Water Transport Across Cell Membranes

Abstract This chapter is focused on the pathways and molecular mechanisms of water transport across the plasma membrane of animal cells. We discuss basic principles of water transport, including diffusion and osmosis and apply these concepts to the case of cell‐membrane water transport. The general conclusion is that diffusion and osmosis can explain water transport and that other mechanisms (e.g. cotransport) are less likely. Concerning the pathways for water movement across the plasma membrane, we consider the lipid bilayer, water‐selective pores (aquaporins – AQPs), nonselective larger pores, ion channels and membrane carriers. We conclude that the lipid bilayers and AQPs are the main pathways for transmembrane water fluxes, by solubility–diffusion and single‐file transport, respectively. Cell‐membrane water permeability varies considerably from cell to cell; high permeability denotes a fluid lipid bilayer and expression of AQPs. Low water permeability occurs when there is no aquaporin expression and membrane is rich in cholesterol. Key Concepts: Water transport across cell membranes occurs bydiffusionandosmosis. Theeffective osmolalityof a biological fluid is determined by the total solute concentrations and the solutes’ permeabilities, relative to water. The cell‐membrane osmotic water permeability varies from cell to cell, depending on the composition of the lipid bilayer and the presence or absence of water pores. The two main pathways for plasma‐membrane water transport are the lipid bilayer and water‐selective pores (aquaporins). Aquaporins are a large family of water pores; some isoforms are water‐selective whereas others are permeable to small solutes. Aquaporin 1 (AQP1), the best studied isoform, is present in the membrane as a tetramer; each monomer has a water pore. The pore of AQP1 is long and narrow (c. 2.8 Å diameter); water molecules lose their hydrogen bonds and permeate in single file. AQP‐mediated water permeability appears to be regulated mostly by controlling the number of molecules present in the membrane. Regulation of the ‘opening’ of the pores has also been proposed for some isoforms. AQP1 genetic deletion in mice causes profound alterations of renal function, consistent with a major physiological role of this protein.