The proton gradient across the mitochondrial inner membrane is central to oxidative phosphorylation and ATP synthesis. During cellular respiration, the electron transport chain (ETC) transfers electrons from NADH and FADH₂ through protein complexes, using the released energy to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates a high concentration of protons outside the matrix, generating both a chemical gradient (difference in proton concentration) and an electrical potential (matrix negative, intermembrane space positive), together forming the proton-motive force.

ATP synthase, a molecular rotary enzyme, harnesses this force. Protons flow back into the matrix through its channel in a process called chemiosmosis, spinning the enzyme’s rotor. This mechanical rotation induces conformational changes in ATP synthase, enabling the condensation of ADP and inorganic phosphate (Pi) into ATP. Each NADH molecule can generate roughly 2.5 ATP, linking the ETC directly to cellular energy supply. This proton gradient thus powers essential biological functions, from muscle contraction to biosynthesis, highlighting its critical role in life’s energy economy.