Overview

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.

Procedure

After lipids and proteins, carbohydrates are the third major component of biological membranes.

Glycoproteins, proteins with bound carbohydrates, comprise nearly ninety percent of membrane carbohydrates. Glycolipids are the remaining ten percent.

Both glycoproteins and glycolipids are found on the outer side of the membrane and perform diverse functions, including acting as antigens and receptors.

Carbohydrates are attached to proteins through N-glycosidic and O-glycosidic bonds.

N-glycosidic bond formation occurs between the amide group of asparagine and a sugar such as N-acetylglucosamine, whereas O-glycosidic bonds form between the hydroxyl group of serine or threonine and a sugar, such as N-acetylgalactosamine.

Bound carbohydrates protect proteins from proteases, change their overall charge and increase their water-binding capacity.

Glycolipids are derived from ceramide– a sphingolipid. Glucocerebroside or galactocerebroside is synthesized when glucose or galactose binds to ceramide.

Gangliosides are formed when an oligosaccharide and sialic acid bind to ceramide. The negatively charged gangliosides regulate the concentration of positively charged calcium ions and thereby help in neurotransmission.