Adenosine triphosphate is composed of three phosphate groups, an adenine ring and a ribose sugar. ATP has been called the energy currency of living systems because it is used to store energy within the cells.
Energy is released when carbohydrates and proteins are broken down within the cell. This energy is used to add a phosphate group to adenosine diphosphate to convert it to ATP, where it is stored until needed. When the cell requires energy, ATP is hydrolyzed to release a free phosphate group and the lower-energy ADP. This reaction is spontaneous, or exergonic, releasing energy that the cell uses to drive nonspontaneous reactions. The ADP produced during the hydrolysis is then available for use the next time it is needed to help capture energy.