Cell Respiration - The Citric Acid Cycle

The citric acid cycle (CAC) is also known as the Krebs cycle. While this metabolic process does not directly require oxygen it would quickly grind to a halt if oxygen were not available to remove electrons released by the oxidation of food molecules in the matrix of the mitochondria. The following are some of the major steps or accomplishments of the CAC.


  1. Requires oxygen


  1. The citric acid cycle frees up electrons which reduce NAD+ to NADH2+ and FAD to FADH2 (notice both carry 2 electrons). NAD and FAD are mobile electron carriers (dinucleotides of adenine)


  1. The citric acid cycle begins with the 6-carbon citric acid which is regenerated each cycle by combining acetyl CoA (2-carbon) with oxaloacetate (4-carbon).


  1. After the cycle is completed oxaloacetate is regenerated. That's why it's considered a "resident" molecule in the cycle.


  1. Summery of main events in CAC for one glucose molecule - two turns of the cycle:


  1. 4 CO2
  2. 6 NADH2+
  3. 2 FADH2
  4. 2 GTP (equivalent of ATP)


  1. The citric acid cycle occurs in the matrix of mitochondria

Modified, Nov. 10, 2005