Adenosine diphosphate (ADP) is a key intermediate in the body's energy metabolism—it serves as the "base" to which energy-producing reactions attach an additional phosphate group, forming adenosine triphosphate (ATP). ATP then diffuses throughout the cell to drive reactions that require energy.
Structurally, ADP 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 two connected phosphate groups to the ribose produces ADP. Schematically, the structure may be depicted as Ad-Ph-Ph, where Ad is adenosine and Ph is a phosphate group.
See also Metabolism.