Biological magnification

Farmers use pesticides and other chemicals to protect crops. Some of these wash into the soil or into ponds and rivers. Plants take them up from the soil with water and minerals. Water plants and animals take them up from the water. This is one way they enter a food chain.

These chemicals do not break down. So they collect a little more at each trophic level. Human beings sit at the top of any food chain, so the most collects in our bodies. That is why grains, vegetables, fruits and even meat carry pesticide residues. Washing cannot always remove them.

A small example

Take a pond chain of 4 steps: algae, then insect larvae, then small fish, then large fish. A pesticide washes into the pond. The algae take up some. Each larva eats many algae, and each fish eats many larvae. The chemical is passed up and kept at every step. So the large fish at level 4 holds the most.

Where marks go

Question 14 of CBSE's 2026-27 Science sample paper, part C, asks which level would hold the most pesticide washed in from farms, and why. The board's Science marking scheme for 2026-27 answers: the large fish, due to biological magnification.

So the answer has two parts, the organism and the reason. Naming the fish without the term answers only the first. See how to write answers that earn full marks.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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