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Our Environment is the last chapter in the Class 10 science book and often the last one students revise. It is short, and it has one small calculation that appears in many papers: the 10 per cent rule. This post works through that calculation, explains biological magnification and the ozone layer with current figures, covers waste, lists the common mistakes and gives a quick practice routine.
What does the syllabus include?
The CBSE Class X Science syllabus for 2026-27 lists, under Unit V, Natural Resources (5 marks): "Eco-system, Environmental problems, Ozone depletion, waste production and their solutions. Biodegradable and non-biodegradable substances." The content below is from Chapter 13 of the NCERT Class 10 Science textbook. Check the current syllabus for your year.
What is a food chain, and what are trophic levels?
An ecosystem has living parts (producers, consumers, decomposers) and non-living parts (such as temperature, rainfall, soil and minerals). A food chain is a series of organisms where each eats the one before. Each step is a trophic level: producers first, then herbivores (primary consumers), then small carnivores (secondary consumers), then large carnivores (tertiary consumers). Decomposers such as bacteria and fungi break down dead remains and return materials to the soil.
In nature, most organisms are eaten by more than one kind of organism, so the real picture is a food web of branching chains.
How much energy moves up each level?
NCERT gives two figures. Green plants in a land ecosystem capture about 1 per cent of the sunlight falling on their leaves as food energy. When an organism is eaten, an average of 10 per cent of the food eaten becomes the eater's body and is available to the next level; the rest is lost, mostly as heat.
Worked example. Sunlight falling on a field delivers 1,000,000 J. How much energy reaches a snake in the chain grass → grasshopper → frog → snake?
Grass (producer, 1 per cent of sunlight): 1,000,000 × 0.01 = 10,000 J.
Grasshopper (10 per cent of grass): 1,000 J.
Frog: 100 J.
Snake: 10 J.
Only 10 J of the original million reaches the snake. That is why NCERT says food chains usually have only three or four steps, why there are more producers than anything else, and why energy flow is one-way: it does not return from the snake to the grass.
What is biological magnification?
Some pesticides and other chemicals are not broken down. Plants absorb them from soil and water, and they build up at each trophic level, because every organism eats many from the level below. NCERT calls this biological magnification. Humans are at the top of many food chains, so these chemicals can reach their highest concentrations in us; NCERT notes that grains, vegetables, fruit and meat carry varying pesticide residues that washing cannot always remove.
How is the ozone layer formed, and is it recovering?
High in the atmosphere, ultraviolet radiation splits oxygen molecules (O₂ → O + O), and the free oxygen atoms join other oxygen molecules to form ozone (O + O₂ → O₃). This ozone layer shields the Earth from UV radiation, which can cause skin cancer. At ground level, NCERT notes, ozone is a deadly poison, so ozone is useful only where it is.
Ozone levels began to fall sharply in the 1980s, linked to chlorofluorocarbons (CFCs) used as refrigerants and in fire extinguishers. The Montreal Protocol, signed in 1987 and in force from 1989, phases out the production and use of ozone-depleting chemicals. The four-yearly report of the Protocol's Scientific Assessment Panel, as reported by the WMO, found that nearly 99 per cent of banned ozone-depleting substances have been phased out. If current policies stay in place, the layer is expected to return to 1980 values by around 2066 over the Antarctic, 2045 over the Arctic and 2040 for the rest of the world.
What is the difference between biodegradable and non-biodegradable waste?
Biodegradable substances, such as vegetable peels and paper, are broken down by bacteria and other decomposers. Non-biodegradable substances, such as many plastics, are not, so they stay in the environment and can harm ecosystems. NCERT's case study on train cups is a good discussion answer: plastic cups were replaced by clay kulhads, but making kulhads on a large scale would remove fertile topsoil, so paper cups followed. Every solution has costs, and a good answer names them.
Which mistakes come up most often?
Reversing food chain arrows.
Applying 10 per cent to sunlight. Plants capture about 1 per cent of sunlight; the 10 per cent rule applies between trophic levels.
Saying energy is recycled. Nutrients are recycled by decomposers; energy flows one way and is lost as heat.
Saying all ozone is good. It protects us in the upper atmosphere but is toxic at ground level.
Food webs need careful reasoning. Griffiths and Grant (1985) tested 200 high-school students and found a clear learning sequence leading up to the hardest skill: working out how a change in one population affects another that is in the same web but not the same chain. They also catalogued specific misconceptions and how often each appeared.
A practice routine for this chapter
Draw a five-organism food web from memory, then trace two separate chains through it.
Do three 10 per cent calculations with different starting energies.
Write the ozone formation equations and three sentences on the Montreal Protocol.
Sort a list of 20 household items into biodegradable and non-biodegradable.
If you want a short plan for your remaining chapters, Fermi's AI tutor works out what you already know and builds a plan for the session. For spacing your revision across the term, see our spaced revision schedule.
Before you close the book, work out how much energy reaches a hawk if grass receives 50,000 J of food energy in a four-level chain.
