The Krebs cycle is essential for energy metabolism because electrons are given off at several places during the reactions of the cycle. At three different points of each cycle, pairs of electrons are assumed by NAD molecules to produce three molecules of reduced NAD, or NADH. In addition, another reaction yields a pair of electrons taken up by the coenzyme FAD. This capture of electrons yields reduced FAD, or FADH2.Also during the Krebs cycle reactions, a reaction occurs in which enough energy is liberated to synthesize a molecule of ATP. Since two turns of the Krebs cycle occur for every glucose molecule, two molecules of ATP are produced. These two ATP molecules are in addition to the two ATP molecules resulting from the reactions of glycolysis.
