Transport Pathways in Biological Membranes

This review deals with transport through biological membranes. The term "mem­ brane" is used in a purely pragmatic sense: a sheet of unstirred material separating two solutions that can be considered well stirred. What unifies the concept of membrane is its physical function as a barrier as well as the methods by which such structures can be studied. The definition therefore covers, for instance, capillary walls and kidney tubule walls as well as plasma membranes. In recent years there has been a tendency to replace the above "classical" definition of a membrane by one based on the appearance of the structure in an electron microscopic preparation. Thus a biological membrane would be something that appears as a dark double line, for instance, a plasma membrane. Such a restriction of the definition is not advisable, however. Even membranes with apparently simple appearance may show evidence of functional sandwich structure. Thus the permeability properties of nerve membranes can be greatly changed by, say, adsorbed inorganic ions (25). Such adsorption, although not chang­ ing the total membrane potential, may modify the potential profile across the membrane, drastically changing the potential shift necessary to trigger electric activity. The permeability may also be modified by the presence of mucopolysaccha­ rides and proteins, if for no other reason than because such substances produce unstirred layers. Functionally, unstirred layers, whether or not supported by such