Permeability of water and polar solutes in lipid bilayers

The three commonly used formalisms to describe water and solute permeation in lipid bilayers (namely, solubility-solute properties, activated rate processes and the thermodynamics of the irreversible process theory) are analyzed in the light of experimental results. These approaches are based on the consideration of the lipid bilayer as a composite membrane containing a hydrocarbon core, an H-bonded interfacial network and a fluctuating structure in which pores can appear. The particular structure of the lipid bilayer (i.e., a hydrophobic-hydrophilic leaflet) makes the permeation process of polar solutions more complicated than that occurring in inert polymeric membranes. Thus, the permeation theories of Fick, Henry and Kedem and Katchalsky should be adapted to introduce interfacial and elastic phenomena. A critical analysis of the experimental results available in the current literature opens the possibility to formulate a broader formalism for permeation in lipid membranes.