ABSTRACT
The main cause of solute or solvent flow across a phospholipid membrane is the transbilayer concentration gradient, the electrostatic potential gradient, or the pressure. The chief transport mechanism, in biological systems at least, is solute diffusion through pores and channels. Incorporation of channels into phospholipid bilayers also may increase the transbilayer water flow appreciably. This is owing to the low resistance of the channel's pore, which is normally relatively hydrophilic, to water penetration. For the permeation of noncharged solutes across a phospholipid bilayer only the latter, nonelectrostatic contributions play an important role. In the simplest approximation this can be seen from the average value of the hydrocarbon partition function. Bilayer permeability for inorganic and small organic ions decreases with ionic size. It also decreases with increasing bilayer hydration, at least when the rate-limiting step occurs in the interfacial region and no specific ion-lipid association takes place. Pores may arise in phospholipid bilayers spontaneously in the defect-rich bilayer regions.
