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.
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.
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.
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