A LevelBiologyCambridge (CIE)Revision Notes12. Energy & RespirationRespirationAerobic Respiration: The Krebs Cycle

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

Exam code: 9700

Phil

Written by: Phil

Reviewed by: Alistair Marjot

Updated on

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

Diagram of the citric acid cycle showing the conversion of oxaloacetate and acetyl CoA into citrate, with carbon dioxide release.
The regeneration of oxaloacetate

Aerobic respiration: the Krebs cycle

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

Diagram of the Krebs cycle showing acetyl CoA, citrate, oxaloacetate, intermediate compound, ATP production, CO2 release, NADH, and FADH2 formation.
The Krebs cycle

Examiner Tips and Tricks

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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    Author
    Reviewer
    Phil

    Author: Phil

    Expertise: Biology Content Creator

    Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.

    Alistair Marjot

    Reviewer: Alistair Marjot

    Expertise: Environmental Systems and Societies & Biology Content Creator

    Alistair graduated from Oxford University 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 & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.