Explain how the link reaction connects glycolysis to the Krebs cycle during aerobic cell respiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Pyruvate produced in glycolysis is transported from the cytoplasm into the mitochondrial matrix.
2. In the matrix, each pyruvate is decarboxylated, releasing CO₂ and oxidised to form an acetyl group.
3. The acetyl group combines with coenzyme A to give acetyl‑CoA, which enters the Krebs cycle.
4. During the decarboxylation, NAD⁺ is reduced to NADH, which later feeds electrons into the electron transport chain for ATP synthesis.
2. In the matrix, each pyruvate is decarboxylated, releasing CO₂ and oxidised to form an acetyl group.
3. The acetyl group combines with coenzyme A to give acetyl‑CoA, which enters the Krebs cycle.
4. During the decarboxylation, NAD⁺ is reduced to NADH, which later feeds electrons into the electron transport chain for ATP synthesis.
Examiner tips
- Use the exact terms: ‘transported’, ‘decarboxylated’, ‘acetyl‑CoA’, ‘NADH’.
- Show the sequence: transport → decarboxylation → acetyl‑CoA formation → NADH production.
- Mention the role of NADH in oxidative phosphorylation to link to ATP production.
- Keep the answer concise – 4 points, one sentence each.
Mark scheme (4 marks)
- Pyruvate produced in glycolysis is transported from the cytoplasm into the mitochondrial matrix.
- Pyruvate is decarboxylated, releasing carbon dioxide, and oxidised to form acetyl groups / acetate.
- The acetyl group combines with coenzyme A to form acetyl CoA, which feeds into the Krebs cycle.
- NAD is reduced to NADH during the link reaction, contributing to ATP production via oxidative phosphorylation / the electron transport chain.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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