Chapter 1 Ion Permeability of Membranes. From Lipid Bilayers to Biological Membranes

Although cells were first observed as early as in the 17th century, it is only over the last 100 years or so that our understanding of the permeability barrier imposed by the plasma membrane of cells started to emerge. The last 50 years, in particular, has seen a marked development in our knowledge of the characteristics of this barrier. The membrane itself comprises in the most part a bilayer of phospholipids, asymmetrically distributed, together with cholesterol and intrinsic and extrinsic proteins. Simple diffusion through the lipid phase can occur for lipid soluble moieties. In other cases, intrinsic membrane proteins provide pathways for solutes and water. Three types of pathways are described: channels, carriers, and pumps. What is known about flux through these proteins, together with their structure and function, is discussed-illustrated with reference to particularly significant pathways. The important part played by the red blood cell as a paradigm for membrane transport is explained. Existing gaps in our understanding of transport across membranes are emphasized. For example, how pumps like the quintessential membrane, the Na+-K+ ATPase, operate remains unknown. In addition, the nature of the residual (or "leak") permeability, which remains when all such pathways are inhibited, is unclear. The residual permeability of biological membranes appears to be about 2 orders of magnitude greater than that of artificial lipid bilayers. An important caveat here is the existence of unknown pathways yet to be described. In this context, a novel permeability with characteristics of K+(Na+)/H+ exchange is described which becomes manifest across the red blood cell membrane when cells are suspended in low ionic strength solution. Future discoveries will add to our understanding of membrane permeability. It is likely that the red blood cell will play an important part in this new chapter. © 2009 Elsevier Inc. All rights reserved.