compared to that of “true” Bacteria. The lists of their biochemical differences from
both Bacteria and Eukarya are long enough that Archaea must have taken a separate
path very early in the evolution of cellular life. Most strikingly distinct are archaeal
cell membranes. All cell membranes are constructed (Fig. 5.2) of molecules with one
hydrophilic and two hydrophobic groups bonded to glycerol. In Archaea, these attach
by ether bonds, while in other organisms attachment is by ester bonds. Archaeal
hydrophobic groups are poly-isoprenes, while in bacteria and eukaryotes they are fatty
acids. Finally, the glycerol in archaeal membranes is consistently a different
stereoisomer from that in other life forms. Membranes are an essential feature of
cellular life. Strong divergence in membrane chemistry between Archaea and other
groups implies a very deep, certainly ancient division. Archaeal membranes are a
single lipid layer, while bacterial membranes are lipid bilayers. Cell membranes also
are the biosynthetic sources of outer-cell coverings such as cell walls. Archaeal cell
walls are never constructed of proper murein (see below) like those of true Bacteria,
although some subgroups are covered with a “pseudo-murein” composed of cross-
linked, ammoniated polycarbonates. More typically, archaea have a protein coat
outside the cell membrane. Some distinctions in archaeal biochemistry, for example
RNA-polymerase and DNA-polymerase forms and functions, imply closer
relationships to Eukarya than to Bacteria.
Fig. 5.2 (a) Chemical structure of the phospholipids that are the main constituents of
bacterial and eukaryote cell membranes. A glycerol backbone is linked by ester bonds
to two long-chain fatty acids and a phosphatidyl group (several moieties can substitute
for the phosphatidyl serine shown). (b) Structure of archaeal membrane lipids. The
hydrophobic moieties are polyisoprenes linked to the glycerol by ether bonds. One
terminal glycerol carbon may carry a hydroxyl group, a sugar or a phosphatidyl
group. In all organisms, the hydrophilic ends form both inner and outer membrane
surfaces; the hydrophobic long-chain fatty acids project into and form the membrane’s
central core layer.