Fig. 4 Respiratory-chain reactions associated with the inner mitochondrial membrane. Mitochondria are enclosed by two membranes; the outer membrane (not shown) is permeable to metabolites but the inner one is not. NAD(P)H in the mitochondrial matrix and in the cytosol can be oxidized by several mitochondrial dehydrogenases. Complex I oxidizes matrix NADH and in so doing pumps protons from the matrix to the cytosolic side of the inner membrane. Other inner-membrane-bound dehydrogenases (indicated by DH) do not pump protons. Inner-membrane dehydrogenases (including complex II, which functions as part of the TCA cycle) transfer electrons (and protons) to ubiquinone (Q), which is reduced to ubiquinol (QH₂) in the process. A mobile Q/QH₂ pool exists in the inner membrane. Complex III oxidizes QH₂ and passes electrons to cytochrome (cyt) c, which in turn passes electrons to complex IV. Electron transport through complexes III and IV is coupled to proton translocation across the inner membrane; a "Q-cycle" associated with Complex III is included in the diagram. Free O₂ is reduced to water by complex IV. The alternative oxidase (alt ox) can also oxidize QH₂, and form water, but this bypasses two sites (complexes III and IV) of proton translocation in the mitochondrial electron transport chain. Protons may enter the matrix through membrane leaks, but F_o-F₁ ATP synthase (similar to the CF_o-CF₁ ATP synthase in Fig. 1) is the main route of proton entry. Apparently, one ADP is phosphorylated when three protons pass through the F₀-F₁ ATP synthase. The ADP required for ATP formation enters the mitochondrial matrix only as ATP exits the matrix through an antiporter. A symporter couples the transport of P_i and H⁺ into the matrix

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