Ik

Succinate Fumarate

FIGURE 19-15 Summary of the flow of electrons and protons through the four complexes of the respiratory chain. Electrons reach Q through Complexes I and II. QH2 serves as a mobile carrier of electrons and protons. It passes electrons to Complex III, which passes them to another mobile connecting link, cytochrome c. Complex IV

then transfers electrons from reduced cytochrome c to O2. Electron flow through Complexes I, III, and IV is accompanied by proton flow from the matrix to the intermembrane space. Recall that electrons from 3 oxidation of fatty acids can also enter the respiratory chain through Q (see Fig. 19-8).

Much of this energy is used to pump protons out of the matrix. For each pair of electrons transferred to O2, four protons are pumped out by Complex I, four by Complex III, and two by Complex IV (Fig. 19-15). The vectorial equation for the process is therefore

The electrochemical energy inherent in this difference in proton concentration and separation of charge represents a temporary conservation of much of the energy of electron transfer. The energy stored in such a gradient, termed the proton-motive force, has two components: (1) the chemical potential energy due to the difference in concentration of a chemical species (H+) in the two regions separated by the membrane, and (2) the electrical potential energy that results from the separation of charge when a proton moves across the membrane without a counterion (Fig. 19-16).

As we showed in Chapter 11, the free-energy change for the creation of an electrochemical gradient by an ion pump is

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