Cell wall
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2008-07-15T21:05:08Z
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[[Image:Chloroplasten.jpg|thumb|250px|[[Plant cell]]s separated by transparent cell walls.]]
A '''cell wall''' is a fairly rigid layer surrounding a [[cell (biology)|cell]], located external to the [[cell membrane]], which provides the cell with structural support, protection, and acts as a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. They are found in [[plant]]s, [[bacteria]], [[fungus|fungi]], [[alga]]e, and some [[archaea]]. [[Animal]]s, and [[protozoa]] do not have cell walls.
The materials in a cell wall varies between species. In plants, the strongest component of the complex cell wall is a [[carbohydrate]] [[polymer]] called [[cellulose]]. In bacteria, [[peptidoglycan]] forms the cell wall. Archaean cell walls have various compositions, and may be formed of [[glycoprotein]] [[S-layer]]s, [[pseudopeptidoglycan]], or [[polysaccharide]]s. Fungi possess cell walls of [[chitin]], and algae typically possess walls constructed of glycoproteins and polysaccharides, however certain algal species may have a cell wall composed of [[silicic acid]]. Often, other accessory molecules are found anchored to the cell wall.
== Properties ==
[[Image:Eukaryota cell strucutre.PNG|thumb|250px|right|Diagram of the plant cell, with the cell wall in green.]]
The cell wall serves a similar purpose in those organisms that possess them. The wall gives cells rigidity and strength, offering protection against mechanical stress. In multicellular organisms, it permits the organism to build and hold its shape ([[morphogenesis]]). The cell wall also limits the entry of large molecules that may be toxic to the cell. It further permits the creation of a stable [[osmotic]] environment by preventing [[osmotic lysis]] and helping to retain water. The composition, properties, and form of the cell wall may change during the [[cell cycle]] and depend on growth conditions.
=== Rigidity ===
In most cells, the cell wall is semi-rigid, meaning that it will bend somewhat rather than holding a fixed shape. This flexibility is seen when plants wilt, so that the stems and leaves begin to droop, or in [[seaweed]]s that bend in [[water current]]s. Wall rigidity seen in healthy plants results from a combination of the wall construction and [[turgor pressure]]. As John Howland states it:
{{cquote|Think of the cell wall as a wicker basket in which a balloon has been inflated so that it exerts pressure from the inside. Such a basket is very rigid and resistant to mechanical damage. Thus does the prokaryote cell (and eukaryotic cell that possesses a cell wall) gain strength from a flexible plasma membrane pressing against a rigid cell wall.<ref name="Howland 2000">{{cite book| last = Howland | first = John L. | year = 2000 | title = The Surprising Archaea: Discovering Another Domain of Life | pages = 69-71 | publisher = Oxford University Press | location = Oxford | isbn = 0-19-511183-4}}</ref>}}
The rigidity of the cell wall thus results in part from inflation of the cell contained. This inflation is a result of the [[osmosis|passive uptake of water]].
Other cell walls are inflexible. In plants, a '''secondary cell wall''' is a thicker additional layer of cellulose. Additional layers may be formed containing [[lignin]] in [[xylem]] cell walls, or containing [[suberin]] in [[cork cambium|cork]] cell walls. These compounds are [[rigid]] and [[waterproof]], making the secondary wall stiff. Both [[wood]] and [[bark]] cells of [[tree]]s have secondary walls. Other parts of plants such as the [[petiole (botany)|leaf stalk]] may acquire similar reinforcement to resist the strain of physical forces.
Certain single-cell [[protist]]s and [[algae]] also produce a rigid wall. [[Diatom]]s build a '''frustule''' from [[silica]] extracted from the surrounding water; [[radiolarian]]s also produce a '''test''' from minerals. Many [[green algae]], such as the [[Dasycladales]] encase their cells in a secreted skeleton of [[calcium carbonate]]. In each case, the wall is rigid and essentially inorganic.
=== Permeability ===
The primary cell wall of most [[plant cell]]s is [[Semipermeable membrane|semi-permeable]] and permit the passage of small molecules and small proteins, with size exclusion estimated to be 30-60 kDa. Key nutrients, especially [[water]] and [[carbon dioxide]], are distributed throughout the plant from cell wall to cell wall in [[apoplast]]ic flow.
==Plant cell walls ==
===Composition===
[[Image:Plant cell wall diagram.svg|thumb|250px|right|Molecular structure of the primary cell wall in plants.]]
