Link Reaction and Krebs Cycle

Link Reaction

  • The link reaction, also known as the Pyruvate Decarboxylase reaction, takes place in the mitochondrial matrix.
  • Each molecule of pyruvate, produced from glycolysis, is transported from the cytoplasm into the mitochondria.
  • In the link reaction, pyruvate is decarboxylated by the enzyme pyruvate decarboxylase. This means a carbon dioxide molecule is removed.
  • The remaining acetyl group is oxidised, forming an acetyl group, and the electrons removed are picked up by NAD to form reduced NAD (NADH).
  • The acetyl group is then combined with a molecule called coenzyme A (CoA) to form acetyl CoA.
  • This prepares the acetyl group for the next stage of respiration, the Krebs cycle.

Krebs Cycle

  • The Krebs cycle also takes place in the mitochondrial matrix.
  • Acetyl CoA from the link reaction combines with a four-carbon compound to form a six-carbon compound, releasing the coenzyme A.
  • The six-carbon compound is decarboxylated and dehydrogenated to form a five-carbon compound, releasing CO2 and transferring electrons to NAD, forming NADH.
  • The five-carbon compound then goes through a series of reactions where it is decarboxylated and dehydrogenated to form a four-carbon compound, releasing another molecule of CO2, another NADH and one molecule of ATP.
  • At the same time, a flavin adenine dinucleotide (FAD) molecule is also reduced to form FADH2.
  • The four-carbon compound can then combine with a new molecule of acetyl CoA to repeat the cycle.
  • The major products of the Krebs Cycle per glucose molecule (2 cycles) are: 4 CO2, 6 NADH, 2 FADH2, and 2 ATP.

Importance of link reaction and Krebs cycle

  • The link reaction and Krebs cycle are key stages in aerobic respiration, providing the cell with a much more efficient way of generating ATP compared to glycolysis alone.
  • The reduced NAD and FAD from these stages pass their electrons to the electron transport chain, where large amounts of ATP are generated in oxidative phosphorylation.
  • These stages also produce small amounts of ATP directly, plus carbon dioxide, the main waste product of respiration.