The Archea lack peptidoglycan and have phospholipids in their
cell membranes held together by ether bonds rather than the ester bonds
found in bacterial and eukaryotic cell membranes. They also have
protein overlaying their cell wall rather than an outer membrane as in
gram-negative cells, though they do stain gram-negative.
Mycoplasmas are the smallest bacterial cells (~2 um). They
have no cell wall and a cell membrane that contains the ring-shaped lipid
cholesterol. Neither gram-positive nor gram-negative cell membranes