How is ATP synthesized in mitochondria?

ATP is synthesized in mitochondria by utilizing the energy from the proton electrochemical gradient. During the process of transferring hydrogen atoms from FMNH2 or FADH2 to oxygen inside the mitochondria. Protons are moved across the inner mitochondrial membrane to the outer side. This movement of protons forms an electric potential and slight pH gradient across the membrane, usually measuring between 200-300 millivolts. The chemical energy of the substrate is transformed into electrical energy. When ADP and Pi attach to ATP synthase, the surplus protons which have accumulated outside the mitochondria return into the mitochondria by passing through the enzyme complex. The energy released is then used to transform ADP and Pi into ATP.