A cell must perform several kinds of work: mechanical work involved in movement of the cell or parts of the cell, transport work in pumping molecules across membranes, and chemical work in driving endergonic reactions to synthesize cellular molecules. The immediate source of the energy to perform this work most often comes from adenosine triphosphate, or ATP.
ATP is a nucleoside triphosphate, the purine base adenine bonded to the sugar ribose, which is connected to a chain of three phosphate groups.
The triphosphate tail of ATP is unstable and the bonds between the phosphate groups can be broken by hydrolysis. Thus, ATP can be hydrolyzed to ADP (adenosine diphosphate) and an inorganic phosphate molecule, releasing 7.3 kilocalories of energy per mole of ATP.
Although the phosphate bonds in ATP are called high-energy bonds, they are actually weak bonds, easily hydrolyzed to yield more products and release energy.
The transformation to a more stable state releases energy. In a cell, this energy can be used to transfer the phosphate group from ATP to another molecule, producing a phosphorylated intermediate that is more reactive. The phosphorylation of other molecules by ATP forms the basis for almost all cellular work.
A cell regenerates ATP at a phenomenal rate,
approximately your body weight every day. The formation of ATP from
ADP and inorganic phosphate is endergonic, with a 
G
of +7.3 kcal/mol. Cellular respiration (the catabolic processing of
glucose and other organic molecules) provides the energy for the
regeneration of ATP. Plants can also produce ATP using light
energy.
The cellular processes of generating ATP are discussed at the following sites:
Modified July 10, 2005
