<P> Oligosaccharides that participate in O - linked glycosylation are attached to threonine or serine on the hydroxyl group of the side chain . O - linked glycosylation occurs in the golgi apparatus, where monosaccharide units are added to a complete polypeptide chain . Cell surface proteins and extracellular proteins are O - glycosylated . Glycosylation sites in O - linked oligosaccharides are determined by the secondary and tertiary structures of the polypeptide, which dictate where glycosyltransferases will add sugars . </P> <P> Glycoproteins and glycolipids are by definition covalently bonded to carbohydrates . They are very abundant on the surface of the cell, and their interactions contribute to the overall stability of the cell . </P> <P> Glycoproteins have distinct Oligosaccharide structures which have significant effects on many of their properties, affecting critical functions such as antigenicity, solubility, and resistance to proteases . Glycoproteins are relevant as cell - surface receptors, cell - adhesion molecules, immunoglobulins, and tumor antigens . </P> <P> Glycolipids are important for cell recognition, and are important for modulating the function of membrane proteins that act as receptors . Glycolipids are lipid molecules bound to oligosaccharides, generally present in the lipid bilayer . Additionally, they can serve as receptors for cellular recognition and cell signaling . The head of the oligosaccharide serves as a binding partner in receptor activity . The binding mechanisms of receptors to the oligosaccharides depends on the composition of the oligosaccharides that are exposed or presented above the surface of the membrane . There is great diversity in the binding mechanisms of glycolipids, which is what makes them such an important target for pathogens as a site for interaction and entrance . For example, the chaperone activity of glycolipids has been studied for its relevance to HIV infection . </P>

What is the purpose of a carbohydrate chain