Adenosine triphosphate

Adenosine triphosphate, or ATP, is a molecule which is often referred to as the energy currency of a cell because of its role as a store and transport of energy for use in metabolism throughout the cell. ATP is composed of three parts: the purine base, adenine; the sugar molecule, ribose; and the chain of three phosphate groups. The readily usable energy is stored in the bond between the last two phosphate groups in the chain; when the last link is removed, the molecule becomes the low energy adenosine diphosphate. ATP is created from ADP through various processes, including glycolysis, the citric acid cycle and the electron transport chain in aerobic respiration, fermentation in anaerobic respiration, the Calvin cycle in plants, and through a membrane-bound hydrogen pump in the mitochondria called ATP synthase.

ATP also fulfils all the criteria of a neurotransmitter.