In oxidative metabolism, the electron pairs liberated through the oxidation of glucose do not pass directly to O2. The direct oxidants are the coenzymes of various oxidoreductases. The reduced forms of these coenzymes, NADH and FADH2, then transfer these electrons into the electron transport system which is located on the mitochondrial matrix. ATP is formed through an energy coupling process that begins with reoxidation of NADH and FADH2. The electrons passing from NADH and FADH2 travel through a sequential oxidation-reduction process involving a long series of redox centers located within a series of protein complexes in the inner mitochondrial membrane. As electrons pass along the chain, protons are expelled from the mitochondrial matrix by various mechanisms. The free energy stored in the resulting concentration and electric potential gradient (proton motive force) drives the synthesis of ATP as the protons flow back to the mitochondrial matrix through the protein complex ATP synthase.
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Conceptual Vocabulary for Oxidative Phosphorylation