The result is a release of energy as the three-strand phosphate bond of ATP is broken into two strands. According to Georgia State University, this process commonly occurs with a water molecule and is written as ATP + H2O ? ADP + Pi. Generally, this reaction is known as hydrolysis. However, for ADP to form into ATP, energy is required. Since energy is released from ATP when the bonds are broken, energy must be put back into the equation to form ATP from ADP and a phosphate molecule. In other words, energy is necessary as a catalyst to start the reaction.