Editing Cell wall (section)

===Archaeal cell walls===
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]],<ref name="White 1995">{{cite book | last = White | first = David | name-list-style = vanc | date = 1995 | title = The Physiology and Biochemistry of Prokaryotes | pages = 6, 12–21 | location = Oxford | publisher = Oxford University Press | isbn = 978-0-19-508439-9 }}</ref> though some [[methanogen]]s have a cell wall made of a similar polymer called [[pseudopeptidoglycan]].<ref name="Howland 2000" /> There are four types of cell wall currently known among the Archaea.

One type of archaeal cell wall is that composed of [[pseudopeptidoglycan]] (also called [[pseudomurein]]). This type of wall is found in some [[methanogen]]s, such as ''[[Methanobacterium]]'' and ''[[Methanothermus]]''.<ref name="Brock 1994">{{cite book | vauthors = Brock TD, Madigan MT, Martinko JM, Parker J | author-link1 = Thomas D. Brock | date = 1994 | title = Biology of Microorganisms | edition = 7th | pages = 818–819, 824 | location = Englewood Cliffs, NJ | publisher = Prentice Hall | isbn = 978-0-13-042169-2 }}</ref> While the overall structure of archaeal ''pseudo''peptidoglycan 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|N-acetylmuramic acid]] is replaced by [[N-Acetyltalosaminuronic acid|N-acetyltalosaminuronic acid]],<ref name="White 1995" /> 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 acid]]s rather than D-amino acids as they are in bacteria.<ref name="Brock 1994" />

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 [[polysaccharide]]s, which may be [[sulfate]]d in the case of ''Halococcus''.<ref name="Brock 1994" /> Structure in this type of wall is complex and not fully investigated.

A third type of wall among the [[Archaea]] consists of [[glycoprotein]], and occurs in the [[hyperthermophile]]s, ''[[Halobacterium]]'', and some [[methanogen]]s. In ''Halobacterium'', the [[protein]]s in the wall have a high content of [[acid]]ic [[amino acid]]s, 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]] [[ion]]s that [[Neutralization (chemistry)|neutralize]] the charge.<ref name="Brock 1994" /> Consequently, ''Halobacterium'' thrives only under conditions with high [[salinity]].

In other Archaea, such as ''[[Methanomicrobium]]'' and ''[[Desulfurococcus]]'', the wall may be composed only of surface-layer [[protein]]s,<ref name="Howland 2000"/> known as an ''[[S-layer]]''. S-layers are common in bacteria, where they serve as either the sole cell-wall component or an outer layer in conjunction with [[polysaccharides]]. Most Archaea are Gram-negative, though at least one Gram-positive member is known.<ref name="Howland 2000" />