(1) A nucleotide made up of adenine, ribose, and two phosphate units; having a molecular formula: C10H15N5O10P2
(2) The product of ATP de-phosphorylation via ATPases to release energy; and can be converted to ATP by phosphorylation (addition of phosphate and energy) via ATP synthases to store energy.
Adenosine diphosphate or ADP is the result when ATP loses one of its phosphate groups when the energy is released to power up many vital cellular processes.
Compare: AMP, ATP.
See also: nucleoside diphosphate.