The Krebs cycle (or citric acid cycle) is a part of cellular respiration. Named after Hans Krebs, it is a series of chemical reactions used by all aerobic organisms to generate energy. Its importance to many biochemical pathways suggests that it was one of the earliest parts of cellular metabolism to evolve.
The diagram below shows how this part of respiration is an ever-repeating cycle which produces ATP and gives off CO2. The ATP is a molecule which carries energy in chemical form to be used in other cell processes. To summarize:
- Two molecules of carbon dioxide are given off
- One molecule of ATP is formed
- Three molecules of NAD+ are combined with hydrogen (NAD+ → NADH)
- One molecule of FAD combines with hydrogen (FAD → FADH2)
Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule. Therefore, at the end of two cycles, the products are: two ATP, six NADH2, two FADH2 two QH2 (ubiquinol) and four CO2.
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