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. Read that again if you did not completely understand.

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 per glucose in aerobic respiration. 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).

All fuel molecules, amino acids, fatty acids, and carbohydrates, are ultimately oxidized in aerobic metabolism by means of the citric acid cycle. Most enter the cycle as acetyl CoA, although there are other points of entry for some exceptions. In addition to serving as the primary pathway in oxidative metabolism for the degradation of nutrient molecules, ultimately to give rise to ATP through electron transport, the citric acid cycle is a major source of precursors for the synthesis of biological molecules.

Although glycolysis takes place in the cytoplasm of eukaryotic cells, the oxidative decarboxylization of pyruvate to form acetyl CoA and the citric acid cycle take place within the mitochondria.

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