The Krebs cycle occurs in the mitochondria of living cells. It's also called the citric acid cycle and the TCA cycle. The Krebs cycle is necessary for metabolism, in which cells change food into energy.
In the Krebs cycle, acetyl-CoA, a form of acetone which is part of the breakdown of fats, proteins and carbohydrates, is gradually changed into carbon dioxide, water and energy. During the process, it combines with oxaloacetic acid to produce citric acid, which is then rearranged to produce other acids, including malic and fumaric acid. The process also produces another generation of oxaloacetic acid, and the cycle starts again. The cycle was discovered by Hans A. Krebs, a German biochemist.