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

                                     figure: The erythrocyte cytoskeleton