ATP is used by cells continuously for a large number of tasks. ATP transfers it's energy to other molecules via hydrolysis by transferring one of it's phosphate groups to the receiving molecule. This process of transferring a phosphate to another molecule is an exergonic reaction called phosphorylation. Phosphorylation is defined as the bonding of a phosphate group to a molecule. The phosphate group is now bonded to the molecule which may be a part of a protein on a muscle cell, a reagent for a chemical compound, or a transport protein. The ATP lost a phosphate group in this reaction and has been reduced to two phosphate groups making ADP, or Adeonisine Diphosphate. The phosphate group attached to the molecule is then detached in another exergonic reaction releasing the energy that is used to create work. In the case of the muscle cell the energy is used to move the motor protein, the transport proteins use the energy to transport solutes up their concentration gradient, reagents lose their phosphate group and use the energy released to bond with other reagents.
ADP is recycled into ATP in the endergonic dehydration reaction phosphorylation. ADP uses the energy released during cellular respiration to bond a free phosphate group (that was released during cellular work) to it's phosphate chain and creates