The chemical equation for ADP to ATP is written as ADP + Pi + energy ? ATP. In other words, adenosine diphosphate plus a phosphate molecule and energy yields adenosine triphosphate.
Normally, this reaction happens in both directions, which means that ATP breaks down into ADP, then ADP rebuilds itself into ATP. This process is mostly dependent upon the addition and subtraction of a phosphate molecule and the energy required to bond phosphate to ADP and release it from ATP. For instance, when ATP is broken down into ADP, it loses one of its phosphates, which yields a diphosphate group. 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.