The Krebs Cycle (OCR A Level Biology): Revision Note

Aerobic Respiration: The Krebs Cycle

• The Krebs cycle (sometimes called the citric acid cycle) consists of a series of enzyme-controlled reactions

• The Krebs cycle takes place in the matrix of the mitochondria

• 2 carbon (2C) Acetyl CoA enters the circular pathway from the link reaction in glucose metabolism

  • Acetyl CoA formed from fatty acids (after the breakdown of lipids) and amino acids enters directly into the Krebs Cycle from other metabolic pathways

• 4 carbon (4C) oxaloacetate accepts the 2C acetyl fragment from acetyl CoA to form the 6 carbon (6C) citrate

  • Coenzyme A is released in this reaction

• Citrate is then converted back to oxaloacetate through a series of oxidation-reduction (redox) reactions

The Krebs Cycle uses acetyl CoA from the link reaction and the regeneration of oxaloacetate to produce reduced NAD, reduced FAD and ATP

Regeneration of Oxaloacetate

• Oxaloacetate is regenerated in the Krebs cycle through a series of redox reactions

• Decarboxylation of citrate

• Releasing 2 CO₂ as waste gas

• Oxidation (dehydrogenation) of citrate

  • Releasing H atoms that reduce coenzymes NAD and FAD

  • 8H + 3NAD + FAD → 3NADH + 3H⁺ + FADH₂

• Substrate-level phosphorylation

  • A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP