Energy is released to be used by a cell when a phosphate group is

removed from ADP to form ATP

added to ADP to form ATP

removed from ATP to form ADP

added to ATP to form ADP

Added to ADP to form ATP.

So here is how you can remember this: Adenosine diphosphate is called ADP, "di" means two. ATP is Adenosine Triphosphate, "tri" means three. So ATP has an additional phosphate.

That's how I learned and remembered this concept. : D