The Role of Electron Transport in Metabolism
Aerobic metabolism is a highly efficient way for an organism to extract energy from nutrients. In eukaryotic cells, the aerobic processes (including conversion of pyruvate to acetyl-CoA, the citric acid cycle, and electron transport) all occur in the mitochondria, while the anaerobic process, glycolysis, takes place outside the mitochondria in the cytosol. The reactions of the electron transport chain take place in the inner mitochondrial membrane.
The energy released by the oxidation of nutrients is used by organisms in the form of the chemical energy of ATP. Production of ATP in the mitochondria give ATP. The production of ATP by oxidative phosphorylation (an endergonic process) is separate from electron transport to oxygen (an exergonic process), but the reactions of the electron transport chain are strongly linked to one another and are tightly coupled to the synthesis of ATP by phosphorylation of ADP. The operation of the electron transport chain leads to pumping of protons (hydrogen ions) across the inner mitochondrial membrane, creating a pH gradient (also called a proton gradient). This proton gradient represents stored potential energy and provides the basis of the coupling mechanism (Figure 20.1). Chemiosmotic coupling is the name given to this mechanism. Oxidative phosphorylation gives rise to most of the ATP production associated with the complete oxidation of glucose.
The NADH and FADH2 molecules generated in glycolysis and the citric acid cycle transfer electrons to oxygen in the series of reactions known collectively as the electron transport chain. The NADH and FADH are oxidized to NAD+ and FAD and can be used again in various metabolic pathways. Oxygen, the ultimate electron acceptor, is reduced to water; this completes the process by which glucose is completely oxidized to carbon dioxide and water.
We have already seen how carbon dioxide is produced from pyruvate, which in turn is produced from glucose by the pyruvate dehydrogenase complex and the citric acid cycle.
The complete series of oxidation–reduction reactions of the electron trans- port chain is presented in schematic form in Figure 20.2. A particularly note- worthy point about electron transport is that, on average, 2.5 moles of ATP are generated for each mole of NADH that enters the electron transport chain, and, on average, 1.5 moles of ATP are produced for each mole of FADH2.
The general outline of the process is that NADH passes electrons to coenzyme Q, as does FADH2, providing an alternative mode of entry into the electron trans-port chain. Electrons are then passed from coenzyme Q to a series of proteins called cytochromes (which are designated by lowercase letters) and, eventually, to oxygen.
Electron transport from one carrier to another creates a proton gradient across the inner mitochondrial membrane.
The proton gradient is coupled to the production of ATP in aerobic metabolism