The permeability properties of "unmodified" thin ( < 100 Angstroms) lipid membranes separating two aqueous phases are those expected of a thin layer of hydrocarbon. Certain molecules interact with these membranes to enormously increase their permeability to ions and hydrophilic nonelectrolytes. Some of these molecules function as cation carriers with high specificity for K+ over Na+; some create aqueous pores approximately 4A in radius; others create strongly voltage-dependent conductance sites that can give rise to action potentials kinetically similar to those of nerve and muscle. These agents produce local modifications occupying a small fraction of the membrane area; ie, the "modified" membrane is a mosaic structure. Permeability and pharmacological data demonstrate that plasma membranes are also mosaic structures. If the Davson-Danielli model of plasma membranes is essentially correct, modified thin lipid membranes are both phenomenologically and structurally an excellent model for biological membranes.
Finkelstein A. Thin Lipid Membranes: A Model for Cell Membranes. Arch Intern Med. 1972;129(2):229–240. doi:10.1001/archinte.1972.00320020073005
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