ATP is a molecule which consists of the nitrogenous base adenine linked to the sugar ribose and which has a chain of three phosphate groups attached to the ribose in a linear fashion. ATP is present in all living cells and serves as an energy source for many metabolic processes; energy is released when ATP is hydrolyzed into ADP. ATP is required for ribonucleic acid synthesis since it is a direct precursor. It is the single most important molecule in all living things since it serves as the currency for energy in biological systems.