The major [[carbohydrate]]s making up the primary (growing) cell wall are [[cellulose]], [[hemicellulose]] and [[pectin]]. The cellulose [[microfibril]]s are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is [[xyloglucan]]. In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by [[glucuronarabinoxylan]], a hemicellulose. Primary cell walls characteristically extend (grow) by a mechanism called [[acid growth]], which involves [[turgor]]-driven movement of the strong cellulose microfibrils within the weaker hemicellulose/pectin matrix, catalyzed by [[expansin]] proteins.
The major [[polymer]]s that make up wood (largely secondary cell walls) include cellulose (35 to 50%), [[xylan]], a type of hemicellulose, (20 to 35%) and a complex phenolic polymer called [[lignin]] (10 to 25%). Lignin penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall. Secondary walls - especially in grasses - may also contain microscopic silicate crystals, which may strengthen the wall and protect it from herbivores.
Plant cells walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim and cross link wall polymers. Small amounts (1-5%) of structural [[protein]]s are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence. Most are [[glycosylation|glycosylated]], contain [[hydroxyproline]] (Hyp) and become cross-linked in the cell wall. These proteins are often concentrated in specialized cells and in cell corners. Cell walls of the [[Epidermis (botany)|epidermis]] and [[endodermis]] may also contain [[suberin]] or [[cutin]], two polyester-like polymers that protect the cell from herbivores.<ref>Laurence Moire, Alain Schmutz, Antony Buchala, Bin Yan, Ruth E. Stark, and Ulrich Ryser (1999). "[http://www.plantphysiol.org/cgi/content/full/119/3/1137 Glycerol Is a Suberin Monomer. New Experimental Evidence for an Old Hypothesis]". ''Plant Physiol.'' 119: 1137-1146</ref> The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age.
Up to three strata or layers may be found in plant cell walls:<ref>{{cite book |last= Buchanan |coauthors= Gruissem, Jones |title= Biochemistry & molecular biology of plants |edition=1st ed. |publisher= American society of plant physiology|year=2000 |isbn=0-943088-39-9}}</ref>
*The [[Lamella (cell biology)|'''middle lamella''']], a layer rich in [[pectin]]s. This outermost layer forming the interface between adjacent plant cells and glues them together.
*The '''primary cell wall''', generally a thin, flexible and extensible layer formed while the cell is growing.
*The '''[[secondary cell wall]]''', a thick layer formed inside the primary cell wall after the cell is fully grown. It is not found in all cell types. In some cells, such as found [[xylem]], the secondary wall contains [[lignin]], which strengthens and waterpoofs the wall.
Cell walls in some plant tissues also function as storage depots for carbohydrates that can be broken down and resorbed to supply the metabolic and growth needs of the plant. For example, endosperm cell walls in the seeds of cereal grasses, nasturtium, and other species, are rich in glucans and other polysaccharides that are readily digested by enzymes during seed germination to form simple sugars that nourish the growing embryo. Cellulose microfibrils are not readily digested by plants, however.
=== Formation ===
The middle [[Lamella (cell biology)|lamella]] is laid down first, formed from the cell plate during [[cytokinesis]], and the primary cell wall is then deposited inside the middle lamella. The actual structure of the cell wall is not clearly defined and several models exist - the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model. However, the primary cell wall, can be defined as composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. The cells are held together and share the gelatinous membrane called the ''middle lamella'', which contains [[magnesium]] and [[calcium]] [[pectate]]s (salts of [[pectic acid]]). Cells interact though [[plasmodesma]](ta), which are inter-connecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall.
In some plants and cell types, after a maximum size or point in development has been reached, a ''secondary wall'' is constructed between the plant cell and primary wall. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer the orientation changes slightly. Cells with secondary cell walls are rigid. Cell to cell communication is possible through ''pits'' in the secondary cell wall that allow plasmodesma to connect cells through the secondary cell walls.
[[Trees]] modify cell walls in their branches to reinforce and support structure.<ref>{{cite book| last = Wilson | first = Brayton F. | year = 1984 | title = The Growing Tree | edition = revised ed. | pages=114-115 | publisher = University of Massachusetts Press | location = Amherst | isbn = 0-87023-424-2 }}</ref> [[Conifer]]s, such as [[pine]], produce thicker cell walls on the undersides of branches to push their branches upwards. The resulting wood is called [[compression wood]]. By contrast, [[hardwood]] trees reinforce the walls on the upper sides of branches to pull their branches up. This is known as [[tension wood]]. Additional thickening may occur in other parts of the plant in [[reaction wood|response]] to mechanical stress.
