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Cell biology
Plant cell diagram
Components of a typical plant cell:
a.Plasmodesmata
b.Plasma membrane
c.Cell wall
1.Chloroplast
d.Thylakoid membrane
e. Starch grain
2. Vacuole
f.Vacuole
g.Tonoplast
h.Mitochondrion
i.Peroxisome
j.Cytoplasm
k. Small membranous vesicles
l.Rough endoplasmic reticulum
3.Nucleus
m.Nuclear pore
n.Nuclear envelope
o.Nucleolus
p.Ribosome
q.Smooth endoplasmic reticulum
r.Golgi vesicles
s.Golgi apparatus(Golgi body)
t.Cytoskeleton

Acell wallis 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 pressureand mechanical stress. While absent in manyeukaryotes,including animals, cell walls are prevalent in other organisms such asfungi,algaeandplants,and are commonly found in mostprokaryotes,with the exception ofmollicutebacteria.

The composition of cell walls varies acrosstaxonomic groups,species,cell type, and thecell cycle.Inland plants,the primary cell wall comprisespolysaccharideslikecellulose,hemicelluloses,andpectin.Often, otherpolymerssuch aslignin,suberinorcutinare anchored to or embedded in plant cell walls.Algaeexhibit cell walls composed ofglycoproteinsandpolysaccharides,such ascarrageenanandagar,distinct from those in land plants. Bacterial cell walls containpeptidoglycan,whilearchaealcell walls vary in composition, potentially consisting of glycoproteinS-layers,pseudopeptidoglycan,or polysaccharides. Fungi possess cell walls constructed from the polymerchitin,specificallyN-acetylglucosamine.Interestingly,diatomshave a unique cell wall composed ofbiogenic silica.[2]

History

A plant cell wall was first observed and named (simply as a "wall" ) byRobert Hookein 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 RudolphiandJ.H.F. Linkproved 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 Münchcoined the termapoplastin order to separate the "living"symplastfrom 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]

Properties

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 stableosmoticenvironments by preventingosmotic lysisand helping to retain water. Their composition, properties, and form may change during thecell cycleand 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 pressurecreates 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 inseaweedsthat 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. Thisinflationis a result of thepassive uptake of water.

In plants, asecondary cell wallis a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed bylignininxylemcell walls, orsuberinincorkcell walls. These compounds arerigidandwaterproof,making the secondary wall stiff. Bothwoodandbarkcells oftreeshave secondary walls. Other parts of plants such as theleaf stalkmay acquire similar reinforcement to resist the strain of physical forces.

Permeability

The primary cell wall of mostplant cellsis 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]

Evolution

Cell walls evolved independently in many groups.

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 synthaseevolved inCyanobacteriaand was part ofArchaeplastidasinceendosymbiosis;secondary endosymbiosisevents transferred it (with thearabinogalactanproteins) further intobrown algaeandoomycetes.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-proteincell wall.[16]They share the 1,3-β-glucan synthesis pathway with plants, using homologous GT48 family1,3-Beta-glucan synthasesto 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 repeatssubject 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 pressuresof several timesatmospheric pressurethat 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 majorcarbohydratesarecellulose,hemicelluloseandpectin.The cellulosemicrofibrilsare 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 betweenpectinand cellulose.[22]This functions to increase cell wall extensibility. The outer part of the primary cell wall of the plant epidermis is usually impregnated withcutinandwax,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 majorpolymersthat make upwood(largely secondary cell walls) include:

  • cellulose, 35-50%
  • xylan,20-35%, a type of hemicellulose
  • 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),arabinogalactanproteins (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 theepidermismay containcutin.TheCasparian stripin theendodermisroots andcorkcells 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-linkwall polymers.

Secondary walls - especially in grasses - may also contain microscopicsilicacrystals, 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 lamellais laid down first, formed from thecell plateduringcytokinesis,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 ofcellulosemicrofibrilsaligned 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 containsmagnesiumandcalciumpectates(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 wallis 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 grittysclereidcells inpearandquincefruit. Cell to cell communication is possible throughpitsin the secondary cell wall that allow plasmodesmata to connect cells through the secondary cell walls.

