y bacterial porins (see Figure 5-14). Ions and most small molecules (up to about 5000 Da) can readily pass through these channel proteins. Although the flow of metabolites across the outer membrane may limit their rate of mitochondrial oxidation, the inner membrane and cristae are the major permeability barriers between the cytosol and the mitochondrial matrix.

Freeze-fracture studies indicate that mitochondrial cristae contain many protein-rich intramembrane particles. Some are the F0F1 complexes that synthesize ATP; others function in transporting electrons to O2 from NADH or other elec

▲ FIGURE 8-7 Summary of the aerobic oxidation of pyruvate and fatty acids in mitochondria. The outer membrane is freely permeable to all metabolites, but specific transport proteins (colored ovals) in the inner membrane are required to import pyruvate (yellow), ADP (green), and Pj (purple) into the matrix and to export ATP (green). NADH generated in the cytosol is not transported directly to the matrix because the inner membrane is impermeable to NAD+ and NADH; instead, a shuttle system (red) transports electrons from cytosolic NADH to NAD + in the matrix. O2 diffuses into the matrix and CO2 diffuses out. Stage 1: Fatty acyl groups are transferred from fatty acyl CoA and transported across the inner membrane via a special carrier (blue oval) and then reattached to CoA on the matrix side.

tron carriers. Various transport proteins located in the inner membrane and cristae allow otherwise impermeable molecules, such as ADP and P(, to pass from the cytosol to the matrix, and other molecules, such as ATP, to move from the matrix into the cytosol. Protein constitutes 76 percent of the total weight of the inner membrane—a higher fraction than in any other cellular membrane. Cardiolipin (diphosphatidyl glycerol), a lipid concentrated in the inner membrane, sufficiently reduces the membrane's permeability to protons that a proton-motive force can be established across it.

Pyruvate is converted to acetyl CoA with the formation of NADH, and fatty acids (attached to CoA) are also converted to acetyl CoA with formation of NADH and FADH. Oxidation of acetyl CoA in the citric acid cycle generates NADH and FADH2. Stage 2: Electrons from these reduced coenzymes are transferred via electron-transport complexes (blue boxes) to O2 concomitant with transport of H+ ions from the matrix to the intermembrane space, generating the proton-motive force. Electrons from NADH flow directly from complex I to complex III, bypassing complex II. Stage 3: ATP synthase, the F0F1 complex (orange), harnesses the proton-motive force to synthesize ATP Blue arrows indicate electron flow; red arrows transmembrane movement of protons; and green arrows transport of metabolites.

Outer mitochondrial membrane (permeable to metabolites) CO2 Intermembrane space

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