Key points#
- Substances that are broken down by biological processes, mainly by microorganisms such as bacteria and saprophytes, are biodegradable. Substances that are not broken down this way are non-biodegradable; they may stay in the environment for a long time and harm living things.
- An ecosystem is made up of all the living organisms in an area together with the non-living components of their environment, interacting with each other. Natural ecosystems include forests, ponds and lakes; gardens and crop fields are human-made (artificial) ecosystems.
- Producers (green plants and blue-green algae) make food by photosynthesis. Consumers depend on producers directly or indirectly: herbivores, carnivores, omnivores and parasites. Decomposers (bacteria and fungi) break down dead remains and waste, returning nutrients to the soil.
- A food chain is a series of organisms where each one feeds on the one before it. Food chains usually have three or four trophic levels because the energy available becomes too small after that.
- In nature, food chains are interconnected and form a food web.
- Energy flow in an ecosystem is unidirectional: energy that passes to the next trophic level does not come back to the previous one.
- Green plants capture only about 1% of the sunlight that falls on their leaves and convert it into food energy.
- Ozone (O₃) at higher levels of the atmosphere shields the Earth from the Sun's UV radiation, but ozone is a deadly poison at ground level.
- Our activities change the environment: overuse of disposable items, packaging and synthetic materials increases non-biodegradable waste.
Definitions and key terms#
| Term | Meaning |
|---|---|
| Ecosystem | Living organisms of an area plus the non-living components, interacting together |
| Biotic components | The living parts: plants, animals, microorganisms |
| Abiotic components | The non-living parts: temperature, rainfall, wind, soil, minerals |
| Trophic level | Each step or level of a food chain where energy is transferred |
| Producers | Organisms that make their own food by photosynthesis (first trophic level) |
| Primary consumers | Herbivores that eat producers (second trophic level) |
| Secondary / tertiary consumers | Carnivores that eat herbivores, or other carnivores |
| Decomposers | Microorganisms that break down dead organisms and waste into simpler substances |
| Food web | A network of interconnected food chains |
| Biological magnification | Increase in concentration of harmful non-biodegradable chemicals at each higher trophic level |
| Ozone depletion | Thinning of the ozone layer, mainly due to chlorofluorocarbons (CFCs) |
How ozone forms (as plain-text equations)
- High-energy UV radiation splits oxygen molecules: O₂ → O + O
- Each free oxygen atom combines with an oxygen molecule: O + O₂ → O₃ (ozone)
Food chains and food webs#
Examples of food chains
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| Ecosystem | Food chain (producer → … → top consumer) | Trophic levels |
|---|---|---|
| Grassland | Grass → grasshopper → frog → snake → eagle | 5 |
| Pond | Phytoplankton → zooplankton → small fish → large fish | 4 |
| Forest | Plants → deer → lion | 3 |
| Garden | Plants → caterpillar → bird | 3 |
Why do decomposers matter?
Without decomposers, dead plants and animals would pile up and the nutrients locked in them would never return to the soil. Decomposers keep nutrients cycling so that producers can use them again.
The 10% law: energy flow#
The energy available at each trophic level is roughly one-tenth of the level below it, because most of the energy is used by organisms for their own life processes or lost as heat. This is often called the ten percent law.
Worked example (illustrative numbers)
If producers in an ecosystem store 20,000 J of energy, how much will reach the tertiary consumers?
| Trophic level | Energy available |
|---|---|
| Producers | 20,000 J |
| Primary consumers (10%) | 2,000 J |
| Secondary consumers (10%) | 200 J |
| Tertiary consumers (10%) | 20 J |
This is why food chains are short, and why there are fewer organisms at the higher trophic levels: very little energy is left for a fifth or sixth level.
Energy flow diagram (described)
| Part of the diagram | What to draw and label |
|---|---|
| Base | The Sun with an arrow to producers (label: about 1% of sunlight captured) |
| Steps | Boxes for producers → herbivores → carnivores → top carnivores, each box smaller than the one below |
| Side arrows | From each box, arrows labelled "heat lost / used in life processes" |
| Decomposers | An arrow from every level to decomposers |
| Key label | "Energy flow is unidirectional"; show roughly 10% passing up at each step |
Biological magnification#
Pesticides and other chemicals are used to protect crops. Some are washed into the soil and water bodies and are absorbed by plants and aquatic organisms. Because these chemicals are non-biodegradable, they are not broken down and they accumulate in the bodies of organisms.
