Glycocalyx is a complex supra-membrane complex that forms a thin membrane on the surface of the plasmalemma of animal cells and bacterial CPM. The term is a combination of the Greek and Latin words glykys callum, which literally means "sweet thick skin." Indeed, glycocalyx plays the role of an additional cell membrane and is built primarily of carbohydrate molecules, but unlike a plasma membrane, it has a non-continuous, but fleecy structure.
general characteristics
Glycocalyx is an additional protective layer on the surface of the cell, formed by molecules of proteins, carbohydrates and lipids attached to the CPM, as well as by the outer parts of the proteins mounted in the membrane. The basis of such a cytological cover is a network of glycosides (glycoproteins and proteoglycans).
Thus, glycocalyx is a highly charged shell enriched with carbohydrate components, which is a combination of biological macromolecules associated with the membrane. This layer serves as an additional barrier between the cell and the environment and performs many functions that are divided into stabilizing, protective and specific.
Glycocalyx is characteristic only of prokaryotic organisms and animals. Membranes of plant cells do not form such a membrane.
Functions
The complete set of glycocalyx functions in cells and at the tissue level of macroorganisms is currently not defined. However, it has already been established that this layer:
- participates in signal transduction from the extracellular medium to the intracellular;
- protects the cytoplasmic membrane from stress and mechanical influences;
- provides adhesive properties of some cells;
- acts as a recognition factor.
In bacteria, glycocalyx provides attachment to the surface, prevents moisture loss when it enters a dry environment, and protects against the action of antibacterial substances. In pathogenic microorganisms, this layer may interfere with the detection of the pathogen by the immune system.
Biochemical composition and structure
The composition of glycocalyx includes:
- proteoglycans (chains of glycosaminoglycans connected to the protein core) —consist of syndicans, glypicans, mimecans, perlacans and biglicans;
- glycosaminoglycans (linear disaccharide polymers of uronic acid and hexosamine) - are 50-90% composed of heparan sulfate, and also include dermatan sulfate, chondroitin sulfate, keratan sulfate and hyaluronan;
- glycoproteins containing acid oligosaccharides and sialic acids;
- various soluble components (proteins, proteoglycans, etc.);
- molecules adsorbed onto the surface of the membrane from the extracellular space.
The structure and exact content of the biochemical components of glycocalyx vary depending on the type of cells, as well as the prevailing physical and mechanical environmental conditions.
The use of special dyes makes it possible to visualize this additional shell using electron microscopy.
Endothelial Hycocalyx
Endothelial glycocalyx is a carbohydrate-rich layer that lines the luminal surface of blood vessels and forms a rather thick (about 500 nanometers) cell-free membrane that functions not only at the cytological, but also at the tissue level. This structure was first discovered by Luft 40 years ago.
It has now been established that endothelial glycocalyx is a key determinant of vascular permeability. With regard to blood flow, it has a partially negative charge, which prevents excessive absorption of cellular albumin. Glycocalyx also serves as a mechanical defense to the endothelium.