This vital substance stands astride the mechanism by which glucose is used as fuel to deliver energy for cellular function. It can be compared to a spark plug in an internal combustion engine, but energy metabolism in mammalian cells has an incredible complexity and our knowledge of this has increased greatly in recent years. Thiamine illustrates this complexity, for one of the most important molecules in the cells is phosphate. When phosphate is added to a biologic substance it provides it with a means of storing energy. The main storage of energy in cells is in the form of adenosine triphosphate, which means that it possesses a high energy potential in electrochemical terms. A lower energy potential would exist in adenosine diphosphate and lower again in monophosphate. Thiamine works in a similar manner. It is "energized" by adding one, then two and finally three phosphate molecules. In each of these different forms the vitamin has a separate and different action in cell chemistry. It must be understood that energy can only be stored by using energy. Put simply, work has to be done (energy utilization) in order to pull back a bow string. The energy is stored in the form of the taut string and the arrow is shot from the bow at the will of the archer, by releasing the bow string. The bow string can be compared to both adenosine and thiamine in its lower or higher energy state. When energy is required for cell function, a phosphate is yielded and the adenosine or thiamine drops down to a lower energy potential. This transfer results in the release of energy which accomplished work. Unphosphorylated thiamine is called "free thiamine" and appears to have no biologic activity at all. When it is in the form of monophosphate its function is poorly understood. It is best known in its diphosphate or pyrophosphate form, for it is used to energize a large number of enzyme systems in the body. An enzyme is a protein which acts as a catalyst in synthesizing a new molecule by a chemical reaction with another molecule, and this action requires energy. Part of this energy is derived from a vitamin or mineral which is known as a cofactor to the enzyme.