Oxidative phosphorylation takes place in and around the membranes of mitochondria in eukaryotic cells. Oxidative phosphorylation is the process of generating adenosine triphosphate, the main energy currency of all cells, by using oxygen as a final electron acceptor. The two main steps of oxidative phosphorylation are electron transport and chemiosmosis, both of which use the movement of charged particles to transfer energy.
Oxidative phosphorylation is the main method whereby eukaryotic cells produce ATP aerobically. The step before oxidative phosphorylation, glycolysis, creates ATP, but in far smaller quantities. Glycolysis also takes place away from the mitochondria. Oxidative phosphorylation starts by using electron transport to move electrons between various molecules in mitochondria. This also serves to draw protons, which are hydrogen nuclei without electrons, across the membrane of the mitochondria. This movement is into an area of higher concentration against the tendencies of diffusion. As such, it requires energy.
Once the electrons stop moving the protons, they are then freed to travel back across the mitochondrial membrane in a process known as chemiosmosis. The process of moving the protons in one direction required energy, and the spontaneous movement back across the membrane releases it. It is this energy which is used to bind a phosphate group to ADP, creating ATP.