A wildlife organisation is studying the carbon cycle in a woodland ecosystem. Explain how respiration and decomposition together return carbon dioxide to the atmosphere from the bodies of dead organisms.

OCR A-Level Biology B — Advancing Biology (H422) — 4.2 Respiration · Explain · 5 marks · View as Markdown

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

When organisms in a woodland die, their bodies contain carbon stored as complex organic molecules. Microorganisms and other decomposers break down this material. The carbon cycle describes how carbon moves between living organisms and the environment.

Model answer (5 marks)

Decomposers such as bacteria and fungi feed on the dead organic matter and break it down into simpler organic compounds.

During decomposition the complex molecules are converted into sugars and other small molecules, releasing carbon compounds in the process.

The decomposers then respire, using the glucose produced from the decomposed material as an energy source.

Respiration by the decomposers produces carbon dioxide, which is released into the atmosphere.

The CO₂ returned to the air can be taken up by plants during photosynthesis, completing the carbon cycle.

Examiner tips

  • Use the word ‘decomposers’ early to show understanding of the role of microorganisms. Include the step that decomposition converts complex molecules into simpler ones. Show that decomposers respire and release CO₂. Mention the final link to photosynthesis to demonstrate the cycle.
  • common_mistakes
  • :
  • Confusing decomposition with photosynthesis or respiration alone. Omitting the step that decomposers break down complex molecules into simpler ones. Failing to state that the CO₂ is released into the atmosphere.

Mark scheme (5 marks)

  1. Decomposers (microorganisms) feed on / break down dead organic matter
  2. Decomposition converts complex organic molecules into simpler organic matter / releases carbon compounds
  3. The decomposers (and other organisms) respire using glucose from the decomposed material
  4. Respiration releases carbon dioxide into the atmosphere / air
  5. This carbon dioxide can then be taken up by plants for photosynthesis, completing the carbon cycle

Key terms in this question

respiration · decomposition · carbon cycle

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