ATP has three phosphates attached to it. In order to form ADP, one of the phosphates is released. To form ADP back into ATP, the phosphate has to be reattached.
Hello! ATP stands for adenosine triphosphate, and ADP stands for adenosine diphosphate. The difference between the two molecules is that ATP has three phosphate group, and ADP has two phosphate groups. ATP is an unstable molecule, which means it will release energy when it becomes reduced to ADP, meaning it will break off one of its phosphate groups. Hope this helps, and let me know if you have any questions! ^-^
