ជញ្ជាំងកោសិកា: Other eukaryotic cell walls

Other eukaryotic cell walls

សារាយ
thumb|250px|right|Scanning electron micrographs of diatoms showing the external appearance of the cell wall
Like plants, algae have cell walls.Sendbusch, Peter V. (2003-07-31). "Cell Walls of Algae ". Botany Online. Retrieved on 2007-10-29. Algal cell walls contain either polysaccharides (such as cellulose (a glucan)) or a variety of glycoproteins (Volvocales) or both. The inclusion of additional polysaccharides in algal cells walls is used as a feature for algal taxonomy.

 Mannans: They form microfibrils in the cell walls of a number of marine green algae including those from the genera, Codium, Dasycladus, and Acetabularia as well as in the walls of some red algae, like Porphyra and Bangia.
 Xylans:
 Alginic acid: It is a common polysaccharide in the cell walls of brown algae.
 Sulfonated polysaccharides: They occur in the cell walls of most algae; those common in red algae include agarose, carrageenan, porphyran, furcelleran and funoran.

Other compounds that may accumulate in algal cell walls include sporopollenin and calcium ions.

The group of algae known as the diatoms synthesize their cell walls (also known as frustules or valves) from silicic acid (specifically orthosilicic acid, H4SiO4). The acid is polymerised intra-cellularly, then the wall is extruded to protect the cell. Significantly, relative to the organic cell walls produced by other groups, silica frustules require less energy to synthesize (approximately 8%), potentially a major saving on the overall cell energy budget and possibly an explanation for higher growth rates in diatoms.<ref>{{Cite journal | doi = 10.1093/plankt/12.6.1117 | author = Furnas, M. J. | year = 1990 | title = In situ growth rates of marine phytoplankton : Approaches to measurement, community and species growth rates | url = | journal = J. Plankton Res. | volume = 12 | issue = 6| pages = 1117–1151 }}</ref>

In brown algae, phlorotannins may be a constituent of the cell walls.

Water molds
The group Oomycetes, also known as water molds, are saprotrophic plant pathogens like fungi. Until recently they were widely believed to be fungi, but structural and molecular evidenceSengbusch, Peter V. (2003-07-31). "Interactions between Plants and Fungi: the Evolution of their Parasitic and Symbiotic Relations ". biologie.uni-hamburg.de. Retrieved on 2007-10-29. has led to their reclassification as heterokonts, related to autotrophic brown algae and diatoms.  Unlike fungi, oomycetes typically possess cell walls of cellulose and glucans rather than chitin, although some genera (such as Achlya and Saprolegnia) do have chitin in their walls.Alexopoulos, C. J., C. W. Mims, & M. Blackwell (1996). Introductory Mycology 4. New York: John Wiley & Sons, 687-688. .  The fraction of cellulose in the walls is no more than 4 to 20%, far less than the fraction of glucans.  Oomycete cell walls also contain the amino acid hydroxyproline, which is not found in fungal cell walls.

Slime molds
The dictyostelids are another group formerly classified among the fungi. They are slime molds that feed as unicellular amoebae, but aggregate into a reproductive stalk and sporangium under certain conditions. Cells of the reproductive stalk, as well as the spores formed at the apex, possess a cellulose wall.Raper, Kenneth B. (1984). The Dictyostelids. Princeton, NJ: Princeton University Press, 99-100. . The spore wall has been shown to possess three layers, the middle of which is composed primarily of cellulose, and the innermost is sensitive to cellulase and pronase.

Prokaryotic cell walls

Bacterial cell walls

thumb|300px|right|Diagram of a typical Gram-positive bacterium. The cell envelope comprises a plasma membrane, seen here in light brown, and a thick peptidoglycan-containing cell wall (the purple layer). No outer lipid membrane is present, as would be the case in Gram-negative bacteria. The red layer, known as the capsule, is distinct from the cell envelope.

Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by unusual peptides containing D-amino acids. Bacterial cell walls are different from the cell walls of plants and fungi which are made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, although L-form bacteria can be produced in the laboratory that lack a cell wall.  The antibiotic penicillin is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse. The lysozyme enzyme can also damage bacterial cell walls.

There are broadly speaking two different types of cell wall in bacteria, called Gram-positive and Gram-negative. The names originate from the reaction of cells to the Gram stain, a test long-employed for the classification of bacterial species.

Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. Most bacteria have the Gram-negative cell wall and only the Firmicutes and Actinobacteria (previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the alternative Gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility, for instance vancomycin can kill only Gram-positive bacteria and is ineffective against Gram-negative pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa.

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,White, David. (1995) The Physiology and Biochemistry of Prokaryotes, pages 6, 12-21. (Oxford: Oxford University Press). . 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.

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.Brock, Thomas D., Michael T. Madigan, John M. Martinko, & Jack Parker. (1994) Biology of Microorganisms, 7th ed., pages 818-819, 824 (Englewood Cliffs, NJ: Prentice Hall). .  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.

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.

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.

In other Archaea, such as Methanomicrobium and Desulfurococcus, the wall may be composed only of surface-layer proteins, 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.

Other cell coverings

Many protists and bacteria produce other cell surface structures apart from cell walls, external (extracellular matrix) or internal.Preisig, H.R. Terminology and nomenclature of protist cell surface structures. In: The Protistan Cell Surface. Protoplasma special edition, 1994.Becker, B. The cell surface of flagellates . In: The Flagellates. Unity, diversity and evolution. Ed.: Barry S. C. Leadbeater and J. C. Green Taylor and Francis, London 2000.Barsanti, Laura; Gualtieri, Paolo (2006). Algae: anatomy, biochemistry, and biotechnology. Florida, USA: CRC Press. Many algae have a sheath or envelope of mucilage outside the cell made of exopolysaccharides. Diatoms build a frustule from silica extracted from the surrounding water; radiolarians, foraminiferans, testate amoebae and silicoflagellates also produce a skeleton from minerals, called test in some groups.  Many green algae, such as Halimeda'' and the Dasycladales, and some red algae, the Corallinales, encase their cells in a secreted skeleton of calcium carbonate.  In each case, the wall is rigid and essentially inorganic. It is the non-living component of cell. Some golden algae, ciliates and choanoflagellates  produces a shell-like protective outer covering called lorica. Some dinoflagellates have a theca of cellulose plates, and coccolithophorids have coccoliths.

An extracellular matrix is also present in metazoans. Its composition varies between cells, but collagens are the most abundant protein in the ECM.