ATP contains three phosphate groups, whereas ADP contains two. ATP stands for "adenosine triphosphate," and ADP stands for "adenosine diphosphate." They both contain the nitrogenous base adenine, the five-carbon sugar ribose and several phosphate groups.
Cells use ATP as an energy source for a variety of biochemical reactions. Humans and other animals take in energy in the form of carbohydrates, lipids and proteins. However, cellular reactions cannot use the energy in these molecules directly. The molecules must first be broken down through cellular respiration and the energy must be converted to ATP.
The third phosphate in ATP is connected to the molecule by a high-energy bond. When this bond is broken, the energy released is used by reactions in the cell as an energy source. These reactions include DNA replication, protein synthesis, cellular transport and muscle contraction. Upon release of the third phosphate group, ATP becomes ADP. In a manner similar to a rechargeable battery, ADP can no longer be used as an energy source. ADP then returns to the site of cellular respiration in the mitochondria. Another phosphate group is then attached to the ADP, producing ATP. This ATP molecule can then be used again by the cell as an energy source.