Adenosine triphosphate, ATP, is the molecule that provides energy that cells can use. It consists of the nitrogen base adenine, the sugar ribose, and three phosphate groups. When a cell needs energy, the bond between the second and third phosphate groups on a molecule of ATP is broken, releasing energy and a phosphate group, and forming adenosine diphosphate, ADP. During cellular respiration, catabolism, and/or the light reactions of photosynthesis, the ADP uses the energy released to combine with a third phosphate group, reforming ATP.
ADP + P + Energy
The first diagram shows the structure of ATP and the second shows the ATP cycle.