: 10,000 → 1000 → 100.
The chemicals mainly responsible for ozone depletion are
1. CO₂
2. O₂
3. CFCs
4. N₂
Show answer
Answer: (c) CFCs. Chlorofluorocarbons.
Bacteria and fungi in an ecosystem are
1. producers
2. decomposers
3. herbivores
4. carnivores
Show answer
Answer: (b) decomposers. They break down dead matter.
In grass → insect → frog → snake → eagle, the highest pesticide concentration will be in
1. grass
2. eagle
3. frog
4. insect
Show answer
Answer: (b) eagle. Biological magnification.
Food chains usually have only 3–4 trophic levels because
1. animals are few
2. plants grow slowly
3. energy available drops sharply at each level
4. decomposers eat everything
Show answer
Answer: (c) energy available drops sharply at each level. About 90% energy is lost at each step.

All chapters of Class 10 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.