Water Permeability of Thin Lipid Membranes

The osmotic permeability coefficient, P(f), and the tagged water permeability coefficient, P(d), were determined for thin (<100 A) lipid membranes formed from ox brain lipids plus DL-alpha-tocopherol; their value of approximately 1 x 10(-3) cm/sec is within the range reported for plasma membranes. It was established that P(f) = P(d). Other reports that P(f) > P(d) can be attributed to the presence of unstirred layers in the experimental determination of P(d). Thus, there is no evidence for the existence of aqueous pores in these thin phospholipid membranes. The adsorption onto the membrane of a protein that lowers its electrical resistance by a factor of 10(3) was found not to affect its water permeability; however, glucose and sucrose were found to interact with the membrane to modify P(f). Possible mechanisms of water transport across these films are discussed, together with the implications of data obtained on these structures for plasma membranes.