Fluid-mosaic model carbohydrates

Fig. 7 Diagrammatic representation of the fluid mosaic model of the cell membrane. The basic structure of the membrane is that of a lipid bilayer in which the lipid portion (long tails) points inward and the polar portion (round head ) points outward. The membrane is penenetrated by transmembrane (or integral) proteins. Attached to the surface of the membrane are peripheral proteins (inner surface) and carbohydrates that bind to lipid and protein molecules (outer surface). (Modified from Ref. 14.)...

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

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.)...

There was good reason to regard carbohydrates as a part of the surface membrane of cells and the evidence for this went back for two decades. Nevertheless, it was not until Singer and Nicolson proposed their fluid mosaic model of membrane structure in 1972 and incorporated carbohydrate specifically at the outer face of the membrane, that the idea was widely accepted. 

Fluid-mosaic model A model of a biological membrane consisting of a phospholipid bilayer, with proteins, carbohydrates, and other Upids embedded in, and on the surface of, the bilayer. 

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. 

Fluid-mosaic model (Section 26.5B) A biological membrane that consists of a phospholipid bilayer with proteins, carbohydrates, and other lipids on the surface and embedded in the bilayer. 

Most of the phospholipids in the lipid bilayer contain unsaturated fatty adds. Because of the kinks in the carbon chains at the cis double bonds, the phospholipids do not fit closely together. As a result, the lipid bilayer is not a rigid, fixed structure, but one that is dynamic and fluid-like. This liquid-like bilayer also contains proteins, carbohydrates, and cholesterol molecules. For this reason, the model of biological membranes is referred to as the fluid mosaic model of membranes. 

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