When energy is released, ATP loses one of its phosphate groups and turns to ADP (ADENOSINE DI-PHOSPHATE). ADP is present in cells and has two phosphate groups firmly attached. The energy from respiration is used to form another phosphate group to each molecule to form ATP.
ATP- ADP + "phosphate" + "energy", and here is an image for this chemical reaction is given. Observe how in ADP, the gamma-PO_4 group is removed and results in release of tremendous energy.
