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Oxidative Phosphorylation

CHAPTER

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Mitochondria, stained green, form a network inside a fibroblast cell (left). Mitochondria oxidize carbon fuels to form cellular energy in the form of ATP.

[(Left) Courtesy of Michael P. Yaffee, Department of Biology, University of California at San Diego.]

OUTLINE

The amount of ATP that human beings require to go about their lives is staggering. A sedentary male of 70 kg (154 lbs) requires about 8400 kJ (2000 kcal) for a day’s worth of activity. To provide this much energy requires 83 kg of ATP. However, human beings possess only about 250 g of ATP at any given moment. The disparity between the amount of ATP that we have and the amount that we require is compensated by recycling ADP back to ATP. Each ATP molecule is recycled approximately 300 times per day. This recycling takes place primarily through oxidative phosphorylation.

We begin our study of oxidative phosphorylation by examining the oxidation–reduction reactions that allow the flow of electrons from NADH and FADH₂ to oxygen. The electron flow takes place in four large protein complexes that are embedded in the inner mitochondrial membrane, together called the respiratory chain or the electron-transport chain.

The overall reaction is exergonic. Importantly, three of the complexes of the electron-transport chain use the energy released by the electron flow to pump protons out of the mitochondrial matrix. In essence, energy is transformed. The resulting unequal distribution of protons generates a pH gradient and a transmembrane electrical potential that creates a proton-motive force. ATP is synthesized when protons flow back to the mitochondrial matrix through an enzyme complex.

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Thus, the oxidation of fuels and the phosphorylation of ADP are coupled by a proton gradient across the inner mitochondrial membrane (Figure 18.1).

FIGURE 18.1Overview of oxidative phosphorylation. Oxidation and ATP synthesis are coupled by transmembrane proton fluxes. Electrons flow from NADH and FADH₂ through four protein complexes to reduce oxygen to water (yellow tube). Three of the complexes pump protons from the mitochondrial matrix to the intermembrane space. The protons return to the matrix by flowing through another protein complex, ATP synthase (red structure), powering the synthesis of ATP.

Respiration

An ATP-generating process in which an inorganic compound (such as molecular oxygen) serves as the ultimate electron acceptor. The electron donor can be either an organic compound or an inorganic one.

Collectively, the generation of high-transfer-potential electrons by the citric acid cycle, their flow through the respiratory chain, and the accompanying synthesis of ATP is called respiration or cellular respiration.