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Oxidative phosphorylation is a biochemical process in biological cells. It is the final step of cellular respiration, after glycolysis and the citric acid cycle: 26 of the total 30 ATP (energy carrier) molecules generated from a single glucose molecule during cellular respiration come from oxidative phosphorylation.

The process takes place in the mitochondrion, in the inner of its two membrane layers. NADH[?] and FADH2[?], electron carrier molecules that were "loaded" during the citric acid cycle, are used in an intricate mechanism (involving NADH-Q reductase, cytochrome c oxidase, and cytochrome reductase) to pump H⁺ across the inner membrane from the mitochondrial matrix (the inside) into the intermembrane space against a proton gradient.

A large protein complex called ATP synthase is embedded in that membrane and enables protons to pass through in both directions; it generates ATP when the proton moves with (down) the gradient, and it costs ATP to pump a proton against (up) the gradient. Because protons have already been pumped into the intermembrane space against the gradient, they now can flow back into the mitochondrial matrix via the ATP synthase, generating ATP in the process. The reaction is:

ADP^3- + H⁺ + P_i ↔ ATP^4- + H₂O

In certain bacteria (which do not have mitochondria and, thus, no cellular respiration), bacteriorhodopsin[?] can take the place of the respiratory chain, taking energy from light. This mechanism is probably a precursor of photosynthesis in algae and plants.

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