ADP is formed from adenosine monophosphate (AMP), a nucleotide. There is continuous synthesis, and use is going on within the cell. Chemical reactions continuously occur inside the cell. These processes generate all the chemicals needed for the cell to function.

Production of nucleotides requires a pentose sugar (ribose), N2 base, and Phosphate group. For the formation of AMP, the N-base is adenine. So adenine, ribose sugar, and phosphate group are needed to form AMP, which includes ADP by taking up another phosphate group.