When inside the mitochondria, pyruvate and fatty acids are broken down to acetyl CoA, which is then metabolized by the citric acid cycle. During the reaction, NAD⁺ is reduced to NADH. In the process of oxidative phosphorylation, high-energy electrons from NADH are passed along the ETC in the inner membrane to O₂. This transport of electrons generates a proton gradient across the mitochondrial inner membrane, which drives the production of ATP by ATP synthase.