The contributions of electron microscopy to membrane biology have been indispensable and, at the same time, disappointing. Membranes were known to exist before the advent of electron microscopy and general principles of their composition and molecular organization had been deduced from permeability and electrical conductivity measurements, polarized light microscopy, and X-ray diffraction. On the other hand, the complexities of the many intracellular membranes and membranous organelles were really not suspected until they were observed by the electron microscopist. One then had further hopes that the high resolution of the electron microscope (theoretically it can resolve atomic distances) would allow the visualization of the molecular architecture of membranes and lead directly to an under standing of structure and function at the molecular level. This expectation has been largely unrealized. Because of the great difficulties encountered in the preparation of biological material, because of the uncertainty of the chemistry of "staining," and because of numerous electron optical artifacts, it has been a major challenge just to rationalize the observed images in terms of the known facts, let alone to utilize the images to expand our knowledge of the molecular structure of biological membranes. The many differences among membranes with respect to function and composition are lost in the universal trilamellar image. Perhaps the one major exception to this, and the major structural contribution of electron microscopy at the molecular level, has come from freeze-etch electron microscopy."

Less than a year before this writing, a Nobel Prize was shared by Albert Claude, Christian de Duve, and George Palade, pioneers in the development of modern cell biology, of which membrane biology is an integral part. For many years, a seemingly unbridgeable gap separated the physiologist working at the organ level from the biochemist studying the molecular composition of cell constituents and the chemical reactions that occur in water-soluble extracts of cells. Physiology has a long history, and the disciplines epitomized by intermediary metabolism and molecular biology progressed rapidly during the 1950s and 1960s. Meanwhile, electron micros copists painstakingly mapped the newly discovered intracellular world of membranes, organelles, microtubules, and microfilaments, and other scien tists developed techniques for the quantitative separation and characteriza tion of these intracellular structures. Thus it finally became possible to localize the many enzymes, and the metabolic activities they catalyze, to recognizable structures whose composition and organization can be studied. We are now well on our way to bridging that gap between biochemistry and physiology-to understanding how the cell functions.

The short period since the publication of Volume 1 of Methods in Membrane Biology has been a time of momentous progress. Calorimetry, electron spin and nuclear magnetic resonance, X-ray diffraction, and freeze-cleavage electron microscopy, reinforced by biochemical analyses and enzymatic studies, have led to universal acceptance of a generalized membrane model. All membrane biologists would agree that a major element of all biological membranes is a bilayer of phospholipids which, in some instances, also contains other lipids, notably sterols and glycolipids. The fatty acid com position of the lipids of most membranes is such that the lipids are above their transition temperatures in their normal environment so that the bilayer is fluid. The microviscosity of the fatty acyl groups decreases pro gressively down the chain so that, at the hydrocarbon interior of the bilayer, the lipid phase has a viscosity approximating that of olive oil at room temperature. As a consequence of this membrane fluidity, a phospholipid molecule is very mobile within the plane of the membrane (moving a distance of about 1-2 fLm in 1 s) but the movement of a phospholipid molecule from one side of the membrane bilayer to the other (flip-flop) is very slow. The lipid bilayer is an essentially inert and rather impermeable struc ture, as shown by many studies with model systems. Proteins, of course, provide the catalytic components of the membranes. as well as playing a significant structural role.

One property common to all cells is transport. Molecules and ions must enter and leave cells by crossing membranes in a controlled manner. The process may take any of several forms: simple diffusion, carrier-mediated diffusion, active transport, or group translocation. There is more than one way to measure each. Transport kinetics, with particular reference to the red blood cell, were discussed in a previous volume. Three chapters deal with the general subject of transport in this volume. Maloney, Kashket, and Wilson summarize the appropriate methodology for studying metabolite and ion transport in bacteria, and Kimmich describes the relevant method ology for the isolated intestinal epithelial cell. The methods described in these two chapters have general application to transport studies in single cells from any source. The approach described in these two complementary articles is extended in the chapter by Hochstadt and her collaborators on the use of isolated membranes from bacterial and mammalian cells for the study of trans port phenomena. If one can prepare a suitable plasma membrane fraction (sealed, impermeable vesicles with the necessary transport components intact), it becomes possible to separate the events of transport from any subsequent metabolism that may occur in the cell. Isolated membrane vesicles are relatively easy to obtain from bacteria, and they are com paratively well studied. Work with similar preparations from cultured mammalian cells is just beginning but has much promise.

Three articles make up Volume 10 of Methods in Membrane Biology. In the first of these, Papahadjopoulos, Poste, and Vail extensively review much of the available data on the fusion of natural membranes, model membranes (liposomes), and natural membranes with liposomes. The authors are led by their review of the experimental methods and their interpretations of the results obtained to a general theory of membrane fusion which they believe is applicable to all systems that have been studied. Arguing that although protein and carbohydrate may serve, in some cases, to bring membranes into sufficiently close proximity for fusion to occur and, in other cases, to remove peripheral and integral proteins from the regions that are to undergo fusion, the authors conclude that membrane fusion per se is solely a property of the lipid bilayer. In their view, all the experimental observations to date can be subsumed under a unifying hypothesis in which membrane fusion is the result of a phase separation in one-half of the membrane bilayer brought about by the interaction - of calcium ions with acidic phospholipids, mostly phosphatidylserine. Where half-membranes already contain sufficient acidic phospholipids, a local increase in calcium ion concentration may suffice to induce fusion (examples might include exocytosis and fusion of intracellular membrane systems). In other cases, natural or experimentally induced events preceding fusion might be necessary to increase the local concentration of the acidic phospholipids in the half-membrane (virus-or fusogenic agent-induced cell-to-cell fusion, or endocytosis, for example).

