Through a set of reactions that occur in the cytosol, energy derived from the partial oxidation of energy-rich carbohydrate molecules is used to form ATP, the chemical energy currency of cells (discussed in Chapter 2). But a much more efficient method of energy generation appeared very early in the history of life. This process is based on membranes, and it enables cells to acquire energy from a wide variety of sources. For example, it is central to the conversion of light energy into chemical bondenergy in photosynthesis, as well as to the aerobicrespiration that enables us to use oxygen to produce large amounts of ATP from food molecules.
