Oxidative phosphorylation is the set of chemical reactions used to produce adenosine triphosphate (ATP). An important part of aerobic respiration, it is perhaps the most fundamental metabolic operation on earth. Different types of organisms have many different ways of organizing oxidative phosphorylation, but the end result is always the same: energy from the next to last step in the series is used to bind a phosphorus atom to adenosine diphosphate (ADP), transforming it into ATP. The potential energy added to the molecule in this reaction is precisely what makes ATP a universally useful source of energy within the cell.
The lead-up to the final step of oxidative phosphorylation involves a series of reduction-oxidation, or redox, reactions. These reactions transfer electrons from one molecule to another, thereby altering the charge of both. This set of operations is called an electron transport chain, because it allows the cell to move energy, in the form of electrons, from storage to a place where it can be readily used. Nicotinamide adenine dinucleotide (NAD+) is a common step near the end this process. The '+' represents a positive charge that allows it to easily accept electrons and become a reduced form called NADH.
The energy of the electrons in NADH is used to power a process called chemiosmosis. Chemiosmosis concentrates the energy of the electrons into potential energy by moving hydrogen ions—protons—across a membrane. This movement creates an energy gradient across the membrane by virtue of the accumulated positive charge on one side. This energy gradient is called the proton-motive force. At this point, the final and most universal step of oxidative phosphorylation can take place.
ATP synthase is the enzyme ultimately responsible for converting ADP to ATP. Part of the protein is embedded in the membrane across which the protons have been driven. ATP synthase provides a route through which the protons can re-enter the cell, but takes advantage of the energy generated when they do so. This operation resembles the way windmills harness differences in pressure and water wheels use changes in potential energy resulting from gravity. The movement of a proton back across the membrane is used to power a change in the shape of the enzyme. If a molecule of ADP is already bound to ATP synthase when this event occurs, the change imposes an additional phosphorus atom upon it. The newly produced ATP molecule is allowed to leave the enzyme and becomes free to provide energy elsewhere in the cell.