Membrane Transport in Biology - Böcker

The contributions of this volume are concerned with transport phenomena in multimembrane systems and in simple epithelia. In addition to the very substan­ tial progress that has been made in the area of transport of fluid and solutes across artifical model membranes in vitro and across simple symmetrical cell membranes, much has been learned from studies of transport phenomena in multi membrane systems of higher complexity to be reviewed in this volume. It should be recalled that many of the fundamental conceptual and methodological problems of transport physiology have been successfully approached and defin­ ed by studying simple epithelia in vitro, and that the direction that research has taken has been affected in a major way by the cellular transport models that have evolved from this approach. Since then striking progress has been made in several areas. Not only have we been witnessing a keen and productive interest in the realtionship between fine structure and transport behavior in multimem­ brane systems but significant advancements have also been made in defining individual active and passive transport operations, in analysing cell ion activities and transport pools, and in describing the differences in transport functions that underly the membrane asymmetry and cell polarization of cells subserving di­ rectional transport.

This Volume forms the cornerstone of this series of four books on Membrane Transport in Biology. It includes chapters that address i) the theoretical basis of investigations of transport processes across biological membranes, ii) some of the experimental operations often used by scientists in this field, iii) chemical and biological properties common to most biological membranes, and iv) planar thin lipid bilayers as models for biological membranes. The themes developed in these chapters recur frequently throughout the entire series. Transport of molecules across biological membranes is a special case of diffu­ sion and convection in liquids. The conceptual frame of reference used by investigators in this field derives, in large part, from theories of such processes in homogeneous phases. Examples of the application of such theories to transport across biological membranes are found in Chapters 2 and 4 of this Volume. In Chapter 2, Sten-Knudsen emphasizes a statistical and molecular approach while, in Chapter 4 Sauer makes heavy use of the thermodynamics of irreversi­ ble processes. Taken together, these contributions introduce the reader to the two sets of ideas which have dominated the thinking of scientists working in this field. Theoretical consideration of a more special character are also included in several other Chapters in Volume I. For example, Ussing (Chapter 3) re-works the flux ratio equation which he introduced into the field of transport across biological membranes in 1949.

This second Volume in the series on Membrane Transport in Biology contains a group of essays on transport across single biological membranes separating the inside and outside of cells or organelles. We have not attempted to include material on all types of plasma and intracellular membranes, but rather have emphasized structures which have been studied relatively thoroughly. Four chapters describe transport of different types of molecules and ions across the plasma membranes of mammalian red cells. Two essays concern the excitable membranes of nerve and muscle cells while the remaining four chapters treat transport across several types of intracellular membranes. Water makes up more than two-thirds of the mass of most living cells. The transport of water between the inside and outside of cells and organelles is important for the function of these structures. As a result of investigations in many laboratories over the past four decades, our picture of the water permea­ bility of the red cell membranes is rather detailed when compared to the water permeability of other biological membranes. In Chapter 1, R. I. Macey describes this picture and also considers the permeability of red cell membranes to non­ electrolytes, including metabolic substrates such as sugars, amino acids, purines and nucleosides.

Well over one decade has passed since the appearance of the original four volumes of Membrane Transport in Biology. Since the publication of the last volume there have been spectacular advances in this field. These advances have been in part the result of the application of exciting new methodologies, and in part the result of new insights into the regulation and integration of transport processes. This volume, as well as a sixth volume, which is in preparation, are intended to cover key areas in which the development has been particularly striking. For many years the trend in studies of membrane transport had been that of increasing specialization with regard to the transporter of interest and of the cell or tissue studied. This trend was supported by the enormous number of publications directed at understanding the cellular physiology of specific organ systems and tissues, and also by the fact that different tissues often seemed to react so differently to the same conditions that mechanisms unique to each appear to be at play. One of the happy developments in recent years has been the realization that this apparent disparity of behaviors in different tissues is based on varying combinations of a limited number of transport mechanisms, all mediated by the same or similar proteins. Some of these transport proteins have already been isolated and analyzed with respect to amino acid sequence whereas others are just entering this phase.