ADP is the precursor to the formation of ATP. The ADP only contain two phosphate and ATP contain three. The process of ATP involves adding one inorganic phosophate to ADP. In order to convert ATP back in ATP, hydrolysis (water) is needed break the inorganic phosphate from ATP. As for the cellular uses of ATP, ATP is used for anything that is being formed or broken down. For example, no ATP works in conjunction with proteins. ATP is also used as regulator because no reaction can take place in the body without ATP. The cell can not function without ATP.