Explain why lactate fermentation allows glycolysis to continue during intense exercise despite the absence of sufficient oxygen.

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).

During intense exercise, skeletal muscle cells can experience a temporary shortage of oxygen relative to their energy demands. Despite this, ATP production continues for a short period through anaerobic pathways.

Model answer (4 marks)

Glycolysis needs NAD⁺ as the electron acceptor; each glucose gives two NADH.
When O₂ is scarce the ETC cannot re‑oxidise NADH, so NAD⁺ levels fall.
Lactate dehydrogenase converts pyruvate to lactate, oxidising NADH to NAD⁺.
The regenerated NAD⁺ keeps glycolysis running, giving a short‑term net gain of 2 ATP per glucose.

Examiner tips

  • State the role of NAD⁺ in glycolysis. Explain why the ETC cannot regenerate NAD⁺ when O₂ is low. Show how lactate dehydrogenase restores NAD⁺. Mention the resulting ATP yield.
  • common_mistakes
  • :
  • Confusing lactate with pyruvate. Saying the ETC works normally. Forgetting that only 2 ATP per glucose are produced.

Mark scheme (4 marks)

  1. Glycolysis requires NAD⁺ as an electron/hydrogen acceptor to oxidise glucose (or glyceraldehyde-3-phosphate), and NAD⁺ is continually consumed (reduced to NADH) during glycolysis.
  2. When oxygen is insufficient, the electron transport chain cannot accept electrons from NADH, so NADH cannot be reoxidised to NAD⁺ by oxidative phosphorylation.
  3. Lactate dehydrogenase converts pyruvate to lactate, simultaneously oxidising NADH back to NAD⁺ (regenerating NAD⁺).
  4. The regenerated NAD⁺ allows glycolysis to continue, sustaining a small but rapid net yield of 2 ATP per glucose molecule.

Key terms in this question

lactate fermentation · glycolysis

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