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IGCSE · Practice questions · Energy flow

In as light, out as heat.

Six original Cambridge-style questions on the flow of energy through living organisms, from sunlight captured by producers to heat released by respiration.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Flows, never cycles.

01
Recall
[2 marks]

State the principal source of energy for biological systems and name the process that brings this energy into a food chain.

  • The Sun (sunlight / light energy). ✓
  • Photosynthesis by producers (green plants). ✓
02
Explain
[4 marks]

Describe the flow of energy through the food chain grass → rabbit → fox, including the forms of energy involved.

  • Light energy from the Sun is absorbed by chlorophyll in the grass and used in photosynthesis. ✓
  • It is stored as chemical energy in organic compounds such as glucose and starch. ✓
  • Chemical energy passes to the rabbit when it eats the grass, and to the fox when it eats the rabbit. ✓
  • At each level some energy is released by respiration and is eventually transferred to the environment as heat. ✓
03
Explain
[3 marks]

Explain why an ecosystem needs a continuous input of energy from the Sun, whereas it does not need a continuous input of carbon.

  • Energy leaves the ecosystem as heat, which producers cannot capture again. ✓
  • So energy flows through the ecosystem in one direction and must be replaced by sunlight. ✓
  • Carbon atoms are recycled between the air, organisms and decomposers, so the same atoms are used again. ✓
04
Analysis
[3 marks]

A student writes: "When the fox respires, the energy from the rabbit is used up." Identify the error and rewrite the statement correctly.

  • Energy cannot be used up (or destroyed). ✓
  • The chemical energy from the rabbit is transferred during respiration. ✓
  • Correct version: respiration releases the chemical energy, which the fox uses for movement and other processes, and it is eventually transferred to the environment as heat. ✓
05
Data
[3 marks]

A field receives 1 000 000 kJ of light energy per m2 per year. The grass stores 10 000 kJ per m2 per year as chemical energy. (a) Calculate the percentage of light energy stored by the grass. (b) Suggest two reasons why this percentage is so low.

  • (a) 10 000 ÷ 1 000 000 × 100 = 1%. ✓
  • (b) Much light misses the leaves or is reflected, or passes through them. ✓
  • Some wavelengths are not absorbed by chlorophyll, and some captured energy is released by the plant's own respiration as heat (any two). ✓
06
Recall
[2 marks]

Dead leaves fall from a tree. State what happens to the energy they contain.

  • Decomposers (bacteria and fungi) feed on the dead leaves and take in the chemical energy. ✓
  • The decomposers respire, and the energy is transferred to the environment as heat. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.