- Cell wall
A cell wall is a tough, flexible and sometimes fairly rigid layer surrounding a cell, located external to the
cell membrane, which provides the cell with structural support, protection, and acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are found in plants, bacteria, fungi, algae, and some archaea. Animals, and protozoado not have cell walls.
The materials in a cell wall vary between species, and in plants and fungi also differ between cell types and developmental stages. In plants, the strongest component of the complex cell wall is a
carbohydrate, the glucose polymercalled cellulose. In bacteria, peptidoglycanforms the cell wall. Archaean cell walls have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls of the glucosaminepolymer chitin, and algae typically possess walls constructed of glycoproteins and polysaccharides. However the diatomshave a cell wall composed of silicic acid. Often, other accessory molecules are found anchored to the cell wall.
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 osmoticenvironment by preventing osmotic lysisand helping to retain water. The composition, properties, and form of the cell wall may change during the cell cycleand depend on growth conditions.
The rigidity of cell walls is often over-estimated. In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerable tensile strength. The apparent rigidity of primary plant tissues is a function of hydraulic
turgor pressureof the cells and not due to rigid cell walls. This flexibility is seen when plants wilt, so that the stems and leaves begin to droop, or in seaweeds that bend in water currents. The rigidity of healthy plants results from a combination of the wall construction and turgor pressure. As John Howland states it:
The rigidity of the cell wall thus results in part from inflation of the cell contained. This inflation is a result of the passive uptake of water.
In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed containing
ligninin xylemcell walls, or containing suberinin cork cell walls. These compounds are rigidand waterproof, making the secondary wall stiff. Both woodand barkcells of trees have secondary walls. Other parts of plants such as the leaf stalk may acquire similar reinforcement to resist the strain of physical forces.
protists and algaealso produce a rigid wall. Diatoms build a frustule from silicaextracted from the surrounding water; radiolarians also produce a test from minerals. Many green algae, such as the Dasycladalesencase their cells in a secreted skeleton of calcium carbonate. In each case, the wall is rigid and essentially inorganic.
The primary cell wall of most
plant cells is semi-permeable and permit the passage of small molecules and small proteins, with size exclusion estimated to be 30-60 kDa. Key nutrients, especially waterand carbon dioxide, are distributed throughout the plant from cell wall to cell wall in apoplastic flow.
Plant cell walls
carbohydrates making up the primary (growing) plant cell wall are cellulose, hemicelluloseand pectin. The cellulose microfibrils 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 expansinproteins. The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutinand wax, forming a permeability barrier known as the plant cuticle.
Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major
polymers 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. The walls of corkcells in the bark of trees are impregnated with suberin, and suberin also forms the permeability barrier in primary roots known as the Casparian strip. Secondary walls - especially in grasses - may also contain microscopic silicacrystals, 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
proteins 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 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 and endodermismay also contain suberinor cutin, two polyester-like polymers that protect the cell from herbivores. [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] 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: [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]
*The middle lamella, a layer rich in
pectins. 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.
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.
The middle 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 magnesiumand calcium pectates (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.
Algal cell walls
Like plants, algae have cell walls. [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.] Algal cell walls contain cellulose and a variety of glycoproteins. The inclusion of additional polysaccharides 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 algaeincluding those from the genera, "Codium", "Dasycladus", and "Acetabularia" as well as in the walls of some red algae, like "Porphyra" and "Bangia".
Alginic acidis 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, furcelleranand funoran.
Other compounds that may accumulate in algal cell walls include
sporopolleninand calcium ions.
The group of
algaeknown 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 [Raven, J. A. (1983). The transport and function of silicon in plants. "Biol. Rev." 58, 179-207.] and possibly an explanation for higher growth rates in diatoms. [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.]
Fungal cell walls
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
chitinand other polysaccharides. [Hudler, George W. (1998). "Magical Mushrooms, Mischievous Molds". Princeton, NJ: Princeton University Press, 7. .] True fungi do not have cellulosein their cell walls, but some fungus-like organisms do.
Not all species of
fungihave cell walls but in those that do, the plasma membraneis followed by three layers of cell wall material. From inside out these are:
chitinlayer ( polymerconsisting mainly of unbranched chains of N-acetyl-D-glucosamine)
* a layer of β-1,3-
* a layer of mannoproteins (
mannose-containing glycoproteins) which are heavily glycosylatedat the outside of the cell.
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 evidence [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.] has led to their reclassification as heterokonts, related to autotrophic brown algaeand 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 comprised by glucans. Oomycete cell walls also contain the amino acid hydroxyproline, which is not found in fungal cell walls.
dictyostelids are another group formerly classified among the fungi. They are slime moulds that feed as unicellular amoebae, but aggregate into a reproductive stalk and sporangiumunder certain conditions. Cells of the reproductive stalk, as well as the spores formed at the apex, possess a cellulosewall.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 cellulaseand pronase.
Prokaryotic cell walls
Bacterial cell walls
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 polysaccharidechains cross-linked by unusual peptides containing D- amino acids. [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 ] Bacterial cell walls are different from the cell walls of plantsand fungiwhich are made of celluloseand chitin, respectively.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 | journal = Clin Microbiol Rev | volume = 16 | issue = 4 | pages = 673 – 87 | year = 2003 | pmid = 14557293] 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 penicillinis able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.
There are broadly speaking two different types of cell wall in bacteria, called
Gram-positiveand Gram-negative. The names originate from the reaction of cells to the Gram stain, a test long-employed for the classification of bacterial species.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 ]
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 Firmicutesand Actinobacteria(previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the alternative Gram-positive arrangement. [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 | journal = Genome Biol | volume = 3 | issue = 2 | pages = REVIEWS0003 | year = 2002 | pmid = 11864374] These differences in structure can produce differences in antibiotic susceptibility, for instance vancomycincan kill only Gram-positive bacteria and is ineffective against Gram-negative pathogens, such as " Haemophilus influenzae" or " Pseudomonas aeruginosa". [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 ]
Archaeal cell walls
Although not truly unique, the cell walls of
Archaeaare unusual. Whereas peptidoglycanis 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). ISBN 0-19-508439-X.] 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). ISBN 0-13-042169-3.] 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 polymerchains of glycancross-linked by short peptideconnections. 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 as yet is 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,Howland, John L. (2000) "The Surprising Archaea: Discovering Another Domain of Life", pages 69-71. (Oxford: Oxford University Press). ISBN 0-19-511183-4.] 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 peptidoglycanand murein. Most Archaea are Gram-negative, though at least one Gram-positive member is known.
Bacterial cell structure
* [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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