AQA A Level Biology

Revision Notes

5.2.5 The Krebs Cycle

The Krebs Cycle

  • The Krebs cycle (sometimes called the citric acid cycle) consists of a series of enzyme-controlled reactions
  • 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-1, downloadable AS & A Level Biology revision notes

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
    • 3 NAD and 1 FAD → 3NADH + H+ and 1 FADH2
  • Substrate-linked phosphorylation
    • A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP

Exam Tip

The Krebs cycle is often referred to as cyclical or circular. This is because the acceptor molecule oxaloacetate is regenerated throughout the reaction so that it can start all over again by adding another acetyl CoA. You may be asked to name the important molecules in the Krebs cycle like oxaloacetate. It is also worth noting how the number of carbon atoms in the substrate molecule changes as the cycle progresses.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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