ADP is combined with a phosphate to form ATP in the reaction ADP+Pi+free energy→ATP+H2O. The energy released from the hydrolysis of ATP into ADP is used to perform cellular work, usually by coupling the exergonic reaction of ATP hydrolysis with endergonic reactions.
Unlike a DNA or RNA nucleotide, the ATP nucleotide has three phosphate groups attached to its ribose sugar. ... The chemical energy freed from the reaction is stored in ATP as chemical bonds between the phosphate groups. An individual ATP molecule doesn't hold a lot of energy