Plasma membrane fluid mosaic model

Figure 5.2 The fluid mosaic model of the plasma membrane.

Figure 4. Structure of the Singer and Nicholson fluid mosaic model for the plasma membrane...

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.)...

Figure 9.24 The fluid-mosaic model of plasma membrane. Phospholipids with darkened heads are on the cytosol side of the bilayer, and the lipids with unfilled heads are on the outer surface. In intrinsic proteins one or more a-helical segments are in contact with the hydrophobic environment of the bilayer. They usually have hydrophobic amino acid side chains. Carbohydrate is indicated by hexagons. The membrane potential (negative inside) is indicated by AV. (Reproduced by permission from Vance DE, Vance JE. Biochemistry of Lipids and Membranes. Menlo Park Benjamin/Cummings, 1985, p. 26.)...

M.S. Bretscher, M.C. Raff, Mammalian Plasma Membrane , Nature, 258,43 (1975) S.J. Singer, G.L. Nicolson, Fluid Mosaic Model of Structiue of Cell Membranes , Science, 175,720 (1972)... 

One observation, which is inconsistent with the simple fluid mosaic model, is the reduced diffusion coefficients of membrane molecules in the plasma membrane compared to model... 

Fig. 2.19 Diagram of the plasma membrane showing its integral proteins (fluid mosaic model) (adapted from S.J. Singer et af, 1972 and H. Knufermann, 1976). 1 external aqueous milieu, 2 internal aqueous milieu, 3 fracture plane of the apolar membrane layer, 4 externally orientated intrinsic protein (ectoprotein), 5 internally orientated intrinsic protein (endoprotein), 6 external extrinsic protein, 7 internal intrinsic protein, 8, 9 membrane-penetrating proteins with hydrophobic interactions in the inside of the membrane (P = polar region), 10 membrane pervaded by glycoprotein with sugar residues (, 11 lateral diffusion (A) and flip-flop (B), 12 hydrophilic region (A) and hydrophobic region (B) of the bilayer membrane...

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. 

Hence, the fluid mosaic model proposes a pattern of organisation which is consistent with the demonstrable physical and biological properties of plasma membranes and with their known chemistry. A major advantage of this model is that it can describe many different membranes and so reinforces the concept of the unit membrane. 

The currently accepted structure of B. is the fluid mosaic model. Lipid molecules and membrane proteins are free to diffuse laterally and to spin within the bilayer in which they are located. However, a flip-flop motion from the inner to the outer surface, or vice versa, is energetically unfavorable, because it would require movement of hydrophilic substituents through the hydrophobic phase. Hence this type of motion is almost never displayed by proteins, and it occurs much less readily than translational motion in the case of lipids. Since there is little movement of material between the inner and outer layers of the bilayer, the two faces of the B. can have different compositions. For membrane proteins, this asymmetry is absolute, and, at least in the plasma membrane, different proportions of lipid classes exist in the two monolayers. Attached carbohydrate residues appear to be located only on the noncytosolic surface. Carbohydrate groups extending from the B. participate in cell recognition, cell adhesion, possibly in intercellular communication, and they also contribute to the distinct immunological character of the cell. 

Although a great deal is known about the chemical composition of the mitochondrial membrane and it is established that the membrane contains a number of catalytic proteins e.g., the ATPase synthetase system, an ion transport molecular machinery and electron transport chain), the topological distribution of these proteins in the membrane is not known. All topological models proposed are at present hypothetical [177]. However, it is accepted that the mitochondrial membrane, like most if not all biological membranes, is of the fluid mosaic model and is composed of a lipid bilayer traversed by proteins (see plasma membrane in Chapter 16). Electron microscopic studies of the freeze-edge fractured faces of the outer and the inner membrane [178] indicate that the proteins are asymmetrically distributed not only when the inner is compared to the outer membrane, but also when the inner and outer faces of each of the fractured membranes are compared (Table 1-3). 

The plasma membrane (Figure 9.2) encapsulates the cell and physically separates the cytoplasm from the external environment. All substances which enter or leave the cell must pass through the plasma membrane which plays an important role in the selective uptake of nutrients from the extracellular medium and the discharge of waste products of metabolism from the cell. The plasma membrane is the most extensively researched and best understood of all cell membranes and its properties have led to the development of models of membrane structure from the fundamental lipid bilayer composed of amphipathic phospholipids (Section 8.5) to the currently most widely accepted model called the fluid mosaic model. 

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