Adenosine triphosphate is the molecule that provides energy to cells for almost all processes that need energy. The energy is generated by removing phosphate groups from the molecule.
Every cell contains ATP. This energy-rich molecule is present in the matrix of both the cell and its nucleus.
Food is broken down into glucose molecules. By burning glucose in the mitochondria of cells, energy is released to make ATP from adenosine diphosphate and free phosphate molecules. This process is called glycolysis. As ATP molecules are used, more are generated in the mitochondria by adding a phosphate group to the ADP molecule. This is a major part of the Krebs cycle.
The ATP molecule contains three phosphate groups linked together and bonded to a molecule of adenine, which is also found in DNA. The chemical reaction of cleaving off each phosphate group from ATP yields energy. The most important bond is between the second and third phosphate groups. The process of taking off the third phosphate group yields 7.3 kilocalories of energy per mole. When additional energy is needed, the cleaving of another phosphate group from ADP to yield adenosine monophosphate provides more energy; however, this reaction gives off less energy than taking a phosphate off ATP.