Acell wall is a structural layer that surrounds somecell types, found immediately outside thecell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functions as a selective barrier.[1] Another vital role of the cell wall is to help the cell withstandosmotic pressure and mechanical stress. While absent in manyeukaryotes, including animals, cell walls are prevalent in other organisms such asfungi,algae andplants, and are commonly found in mostprokaryotes, with the exception ofmollicute bacteria.
A plant cell wall was first observed and named (simply as a "wall") byRobert Hooke in 1665.[3] However, "the dead excrusion product of the living protoplast" was forgotten, for almost three centuries, being the subject of scientific interest mainly as a resource for industrial processing or in relation to animal or human health.[4]
In 1804,Karl Rudolphi andJ.H.F. Link proved that cells had independent cell walls.[5][6] Before, it had been thought that cells shared walls and that fluid passed between them this way.
The mode of formation of the cell wall was controversial in the 19th century.Hugo von Mohl (1853, 1858) advocated the idea that the cell wall grows by apposition.Carl Nägeli (1858, 1862, 1863) believed that the growth of the wall in thickness and in area was due to a process termed intussusception. Each theory was improved in the following decades: the apposition (or lamination) theory byEduard Strasburger (1882, 1889), and the intussusception theory byJulius Wiesner (1886).[7]
In 1930,Ernst Münch coined the termapoplast in order to separate the "living"symplast from the "dead" plant region, the latter of which included the cell wall.[8]
By the 1980s, some authors suggested replacing the term "cell wall", particularly as it was used for plants, with the more precise term "extracellular matrix", as used for animal cells,[9][4]: 168 but others preferred the older term.[10]
Diagram of the plant cell, with the cell wall in green.
Cell walls serve similar purposes in those organisms that possess them. They may give cells rigidity and strength, offering protection against mechanical stress. The chemical composition and mechanical properties of the cell wall are linked with plant cell growth andmorphogenesis.[11] In multicellular organisms, they permit the organism to build and hold a definite shape. Cell walls also limit the entry of large molecules that may be toxic to the cell. They further permit the creation of stableosmotic environments by preventingosmotic lysis and helping to retain water. Their composition, properties, and form may change during thecell cycle and depend on growth conditions.[11]
Rigidity of cell walls
In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerabletensile strength. The apparent rigidity of primary plant tissues is enabled by cell walls, but is not due to the walls' stiffness. Hydraulicturgor pressure creates this rigidity, along with the wall structure. The flexibility of the cell walls is seen when plants wilt, so that the stems and leaves begin to droop, or inseaweeds that bend inwater currents. As John Howland explains
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.[12]
The apparent rigidity of the cell wall thus results from inflation of the cell contained within. Thisinflation is a result of thepassive uptake of water.
In plants, asecondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed bylignin inxylem cell walls, orsuberin incork cell walls. These compounds arerigid andwaterproof, making the secondary wall stiff. Bothwood andbark cells oftrees have secondary walls. Other parts of plants such as theleaf stalk may acquire similar reinforcement to resist the strain of physical forces.
Permeability
The primary cell wall of mostplant cells is freely permeable to small molecules including smallproteins, with size exclusion estimated to be 30-60kDa.[13] The pH is an important factor governing the transport of molecules through cell walls.[14]
Thephotosyntheticeukaryotes (so-called plant and algae) is one group with cellulose cell walls, where the cell wall is closely related to the evolution ofmulticellularity, terrestrialization and vascularization. TheCesA cellulose synthase evolved inCyanobacteria and was part ofArchaeplastida sinceendosymbiosis;secondary endosymbiosis events transferred it (with thearabinogalactan proteins) further intobrown algae andoomycetes. Plants later evolved various genes from CesA, including the Csl (cellulose synthase-like) family of proteins and additional Ces proteins. Combined with the various glycosyltransferases (GT), they enable more complex chemical structures to be built.[15]
Fungi use achitin-glucan-protein cell wall.[16] They share the 1,3-β-glucan synthesis pathway with plants, using homologous GT48 family1,3-Beta-glucan synthases to perform the task, suggesting that such an enzyme is very ancient within the eukaryotes. Their glycoproteins are rich inmannose. The cell wall might have evolved to deter viral infections. Proteins embedded in cell walls are variable, contained intandem repeats subject tohomologous recombination.[17] An alternative scenario is that fungi started with achitin-based cell wall and later acquired the GT-48 enzymes for the 1,3-β-glucans viahorizontal gene transfer. The pathway leading to 1,6-β-glucan synthesis is not sufficiently known in either case.[18]
Plant cell walls
The walls of plant cells must have sufficient tensile strength to withstand internalosmotic pressures of several timesatmospheric pressure that result from the difference in solute concentration between the cell interior and external solutions.[1] Plant cell walls vary from 0.1 to several μm in thickness.[19]
Layers
Cell wall in multicellular plants – its different layers and their placement with respect to protoplasm (highly diagrammatic)Molecular structure of the primary cell wall in plants
Up to three strata or layers may be found in plant cell walls:[20]
Theprimary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing.
