Link reaction and Krebs cycleEdexcel International A Level Biology: Revision notes

Glycolysis in the cytoplasm converts glucose to pyruvate. In aerobic conditions, pyruvate is actively transported into a mitochondrion, where the link reaction and the Krebs cycle take place in the matrix.

Together they complete the oxidation of glucose:

• the carbon is removed as carbon dioxide (decarboxylation)
• the hydrogen is removed and used to reduce coenzymes (dehydrogenation)
• a little ATP is made by substrate-level phosphorylation

The reduced coenzymes then pass to the electron transport chain on the inner mitochondrial membrane.

The link reaction connects glycolysis to the Krebs cycle. For each pyruvate (3C):

• decarboxylation: one carbon is removed as carbon dioxide
• dehydrogenation: hydrogen is removed and accepted by NAD, forming reduced NAD
• the remaining two-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A

No ATP is made. Because one glucose gives two pyruvate, the link reaction gives 2 CO₂ and 2 reduced NAD per glucose.

In the Krebs cycle the acetyl group from acetyl coenzyme A combines with a four-carbon compound to form a six-carbon compound. The cycle then goes through a series of reactions in which this is converted back to the four-carbon compound (names of the intermediates are not required).

In each turn:

• 2 carbon dioxide are released by decarboxylation
• 3 reduced NAD and 1 reduced FAD are made by dehydrogenation
• 1 ATP is made by substrate-level phosphorylation

The four-carbon compound is regenerated, so the cycle can continue to accept acetyl groups.

Each glucose gives two pyruvate, so the link reaction occurs twice and the Krebs cycle turns twice.

• Link reaction: 2 CO₂, 2 reduced NAD
• Krebs cycle: 4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP
• Glycolysis: 2 reduced NAD, net 2 ATP

Worked example: 0.0020 mol pyruvate gives 3 × 0.0020 = 0.0060 mol CO₂, which is 0.0060 × 24 000 = 144 cm³ at room temperature and pressure.

Neither the link reaction nor the Krebs cycle uses oxygen directly. However, they need a supply of oxidised NAD and FAD to accept hydrogen. These are regenerated when the reduced coenzymes pass their hydrogen to the electron transport chain, which needs oxygen as the final acceptor.

Without oxygen, the coenzymes stay reduced, so the link reaction and the Krebs cycle stop. Cells are then restricted to glycolysis and anaerobic respiration.