The chemical energy carried in carbohydrates, lipids, and proteins is harnessed by cells to do work. But cells do not use this energy all at once. Instead, through the series of chemical reactions described in Chapter 7, they package this energy into a chemical form that is readily accessible to the cell. One form of chemical energy is adenosine triphosphate, or ATP, shown in Fig. 6.4. The chemical energy in the bonds of ATP is used in turn to drive many cellular processes, such as muscle contraction, cell movement, and membrane pumps. In this way, ATP serves as a go-between, acting as an intermediary between fuel molecules that store a large amount of potential energy in their bonds and the activities of the cell that require an input of energy.

ATP is composed in part of adenosine, which is made up of the base adenine and the five-carbon sugar ribose. The ribose is attached to triphosphate, or three phosphate groups (Fig. 6.4). Its chemical relatives are ADP (adenosine diphosphate) and AMP (adenosine monophosphate), with two phosphate groups and one phosphate group, respectively. The use of ATP as an energy source in nearly all cells reflects its use early in the evolution of life on Earth.

The chemical energy of ATP is held in the bonds connecting the phosphate groups. At physiological pH, these phosphate groups are negatively charged and have a tendency to repel each other. The chemical bonds connecting the phosphate groups therefore store chemical energy. This energy is released when new, more stable bonds are formed that contain less chemical energy (section 6.4). The released energy in turn can be harnessed to power the work of the cell.