PLASMA MEMBRANE LIPIDS
Biochemistry, Properties, Structure and Classification of Lipids of Plasma Membrane
“Good fences make good neighbors”
Robert Frost, “Mending Wall”, 1914
Biological membrane system: Biological membranes are highly dynamic two layers thick sheath like structures formed by the non-covalent assemblage of lipids, proteins and carbohydrates. They form closed boundaries between different compartments of the cells such as separation of nucleoplasm from the cytoplasm by nuclear membrane in all eukaryotes. They act as barriers to the passage of polar molecules and ions. The thickness of membrane varies among different classes of organisms and the average width ranges from 60 Å (6 nm) to 100 Å (10 nm) in most of the cases.
Even though membrane contains carbohydrates and proteins, the major structural components of bio-membrane are a special class of lipids called membrane lipids. In storage lipids (triglycerides) the three –OH groups of glycerol moieties are esterified by three fatty acids and thus they are completely non-polar. However in membrane lipids, the hydroxyl group at C1 and C2 are esterified with fatty acids and the remaining third –OH group (at C3) will combine to a polar molecule. Thus membrane lipids are amphipathic because they have hydrophilic head (polar) at one end and hydrophobic tail (nonpolar) at the other end. The long hydrocarbon chain of fatty acids forms the hydrophobic part. The hydrophilic moieties of membrane lipids are of different types and which may be as simple as –OH or may be much complex like carbohydrates or amino acids or their derivatives. We commonly call the polar part of membrane lipid as ‘Head’ group and the nonpolar part as ‘Tail’ group.
The hydrophobic interactions of the nonpolar parts among themselves and the hydrophilic interaction with water are responsible for the packing of lipids in the membrane. These hydrophilic and hydrophobic interactions are also responsible for the bilayer organization of membranes in the cells.
Classification of membrane lipids:
Membrane lipids are classified based on the properties of ‘Head’ group. The membrane lipids of Prokaryotes, Eukaryotes and Archaebacteria are grouped into four major categories:
4. Archaebacterial ether lipids