This is a process that occurs on the membrane of the mitochondria. Here the electrons are passed from one electron carrier to the next in a chain know as the electron transport chain. With each carrier being a lower energy state than the next, the excess energy is used to pump protons from the matrix across the membrane to create a concentration gradient. For the protons to move down their membrane gradient they have to pass through a hydrophillic transmembrane protein known as ATP synthase as the rest of the membrane is impermeable. The protein then makes use of the energy released to phosphorylate an ADP molecule to form ATP.