Glycocalyx

The glycocalyx is a system of protein-linked (glycoproteins) and lipid-linked carbohydrates that span the plasma membrane of a eukaryotic cell forming a slimy layer. The transmembrane proteins (a form of integral membrane protein) - forming the glycoproteins - are the main way that the carbohydrate layer is held in place in the layer.

The layer also contains proteoglycans., which contain a protein core with long polysaccharide chains covalently linked to it.

The glycocalyx has a number of important functions:

• Cell signalling: specific patterns are recognised within the glycocalyx and therefore binding of molecules can occur. These are known as the glycosylation pattern. The glycocalyx can also bind and release growth factors.
• Protecting the cell: this is because the glycocalyx acts as a slimy layer so that movement is easier for the cell. Also, it acts as a barrier against substances that may otherwise be permeable to the cell and not required within it.
• Adhesion with other cells: proteins in the cell surface of other cells will bind to the carbohydrates involved in making up the glycocalyx.

The formation of the glycocalyx occurs by the movement of sugars within intracellular membrane-bound organelles to the plasma membrane via vesicles: in the Golgi apparatus the proteins are modified and sugars or other molecules are added. The proteins leave the Golgi in vesicles: the glycocalyx is made via the insertion of these vesicles into the plasma membrane where the sugars, lipids and carbohydrates are on the outside of the cell.

Each cell type has its own, specific, glycosylation pattern. The glycosylation pattern depends on the membrane association proteins and the carbohydrate attached. Carbohydrates can be attached through the nitrogen in asparagine or the oxygen in serine and threonine. The carbohydrates that are attached can form complex, very diverse branches. The diversity is the basis of interactions and coordination between cells, other cells and their environment.