Explain why oxygen is described as the final electron acceptor in aerobic cell respiration.

IB DP Biology Standard 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)

Oxygen is the final electron acceptor because the electrons are transferred through the electron transport chain on the inner mitochondrial membrane via a series of carrier proteins. At the end of the chain, the final carrier (cytochrome oxidase) passes the electrons to oxygen. Oxygen then combines with the electrons and protons (H⁺) to form water. If oxygen is absent, the electrons cannot be accepted, the chain stalls, the proton gradient cannot be maintained and ATP synthesis by ATP synthase stops, so aerobic respiration cannot continue.

Examiner tips

Use the phrase ‘final electron acceptor’ and mention the electron transport chain and cytochrome oxidase.
Explain the consequence of oxygen absence – chain stalls, no proton gradient, no ATP synthesis.
Show the reaction O₂ + 4e⁻ + 4H⁺ → 2H₂O to link electrons and protons to water formation.
Keep the answer concise, 4 points, no extra words.

Common mistakes

Forgetting to mention the proton gradient or ATP synthase.
Saying oxygen is just a ‘source of energy’ rather than the electron acceptor.
Mixing up anaerobic respiration or not specifying the inner mitochondrial membrane.

Mark scheme (4 marks)

Electrons are passed along the electron transport chain (on the inner mitochondrial membrane) through a series of carrier proteins/molecules.
Oxygen accepts the electrons at the end of the electron transport chain (from the final carrier/cytochrome oxidase).
Oxygen combines with electrons and protons (H⁺) to form water.
If oxygen is absent, electrons cannot be passed along the chain, so the proton gradient cannot be maintained and ATP synthesis via ATP synthase stops / aerobic respiration cannot continue.

Key terms in this question

final electron acceptor