phosphorylation, chemical process in which a phosphate group is added to an organic molecule. In living cells phosphorylation is associated with respiration, which takes place in the cell's mitochondria, and photosynthesis, which takes place in the chloroplasts. The energy released during metabolic or photosynthetic processes is captured in the energy-rich phosphate bonds of certain molecules, most commonly in the high-energy bonds of adenosine triphosphate (ATP). In the process of oxidative phosphorylation ATP formation is associated with respiratory uptake of oxygen. In this process a cell substance known as NADH (one of a variety of coenzymes) donates hydride ions (one proton and two electrons) to the first of a series of enzymes in the so-called electron transport chain. As the hydride ion is passed from one enzyme to another in the chain, energy is made available to power the formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate. At the end, or lowest energy level, of the electron transport chain, the hydride ion combines with oxygen and a proton (hydrogen ion) to form a water molecule. The phosphorylation process is linked to cell metabolism in that metabolic degradation of food, e.g., glucose, allows formation of the coenzyme NADH. The electron transport enzymes are complex aggregates of cytochromes, i.e., proteins with iron-containing heme groups, and various coenzymes. The precise mechanisms by which chemical energy is coupled to ATP synthesis are not yet understood. In photosynthetic phosphorylation, or photophosphorylation, substances such as the reduced coenzyme NADPH also donate hydride ions to an electron transport system so that ATP is synthesized from ADP and inorganic phosphate; in photophosphorylation, however, the coenzyme is produced from chemical reactions initiated by illuminated photosynthetic pigments instead of from metabolism of food molecules. The net result in phosphorylation of ADP is the formation of the high-energy molecule ATP, which the cell can use as a kind of universal energy currency to power many important cell processes, such as protein synthesis. Other phosphorylation reactions occur in cells, some without mediation by the electron transport chain, e.g., ATP is formed from ADP and inorganic phosphate in a reaction coupled to the oxidation of glyceraldehyde phosphate to phosphoglyceric acid.