Adenosine triphosphate (ATP) is an organic compound composed of the nitrogenous base adenine, the sugar ribose, and three phosphate groups arranged in tandem. ATP serves as the major energy source within the cell where it is used to drive a number of biological processes such as photosynthesis, vesicle trafficking, ionic symporters/antiporters (e.g. Na+/K+ pump), muscle contraction, and protein synthesis. It is broken down by hydrolysis to yield adenosine diphosphate (ADP), inorganic phosphate, and energy. ADP can be further broken down to yield adenosine monophosphate (AMP), an additional phosphate ion, and more energy. When the phosphate group and energy are used to drive other reactions, such as the synthesis of uridine diphosphate (UDP), an RNA precursor, from uridine monophosphate (UMP), the pair of reactions are said to be coupled. New ATP can be generated by two distinct processes, substrate level phosphorylation and oxidative phosphorylation. The latter is the end point of cellular respiration via the oxidation of glucose.
Extracellularly, ATP has been found to act as a neurotransmitter. ATP receptors are widespread throughout the body. On its own it is known to have effects on the arteries, intestines, lungs, and bladder. It may also be released in tandem with other neurotransmitters.
The structure of ATP has an ordered carbon compound as a backbone, but the part that is critical is the triphosphate moiety. Three phosphate groups are connected by oxygens to one another. There are also side oxygens connected to the phosphorus atoms. Under normal conditions in the body, each of these oxygens has a negative charge, the negative charges repel each other resulting in stored potential energy.