Respiration: (d) the link reaction and its site in the cell

The idea

The link reaction joins glycolysis to the Krebs cycle and takes place in the mitochondrial matrix. Pyruvate made in the cytoplasm is moved into the mitochondrion. There it is decarboxylated, losing a carbon as carbon dioxide, and dehydrogenated, with the removed hydrogen reducing NAD to reduced NAD. The remaining 2-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A, which then delivers the acetyl group into the Krebs cycle. Per pyruvate the link reaction removes one carbon dioxide and produces one reduced NAD; because glycolysis gives two pyruvate per glucose, the totals per glucose are 2 carbon dioxide and 2 reduced NAD. No ATP is made directly in the link reaction.

What earns the marks

• The link reaction occurs in the mitochondrial matrix.
• Pyruvate is decarboxylated, releasing carbon dioxide (one carbon removed).
• Pyruvate is dehydrogenated, and the hydrogen reduces NAD to form reduced NAD.
• The remaining 2-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A.
• Per glucose, two pyruvate give 2 carbon dioxide and 2 reduced NAD, with no ATP produced directly.

Common mistakes and how to put them right

• Placing the link reaction in the cytoplasm.

  Put it right: Correct it to the mitochondrial matrix.

• Claiming the link reaction produces ATP directly.

  Put it right: It produces reduced NAD and carbon dioxide but no ATP.

• Saying the whole pyruvate enters the Krebs cycle.

  Put it right: Only the 2-carbon acetyl group is passed on via acetyl coenzyme A.

Exam tips

• Name the site (mitochondrial matrix) before describing the chemistry.
• Use the paired terms decarboxylation (loses CO₂) and dehydrogenation (reduces NAD) to structure the answer.
• Scale up to per glucose by doubling the per-pyruvate products.