Chapter 14 - Electron Transport and Oxidative Phosphorylation

Abstract

The energy requirements of aerobic cells are met by the energy released in the oxidation of carbohydrates, fatty acids, and amino acids by molecular oxygen. The numerous coupled redox reactions are tightly linked and take place in the inner mitochondrial membrane. This reaction sequence is called the respiratory chain electron transport. Electrons in substrate or cofactor begin with a high potential energy and end at oxygen with a lower potential energy. During this electron flow, a portion of the free energy liberated is conserved by an energy-transducing system, through which electrical energy is changed to chemical energy. Since the energy is conserved in the terminal phosphoanhydride bond of ATP through phosphorylation of ADP to ATP, the overall coupled process is known as oxidative phosphorylation. The three hypotheses that have been proposed to explain how the mechanism of energy conservation is coupled to electron transport are the chemical, conformational, and chemiosmotic hypotheses. The chemical hypothesis proposes that the energy is conserved by the formation of high-energy intermediates as reducing equivalents pass from one carrier to the next. The conformational hypothesis proposes that the energy-yielding steps generate protein conformational changes that are used in ATP synthesis. According to the chemiosmotic hypothesis, an electrochemical gradient (pH gradient), generated across the inner mitochondrial membrane by the passage of reducing equivalents along the respiratory chain, provides the driving force for the synthesis of ATP.