Adenosine triphosphate (ATP) is often described as the body's "energy currency." Energy-producing metabolic reactions store their energy in the form of ATP, which can then drive energy-requiring syntheses and other reactions anywhere in the cell.

Structurally ATP consists of the purine base adenine (a complex, double-ring molecule containing five nitrogen atoms) attached to the five-carbon sugar ribose; this combination is known as adenosine. Attaching a string of three connected phosphate groups to the ribose produces ATP. Schematically, one may depict the structure of ATP as Ad-Ph-Ph-Ph, where Ad is adenosine and Ph is a phosphate group. If only two phosphate groups are attached, the resulting compound is adenosine diphosphate (ADP).

The final step in almost all the body's energy-producing mechanisms is attachment of the third phosphate group to ADP. This new phosphate-phosphate bond, known as a high-energy bond, effectively stores the energy that has...