12.2.4—Link reaction

The link reaction connects glycolysis to the Krebs cycle. For each pyruvate that enters the mitochondrial matrix under aerobic conditions, carbon is removed as carbon dioxide, hydrogen is transferred to NAD, and the remaining two-carbon acetyl group is carried by coenzyme A as acetyl-CoA.

1. Entry: Pyruvate from glycolysis is transported into the mitochondrial matrix when aerobic processing can continue. The link reaction itself is a matrix process.
2. Decarboxylation: Enzymes remove one carbon from pyruvate and release it as carbon dioxide. This converts the three-carbon input into a two-carbon remainder.
3. Oxidation: The remaining carbon compound is dehydrogenated. Hydrogen/electrons are accepted by NAD, forming reduced NAD and storing transferable reducing power.
4. Acetyl-CoA formation: Coenzyme A binds to the two-carbon acetyl group. The product acetyl-CoA is the form that carries this group into the next stage.
5. Bridge: Acetyl-CoA supplies the acetyl group to the Krebs cycle. The cycle details are separate; this card ends at the hand-off.

Per-pyruvate output: one carbon dioxide, one reduced NAD and one acetyl-CoA are formed in the source-supported link-reaction account. If the question changes the basis to one glucose, remember that glycolysis produces two pyruvate, but do not confuse this doubling with a new link-reaction step.

The link reaction is a bridge, not a cycle and not glycolysis. Its defining outputs are carbon dioxide, reduced NAD and acetyl-CoA; it is not the stage that explains the full Krebs-cycle sequence or the final ATP yield.