==Algal cell walls==
[[Image:Diatoms.png|thumb|250px|right|[[Scanning electron microscope|Scanning electron]] [[micrograph]]s of [[diatom]]s showing the external appearance of the cell wall]]
Like plants, algae have cell walls.<ref>Sendbusch, Peter V. ([[2003-07-31]]). "[http://www.biologie.uni-hamburg.de/b-online/e26/26d.htm Cell Walls of Algae]". ''Botany Online''. Retrieved on [[2007-10-29]].</ref> Algal cell walls contain cellulose and a variety of [[glycoprotein]]s. The inclusion of additional [[polysaccharide]]s in algal cells walls is used as a feature for algal taxonomy.
* Manosyl 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''.
* Xylanes
* [[Alginic acid]] is a common polysaccharide in the cell walls of [[brown algae]]
* Sulfonated polysaccharides 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|calcium ions]].
The group of [[algae]] known as the [[diatom]]s synthesize their cell walls (also known as frustules or valves) from [[silicic acid]] (specifically orthosilicic acid, H<sub>4</sub>SiO<sub>4</sub>). The acid is [[polymer]]ised 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<ref>Raven, J. A. (1983). The transport and function of silicon in plants. ''Biol. Rev.'' '''58''', 179-207.</ref> and possibly an explanation for higher growth rates in diatoms.<ref>Furnas, M. J. (1990). "''In situ'' growth rates of marine phytoplankton : Approaches to measurement, community and species growth rates". ''J. Plankton Res.'' '''12''', 1117-1151.</ref>
== Fungal cell walls ==
[[Image:Chitin.svg|thumb|Chemical structure of a unit from a [[chitin]] polymer chain.]]
There are several groups of organisms that may be called "fungi". Some of these groups have been transferred out of the Kingdom Fungi, in part because of fundamental biochemical differences in the composition of the cell wall. Most true fungi have a cell wall consisting largely of [[chitin]] and other [[polysaccharide]]s.<ref>Hudler, George W. (1998). ''Magical Mushrooms, Mischievous Molds''. Princeton, NJ: Princeton University Press, 7. [[Special:Booksources/0691028737|ISBN 0-691-02873-7]].</ref> True fungi do not have [[cellulose]] in their cell walls, but some fungus-like organisms do.
=== True fungi ===
Not all species of [[fungi]] have cell walls but in those that do, the [[plasma membrane]] is followed by three layers of cell wall material. From inside out these are:
* a [[chitin]] layer ([[polymer]] consisting mainly of unbranched chains of [[N-Acetylglucosamine|N-acetyl-D-glucosamine]])
* a layer of β-1,3-[[glucan]]
* a layer of mannoproteins ([[mannose]]-containing [[glycoproteins]]) which are heavily [[glycosylated]] at the outside of the cell.
=== Fungus-like protists ===
The group [[Water mould|Oomycetes]], also known as water molds, are [[saprotroph]]ic [[Plant pathology|plant pathogens]] like fungi. Until recently they were widely believed to be fungi, but [[organelle|structural]] and [[molecular biology|molecular]] evidence<ref>Sengbusch, Peter V. ([[2003-07-31]]). "[http://www.biologie.uni-hamburg.de/b-online/e33/33.htm Interactions between Plants and Fungi: the Evolution of their Parasitic and Symbiotic Relations]". ''biologie.uni-hamburg.de''. Retrieved on [[2007-10-29]].</ref> has led to their reclassification as [[heterokont]]s, related to [[autotroph]]ic [[brown algae]] and [[diatom]]s. Unlike fungi, oomycetes typically possess cell walls of cellulose and [[glucan]]s rather than chitin, although some genera (such as ''[[Achlya]]'' and ''[[Saprolegnia]]'') do have chitin in their walls.<ref name="Alexopoulos 1996">Alexopoulos, C. J., C. W. Mims, & M. Blackwell (1996). ''Introductory Mycology'' '''4'''. New York: John Wiley & Sons, 687-688. [[Special:Booksources/0471522295|ISBN 0-471-52229-5]].</ref> The fraction of cellulose in the walls is no more than 4 to 20%, far less than the fraction comprised by glucans.<ref name="Alexopoulos 1996" /> Oomycete cell walls also contain the [[amino acid]] [[hydroxyproline]], which is not found in fungal cell walls.
