Unique features of Archaea

Although Archaea have similarities to both eubacteria and to eukaryotes, they have some biochemical features that are unique.

Phospholipid chirality
The stereoisomer of glycerol that is a constituent of the membrane phospholipids of Archaea is L-glycerol while that found in the membrane phospholipids of eubacteria and eukaryotes is D-glycerol.

Phospholipid side chains
In eubacteria and eukaryotes, the two fatty acid side chains attached to the glycerol of the phospholipid are linked to the glycerol by ester bonds. In the Archaea the side chains are attached by ether bonds.  Moreover, while fatty acids (usually 16 or 18 carbon or even longer) constitute the side chains of the phospholipids of eubacteria and eukaryotes, the phospholipid side chains of Archaea are 20 carbon branched isoprenes. The branches can link the side chains together and can be intermolecular or intramolecular. The branches can link two phospholipids in the different monolayers of the bilayer giving rise to transmembrane phospholipids. The isoprenes can also bend around to form a five-carbon ring structure which may stabilize the membrane of these organisms in the extreme environments that they inhabit.