Question

Discuss the location, general characteristics, and relative abundance of glycolipids and glycoproteins in cellular membranes

Describe the general functions of the glycolipids and glycoproteins

Compare the mobility of membrane proteins to that of membrane lipids and explain the factors that influence the mobility of membrane proteins

Answer 1

The plasma membrane is a phospholipid bilayer composed of two layers of amphipathic phospholipids molecules whose polar head groups are in contact with aqueous phase and non polar tails face each other, corming the hydrophobic interior of the membrane.

Two types of phospholipids occur in bilayer - Glycerophospholipids (two fatty acids joined to glycerol) and shingophospholipids (a single fatty acid joined to sphingosine). Glycerophospholipids is the major lipid present in the membrane. In addition, the palsma membrane also contains- glycolipids and sterols.

Glycolipids contain sugar residues covalently attached to the lipid. These are derived from glycerolipids or sphingolipids. The simplest glycolipid called cerebroside, contains a single sugar residue, either glucose or galactose. Complex glycolipid is gangliosides, a branched chain of max. seven sugar residues. Glycolipids are found in the outer leaflet of the plasma membrane, with their carbohydrate moiety exposed to the cell surface.

Glycoproteins are the integral membrane protein, found on the surface of the lipid bilayer of cell membranes. Carbohydrate groups are covalently attached to proteins forms glycoproteins. In this, sugar are attached to the amide nitrogen atom in the side chain of asparagine, N-linkage or to oxygen atom in the side chain of threonine, O-linkage. They are important for cell-to-cell recognition and adhesion and serves as receptors for ligands.

They both stabilise the structure of plasma membrane.

Lipid molecules can rotate freely and diffuse laterally - rotational motion and lateral motion respectively, within each leaflet. Lateral movement of proteins is seen by fluorescence recovery after photobleaching (FRAP). The transition of a molecule from one membrane to the other is termed as transverse diffusion or flip-flop.This transition depends on phospholipid acyl chain length and degree of unsaturation. With the number of unsaturated fatty acids, the fluidity of bilayer increases at low temperature. Lipids with short fatty acyl chain increases the transition rate at low temperature.