Explain how acetyl-CoA links the products of glycolysis to the Krebs cycle, including the fate of carbon atoms and the significance of the coenzyme involved.

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)

Pyruvate produced in glycolysis is transported into the mitochondrial matrix where it undergoes the link reaction (pyruvate oxidation). In this step one carbon of the three‑carbon pyruvate is released as CO₂ (decarboxylation) and the remaining two‑carbon acetyl group is transferred to coenzyme A, forming acetyl‑CoA. The acetyl‑CoA then donates its acetyl group to the four‑carbon molecule oxaloacetate, producing the six‑carbon citrate that enters the Krebs cycle. Coenzyme A is released after the transfer and can accept another acetyl group, allowing continuous entry of acetyl units into the cycle. During the link reaction NAD⁺ is reduced to NADH, providing reducing power for the electron transport chain.

Examiner tips

  • Mention the link reaction and mitochondrial matrix
  • State the decarboxylation of one carbon as CO₂
  • Explain the transfer of the acetyl group to oxaloacetate forming citrate
  • Highlight CoA’s role as a carrier and the production of NADH

Common mistakes

  • Confusing the location of the link reaction (cytosol instead of matrix)
  • Failing to note the removal of one carbon as CO₂
  • Omitting the regeneration of CoA or the production of NADH

Mark scheme (4 marks)

  1. Pyruvate (from glycolysis) is converted to acetyl-CoA during pyruvate oxidation / the link reaction, which occurs in the mitochondrial matrix.
  2. One carbon atom is removed as CO₂ (decarboxylation), so the two-carbon acetyl group is transferred to CoA to form acetyl-CoA.
  3. The acetyl group is donated to oxaloacetate (a four-carbon compound) to form citrate (a six-carbon compound), entering the Krebs cycle.
  4. CoA is regenerated / released and is significant because it acts as a carrier molecule that can accept another acetyl group, allowing continuous entry of acetyl groups into the Krebs cycle; also, NAD⁺ is reduced to NADH during pyruvate oxidation, providing reducing power for the electron transport chain.

Key terms in this question

acetyl-CoA · Krebs cycle

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