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Through the process of glycolysis, one molecule of glucose breaks down to form two molecules of pyruvate. Depending on the microcellular environment (specifically, oxygen availability, energy demand, and the presence or absence of mitochondria), pyruvate has several separate fates: In mitochondria-containing cells, pyruvate can enter the citric acid cycle within the mitochondrial matrix and undergo oxidative phosphorylation. Aptly named due to its dependence on oxygen as the final electron acceptor, oxidative phosphorylation cannot take place in the absence of oxygen. Moreover, as the enzymes of both the citric acid cycle and electron transport chain are within the mitochondria, cells lacking mitochondria (e.g., erythrocytes) cannot rely on oxidative phosphorylation for energy production. In erythrocytes and oxygen-deprived tissue, pyruvate remains within the cytoplasm and converts to lactate, a process referred to as anaerobic glycolysis. This final reaction allows for the regeneration of NAD+, a cofactor that must be available in high enough intracellular concentrations for the earlier reactions of glycolysis to remain favorable. Compared to oxidative phosphorylation, however, anaerobic glycolysis is significantly less efficient, providing a net production of only 2 ATP per glucose molecule (versus 32 ATP per glucose molecule produced during oxidative phosphorylation).[1]