The [[dictyostelid]]s are another group formerly classified among the fungi. They are [[slime mould]]s that feed as unicellular [[amoeba]]e, but aggregate into a reproductive stalk and [[sporangium]] under certain conditions. Cells of the reproductive stalk, as well as the [[spore]]s formed at the apex, possess a [[cellulose]] wall.<ref name="Raper 1984">Raper, Kenneth B. (1984). ''The Dictyostelids''. Princeton, NJ: Princeton University Press, 99-100. [[Special:Booksources/0691083452|ISBN 0-691-08345-2]].</ref> 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]].<ref name="Raper 1984" />
==Prokaryotic cell walls==
=== Bacterial cell walls ===<!-- This section is linked from [[Bacteriocin]] -->
[[Image:Average prokaryote cell- en.svg|thumb|300px|right|Diagram of a typical [[gram-negative]] bacterium, with the thin cell wall sandwiched between the yellow outer membrane and the thin red plasma membrane]]
[[Image:Gram-positive cellwall-schematic.png|thumb|Schematic of typical [[gram-positive]] cell wall showing arrangement of N-Acetylglucosamine and N-Acetlymuramic acid]]
{{further|[[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 [[peptide]]s containing D-[[amino acid]]s.<ref>{{cite journal | author = van Heijenoort J | title = Formation of the glycan chains in the synthesis of bacterial peptidoglycan | url=http://glycob.oxfordjournals.org/cgi/content/full/11/3/25R | journal = Glycobiology | volume = 11 | issue = 3 | pages = 25R – 36R | year = 2001 | pmid = 11320055 | doi = 10.1093/glycob/11.3.25R }}</ref> Bacterial cell walls are different from the cell walls of [[plants]] and [[fungi]] which are made of [[cellulose]] and [[chitin]], respectively.<ref name=Koch>{{cite journal | author = Koch A | title = Bacterial wall as target for attack: past, present, and future research | url=http://cmr.asm.org/cgi/content/full/16/4/673?view=long&pmid=14557293 | doi = 10.1128/CMR.16.4.673-687.2003 <!--Retrieved from url by DOI bot--> | journal = Clin Microbiol Rev | volume = 16 | issue = 4 | pages = 673 – 87 | year = 2003 | pmid = 14557293}}</ref> 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. The antibiotic [[penicillin]] is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.<ref name=Koch/>
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.<ref name=Gram>{{cite journal | last = Gram | first = HC | authorlink = Hans Christian Gram | year = 1884 | title = Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten | journal = Fortschr. Med. | volume = 2 | pages = 185–189 }}</ref>
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 [[lipopolysaccharide]]s and [[lipoprotein]]s. 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.<ref>{{cite journal | author = Hugenholtz P | title = Exploring prokaryotic diversity in the genomic era | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=11864374 | doi = 10.1186/1471-2148-1-8 <!--Retrieved from url by DOI bot--> | journal = Genome Biol | volume = 3 | issue = 2 | pages = REVIEWS0003 | year = 2002 | pmid = 11864374}}</ref> 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]]''.<ref>{{cite journal | author = Walsh F, Amyes S | title = Microbiology and drug resistance mechanisms of fully resistant pathogens. | journal = Curr Opin Microbiol | volume = 7 | issue = 5 | pages = 439-44 | year = 2004 | pmid = 15451497 | doi = 10.1016/j.mib.2004.08.007 }}</ref>
=== 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">White, David. (1995) ''The Physiology and Biochemistry of Prokaryotes'', pages 6, 12-21. (Oxford: Oxford University Press). ISBN 0-19-508439-X.</ref> with the exception of one group of [[methanogen]]s.<ref name="Howland 2000" /> In that group, the peptidoglycan is a modified form very different from the kind found in bacteria.<ref name="White 1995" /> 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">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). ISBN 0-13-042169-3.</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 [[bacteria]]l 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 as yet is 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 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">Howland, John L. (2000) ''The Surprising Archaea: Discovering Another Domain of Life'', pages 69-71. (Oxford: Oxford University Press). ISBN 0-19-511183-4.</ref> 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 [[peptidoglycan]] and [[murein]]. Most Archaea are Gram-negative, though at least one Gram-positive member is known.<ref name="Howland 2000" />
==See also==
* [[Bacterial cell structure]]
* [[Plant cell]]
==References==
{{reflist|2}}
==External links==
{{wiktionary}}
* [http://micro.magnet.fsu.edu/cells/plants/cellwall.html Cell wall ultrastructure]
* [http://www.palaeos.com/Fungi/FPieces/CellWall.html The Cell Wall]
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{{Organelles}}
{{Botany}}
[[Category:Cell anatomy]]
[[Category:Plant physiology]]
[[Category:Organelles]]
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