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The amount of ATP that human beings need in order to go about their lives is staggering. A sedentary male of 70 kg (154 lb) requires about 8400 kJ (2000 kcal) for a day’s worth of activity. This much energy requires 83 kg of ATP. However, human beings possess only about 250 g of ATP, less than 1% of the daily required amount. The disparity between the amount of ATP that we have and the amount that we require is solved by recycling spent ATP back to usable ATP. Each ATP molecule is recycled from ADP approximately 300 times per day. This recycling takes place primarily through oxidative phosphorylation, in which ATP is formed as a result of the transfer of electrons from NADH or FADH₂ to O₂ by a series of electron carriers. This process, which takes place in mitochondria, is the major source of ATP in aerobic organisms. For example, oxidative phosphorylation generates 26 of the 30 molecules of ATP that are formed when 1 molecule of glucose is completely oxidized to CO₂ and H₂O.

Oxidative phosphorylation is the culmination of the series of energy transformations as presented in Section 8, called cellular respiration or, simply, respiration, in their entirety. Carbon fuels are first oxidized in the citric acid cycle to yield high-transfer-potential electrons. In oxidative phosphorylation, high-transfer-potential electrons flow through a series of large protein complexes embedded in the inner mitochondrial membrane, called the respiratory chain, to reduce oxygen to water. The electron flow through these complexes is a series of highly exergonic oxidation–reduction reactions that power the pumping of protons from the inside of the mitochondria to the outside, establishing a proton gradient, called the proton-motive force. The final phase of oxidative phosphorylation is carried out by an ATP-synthesizing assembly that is driven by the flow of protons back into the mitochondrial matrix. Oxidative phosphorylation vividly shows that proton gradients are an interconvertible currency of free energy in biological systems.

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We begin this section with an examination of how the electron-transport chain harnesses the energy released when electrons flow from NADH and FADH₂ to oxygen to generate a proton gradient. We will then see how the energy inherent in the proton gradient is converted into ATP. Finally, we will see how the process of oxidative phosphorylation is regulated and how transporters facilitate the movement of biochemicals between the mitochondria and the cytoplasm.

✓ By the end of this section, you should be able to: