Figure 1. Mitochondria–cytosol interactions. Represented are the minimum exchanges between mitochondrial and cytosolic compartments. In most mitochondria, a variety of other oxidizable substrates, mainly Krebs cycle intermediates or their transamination partners, also traverse the inner membrane. Transport of substrate anions is catalysed by carrier proteins and is driven by either the membrane potential or pH component of the protonmotive force (see text).

Figure 2. A chemiosmotic overview of oxidative phosphorylation. Electron transport from NADH to  O₂ creates a proton gradient (the ‘protonmotive force’ of Mitchell), which drives ATP synthesis through a membrane-spanning, proton-translocating ATP synthase. No statement is made of the stoichiometry of H⁺ pumping per redox loop, or of the ATP-synthase (see text).

Figure 3. Redox potentials of components of the mitochondrial respiratory chain. The standard reduction potentials (E₀) at pH 7 are presented as blue bars for the redox centres that form the respiratory chain. The direction of electron flow is actually determined by the observed reduction potentials (E_h; see text) but these are not known in every case and are condition-dependent: some values are presented using green-coloured bars. It can be seen that there are three regions where there is a large E: in reactions catalysed by Complexes I, III and IV (dashed areas). The energy released by electrons as they traverse these steps is used to pump protons and create the protonmotive force. There is no large drop in E₀, or E_h, in Complex II, and there is no proton pumping by this complex. The appropriate redox couple for each cytochrome (cyt.) is the  Fe²⁺/ Fe³⁺ of the haem prosthetic group. Only some of the Fe–S centres that have been identified in Complexes I and II, in studies involving electron paramagnetic resonance spectroscopy and potentiometry, are represented. UQ = ubiquinone.

Figure 4. A model of the ‘binding-change’ mechanism of ATP synthase, as proposed by Boyer. The top panel depicts the catalytic sites of the soluble (F₁) portion of the ATP synthase. Rotation of the asymmetric protein, which forms the ‘stalk’, imparts conformational strain to the subunits, which bear the catalytic sites. This causes them to alternate between tight-binding (T), loose-binding (L) and open (O) conformations. ATP synthesis occurs spontaneously in the T environment, but then energy is required to generate the O conformation and release the ATP. The bottom panel presents a mechanism whereby the flow of protons through a channel in the F₀ portion of the ATP synthase causes the whole ‘collar’ of c subunits and the ‘stalk’ to spin relative to the catalytic subunits. From Cross and Duncan (1996) Subunit rotation in F₀F₁-ATP synthases as a means of coupling proton transport through F₀ to the binding changes in F₁. Journal of Bioenergetics and Biomembranes 28: 403–408; reproduced by permission of the Plenum Publishing Corporation.

Figure 5. Respiratory control of  O₂ uptake by mitochondria. Oxygen uptake by mitochondria added to a medium containing an oxidizable substrate plus inorganic phosphate is stimulated by ADP. When all of the ADP has been phosphorylated, the rate of respiration returns to a ‘resting’ value determined by the intrinsic leakage of protons across the inner membrane. Addition of an uncoupling agent dissipates the protonmotive force and allows a maximal rate of respiration.