Water Permeability

Abstract

The plasma membrane of animal cells constitutes a selectively permeable barrier, the properties of which determine intracellular solute composition (e.g., Stein 1990). Characterising these properties has been a longstanding objective of biologists, and it is now recognised that a large array of membrane proteins regulates the exchange of ions and solutes between the cell and its extracellular environment. The passage of water across the lipid bilayer can also occur and, as the chief solvent for body fluids, this movement is axiomatic to life. The characterisation of water movement across cell membranes has relied heavily on the use of the erythrocyte as a paradigm (e.g., Rich et al. 1967). We now know that the swelling and subsequent haemolysis observed when erythrocytes are suspended in hypotonic media predominantly reflects diffusion of water through protein channels, termed aquaporins. In this chapter, we will consider the evidence that supports this assertion, and review the structure and function of the aquaporin family. The importance of aquaporin expression for erythrocyte function will also be discussed, both in terms of its conventional role, and, in the light of recent evidence, as a channel which can facilitate gas carriage.