ATP (or adenosine triphosphate) is a molecule that stores energy needed in phosphate bonds. ATP is synthesized in mitochondria. ATP is composed of the nucleotide adenosine which is attached to a three-phosphate tail by a ribose group. ATP is used to drive all cellular processes that require energy. The energy that is derived from the breakdown of carbohydrates, fats, or proteins is used to make ATP. ATP can power some reactions by transforming into ADP. The ADP also regain an extra phosphate group to become ATP again. This cycle could go on forever. In order to climb the stairs to the second level of the Eiffel Tower, you need (A LOT) of ATP, to give you the energy to contract your muscles in order to make it all the way.