The most ATP during cellular respiration is made at the inner membrane of the mitochondria, specifically through the electron transport chain embedded in the inner mitochondrial membrane. This process, known as oxidative phosphorylation, uses a proton gradient created across the inner membrane to power ATP synthase, which produces the bulk of ATP molecules. The mitochondrial matrix contains enzymes for the citric acid cycle that provide the high-energy electrons used in the electron transport chain, but the actual synthesis of the majority of ATP occurs at the inner membrane's folds called cristae.
