Describe how the carbon cycle returns carbon dioxide to the atmosphere, referring to the roles of living organisms and combustion.

OCR GCSE Biology A: Gateway Science (J247) — B4.1 Ecosystems · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Peat bogs store vast amounts of carbon in partially decomposed plant material. In some countries, peat is drained and burnt as a fuel, while in others it is left undisturbed. Scientists are concerned about the impact of disturbing peat bogs on atmospheric carbon dioxide levels.

Model answer (4 marks)

1. Respiration by living organisms releases CO₂ into the atmosphere.
2. Decomposers and microorganisms break down dead organic matter, releasing CO₂.
3. Combustion of organic matter, fossil fuels and peat releases CO₂.
4. Photosynthesis by producers removes CO₂ from the atmosphere.

Examiner tips

Use the exact terms: respiration, decomposers, combustion, photosynthesis. Show the cycle: living organisms → CO₂ → atmosphere → plants → CO₂ removed. Keep each point short and to the point.
Include the word "atmosphere" in each relevant point to show the return to the air.

Common mistakes

Confusing photosynthesis with respiration. Missing the role of decomposers. Using vague phrases like "break down" without specifying microorganisms.

Mark scheme (4 marks)

Respiration by living organisms releases carbon dioxide into the atmosphere
Decomposers / microorganisms break down dead organic matter, releasing carbon dioxide
Combustion of organic matter / fossil fuels / peat releases carbon dioxide
Photosynthesis removes carbon dioxide from the atmosphere (by producers / plants)

Key terms in this question

carbon cycle · combustion