Cell-Surface Carbohydrates

The surface of many cells contains short oligosaccharides bonded to the proteins of cell membranes. These proteins bonded to carbohydrates are called glycoproteins. The formation of glycoprotein takes place when the anomeric carbon of carbohydrate is linked to the -NH₂ or -OH group of cell membrane protein. The percentage of carbohydrates in glycoproteins vary between 1% - 80%.

The presence of carbohydrates on the surface of a cell enables the cell to recognize other cells, function as a point of attachment for viruses, toxins, and other cells. Hence, the carbohydrates present on the cell surface play distinct roles including causing infections, preventing infections, fertilization, blood clotting, and septic shock.

The difference in types of carbohydrates present on the surfaces of red blood cells result in different blood types i.e. A, B, and O. Each blood type has a carbohydrate of specific structure as shown below.

In the human body proteins called antibodies are synthesized by the immune system of the cells which respond to unfamiliar substances called antigens. If blood of different types is transferred to the human body, it is taken as antigen by the immune system. For this reason, different blood types cannot be transferred to the human body. If a body with blood type A, B, or AB is donated with blood type O it is not taken as foreign substance because the type O blood contains the same components as that contained by type A and type B blood types. For this reason, blood group AB is a universal acceptor while blood group O is termed as universal donor.

The presence of different carbohydrates on the surface of the cells allows the cell to recognize other cells. Cells recognize other cells by binding to them and recognizing specific carbohydrates present on its surface. The carbohydrates present on the exterior part of the cell membrane vary in different species. For example, in reproduction the binding of sperm cell to egg is assisted by the presence of glycoproteins present on the surfaces of each membrane. The glycoprotein called zona pellucida surrounding the egg prevents polyspermy process.

In different diseases the bacteria, viruses, and parasites exploit the cell surface carbohydrates and enter into cells. For example, different bacteria infect distinct types of cells in the human body. Escherichia coli (E. coli) causes urinary tract infections whereas, Streptococci infects respiratory tract and skin infections. This selectivity is due to the presence of fimbriae on bacteria which allows it to bind to certain surface carbohydrates.