Glycocalyx
The glycocalyx or mucus also called capsule shell is a layer on the outer surface of the cell membrane in eukaryotic cells, but also on the outside of the cell wall in prokaryotic cells. It consists of polysaccharides covalently linked to the membrane proteins ( glycoproteins ), and membrane lipids ( glycolipids ) are attached. Capsule and mucus sheath, however, differ in that the capsule to the cell wall is well established, whereas this is not true of the slime sheath.
General
Living cells, whether prokaryote or eukaryote, to a membrane of the cell or plasma membrane wrapped. It represents something of a semi-permeable boundary to " external environment " dar. Almost all biological membranes on their surface a different thick layer of sugar residues which is also referred to as surface coat. The glycocalyx consists of oligosaccharides ( then glycolipids ) are covalently bound to almost all the membranous proteins ( be glycoproteins ), but also in a smaller number of lipids. For sugar radicals are only a limited number of sugars, namely glucose, fucose, mannose, glucosamine, galactosamine and sialic acids. The sialic acids on the cell membrane surface cause a negative charge (see cathode). The glycocalyx is regularly renewed. Note that no glycocalyx is present in plants.
Its thickness can vary, and they may occur in gram-positive and gram-negative bacteria ( Gram stain ). The capsule can greatly increase the volume of the bacterium. This will retain all other cell wall layers of the capsule layer.
In biomembranes of branched sugar depends to 90% of membrane proteins and 10% of glycolipids.
In particular, the glycocalyx of animal cells or tissue can be five phylogenetically highly conserved molecules or molecule families for intercellular communication (cell identification, contact and adhesion ) are significant, differ: immunoglobulins, integrins, cadherins, and cell adhesion molecules selectin.
A large number of members of each molecule family related to the microfilament system of the cells in a relationship or are coupled to GTP - binding forming proteins from the cell membrane, others are with the MAP kinase pathway in conjunction.
Importance
These capsules are single-celled organisms such as the bacteria:
- As protection against desiccation (dehydration )
- As protection against phagocytosis
- Attachment to surfaces (eg on the enamel, see also dental plaque )
- A pathogenetic
In multicellular organisms approximately determines the glycocalyx of erythrocyte blood group and includes the Rhesus factor. In addition, the glycocalyx is an important part of Blutgefäßwand.Sie is also the focus in the development of septic disease.
Composition
The glycocalyx is formed from sucrose. In the synthesis, there are two different possibilities. Depending on how the composition is carried out, a distinction is made between:
Biosynthesis
To activate a uridine diphosphate sugar is produced with consumption of UTP ( uridine triphosphate ). Upon hydrolysis of the phosphate ester pyrophosphate is released, so that sufficient energy is available for the formation of a glycosidic linkage between the monosaccharides is available. The enzyme catalyzing the levan or dextran sucrase.