Coupling of Electron Transport to ATP Production

According to the chemiosmotic theory, the electron-transport system, powered by the transport of electrons, pumps protons (H+) from the mitochondrial matrix into the space between the inner and outer mitochondrial membranes. The electron-transport system is grouped into three complexes that serve as proton pumps (fig. 5.10). The first pump (the NADH-coenzyme Q reductase complex) transports four H+ from the matrix to the intermembrane space for every pair of electrons moved along the electron-transport system. The second pump (the cy-tochrome c reductase complex) also transports four protons into the intermembrane space, and the third pump (the cytochrome c oxidase complex) transports two protons into the intermembrane space. As a result, there is a higher concentration of H+ in the intermembrane space than in the matrix, favoring the diffusion of H+ back out into the matrix. The inner mitochondrial membrane, however, does not permit diffusion of H+, except through structures called respiratory assemblies.

The respiratory assemblies consist of a group of proteins that form a "stem" and a globular subunit. The stem contains a channel through the inner mitochondrial membrane that permits the passage of protons (H+). The globular subunit, which protrudes into the matrix, contains an ATP synthase enzyme that is capable of catalyzing the reaction ADP + Pi ^ ATP when it is activated by the diffusion of protons through the respiratory assemblies and into the matrix (fig. 5.10). In this way, phosphory-lation (the addition of phosphate to ADP) is coupled to oxidation (the transport of electrons) in oxidative phosphorylation.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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