12.7: Membrane Carbohydrates
The plasma membrane is a dynamic barrier composed of lipids, proteins, and carbohydrates. It is the epicenter of many cellular processes required for cell growth and survival. Carbohydrates have unique structural and chemical properties that help the plasma membrane to carry out its functions effectively.
Membrane carbohydrates do not have any hydrophobic region and are exclusively located on the cell's outer surface. The addition of sugar molecules or glycosylation of proteins happens in lumens of both the endoplasmic reticulum and the Golgi apparatus, whereas glycosylation of lipids occurs only in the lumen of the Golgi apparatus. Therefore, when the vesicles bud off from the Golgi apparatus, the glycolipids and glycoproteins are present on the inner layer. However, when the vesicles fuse with the plasma membrane, the inner layer and associated glycolipids and glycoproteins are exposed to the exoplasmic side; the reason why glycoproteins and glycolipids are exclusively found on the outer layer. Some eukaryotic and bacterial cells secrete a large amount of carbohydrate outside the cell, forming a sugar coat around the cell, known as glycocalyx.
Carbohydrates are structurally more diverse than both proteins and DNA. In proteins and DNA, mainly peptide and phosphodiester covalent bonds are observed, respectively. In contrast, each sugar molecule can form multiple types of covalent bonds and allows the formation of different oligosaccharides structures from the same sugar molecules. The structural diversity of carbohydrates helps to provide each cell type with a distinct identity and plays an essential role in diverse cellular functions. For example, depending on the antigen present on the surface of human red blood cell (RBC), the blood group is classified into the four types – A, B, AB, and O. A person with the A blood group has the A antigen present on their RBC and carries antibodies for the B antigen. A person with the B blood group has the B antigen present on their RBC and carries antibodies for the A antigen. A person with the AB blood group has both the A and B antigens present on their RBCs and does not have antibodies for either antigen. On the other hand, the RBCs of a person with the O blood group do not have either A or B antigens, and this person carries antibodies for both of them.