Electron tomography suggests that the proton pumps of the respiratory chain are located in the membrane regions at either side of the dimer rows. Protons pumped into the crista space by these respiratory-chain complexes are thought to diffuse very rapidly along the membrane surface, with the ATP synthase rows creating a proton “sink” at the cristae tips (Figure 14–33). In vitro studies suggest that the ATP synthase needs a proton gradient of about 2 pH units to produce ATP at the rate required by the cell, irrespective of the membrane potential. The H+ gradient across the inner mitochondrial membrane is only 0.5 to 0.6 pH units. The cristae thus seem to work as proton traps that enable the ATP synthase to make efficient use of the protons pumped out of the mitochondrial matrix. As we shall see in the next section, this elaborate arrangement of membrane protein complexes is absent in chloroplasts, where the H+ gradient is much higher.
Special transport proteins Exchange atp and aDp through the Inner Membrane
Like all biological membranes, the inner mitochondrial membrane contains numerous specific transport proteins that allow particular substances to pass through. One of the most abundant of these is the ADP/ATP carrier protein (Figure 14–34). This carrier shuttles the ATP produced in the matrix through the inner membrane to the intermembrane space, from where it diffuses through the outer mitochondrial membrane to the cytosol. In exchange, ADP passes from the cytosol into the matrix for recycling into ATP. ATP4– has one more negative charge than ADP3–, and the exchange of ATP and ADP is driven by the electrochemical gradient across the inner membrane, so that the more negatively charged ATP is pushed out of the matrix, and the less negatively charged ADP is pulled in. The ADP/ATP carrier is but one member of a mitochondrial carrier family: the inner
CYTOSOL pH 7.4 inner membraneMATRIX pH 7.9 ATP synthase
Figure 14–32 Dimers of mitochondrial ATP synthase in cristae membranes.
a three-dimensional map of a small mitochondrion obtained by electron microscope tomography shows that atp synthases form long paired rows along cristae ridges. the outer membrane is gray, the inner membrane and cristae membranes have been colored light blue. Each head of an atp synthase is indicated by a yellow sphere. (B) a three-dimensional map of a mitochondrial atp synthase  dimer in the crista membrane obtained by subtomogram averaging, with fitted x-ray structures (Movie 14.7). (a, from
K.
Davies et al., Proc. Natl Acad. Sci. USA 108:14121–14126, 2011. With permission from the National academy of Sciences;  B, from K. Davies et al., Proc. Natl Acad. Sci. USA 109:13602–13607, 2012. With permission from the National academy of Sciences.)
Figure 14–33 ATP synthase dimers at cristae ridges and ATP production. at the crista ridges, the atp synthases (yellow) form a sink for protons (red). the proton pumps of the electron-transport chain (green) are located in the membrane regions on either side of the crista. as illustrated, protons tend to diffuse along the membrane from their source to the proton sink created by the atp synthase. this allows efficient atp production despite the small h+ gradient between the cytosol and matrix. Red arrows show the direction of the proton flow.