Explain how the carbon cycle returns carbon dioxide to the atmosphere after a plant dies and is broken down.

Edexcel GCSE Biology (1BI0) — 9.1 Ecosystems · Explain · 4 marks · View as Markdown

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

When plants die, their bodies contain carbon compounds built up during photosynthesis. These carbon compounds must be broken down before the carbon can re-enter the atmosphere.

Model answer (4 marks)

Decomposers break down the dead plant material, releasing enzymes that catalyse the conversion of complex organic molecules into simpler compounds. The decomposers (and other organisms that feed on the dead material) then respire these carbon compounds, producing carbon dioxide as a waste product. The CO₂ is released into the atmosphere, completing the cycle.

Examiner tips

Use the word "decomposers" first to show understanding of the key players.
Show the sequence: decomposition → enzyme action → respiration → CO₂ release.
Include the word "respire" to link to the final release of CO₂.
Mention that CO₂ returns to the atmosphere to finish the cycle.

Common mistakes

Forgetting to mention decomposers or enzymes.
Using vague terms like "break down" without linking to respiration.
Not stating that CO₂ is released into the atmosphere.

Mark scheme (4 marks)

Decomposers break down the dead plant material / dead organic matter
Decomposers release enzymes that catalyse the breakdown of complex organic molecules into simpler organic matter / compounds
The decomposers (and other organisms that feed on the dead material) respire the carbon compounds
Respiration releases carbon dioxide back into the atmosphere

Key terms in this question

carbon cycle