Cells release stored energy by transferring a phosphate group from adenosine triphosphate, or ATP, to a receptor in another biological molecule. This process results in a change in a cellular mechanism and a remnant adenosine diphosphate, or ADP, which must be replenished via cellular metabolism. This process is shared by every form of life yet discovered.
The molecule ADP is actually an ion, with one fewer electron than would be required for a neutral charge. Nonetheless, it is a more stable molecule than ATP, which is why the cell must use energy to create ATP from it. Because this reaction requires energy to complete, the bond with the extra phosphate group contains that energy, and transferring the phosphate to a more favorable bond releases it.
One important example of cells releasing energy from ATP molecules is in fibers of the proteins myosin and actin. These important, large protein molecules work together, both to cause muscles to contract and to complete the process of cell division.
Myosin is the larger of the two proteins and acts on actin to produce movement. It does this by binding to actin with a group known as the myosin head. Each myosin molecule actually has two heads, one which has an actin binding site, and the other which has an ATP binding site. The ATP releases a phosphate group to the ATP binding site, which causes the myosin heads to change shape, pulling the actin group along.