Cells use adenosine triphosphate (ATP) for the capture and transfer of the free energy they require to do chemical work. ATP operates as a kind of “energy currency.” Just as it is more efficient and convenient for you to trade money for a lunch in a café than to trade your actual labor, it’s useful for cells to have a single currency for transferring energy between different reactions and cell processes. So some of the free energy that is released by exergonic reactions is captured in the formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate (HPO₄^2–, which is commonly abbreviated to P_i). The ATP can then be hydrolyzed in the cell to release free energy to drive endergonic reactions. (In some reactions, guanosine triphosphate [GTP] is used as the energy transfer molecule instead of ATP, but we will focus on ATP here.)

ATP has another important role in the cell beyond its use as an energy currency: it can be converted into a building block for nucleic acids (see Chapter 4). The structure of ATP is similar to that of other nucleoside triphosphates, but two things about ATP make it especially useful to cells: