In cellular respiration, the chemical energy stored in a molecule of glucose is used to produce ATP in two different ways (Fig. 7.1). In the first, a phosphorylated organic molecule directly transfers a phosphate group to ADP, as we saw in Chapter 6. In this case, there are two coupled reactions carried out by a single enzyme: the hydrolysis of a phosphorylated organic molecule and the addition of a phosphate group to ADP. The hydrolysis reaction releases enough free energy to drive the synthesis of ATP. This way of generating ATP is called substrate-level phosphorylation because a phosphate group is transferred to ADP from an enzyme substrate, in this case an organic molecule.

Substrate-level phosphorylation produces only a small amount of the total ATP generated in cellular respiration, about 12% if the fuel molecule is glucose. Most of the ATP produced during cellular respiration (the remaining 88%) is produced in a different way (Fig. 7.1). In this case, the chemical energy of organic molecules is transferred first to electron carriers. The role of these electron carriers is exactly what their name suggests—they carry electrons (and energy) from one set of reactions to another. In cellular respiration, electron carriers transport electrons released during the catabolism of organic molecules to the respiratory electron transport chain. Electron transport chains in turn transfer electrons along a series of membrane-associated proteins to a final electron acceptor and in the process harness the energy released to produce ATP. In aerobic respiration, oxygen is the final electron acceptor, resulting in the formation of water. This way of generating ATP is called oxidative phosphorylation.

How electron movement through the electron transport chain is coupled to ATP synthesis is central to the energy economy of most cells and is discussed later in the chapter. Electron transport chains are used in respiration to harness energy from fuel molecules such as glucose and in photosynthesis to harness energy from sunlight (Chapter 8).