Final answer:
Energy is used to add a phosphate group to ADP forming ATP, whereas the breakdown of ATP into ADP and inorganic phosphate releases energy.
Step-by-step explanation:
When a phosphate group is added to a molecule of ADP to form ATP, energy is used. The process of adding a phosphate group to ADP, known as phosphorylation, requires a significant input of energy due to the repulsion between the negatively charged phosphate groups. Despite the instability of ATP, this molecule is crucial as a temporary storage of energy. The breakdown of ATP into ADP and inorganic phosphate, a process called hydrolysis, is what actually releases energy that cells can use to perform various functions such as muscle contraction, nerve impulse transmission, and synthesis of macromolecules.