<P> Although not truly unique, the cell walls of Archaea are unusual . Whereas peptidoglycan is a standard component of all bacterial cell walls, all archaeal cell walls lack peptidoglycan, with the exception of one group of methanogens . In that group, the peptidoglycan is a modified form very different from the kind found in bacteria . There are four types of cell wall currently known among the Archaea . </P> <P> One type of archaeal cell wall is that composed of pseudopeptidoglycan (also called pseudomurein). This type of wall is found in some methanogens, such as Methanobacterium and Methanothermus . While the overall structure of archaeal pseudopeptidoglycan superficially resembles that of bacterial peptidoglycan, there are a number of significant chemical differences . Like the peptidoglycan found in bacterial cell walls, pseudopeptidoglycan consists of polymer chains of glycan cross-linked by short peptide connections . However, unlike peptidoglycan, the sugar N - acetylmuramic acid is replaced by N - acetyltalosaminuronic acid, and the two sugars are bonded with a β, 1 - 3 glycosidic linkage instead of β, 1 - 4 . Additionally, the cross-linking peptides are L - amino acids rather than D - amino acids as they are in bacteria . </P> <P> A second type of archaeal cell wall is found in Methanosarcina and Halococcus . This type of cell wall is composed entirely of a thick layer of polysaccharides, which may be sulfated in the case of Halococcus . Structure in this type of wall is complex and not fully investigated . </P> <P> A third type of wall among the Archaea consists of glycoprotein, and occurs in the hyperthermophiles, Halobacterium, and some methanogens . In Halobacterium, the proteins in the wall have a high content of acidic amino acids, giving the wall an overall negative charge . The result is an unstable structure that is stabilized by the presence of large quantities of positive sodium ions that neutralize the charge . Consequently, Halobacterium thrives only under conditions with high salinity . </P>

What is chemical composition of cell wall in plants and fungi