The Fluid-Mosaic Model of Membrane Structure

How do proteins and the lipid bilayer interact with each other in membranes  

These proteins tend to have a specihc orientation in the membrane. The term fluid mosaic implies that the same sort of lateral motion that we have already seen in lipid bilayers also occurs in membranes. The proteins float in the lipid bilayer and can move along the plane of the membrane. 

Membrane proteins can be seen embedded in the lipid bilayer. 

Because the driving force behind the formation of lipid bilayers is the exclusion of water from the hydrophobic region of lipids, and not some enzymatic process, artificial membranes can be created in the lab. Liposomes are stable structures based on a lipid bilayer that form a spherical vesicle. These vesicles can be prepared with therapeutic agents on the inside and then used to deliver the agent to a target tissue. 

Researchers have developed a skin lotion to counteract the effects of UV light. The lotion contains liposomes filled with a DNA-repair enzyme from a virus, called T4 endonuclease V. The liposomes penetrate the skin cells. Once inside, the enzymes make their way to the nucleus, where they attack pyrimidine dimers and start a DNA-repair mechanism that the normal cellular processes can complete. The skin lotion, marketed by AGI 

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FIGURE 9.6 The fluid mosaic model of membrane structure proposed by S. J. Singer and G. L. Nicolsou. In this model, the lipids and proteins are assumed to be mobile, so that they can move rapidly and laterally in the plane of the membrane. Transverse motion may also occur, but it is much slower. 

Figure 41-7. The fluid mosaic model of membrane structure. The membrane consists of a bimolecu-lar lipid layer with proteins inserted in it or bound to either surface. Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called transmembrane proteins, while others are embedded in either the outer or inner leaflet of the lipid bilayer. Loosely bound to the outer or inner surface of the membrane are the peripheral proteins. Many of the proteins and lipids have externally exposed oligosaccharide chains. (Reproduced, with permission, from Junqueira LC, Carneiro J Basic Histology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

THE FLUID MOSAIC MODEL OF MEMBRANE STRUCTURE IS WIDELY ACCEPTED... 

Our knowledge of biological membrane ultrastructure has increased considerably over the years as a result of rapid advances in instrumentation. Although there is still controversy over the most correct biological membrane model, the concept of membrane structure presented by Davson and Danielli of a lipid bilayer is perhaps the one best accepted [12,13]. The most current version of that basic model, illustrated in Fig. 7, is referred to as the fluid mosaic model of membrane structure. This model is consistent with what we have learned about the existence of specific ion channels and receptors within and along surface membranes. 

Fig. 1.4 The fluid mosaic model of membrane structure proposed by Singer and Nicolson [28]. (Reprinted from Fig. 1 of ref. 40 with permission from Wiley-VCH.)...

Fig. 2.3. Diagrammatic representation of the molecular organisation of the tegument plasma membrane (based on the fluid mosaic model of membrane structure of Singer Nicolson (1972)). The carbohydrate moieties of the membrane glycoproteins and glycolipids are exposed on the external face as the glycocalyx. (After Smyth Halton, 1983.)...

The basic tenets of the fluid mosaic model of membrane structure shown in Figure III-44 are widely accepted. The phospholipid bilayer is the basic structural feature of the membrane, and proteins or multiprotein complexes are viewed as island embedded in a sea made up by the lipid bilayer. Membranes are fluid in the sense that lipids and proteins can diffuse freely in the plane of the... 

Figure III-44 The fluid mosaic model of membrane structure.

Membrane fluidity is one of the most important assumptions of the fluid mosaic model of membrane structure. One measure of membrane fluidity, the ability of membrane components to diffuse laterally, can be demonstrated when cells from two different species are fused to form a heterokaryon (Figure 1 ID). (Certain viruses or chemicals are used to promote cell-cell fusion.) The plasma membrane proteins of each cell type can be tracked because they are labeled with different fluorescent markers. Initially, the proteins are confined to their own side of the heterokaryon membrane. As time passes, the two fluorescent markers intermix, indicating that proteins move freely in the lipid bilayer. 

The fluid mosaic model of membrane structure has been very useful in explaining membrane behavior. However, the description of membrane as proteins floating in a phospholipid sea is oversimplified. Describe some components of membrane that are restricted in their lateral motion. 

The fluid mosaic model of membrane structure pictures biological membranes that are composed of lipid bilayers in which proteins are embedded. Membrane lipids contain polar head groups and nonpolar hydrocarbon tails. The hydrocarbon tails of phospholipids are derived from saturated and unsaturated long-chain fatty acids containing an even number of carbon atoms. The lipids and proteins diffuse rapidly in the lipid bilayer but seldom cross from one side to the other. 

According to the fluid-mosaic model of membrane structure ( ), cell membranes consist of a fluid phospholipid bilayer. Embedded within this bilayer are globular proteins essential to membrane function. A large class of phospholipids present in membranes are phosphoglycerides (1 ). The fatty acid in the number 2 position is often unsaturated ( ). In plants, the unsaturated fatty acid is frequently linolenic acid (1 ) with 3 double bonds (18 3 i 15) ... 

Figure 8.11 The fluid-mosaic model of membrane structure. Phosphoglycerides are the chief lipid component. They are arranged in a bilayer. Proteins float like icebergs in a sea of lipid.

Fatty acids have important roles in membrane structure and there are several ways by which they can potentially influence the functions of membrane proteins (and indeed some intracellular proteins). The fluid mosaic model of membrane structure describes biological membranes as dynamic and responsive structures. It is now also recognized that domains exist in membranes, where lipid-protein and lipid-lipid interactions may be highly... 

Singer, S. J., 1976, The fluid mosaic model of membrane structure, in The Structure of Biological Membranes (S. Abrahamsson and I. Pascher, eds.), p. 443, Plenum, New York. 

Israelachvili, J. H., 1977, Refinement of the fluid-mosaic model of membrane structure, Biochim. Biophys. Acta 469 221. 

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