CIE A Level Biology (9700) 2019-2021

Revision Notes

12.2.5 Aerobic Respiration: The Krebs Cycle

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

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.

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
    • 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

It is a good idea to learn the Krebs cycle in detail. 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.

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Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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