Explain how oxygen acts as the final electron acceptor in aerobic cell respiration and why its absence prevents continued ATP production via oxidative phosphorylation.

IB DP Biology Higher Level (2023 syllabus) — C1.2 Cell respiration · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Electrons are transferred through the ETC by carrier proteins, losing energy at each step.
Oxygen accepts the final electrons (and protons) at the end of the chain, forming water.
If oxygen is absent, electrons cannot leave the ETC; carriers remain reduced and the chain is blocked.
Without electron flow the proton gradient cannot be maintained, so ATP synthase cannot produce ATP by oxidative phosphorylation.

Examiner tips

  • Use the word ‘accepts’ to show oxygen’s role as final electron acceptor. Show the chain blockage and loss of proton gradient. Mention water formation as evidence of oxygen’s role. Keep answer concise to fit 4 marks.

Common mistakes

  • Forgetting to mention protons/H⁺ are also taken up by oxygen. Stating that ATP production stops without oxygen but not explaining the proton gradient. Using vague terms like ‘stop’ instead of ‘block’ the ETC.

Mark scheme (4 marks)

  1. Electrons pass along the electron transport chain (ETC) through carrier proteins, losing energy at each step.
  2. Oxygen accepts electrons (and protons/H⁺) at the end of the ETC, forming water.
  3. Without oxygen, electrons accumulate / cannot leave the ETC, so the chain becomes blocked / reduced carriers cannot be re-oxidised.
  4. Because the proton gradient across the inner mitochondrial membrane cannot be maintained, ATP synthase cannot synthesise ATP via oxidative phosphorylation.

Key terms in this question

oxidative phosphorylation

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