The Mitochondrial membranes

1. Cristae. The outer membrane is smooth while the inner membrane is thrown up into folds and tubules.

2. Electron transfer and ATP synthesis. The electron transfer and ATP synthesizing systems are located only in the inner membrane. Since the matrix contains the enzymes of the Krebs cycle, the inner mitochondrial membrane and the matrix together constitute the respiratory unit.

3. Lipids. The outer membrane contains more phospholipids (X3) and more cholesterol (X6) than the inner membrane. Phosphatidylcholine is the predominant lipid of the outer membrane. The inner membrane on the other hand contains most of the diphosphatidylglycerol of the mitochondrion.

4. Proteins. The outer membrane contains less than 10% of the total protein of the mitochondrion while the inner membrane contains as much as 60%. The outer membrane contains about 14 different proteins with molecular weights ranging from 12,000 to 220,000. The inner membrane

5. Enzymes. The outer membrane has a variety of enzymes, but none closely associated with respiration or oxidative phosphorylation. As mentioned previously, the electron transfer and ATP-synthesizing enzymes are located in the inner membrane.

6. Sialic acid. The outer membrane has X4 to X5 as much sialic acid as the inner membrane. The sialic acid is associated with glycolipid and glycoprotein.

7. Ion transport components. The inner membrane contains several ion-transport components for transporting ions such as phosphate, ADP, ATP, dicarboxylate, tricarboxylate, glutarate, α-ketoglutarate, aspartate, pyruvate, citrulline, ornithine, bicarbonate + CO2 , calcium, magnesium and sodium.

8. Diffusion. Substances upto MW 10,000 daltons can pass through the outer membrane by passive diffusion. On the other hand, except for uncharged molecules of MW<100 daltons, most substances cannot pass through the inner membrane by simple diffusion.