Unlike the outer mitochondrial membrane, the inner mitochondrial membrane is a diffusion barrier to ions and small molecules, just like the bacterial inner membrane. However, selected ions, most notably protons and phosphate, as well as essential metabolites such as ATP and ADP, can pass through it by means of special transport proteins.
The inner mitochondrial membrane is highly differentiated into functionally distinct regions with different protein compositions. As discussed in Chapter 10, the lateral segregation of membrane regions with different protein and lipid compositions is a key feature of cells. In the inner mitochondrial membrane, the boundary membrane region is thought to contain the machinery for protein import, new membrane insertion, and assembly of the respiratory-chain complexes. The membranes of the cristae, which are continuous with the boundary membrane, contain the ATP synthase enzyme that produces most of the cell’s ATP; they also contain the large protein complexes of the respiratory chain—the name given to the mitochondrion’s electron-transport chain.
At the cristae junctions, where the membranes of the cristae join the boundary membrane, special protein complexes provide a diffusion barrier that segregates the membrane proteins in the two regions of the inner membrane; these complexes are also thought to anchor the cristae to the outer membrane, thus maintaining the highly folded topology of the inner membrane. Cristae membranes have one of the highest protein densities of all biological membranes, with a lipid content of 25% and a protein content of 75% by weight. The folding of the inner membrane into cristae greatly increases the membrane area available for oxidative phosphorylation. In highly active cardiac muscle cells, for example, the total area of cristae membranes can be up to 20 times larger than the area of the cell’s plasma membrane. In total, the surface area of cristae membranes in each human body adds up to roughly the size of a football field.
the Citric acid Cycle in the Matrix produces NaDh
Together with the cristae that project into it, the matrix is the major working part of the mitochondrion. Mitochondria can use both pyruvate and fatty acids as fuel. Pyruvate is derived from glucose and other sugars, whereas fatty acids are derived from fats. Both of these fuel molecules are transported across the inner mitochondrial membrane by specialized transport proteins, and they are then converted to the crucial metabolic intermediate acetyl CoA by enzymes located in the mitochondrial matrix (see Chapter 2).
in medium of low osmolarity the infux of water causes the mitochondrion to swell and the outer membrane to rupture, releasing the contents of the intermembrane space; the inner membrane remains intact FOOD MOLECULES FROM CYTOSOL
The acetyl groups in acetyl CoA are oxidized in the matrix via the citric acid cycle, also called the Krebs cycle (see Figure 2–57 and Movie 2.6). The oxidation of these carbon atoms in acetyl CoA produces CO2, which diffuses out of the mitochondrion to be released to the environment as a waste product. More importantly, the citric acid cycle saves a great deal of the bond energy released by this oxidation in the form of electrons carried by NADH. This NADH transfers its electrons from the matrix to the electron-transport chain in the inner mitochondrial membrane, where—through the chemiosmotic coupling process described previously (see Figures 14–2 and 14–3)—the energy that was carried by NADH electrons is converted into phosphate-bond energy in ATP. Figure 14–10 outlines this sequence of reactions schematically.