FIGURE 1 (A) Monolayer formed by phospholipids at an air-water interface. (B) A phospholipid bilayer separating two aqueous compartments. (C) A bimolecular lipoprotein membrane.
layer (Fig. 1A) of olive oil on water have not significantly improved on Franklin's estimate.
In 1925, Gorter and Grendel, stimulated by the findings of Franklin and Overton, performed a series of studies that had a major impact on all subsequent thinking dealing with membrane structure. These investigators extracted the lipids from erythrocyte membranes of a variety of species and calculated the area covered by these lipids when spread on water to form a monomolecular layer. They also approximated the total area of the membranes from which the lipid was extracted and concluded that the area of the monomolecular layer was twice the membrane area; that is, there was sufficient lipid to form a double or bimolecular layer of lipid around the cells, with each layer being about 25 A thick (Fig. IB).
In 1935, Davson and Danielli modified the model proposed by Gorter and Grendel by including protein in the membrane structure. The bimolecular lipoprotein model they proposed is illustrated in Fig. 1C. The essential features of this model are (1) a bimolecular lipid core that is 50 A (5 nm) thick and corresponds to the Gorter-Grendel model; and (2) inner and outer protein layers attached to the polar head groups of the lipids by ionic interactions. This model enjoyed 25 years of essentially universal acceptance.
Current Concepts: The Fluid-Mosaic Membrane
Since 1945, the application of increasingly sophisticated analytical and ultrastructural techniques to the study of the composition and structure of biologic membranes has met with remarkable success. First, the early notion that biologic membranes are made up of a mixture of lipids and proteins has been firmly established for all such barriers throughout the animal and plant kingdoms. The proportions of these two components differ among different cell membranes. In general, membranes that primarily serve as insulators between the intracellular and extracellular compartments and have few metabolic functions (e.g., myelin) are made up of a relatively high proportion of lipids compared to proteins. On the other hand, membranes that surround metabolic factories (e.g., hepatocytes, mitochondria) are relatively rich in protein content compared to lipid content.
Second, it is now generally accepted that the lipids that comprise biologic membranes are primarily from the group referred to as phospholipids. While cholesterol is present in the membranes of many eurokaryotic cells, it is not found in most prokaryotic cells. All phospholipids are derivatives of phosphatidic acid, which consists of a phosphorylated glycerol backbone to which two fatty acid tails are attached by ester bonds. The most prevalent phospholipids found in biologic membranes, such as phosphatidylcholine, phosphatidyl-serine, phosphatidylinositol, and phosphatidylethanol-amine, result from esterification of the free phosphate group of phosphatidic acid with the hydroxyl groups of choline, serine, inositol, and ethanolamine, respectively. The important point is that all phospholipids contain a water-soluble (hydrophilic) head group (i.e., the phos-phorylated glycerol backbone and its conjugates) and two water-insoluble (hydrophobic or lipophilic) tails. Such molecules are referred to as amphiphatic (or amphipathic). The prefix amphi- derives from both the Greek and Latin and means ''having two sides''; e.g., amphibians live in air (on land) and in water. When these compounds are poured onto water, their hydro-philic head groups enter the aqueous phase and their hydrophobic tails simply wave in the air (Benjamin Franklin's observation; see Fig. 1A). But, when these molecules are confronted with two aqueous compartments, they will spontaneously form a bilayer (per the Gorter-Grendel model; see Fig. IB), with their water-soluble head groups immersed in the two aqueous phases and their lipid tails forming a hydrophobic core.
Finally, perhaps the most revolutionary advance in this area has been our understanding of the way in which these proteins and lipids are assembled in
biological membranes. It is now clear that the phospholipids and cholesterol form an ''oily'' fluid bilayer in which the adherent proteins are free to float around at will. Some of these proteins span the thickness of the bilayer; these so-called integral proteins have hydro-phobic middles and hydrophilic ends, so that their ends protrude into the intracellular and extracellular watery compartments, while their middles are glued by hydro-phobic bonds within the oil (Fig. 2). Other proteins are electrostatically attached to either the inner or outer surface of the bilayer; they are referred to as peripheral proteins. In stark contrast to the Davson-Danielli bimolecular lipoprotein model (Fig. 1C), which has a rather static appearance, this fluid-mosaic model is dynamic and possesses structural features that afford avenues for communication between the extracellular milieu and the intracellular compartments by virtue of signal transduction and the selective exchange of solutes. In short, the fluid-mosaic model provides an ideal basis for the correlation of membrane structure and function.
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