The movement of water across cell membranes has been an active area of research for more than 100 years and is of fundamental importance in the normal water metabolism of all terrestrial animals. The objective of this review is to integrate recent data obtained from the isolation and molecular cloning of water channel proteins, with functional information provided by biophysical measurements of membrane water transport. Whereas the water permeability of most cell membranes can be accounted for by the diffusion of water across the lipid bilayer, other cells, including the erythrocyte as well as certain cells in renal epithelia, possess specialized water channels. Water channels are composed of specialized proteins that create highly selective aqueous pores across cell membranes. Data concerning the distribution, permeability, and function of these various water channels will greatly enhance our knowledge of how water is transported across cell membranes.
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