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

• 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

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