Cells store energy in a molecule called adenosine triphosphate (abbreviated as ATP). The adenosine molecule has three phosphate groups attached to it (triphosphate means three phosphates) which are held together by high energy bonds. If one of these bonds is broken, a great amount of energy is released which can be used in an endergonic reaction. Also, the ATP no longer has three phosphate groups; now it has only two, so it is called adenosine diphosphate (di means two). This is abbreviated as ADP. Occasionally, another phosphate bond is broken, releasing more energy and leaving the ADP with only one phosphate left. It is now AMP: adenosine monophosphate, since mono means one.
When food is broken down, energy is released as the food molecules' bonds are broken. This energy can be used to reform the bonds between the phosphate groups, so that ATP can be recreated. The process by which ATP is synthesized is accomplished differently in anaerobic and aerobic organisms; more energy can be obtained when oxygen is present and the process proceeds aerobically. The next few sections discuss the different methods by which ATP can be synthesized, but let's go through an overview here.
The main source of energy for living organisms is a sugar called glucose. In breaking down glucose, the energy in the glucose molecule's chemical bonds is released and can be harnessed by the cell to form ATP molecules. The process by which this occurs consists of several stages. The first, which is discussed in the next section, is called glycolysis (the prefix glyco refers to glucose, and lysis means to split), in which the glucose molecule is broken down into two smaller molecules called pyruvic acid. The other stages are different for anaerobes and aerobes and will be discussed separately.
