In the first step of the citric acid cycle, a two-carbon atom acetyl group carried by coenzyme A combines with oxaloacetate, which carries four carbon atoms, to form citrate, a six-carbon molecule. This acetyl group is formed when pyruvate, a product of glycosis, is converted to acetyl-CoA, a two-carbon group acetyl group attached to coenzyme-A. This reaction is called the Transition Reaction. It occurs just before the citric acid cycle. Like the citric acid cycle, it occurs twice for each glucose molecule.
During each turn of the cycle, three molecules of the coenzyme NADH are formed when the coenzyme NAD+ accepts two electrons and one hydrogen ion. As the cycle turns twice for each molecule of glucose, six molecules of NADH are formed. Also, one molecule of FADH2 is formed in each turn of the cycle when the coenzyme FAD accepts two electrons and two hydrogen ions to become FADH2, producing two molecules of FADH2 in total.
The NADH and FADH2 react in the electron transport chain to form ATP. The nucleotide ADP (adenosine diphosphate) accepts a phosphate group and becomes ATP (adenosine triphosphate). The energy that is released when ATP is converted to ADP is coupled to energy-requiring processes such as synthesizing carbohydrates and proteins. In muscle cells, the energy is used for muscle contractions and for the conduction of nerve impulses in nerve cells.
Four molecules of carbon dioxide are produced during the entire citric acid cycle. Carbon dioxide diffuses out of the cell and, in humans and other air-breathing animals, enters the bloodstream where it is taken to the lungs and exchanged. Water (H2O) is also produced as a byproduct of the cycle. The water may remain in the cell or be excreted.