In all organisms cell membranes are made of phospholipids. These amphipathic molecules possess
both a polar part that will dissolve in water (the phosphate "head"), and a non-polar part that will
not dissolve in water (the lipid tail) connected by a glycerol group. In water, phospholipids cluster
together, with the polar phosphate heads facing the water and the non-polar lipid tails facing away
from the water which causes them to assemble into layers.
The phospholipids in the membranes of archaea are unusual in four ways:
1. Bacteria and eukaryotes have membranes composed mainly of glycerol-ester lipids, whereas
archaea have membranes composed of glycerol-ether lipids. The difference between these two
types of phospholipid is the type of bond that joins the lipids to the glycerol group. Ether bonds are
more chemically-resistant then ester bonds, which might contribute to the ability of some archaea
to survive at extremes of temperature and in very acidic or alkaline environments.
2. Archaeal lipids are unique because the stereochemistry of the glycerol group is the reverse of
that found in other organisms. The glycerol group can occur in two forms that are mirror images of
one another, which may be called the right-handed and left-handed forms; in chemical terms these
forms are called enantiomers.
3. The lipid tails of the phospholipids of archaea are chemically different from those in other
organisms. Archaeal lipids are based upon the isoprenoid sidechain and are long chains with
multiple side-branches. This is in contrast to the fatty acids found in other organisms' membranes,
which have straight chains with no branches or rings. These branched chains may help prevent
archaean membranes from becoming leaky at high temperatures.
4. In some archaea the phospholipid bilayer is replaced by a single monolayer. In effect, the
archaea have fused the tails of two independent phospholipid molecules into a single molecule
with two polar heads; this fusion may make their membranes more rigid and better able to resist
harsh environments.
Top: an archaeal phospholipid, 1 isoprene sidechain, 2 ether linkage, 3 L-glycerol, 4 phosphate
group. Middle: a bacterial and eukaryotic phospholipid: 5 fatty acid, 6 ester linkage, 7 D-glycerol,
8 phosphate group. Bottom: 9 lipid bilayer of bacteria and eukaryotes, 10 lipid monolayer of some
archaea.