ABSTRACT
O2 is the terminal electron acceptor in the electron transport chain on the inner membrane of the mitochondrion. Reduced cofactors (three molecules of reduced nicotinamide adenine dinucleotide (NADH) and one of reduced flavine adenine dinucleotide (FADH2)) are produced at the four oxidative steps in the tricarboxylic acid (TCA) cycle per molecule of acetyl-CoA oxidized to CO2. Other sources of reduced nucleotides in the mitochondrion are oxidation of pyruvate to CO2 and β-oxidation of fatty acids. It is during the redox reactions that take place as electrons flow down the electron transport chain from the reduced cofactors to oxygen that sufficient energy is released to drive ATP synthesis. In terms of efficiency, approximately 40% of the energy released from the redox reactions is stored in the high-energy phosphate bonds of ATP, while the rest is released as heat and helps maintain body temperature at 37°C, well above ambient temperature. The high-energy phosphate bonds are used in other energy-requiring processes in the cell, including maintenance of transmembrane ionic gradients, movement and enzyme reactions.