Thesecondary 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. Some cells, such as the conducting cells inxylem, possess a secondary wall containinglignin, which strengthens and waterproofs the wall.
Themiddle lamella, a layer rich inpectins. This outermost layer forms the interface between adjacent plant cells and glues them together.
Composition
In the primary (growing) plant cell wall, the majorcarbohydrates arecellulose,hemicellulose andpectin. The cellulosemicrofibrils 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 isxyloglucan.[21] In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronoarabinoxylan, another type of hemicellulose. Primary cell walls characteristically extend (grow) by a mechanism calledacid growth, mediated byexpansins, extracellular proteins activated by acidic conditions that modify the hydrogen bonds betweenpectin and cellulose.[22] This functions to increase cell wall extensibility. The outer part of the primary cell wall of the plant epidermis is usually impregnated withcutin andwax, forming a permeability barrier known as theplant cuticle.
Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The majorpolymers that make upwood (largely secondary cell walls) include:
lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.
Photomicrograph of onion root cells, showing the centrifugal development of new cell walls (phragmoplast)
Additionally, structuralproteins (1-5%) 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 areglycosylated, containhydroxyproline (Hyp) and become cross-linked in the cell wall. These proteins are often concentrated in specialized cells and in cell corners. Cell walls of theepidermis may containcutin. TheCasparian strip in theendodermis roots andcork cells of plant bark containsuberin. Both cutin and suberin are polyesters that function as permeability barriers to the movement of water.[23] The relative composition of carbohydrates, secondary compounds and proteins varies between plants and between the cell type and age. Plant cells walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim andcross-link wall polymers.
Secondary walls - especially in grasses - may also contain microscopicsilica crystals, which may strengthen the wall and protect it from herbivores.
Cell walls in some plant tissues also function as storage deposits 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[24]: 228 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.
Formation
Themiddle lamella is laid down first, formed from thecell plate duringcytokinesis, and the primary cell wall is then deposited inside the middle lamella.[clarification needed] 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 ofcellulosemicrofibrils aligned at all angles. Cellulose microfibrils are produced at the plasma membrane by thecellulose synthase complex, which is proposed to be made of a hexameric rosette that contains three cellulose synthase catalytic subunits for each of the six units.[25] Microfibrils are held together by hydrogen bonds to provide a high tensile strength. The cells are held together and share the gelatinous membrane (the middle lamella), which containsmagnesium andcalciumpectates (salts ofpectic acid). Cells interact thoughplasmodesmata, 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, asecondary wall is constructed between the plasma membrane and primary wall.[26] Unlike the primary wall, the cellulose microfibrils are aligned parallel in layers, the orientation changing slightly with each additional layer so that the structure becomes helicoidal.[27] Cells with secondary cell walls can be rigid, as in the grittysclereid cells inpear andquince fruit. Cell to cell communication is possible throughpits in the secondary cell wall that allow plasmodesmata to connect cells through the secondary cell walls.
Fungal cell walls
Chemical structure of a unit from achitin polymer chain
There are several groups of organisms that have been called "fungi". Some of these groups (Oomycete andMyxogastria) 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 ofchitin and otherpolysaccharides.[28] True fungi do not havecellulose in their cell walls.[16]
In fungi, the cell wall is the outer-most layer, external to theplasma membrane. The fungal cell wall is a matrix of three main components:[16]
glucans: glucosepolymers that function to cross-linkchitin orchitosan polymers. β-glucans are glucose molecules linked via β-(1,3)- or β-(1,6)- bonds and provide rigidity to the cell wall while α-glucans are defined by α-(1,3)- and/or α-(1,4) bonds and function as part of the matrix.[16]
proteins: enzymes necessary for cell wall synthesis and lysis in addition to structural proteins are all present in the cell wall. Most of the structural proteins found in the cell wall areglycosylated and containmannose, thus these proteins are called mannoproteins ormannans.[16]
The group ofalgae known as thediatomssynthesize their cell walls (also known asfrustules or valves) fromsilicic acid. 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[30] and possibly an explanation for higher growth rates in diatoms.[31]
The groupOomycetes, also known as water molds, aresaprotrophicplant pathogens like fungi. Until recently they were widely believed to be fungi, butstructural andmolecular evidence[33] has led to their reclassification asheterokonts, related toautotrophicbrown algae anddiatoms. Unlike fungi, oomycetes typically possess cell walls of cellulose andglucans rather than chitin, although some genera (such asAchlya andSaprolegnia) do have chitin in their walls.[34] The fraction of cellulose in the walls is no more than 4 to 20%, far less than the fraction of glucans.[34] Oomycete cell walls also contain theamino acidhydroxyproline, which is not found in fungal cell walls.