Volume 3 continues the approach carried out in the first two volumes of this se ries of publishing articles on membrane methodology which include, in addition to procedural details, incisive discussions of the ap plications of the methods and of their limitations. Wh at is the theoretical basis of the method, how and to what problems can it be applied, how does one interpret the results, what has thus far been achieved by the method, what lies in the future-these are the questions the authors have tried to answer. No area of membrane biology engages the interest of more investigators than studies of the plasma membrane. Four chapters in this volume are concerned with one or more aspects of the cell surface. Fundamental to all studies of the cell surface are the isolation and characterization of pure plasma membranes. Many preparations described in the literature are inadequate or are inadequately characterized. In the first chapter, Neville discusses the theoretical and practical bases of tissue fractionation, empha sizes the variations in enzyme content among plasma membranes from different sources, offers guidance in the choice of the proper criteria for assessing membrane purity, and suggests the best markers for detecting the possible presence of contaminating organelles. To review in detail each of the many preparations of plasma membranes that have been published is impossible.

Examination of the tables of contents of journals - biochemical, molecular biological, ultrastructural, and physiological-provides convincing evidence that membrane biology will be in the 1970s what biochemical genetics was in the 1960s. And for good reason. If genetics is the mechanism for main taining and transmitting the essentials of life, membranes are in many ways the essence of life. The minimal requirement for independent existence is the individualism provided by the separation of "life" from the environment. The cell exists by virtue of its surface membran . One might define the first living organism as that stage of evolution where macromolecular catalysts or self-reproducing polymers were first segregated from the surrounding milieu by a membrane. Whether that early membrane resembled present cell membranes is irrelevant. What matters is that a membrane would have provided a mechanism for maintaining a local concentration of molecules, facilitating chemical evolution and allowing it to evolve into biochemical evolution. That or yet more primitive membranes, such as a hydrocarbon monolayer at an air-water interface, could also have provided a surface that would facilitate the aggregation and specific orientation of molecules and catalyze their interactions. If primitive membranes were much more than mere passive barriers to free diffusion, how much more is this true of the membranes of contemporary forms of life. A major revolution in biological thought has been the recogni tion that the cell, and especially the eukaryotic cell, is a bewildering maze of membranes and membranous organelles."

The purposes of this senes were discussed in the preface to Volume I: to present "a range of methods . . . from the physical to the physiological . . . in sufficient detail for the reader to use them in his laboratory" and also to describe "the theoretical backgrounds of the methods and their limita tions in membrane biology" so that the reader will be enabled "to evaluate more critically and to understand more fully data obtained by methods foreign to [his] usual experiences. " The chapter by Lee, Birdsall, and Metcalfe with which Volume 2 begins accomplishes these twin goals with a thorough description of the application of nuclear magnetic relaxation measurements to membrane biology together with a lucid and succinct integration of the results of such studies into present concepts of the organi zation of membrane lipids. This then permits speculation on the physical basis of membrane permeability. The powerful tool of NMR spectroscopy will have even fuller application with the development of techniques, al ready partially exploited, for l3C-Iabeling of specific carbon atoms in lipid molecules and with extension of the observations to membrane proteins. The following two chapters, by Glick and by Laine, Stellner, and Hako mori, describe the isolation and characterization of membrane glycoproteins and membrane glycolipids, respectively.

Although not the only volume in this series in which lipids are discussed, the present volume is devoted entirely to methods for the study of membrane lipids. Even now, when membrane proteins are properly receiving so much attention, this emphasis on membrane lipids is appropriate. Essentially all of the phospholipids and sterols of cells are in membranes. Moreover, although membrane proteins are certainly of utmost importance, the more we learn about the functional properties of membrane proteins, the more we appreciate the unique features of phospholipids, without which biological membranes would be impossible. The hydrophobic-hydrophilic duality of phospholipids allows, indeed requires, their association, in an aqueous environment, into an essentially two-dimensional membrane-only molec ularly thick in one dimension but relatively infinite in the other two; a structure composed of small molecules, not covalently linked, and therefore, infinitely mobile and variable, but yet a structure with great stability and one largely impermeable to most biomolecules. These membrane-forming properties are shared by many amphipathic polar lipids-phospholipids, glycolipids, and sphingolipids-that differ significantly from each other in the nature of their polar head groups and their fatty acids. These variations in structure allow a range of specific interactions among membrane lipids and between lipids and proteins and also provide for membranes of variable, but controlled, fluidity. In this way, phospholipids provide an appropriate milieu for functional membrane proteins and also significantly modulate their catalytic activities.