Membrane carbohydrate

The  extracellular   surface  of  the  plasma  membrane   is  frequently  covered  with  a protective  coat  of carbohydrate. A Sugar  residues  of this  carbohydrate  coat can  be  attached  either  to  certain  lipids  like  as  the  glycosphingolipids,  or  to  the  polypeptide  chains  of  peripheral  or  integral  membrane proteins. These glycoproteins and glycolipids are abundant in the plasma membrane  of eukaryotic  cells  but are virtually  absent  from  most  intracellular membranes,  particularly  the chloroplast lamellae and the inner mitochondrial  membrane. In glycoproteins the sugar residues can be attached to the polypeptide chain either through the hydroxyl group in the side-chain of Ser or Thr residues as O-linked oligosaccharides, or through the amide set in the side-chain of Asn as N-linked oligosaccharides. The carbohydrate on the extracellular face of the membrane not only serves a protective role but is also included in intercellular recognition and in preserve the asymmetry of the membrane.

                                                                   Figure: The erythrocyte cytoskeleton