48 Aerobic Respiration: Oxidative Phosphorylation
You have just read about two stages in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these stages. Rather, it derives from a process that begins with passing electrons through a series of chemical reactions to a final electron acceptor, oxygen. This is the only place in aerobic respiration where O2 is actually required. These reactions take place in specialized protein complexes located in the inner membrane of the mitochondria of eukaryotic organisms and on the inner part of the cell membrane of prokaryotic organisms. The energy of the electrons is used to generate ATP. The entirety of this process is called oxidative phosphorylation.
During oxidative phosphorylation:
- The energy from NADH and FADH2 is used up.
- Oxygen gas is converted into water.
- 30-36 ATP are recharged from ADP
More Details
The electron transport chain (Figure 1a) is the last component of aerobic respiration and is the only part of metabolism that actually uses atmospheric oxygen. Oxygen continuously diffuses into plants for this purpose. In animals, oxygen enters the body through the respiratory system. Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced. There are four complexes composed of proteins, labeled I through IV in Figure 1c, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes. As the electron travels through the electron transport chain, the energy that it contained is stored as potential energy by the cell. Specifically, the energy in the electrons is used pump hydrogen ions (H+) across the inner mitochondrial membrane into the intermembrane space, creating an electrochemical gradient. You can think of this gradient as being like water behind a dam – the potential energy stored can be used to turn a turbine and generate energy (electricity in the case of the dam, ATP in the case of the electron transport chain).
Electrons from NADH and FADH2 are passed to protein complexes in the electron transport chain. As they are passed from one complex to another (there are a total of four), the electrons lose energy, and some of that energy is used to pump hydrogen ions from the mitochondrial matrix into the intermembrane space. In the fourth protein complex, the electrons are accepted by oxygen, the terminal acceptor. The oxygen with its extra electrons then combines with two hydrogen ions, further enhancing the electrochemical gradient, to form water.