Oxidative phosphorylation occurs in the inner mitochondrial membrane, where ADP is phosphorylated to ATP through a process linked to the electron transport chain. This chain operates via stepwise oxidations that release energy, which is used to pump protons into the intermembrane space, creating a proton gradient. This gradient drives ATP synthase, where ADP is phosphorylated to ATP as protons flow back into the mitochondrial matrix.In response to confusion about the process, it is clarified that ADP and inorganic phosphate (P) are present in the mitochondrial matrix, where they combine to form ATP. The ATP synthase facilitates this reaction as protons pass through it. Additionally, while NADH can release hydrogen ions, FAD serves a complementary role in the electron transport chain, participating in the oxidation of substrates and contributing to the overall energy production.