These are the stages of cellular respiration: a preview. Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and the electron transport chain and oxidative phosphorylation. Glycolysis occurs in the cytoplasm. It begins catabolism by breaking glucose into two molecules of pyruvate. The citric acid cycle occurs in the mitochondrial matrix. It completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide. Several steps in glycolysis and the citric acid cycle are redox reactions in which dehydrogenase enzymes transfer electrons from substrates to NAD + , forming NADH. NADH passes these electrons to the electron transport chain. In the electron transport chain, the electrons move from molecule to molecule until they combine with molecular oxygen and hydrogen ions to form water. As they are passed along the chain, the energy carried by these electrons is transformed in
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