Carrier-Mediated Ion Transport Across Thin Lipid Membranes

Abstract

The molecular basis of ion transport across biological membranes is largely unknown at present. Its study is complicated above all by the complex nature of living systems, which allows the application of suitable physical methods only to a small number of especially favorable systems. Even in these cases, interpretation of experimental results is usually hampered by the presence of several other transport systems, even when these can be partially eliminated by the use of special blocking agents. A different approach, designed to circumvent these difficulties, consists in a transfer of the transport system under study to model membranes. Planar lipid membranes, which are formed according to a technique originally developed by MUELLER et al. (1962), seem to be especially appropriate for the study of ion transport, since they easily allow for the application of an electric voltage difference across the membrane and the measurement of the electric current. In addition, as is shown in detail in the previous article, the resistance of an unmodified black film assumes rather high values, typically 108 Ωcm2. Therefore, the incorporation of relatively few, and in some cases even of single “transport entities” into a black film may yield a significant change in the membrane resistance. The isolation of special transport proteins from biological membranes and their active reconstitution into black films have proved to involve extreme difficulties, and the results must be considered as rather modest up to now.