Adenosine triphosphate (ATP) is the source of energy for use and storage at the cellular level. The structure of ATP is a nucleoside triphosphate, consisting of a nitrogenous base (adenine), a ribose sugar, and three serially bonded phosphate groups. ATP is commonly referred to as the "energy currency" of the cell, as it provides readily releasable energy in the bond between the second and third phosphate groups. In addition to providing energy, the breakdown of ATP through hydrolysis serves a broad range of cell functions, including signaling and DNA/RNA synthesis. ATP synthesis utilizes energy obtained from multiple catabolic mechanisms, including cellular respiration, beta-oxidation, and ketosis.