Explain the role of decomposers in the carbon cycle.

AQA A-Level Biology (7402) — 3.5.4 Nutrient Cycles · Explain · 5 marks · View as Markdown

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

Dead organisms and waste materials contain carbon compounds. These materials must be broken down before carbon can be returned to the atmosphere.

Model answer (5 marks)

Decomposers such as bacteria and fungi digest dead organisms and waste material. They release extracellular enzymes that break down complex organic compounds into simpler substances. During this process carbon is released as carbon dioxide, which the decomposers then respire. The CO₂ is returned to the atmosphere, making carbon available again for photosynthesis.

Examiner tips

Use the term ‘decomposers’ and specify bacteria and fungi; mention extracellular digestion and respiration; link CO₂ release to the atmosphere and photosynthesis.
Show the causal chain: digestion → CO₂ release → atmosphere → photosynthesis.
Use correct UK spelling (e.g., ‘decomposers’, ‘respiration’).

Common mistakes

Confusing decomposers with producers; omitting the role of enzymes; failing to mention CO₂ release or its return to the atmosphere.

Mark scheme (5 marks)

Decomposers (bacteria and fungi) break down / digest dead organisms and waste material
Decomposers release enzymes to break down organic compounds (extracellular digestion)
Carbon compounds in dead material are broken down into simpler substances / carbon is released as carbon dioxide
This release of carbon dioxide occurs through respiration by the decomposers
This returns carbon (as carbon dioxide) to the atmosphere, making it available for photosynthesis

Key terms in this question

decomposers · carbon cycle