chemiosmosis

Meaning: In mitochondria, the electron transport chain uses energy from electrons donated by NADH and FADH₂ to pump H⁺ from the matrix into the intermembrane space. This builds a proton-motive force, made of a pH difference and an electrical difference across the inner membrane. H⁺ then flows back into the matrix through ATP synthase, and the energy released drives ADP + Pᵢ → ATP. Chloroplasts do the same, with H⁺ moving from the thylakoid lumen to the stroma.

On the MCAT: A passage adds an uncoupler or an inhibitor and asks what happens. An uncoupler makes the inner membrane leaky to H⁺, so the gradient collapses, ATP synthesis falls, O₂ consumption continues or rises, and the energy is released as heat. Blocking ATP synthase lets the gradient build and slows electron transport. A chain inhibitor stops the pumping, so the gradient and ATP both fall. Expect to be asked which side is more acidic (the intermembrane space).

Common confusion: Despite the name, the moving species is H⁺, not water. Oxygen and the chain do not make ATP directly; they build the gradient, and ATP synthase uses it. Substrate-level phosphorylation, as in glycolysis, does not use a gradient.