The Citric Acid Cycle including the initial pyruvate mobilization is a series of reactions oxidizing the pyruvate from glycolysis to form three molecules of CO2. Through the process NAD+ and FAD act as electron acceptors. These electrons are eventually passed to their final acceptor O2 by means of the electron transport chain. While direct substrate phosphorylation does occur to an extent in the citric acid cycle, the processes of the electron transport chain yield a great deal of energy, forming most of the ATPs. Oxidative metabolism yields far more energy than glycolysis and fermentation because the ultimate electron acceptor is oxygen, rather than organic carbon. While glycolysis and fermentation alone yields only two ATP, the entirety of oxidative metabolism, including glycolysis, oxidation of pyruvate, and the citric acid cycle will yield thirty-six molecules of ATP per molecule of glucose (38 in aerobic bacteria).