Glycocalyx plasma membrane

The plasma membrane contains a small amount of carbohydrate (2 to 10% of the mass of the membrane) on the outer surface. This carbohydrate is found attached to most of the protein molecules, forming glycoproteins, and to some of the phospholipid molecules (<10%), forming glycolipids. Consequently, the external surface of the cell has a carbohydrate coat, or glycocalyx. 

There is still some discordance about markers for ATI cells [17, 18]. Varieties of glycocalyx appear to be expressed in ATI versus ATII cells, which can be distinguished by the binding of specific lectins. Lycopersicon esculen-tum lectin, for example, appears to adhere specifically to the apical plasma membrane of ATI cells [19, 20], Other lectin markers that have been reported in AEC typing are Ricinus communis agglutinin, Erythrina cristagalli lectin, soybean lectin and Bauhinia purpurea agglutinin (all for the ATI phenotype),... 

Figure 10.4 Lipid globule in human milk surrounded by a membrane which has a typical unit or bilayer appearance. This membrane is separated from the globule core by a layer of intensely stained coat material. The material on the outer face of the membrane has an appearance typical of the glycocalyx observed on plasma membranes, (b) A higher magnification of a region of membrane of the lipid globule shown in (a). Note that in certain regions a bilayer-like membrane is indiscernible. Bars = 0.1/im, magnification" (a) x 75,000, (b) x 137,000. Prints are of a micrograph published previously (Freudenstein et al. 1979) and are included with permission of Academic Press, Inc.

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.)...

Adsorption-mediated transport is triggered by an electrostatic interaction between a positively charged substance, usually a charge moiety of a peptide, and the negatively charged plasma membrane surface (i.e., glycocalyx) [58]. 

The plasma membrane of eukaryotic cells performs several vital functions. As with all plasma membranes, it provides shape and some mechanical strength and protection, as well as a permeability barrier, to the cell. The relatively thick glycocalyx assists in these functions. The plasma membrane, which possesses a diversity of channel complexes that transport ions and molecules and receptors that bind signal molecules, is also involved in various types of transport and signaling processes. 

In summary, a lipid molecule on its route from the luminal bulk phase into the intracellular compartment of an enterocyte has to overcome two unstirred water layers and one plasma membrane of lipid bilayer structure. The unstirred water layer on the luminal side partly coincides with the mucus gel and the glycocalyx relatively little is known of the importance of these diffusional barriers. 

Some of the proteins and lipids on the external surface of the membrane contain short chains of carbohydrates (oligosaccharides) that extend into the aqneons medium. Carbohydrates therefore constitute 2 to 10% of the weight of plasma membranes. This hydrophilic carbohydrate layer, called the glycocalyx, protects the cell against digestion and restricts the nptake of hydrophobic componnds. 

Crane (1966) has theorized that the brush border plasma membrane is the site of a mosaic of the enzymes associated with the microvillus. This was based on experiments of Eichholz and Crane (1965) who recovered a fraction of pure microvillous membranes by density gradient centrifugation of brush border homogenate which possessed the total activities of alkaline phosphatase, maltas and sucrase and various peptidases. Evidence in favor of this idea was also collected by Johnson (1967) who demonstrated the presence of knobs 60 A in diameter on the glycocalyx of the luminal side of the plasma membrane which contained the brush border invertase and maltase (see Fig. 6). These knobs could be removed entirely from the microvilli of hamster intestine by papain digestion the remaining membrane, however, still has the alkaline phosphatase incorporated into it (Eichholz, 1969 Oda and Seki, 1966). 

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