It has already been noted that carbohydrate is present in the plasma membrane as short, sometimes branched chains of sugars attached either to exterior peripheral proteins (forming glycoproteins) or to the polar ends of phospholipid molecules in the outer lipid layer (forming glycolipids).
No membrane carbohydrate is located at the interior surface.
The oligosaccharide chains of membrane glycoproteins and glycolipids are formed by various combinations of six principal sugars D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (also called sialic acid), N-acetyl-D-glucosamine, and N-acetyl-D- galactosamine. All of these may be derived from glucose.
Possible Functions of Membrane Carbohydrate:
Several roles have been suggested for the carbohydrate present on the outer surface of the plasma membrane. One possibility is that because they are highly hydrophilic, the sugars help to orient the glycoproteins (and glycolipids) in the membrane so that they are kept in contact with the external aqueous environment and are unlikely to rotate toward the membrane’s hydrophobic interior.
Certain plasma transport proteins, hormones, and enzymes are glycoproteins, and in these molecules, carbohydrate is important to physiological activity. It would therefore not be inappropriate to expect that in certain glycoproteins of the plasma membrane the carbohydrate moiety is basic to either enzymatic or some other activity.
Carbohydrate chains of surface glycoproteins are clearly responsible for the various human blood types (e.g., ABO types, MN types, etc.) and other tissue types. That is, the sugar sequence and the arrangement of the sugar chains in the membranes of blood cells of an individual with type A blood differ from those of an individual with type B blood, and so on.
The carbohydrate is responsible for cell type specificity and is therefore fundamental to the specific antigenic properties of cell membranes. These antigenic properties are linked in some manner to the body’s immune system and the capability of that system to distinguish between cells that should be present in the organism (native cells) and foreign cells. Foreign cells (such as bacteria or other microorganisms, transplanted tissue, or transfused blood) may be recognized as foreign because their membrane glycoproteins contain different carbohydrate markers than those present in the individual’s own tissues.
Such a situation triggers the immune response. In contrast, an individual’s own plasma membrane carbohydrate organization is recognized as being native (referred to as “recognition of self”) and does not normally trigger an immunological response. Of course, neither does blood transfusion nor tissue transplantation if the carbohydrate organization in the membranes of the “donor’s” and “recipient’s” cells is the same. Cell- specific membrane carbohydrate organization is considered further in connection with the actions of lectins and antibodies.
Oppenheimer, Roseman, Roth, and others have clearly implicated surface carbohydrate in the adhesion of a cell to its neighbors in a tissue; presumably, the carbohydrate acts as an adhesive maintaining the integrity of the tissue by linking neighboring cells together.
Density-dependent inhibition, the phenomenon in which cells grown in culture stop dividing when their numbers attain a certain level, may be attributable to a mechanism triggered by interaction of carbohydrates on neighboring cells. Carbohydrate is also present at the surface of cells as glycolipid, especially glycosphingolipids. Although the role of this glycolipid is uncertain, it is believed to function in cell-to-cell recognition.