Aerobic Respiration: The Krebs Cycle (Cambridge (CIE) A Level Biology)

Aerobic Respiration: Outline of the Krebs Cycle

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

• Acetyl CoA (2C) enters the circular pathway via the link reaction

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

• Citrate is then converted back to oxaloacetate through a series of small reactions

Entry of Acetyl CoA into the Krebs Cycle Diagram

The regeneration of oxaloacetate

Aerobic Respiration: The Krebs Cycle

The steps in the Krebs cycle

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

• Decarboxylation of citrate

  • Releasing 2 CO₂ as waste gas

• Dehydrogenation of citrate

  • Releasing H atoms that reduce coenzymes NAD and FAD

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

• Substrate-level phosphorylation

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

Krebs Cycle Diagram

The Krebs cycle