Membrane Permeability

You have learned that the membrane is selectively permeable, and that the hydrophobic region of the membrane acts as a selective-permeability barrier that allows hydrophobic molecules to diffuse across the bilayer at relatively rapid rates while serving as an almost impenetrable barrier to hydrophilic molecules. Many polar and charged molecules and ions therefore rely on the processes of facilitated diffusion and active transport to move across the membrane. That is not the whole story, however. Because the phospholipids are constantly moving about, small gaps open between the molecules for short periods of time, allowing small molecules, such as water and carbon dioxide, to move through. The rate of diffusion of a substance across a lipid bilayer parallels the solubility of that substance in a nonpolar solvent relative to its solubility in water.

Hydrophobic molecules are more soluble in nonpolar solvents than they are in water; therefore they diffuse across lipid bilayers fairly rapidly. Polar and ionic substances, on the other hand, are much less soluble in nonpolar solvents than they are in water. As a result, they diffuse slowly across the hydrophobic region of the membrane. As you can see from the diagram below however, water is a surprising exception to this rule that polar molecules diffuse slowly across lipid membranes. Why? The diffusion of many substances depends on its concentration. In living cells, there is a very high concentration of water (e.g., 55.5M) surrounding the bilayer, which facilitates the diffusion of water molecules. Therefore, even though water is a relatively polar substance, it diffuses more rapidly than other polar substances as a result of its abundance.