The Krebs Cycle (AQA A Level Biology): Revision Note

Aerobic respiration: the Krebs cycle

• The Krebs cycle, sometimes called the citric acid cycle, involves a series of enzyme-controlled reactions

• During the Krebs cycle:

  1. acetyl coenzyme A releases a two-carbon molecule, which reacts with a four-carbon molecule to produce a six-carbon molecule

     • 4C molecule = oxaloacetate

     • 6C molecule = citrate

     • Coenzyme A is freed up to return to the link reaction, where it can combine with more acetate

  2. a series of oxidation-reduction reactions occur, during with the 6C molecule is converted back into a 4C molecule; during these reactions:

     • coenzymes NAD and FAD are reduced

     • ATP is produced by substrate-level phosphorylation

     • carbon dioxide is lost

• The reduced coenzymes produced during the Krebs cycle will enter the next stage of respiration

  • Reduced NAD and reduced FAD carry hydrogen ions and electrons which will be used in oxidative phosphorylation

• Other respiratory substrates may enter the Krebs cycle, e.g. the breakdown products of:

  • lipids

  • amino acids