Explain why anaerobic cell respiration produces far less ATP per glucose molecule than 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)

In aerobic respiration the glucose is fully oxidised to CO₂ and H₂O, so the energy released is used to re‑oxidise NADH and FADH₂ in the electron transport chain, creating a large proton gradient that drives ATP synthesis by chemiosmosis – giving about 30–32 ATP per glucose.
In anaerobic respiration the pyruvate is only partially oxidised (to lactate or ethanol), so most of the energy in glucose remains in the end products. NADH is re‑oxidised by reducing pyruvate rather than by the electron transport chain, so ATP is produced only by substrate‑level phosphorylation in glycolysis – a net of 2 ATP per glucose.

Examiner tips

  • Use the word ‘fully oxidised’ for aerobic and ‘partially oxidised’ for anaerobic; mention the electron transport chain and chemiosmosis; state the net ATP numbers; keep it concise.
  • Show the contrast between ATP from oxidative phosphorylation vs substrate‑level phosphorylation.

Common mistakes

  • Confusing the number of ATP produced in glycolysis with the total from aerobic respiration; not mentioning the electron transport chain; using ‘more ATP’ without explaining the mechanism.

Mark scheme (4 marks)

  1. In aerobic respiration, pyruvate (and acetyl groups) are fully oxidised / broken down to CO₂ and water, releasing much more energy for ATP synthesis.
  2. In anaerobic respiration, pyruvate is only partially oxidised / converted to lactate or ethanol and CO₂, so much of the potential chemical energy in glucose remains locked in these products.
  3. Aerobic respiration re-oxidises NADH (and FADH₂) via the electron transport chain, generating a large proton gradient that drives ATP synthesis by chemiosmosis, yielding the majority of ATP.
  4. In anaerobic respiration, the electron transport chain is not used / NADH is re-oxidised by reducing pyruvate instead, so ATP is produced only from substrate-level phosphorylation in glycolysis (net 2 ATP per glucose).

Key terms in this question

anaerobic cell respiration · aerobic cell respiration

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