Slime molds
Thedictyostelids are another group formerly classified among the fungi. They areslime molds that feed as unicellularamoebae, but aggregate into a reproductive stalk andsporangium under certain conditions. Cells of the reproductive stalk, as well as thespores formed at the apex, possess acellulose wall.[35] The spore wall has three layers, the middle one composed primarily of cellulose, while the innermost is sensitive tocellulase andpronase.[35]
Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls are made ofpeptidoglycan (also called murein), which is made frompolysaccharide chains cross-linked by unusualpeptides containing D-amino acids.[36] Bacterial cell walls are different from the cell walls ofplants andfungi which are made ofcellulose andchitin, respectively.[37] 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, althoughL-form bacteria can be produced in the laboratory that lack a cell wall.[38] The antibioticpenicillin is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse.[37] Thelysozyme enzyme can also damage bacterial cell walls.
There are broadly speaking two different types of cell wall in bacteria, calledgram-positive andgram-negative. The names originate from the reaction of cells to theGram stain, a test long-employed for the classification of bacterial species.[39]
Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan andteichoic acids.
Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containinglipopolysaccharides andlipoproteins. Most bacteria have the gram-negative cell wall and only theBacillota andActinomycetota (previously known as the low G+C and high G+C gram-positive bacteria, respectively) have the alternative gram-positive arrangement.[40]
Although not truly unique, the cell walls ofArchaea are unusual. Whereaspeptidoglycan is a standard component of all bacterial cell walls, all archaeal cell walls lackpeptidoglycan,[42] though somemethanogens have a cell wall made of a similar polymer calledpseudopeptidoglycan.[12] There are four types of cell wall currently known among the Archaea.
One type of archaeal cell wall is that composed ofpseudopeptidoglycan (also calledpseudomurein). This type of wall is found in somemethanogens, such asMethanobacterium andMethanothermus.[43] While the overall structure of archaealpseudopeptidoglycan superficially resembles that of bacterial peptidoglycan, there are a number of significant chemical differences. Like the peptidoglycan found in bacterial cell walls, pseudopeptidoglycan consists ofpolymer chains ofglycan cross-linked by shortpeptide connections. However, unlike peptidoglycan, the sugarN-acetylmuramic acid is replaced byN-acetyltalosaminuronic acid,[42] and the two sugars are bonded with aβ,1-3 glycosidic linkage instead ofβ,1-4. Additionally, the cross-linking peptides areL-amino acids rather than D-amino acids as they are in bacteria.[43]
A second type of archaeal cell wall is found inMethanosarcina andHalococcus. This type of cell wall is composed entirely of a thick layer ofpolysaccharides, which may besulfated in the case ofHalococcus.[43] Structure in this type of wall is complex and not fully investigated.
A third type of wall among theArchaea consists ofglycoprotein, and occurs in thehyperthermophiles,Halobacterium, and somemethanogens. InHalobacterium, theproteins in the wall have a high content ofacidicamino acids, giving the wall an overall negative charge. The result is an unstable structure that is stabilized by the presence of large quantities of positivesodiumions thatneutralize the charge.[43] Consequently,Halobacterium thrives only under conditions with highsalinity.
In other Archaea, such asMethanomicrobium andDesulfurococcus, the wall may be composed only of surface-layerproteins,[12] known as anS-layer. S-layers are common in bacteria, where they serve as either the sole cell-wall component or an outer layer in conjunction withpolysaccharides. Most Archaea are Gram-negative, though at least one Gram-positive member is known.[12]
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^Roberts K (October 1994). "The plant extracellular matrix: in a new expansive mood".Current Opinion in Cell Biology.6 (5):688–94.doi:10.1016/0955-0674(89)90074-4.PMID7833049.
^Hogan CM (2010)."Abiotic factor". In Monosson E, Cleveland C (eds.).Encyclopedia of Earth. Washington DC: National Council for Science and the Environment. Archived fromthe original on 2013-06-08.
^Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB (2008).Biology (8th ed.). Pearson Benjamin Cummings. pp. 119.ISBN978-0-8053-6844-4.
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External links
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