The "Krebs cycle" is a series of chemical reactions used by all aerobic organisms in their energy conversion processes. It is important to many biochemical pathways. This 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 GTP 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 NADH, two FADH2 two QH2 (ubiquinol) and four CO2.