Oxidative phosphorylation is one of the ATP generating processes in the cell. Understanding of this operation is based on chemiosmotic hypothesis (or Mitchell hypothesis) which clarifies how the free energy from electron transport chain across the inner mitochondrial membrane is used to produce ATP.
Mitochondrial inner membrane is embedded with four electron transferring complexes which work together as electron transport chain. Each complex is adapted to accept, oxidize and pass electrons to the next carrier of the chain. Main electron donors are NADH or FAD(2H). Firstly electrons from NADH are transported by complex I (NADH dehydrogenase), then CoQ (coanzyme Q), complex III (cytochrome b-c1 complex), cytochrome c, and complex IV (cytochrome oxydase). While electrons are transferring, the same complexes works as proton pumps and pump protons from the mitochondrial matrix to the intermembrane space where electrochemical potential gradient (Δp) are created. This gradient generates energy essential for ATP synthesis to be driven. Δp stimulates protons to reenter into the mitochondrial matrix through ATP synthase, which forms ATP from ADP and Pi
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