There are many different sources of energy, including sunlight, wind, electricity, and fossil fuels. The food we eat also contains energy. Energy can be defined as the capacity to do work. For a cell, work involves processes we discussed in earlier chapters, such as synthesizing DNA, RNA, and proteins, pumping substances across the plasma membrane, and moving vesicles between various compartments of a cell.

Although there are many different sources of energy, energy comes in just two major forms. In this section, we consider these two forms of energy. In addition, we focus on how energy is held in chemical bonds of molecules such as glucose and ATP.