ATP is the universal energy currency in cells. It has three phosphate groups. The bonds between the three phosphate groups are unstable and have a low activation energy so can be rapidly hydrolysed. The hydrolysis reactions are energy yielding so they can be coupled with energy-requiring reactions.ATP is synthesised by adding an inorganic phosphate group to ADP. This occurs in three ways - photophosphorylation in chloroplasts during photosynthesis, oxidative phosphorylation in mitochondria during aerobic respiration and substrate level phosphorylation in respiration. ATP is an immediate energy source. Cells maintain a few seconds supply. ATP cannot be stored so it is continuously made.