The main energy source cells use to power the cell's biochemical reactions is adenosine triphosphate (ATP). ATP is often called the "energy currency" of the cell. It is a nucleoside triphosphate consisting of the nitrogenous base adenine, the five-carbon sugar ribose, and three serially bonded phosphate groups.
During a series of biochemical reactions that are known as cellular respiration, the chemical energy stored in glucose molecules is converted into ATP. ATP is then used to provide the energy needed for all other cellular functions.
When a molecule of ATP is not being used, the energy it has stored in the bond between its second and third phosphate groups is released. This is because the phosphate groups are negatively charged, and like charges repel each other. The release of this energy is also an exothermic reaction, causing the formation of adenosine diphosphate (ADP) and a free phosphate group.
Cells use the energy released from ATP to accomplish three broad types of tasks: to drive metabolic reactions that would not otherwise occur; to transport needed substances across membranes; and to do mechanical work, such as contracting muscles. Without ATP, life could not exist.
The potential energy stored in a molecule of ATP is found in the high-energy bonds that connect its phosphate groups. The energy in these bonds is similar to the kinetic energy that is stored when a tightly-coiled spring is compressed. To get the energy back out, a protein or other enzyme breaks the bond between the phosphate groups. This releases the energy, just as a spring uncoils to generate force and movement. Because ATP can be broken down and its energy recycled, it is an efficient energy carrier for the cell.