At each trophic level, an organism eats many organisms from the level below, so the chemical gets more concentrated as it moves up the food chain. Humans, who are at the top of many food chains, can end up with the highest concentration. This is why food grains, vegetables, fruits and even meat can contain pesticide residues.
Worked example (hypothetical numbers)
A non-biodegradable pesticide is found in a pond at a very low level. Using illustrative numbers, show how its concentration rises.
⇆ Swipe sideways to see the whole table
| Level | Organism | Relative concentration |
|---|---|---|
| Water | – | 1 unit |
| Producer | Algae | 10 units |
| Primary consumer | Small fish | 100 units |
| Secondary consumer | Large fish | 1,000 units |
| Top consumer | Fish-eating bird | 10,000 units |
These numbers are only for understanding the trend. Real concentrations depend on the chemical and the ecosystem. The point to write in the exam is: the concentration increases at every higher trophic level, and the top consumer is affected most.
Ozone layer and its depletion#
- Ozone is a molecule made of three oxygen atoms (O₃). In the upper atmosphere it absorbs harmful UV radiation, which can cause skin cancer in humans.
- The amount of ozone in the atmosphere began to drop sharply in the 1980s. This decrease has been linked to synthetic chemicals such as chlorofluorocarbons (CFCs), which are 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. It is now mandatory for manufacturers to make CFC-free refrigerators.
Managing the garbage we produce#
- Improved lifestyles and changes in attitude have led to more waste, much of it in disposable packaging.
- Separate waste at home into biodegradable (kitchen and garden waste, paper) and non-biodegradable (plastics, glass, metal cans) so each can be handled properly.
- Biodegradable waste can be composted; recyclable materials can be sent for recycling.
- Reduce the use of disposable items; reuse bags, containers and bottles; prefer products with less packaging.
Biodegradable vs non-biodegradable
⇆ Swipe sideways to see the whole table
| Feature | Biodegradable | Non-biodegradable |
|---|---|---|
| Broken down by microorganisms? | Yes | No |
| Examples | Vegetable peels, paper, cotton cloth, leaves, dung | Plastics, polythene, glass, DDT, metal cans |
| Effect if dumped | Decomposes, may smell during decay | Persists for a long time, can enter food chains |
| Best handling | Composting | Reduce, reuse, recycle |
Important questions#
These practice questions are written by our team in the CBSE board style. For the official format, see the 2026-27 CBSE sample paper for Science.
- What is an ecosystem? Name its two main components with two examples each. (2 marks)
- Why are decomposers called the cleaning agents of the environment? (2 marks)
- Why do food chains generally have only three or four trophic levels? (2 marks)
- If 10,000 J of solar energy is stored by plants, how much energy will be available to a peacock in the food chain plants → mice → snakes → peacock? (2 marks) [Answer: plants 10,000 J → mice 1,000 J → snakes 100 J → peacock 10 J.]
- What is biological magnification? Which organisms in a food chain will have the maximum concentration of a non-biodegradable chemical, and why? (3 marks)
- How is ozone formed in the upper atmosphere? Why is the depletion of the ozone layer a cause for concern? What step was taken in 1987 to limit it? (3 marks)
- Give two differences between biodegradable and non-biodegradable substances with two examples each. (3 marks)
- Case-based: a village pond receives water from nearby fields sprayed with a pesticide. Fish-eating birds near the pond start laying eggs with thin shells. Explain the link using biological magnification. (4 marks) For more questions in this style, see our guide to competency based questions for Class 10.
Common mistakes#
- Writing "energy flow is cyclic". Nutrients cycle; energy flows in one direction only.
- Forgetting the starting level in 10% calculations. Count the levels carefully: producers are the first trophic level.
- Saying biological magnification happens with all chemicals. It happens with non-biodegradable chemicals that are not broken down or excreted easily.
- Calling ozone harmful everywhere. Ozone in the upper atmosphere protects us; at ground level it is a poison.
- Missing arrows or labels. In food chains, arrows point from the food to the eater (grass → grasshopper), showing the direction of energy flow.
Next steps#
This chapter is short and scoring, so revise it at least twice. Go back to the Class 10 Science notes hub for other chapters, read the notes of Electricity Class 10 CBSE for the numerical-heavy Physics chapter, and follow the CBSE Class 10 preparation plan for your revision timetable. Then check yourself with chapter-wise practice quizzes.