The formation of ATP from ADP is an endergonic reaction, which means that it requires an input of energy in order to occur. This energy is typically provided by the hydrolysis of another ATP molecule, which is a process that releases energy. The overall reaction can be represented as follows:

ADP + Pi + energy → ATP

where ADP is adenosine diphosphate, Pi is inorganic phosphate, and ATP is adenosine triphosphate.