Unique features of archaebacterial plasma membrane
Archaebacteria differ from all other organisms in cell membrane components. Cell wall is followed by cell membrane that serves as a barrier between the cell and its environment.
The four major features that make archaebacterial cell membrane distinct are:
1.Chirality of glycerol : Cell membrane is made up of phospholipids. A phospholipid molecule consists of a glycerol molecule which has a phosphate added to one end, and two side chains attached at the other end. The basic unit from which cell membranes are built is the phospholipid. In a cell membrane, the glycerol and phosphate end are present on the outer surface of the membrane, with the long side chains sandwiched in the middle (see illustration above). This layering creates an effective chemical barrier around the cell and helps maintain chemical equilibrium.
While bacteria and eukaryotes have D-glycerol in their membranes, archaeans have L-glycerol in theirs. More than a mere geometric difference, the enzymes involved in the synthesis of D and L glycerol will be different.
(2) Isoprenoid chains: In bacteria and eukaryotes, side chains are fatty acids of 16-18 carbon atoms.
In archaebacteria, side chains in the phospholipids are made of isoprene units of 20 Carbon atoms. Isoprene units are the building blocks of terpenes. Common terpenes include carotenes, cholesterol, estrogen etc.
(3) Ether linkage: In all organisms, ester linkage bonds the glycerol to the side chain. In archae bacteria, isoprene side chains are attached to the glycerol by ether linkage.
(4) Branching of side chains:Isoprene units in archae bacteria are more flexible and forms many side branches of the main chain. The fatty acids of bacteria and eukaryotes do not have these side branches.
Another interesting property of the side branches is their ability to form carbon rings. This happens when one of the side branches curls around and bonds with another atom down the chain to make a ring of five carbon atoms. This ring formation is common among species surviving in high temperatures, thus thought to be contributing to the structural stability of the membrane.
In eukaryotes, cholesterol is responsible for stabilizing membrane. Interestingly, cholesterol is also a terpene made of isoprene units.