Water Movement through Membrane Channels

This chapter reviews the findings from water permeability studies on both unmodified and channel-modified planar lipid bilayers, with particular emphasis on the latter. It discusses their implications for water transport across plasma membranes. The only direct data on water transport through biological-like channels come from studies on the channels formed in planar lipid bilayer membranes by gramicidin A and the polyene antibiotics nystatin and amphotericin B. From the water permeability coefficients for single nystatin or gramicidin A channels, one can estimate the number of channels required in plasma membranes to account for their water permeability. The general conclusion is that there are too few channels in most plasma membranes to represent a significant pathway for water movement, and, therefore, by implication, the major route for water transport across most plasma membranes is through their lipid bilayers. The interesting topic of water-ion interaction within channels is also highlighted in the chapter.