Water Transport Across Cell Membranes


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 by diffusion and osmosis.

  • The effective osmolality of 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.

Keywords: water transport; diffusion; osmosis; permeability; pores; cell membranes; aquaporin

Figure 1.

Water diffusion across a lipid membrane separating solutions of identical compositions and permeable to water, but not solute (semipermeable membrane). Solutes denoted by blue dots, selected water molecules by open circles. The box on the top left side denotes water self‐diffusion (i.e. random motion by thermal agitation). Water diffusion results in collisions of water molecules with the membrane and permeation by solubility diffusion. In as much as the solute concentrations are the same on both sides, the water chemical potentials are also the same. Thus, the unidirectional fluxes are the same in both directions (middle box) and the net flux is zero. When the water chemical potentials differ, there is a net water flux from the side of higher water chemical potential (lower solute concentration).

Figure 2.

Osmotic equilibrium. A ‘cell’ filled with a concentrated solution of impermeant solute (blue dots) is suspended in a dilute solution of the same solute. The differences in solute concentration create a difference in osmotic pressure across the membrane (Δπ), causing a net flow of water (open circles) into the cell, raising its hydrostatic pressure (measured from the difference in fluid height, ΔP). Osmotic equilibrium is reached when ΔP=Δπ (see text).

Figure 3.

Proposed mechanism of osmotic water flow across a porous membrane. The driving force is a difference in osmotic pressure (solute present on the right side only). Top: because the solute cannot enter the pore, there is a sharp fall in water concentration (Cw) at the solution‐facing mouth of the pore. Middle: this creates an imbalance in water fluxes across the interface: more water molecules leave than enter the pore, and thus the hydrostatic pressure (P) falls within the pore, creating a driving force for water flux from left to right. Bottom: the net driving force is the water chemical potential (μw), proportional to the sum of hydrostatic and osmotic pressures.

Figure 4.

Proposed water transport pathways across plasma membranes. (a) Solubility–diffusion across the phospholipid bilayer. The water permeability increases with the fluidity of the membrane. (b) Permeation via narrow pores (aquaporins) by single file. (c) Permeation via wide pores by viscous flow governed by Pouiseuille's law. (d) Permeation via ion channels: single file of water molecules separated by ions. (e) Permeation through carriers: water is contained in the carrier together with substrate(s) and cotransported by a conformational change. As discussed in the text, permeation through the lipid bilayers and aquaporins are the two main mechanisms. Open circles, water molecules and filled circles, solute molecules.

Figure 5.

Ribbon representation of a human AQP1 monomer viewed from the extracellular side. The amino acid residues surrounding the pore are shown in spherical representation (yellow), and are (starting clockwise from the asterisk): Asn76, Asn192, Leu75, Phe24, Ile60, Val176, Leu149 and Ile191. The minimum diameter of the pore is approximately 2.8 Å (a water molecule's molecular diameter is approximately 2.75 Å). In the plasma membrane AQP1 forms tetramers, with each subunit contributing a water pore, see text.



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Further Reading

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Wang Y, Shaikh SA and Tajkhorshid E (2010) Exploring transmembrane diffusion pathways with molecular dynamics. Physiology 25: 142–154.

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Reuss, Luis(Jan 2012) Water Transport Across Cell Membranes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020621.pub2]