Fungal cell walls

Chemical structure of a unit from achitinpolymer chain

There are several groups of organisms that have been called "fungi". Some of these groups (OomyceteandMyxogastria) 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 ofchitinand otherpolysaccharides.[28]True fungi do not havecellulosein 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]

Other eukaryotic cell walls

Algae

Scanning electronmicrographsofdiatomsshowing the external appearance of the cell wall

Like plants, algae have cell walls.[29]Algal cell walls contain eitherpolysaccharides(such as cellulose (aglucan)) or a variety ofglycoproteins(Volvocales) or both. The inclusion of additionalpolysaccharidesin algal cells walls is used as a feature for algaltaxonomy.

Other compounds that may accumulate in algal cell walls includesporopolleninandcalcium ions.

The group ofalgaeknown as thediatomssynthesizetheir cell walls (also known asfrustulesor 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]

Inbrown algae,phlorotanninsmay be a constituent of the cell walls.[32]

Water molds

The groupOomycetes,also known as water molds, aresaprotrophicplant pathogenslike fungi. Until recently they were widely believed to be fungi, butstructuralandmolecularevidence[33]has led to their reclassification asheterokonts,related toautotrophicbrown algaeanddiatoms.Unlike fungi, oomycetes typically possess cell walls of cellulose andglucansrather than chitin, although some genera (such asAchlyaandSaprolegnia) 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

Thedictyostelidsare another group formerly classified among the fungi. They areslime moldsthat feed as unicellularamoebae,but aggregate into a reproductive stalk andsporangiumunder certain conditions. Cells of the reproductive stalk, as well as thesporesformed at the apex, possess acellulosewall.[35]The spore wall has three layers, the middle one composed primarily of cellulose, while the innermost is sensitive tocellulaseandpronase.[35]

Prokaryotic cell walls

Bacterial cell walls

Illustration of a typicalgram-positive bacterium.The cell envelope comprises aplasma membrane,seen here in light brown, and a thickpeptidoglycan-containing cell wall (the purple layer). Noouter lipid membraneis present, as would be the case ingram-negative bacteria.The red layer, known as thecapsule,is distinct from the cell envelope.
Further information:Cell envelopeandBacterial cell structure

Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls are made ofpeptidoglycan(also called murein), which is made frompolysaccharidechains cross-linked by unusualpeptidescontaining D-amino acids.[36]Bacterial cell walls are different from the cell walls ofplantsandfungiwhich are made ofcelluloseandchitin,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 bacteriacan be produced in the laboratory that lack a cell wall.[38]The antibioticpenicillinis able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse.[37]Thelysozymeenzyme can also damage bacterial cell walls.

There are broadly speaking two different types of cell wall in bacteria, calledgram-positiveandgram-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 containinglipopolysaccharidesandlipoproteins.Most bacteria have the gram-negative cell wall and only theBacillotaandActinomycetota(previously known as the low G+C and high G+C gram-positive bacteria, respectively) have the alternative gram-positive arrangement.[40]

These differences in structure produce differences in antibiotic susceptibility. Thebeta-lactam antibiotics(e.g.penicillin,cephalosporin) only work against gram-negative pathogens, such asHaemophilus influenzaeorPseudomonas aeruginosa.Theglycopeptide antibiotics(e.g.vancomycin,teicoplanin,telavancin) only work against gram-positive pathogens such asStaphylococcus aureus[41]

Archaeal cell walls

Although not truly unique, the cell walls ofArchaeaare unusual. Whereaspeptidoglycanis a standard component of all bacterial cell walls, all archaeal cell walls lackpeptidoglycan,[42]though somemethanogenshave 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 asMethanobacteriumandMethanothermus.[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 ofpolymerchains ofglycancross-linked by shortpeptideconnections. However, unlike peptidoglycan, the sugarN-acetylmuramic acidis 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 acidsrather than D-amino acids as they are in bacteria.[43]

A second type of archaeal cell wall is found inMethanosarcinaandHalococcus.This type of cell wall is composed entirely of a thick layer ofpolysaccharides,which may besulfatedin the case ofHalococcus.[43]Structure in this type of wall is complex and not fully investigated.

A third type of wall among theArchaeaconsists ofglycoprotein,and occurs in thehyperthermophiles,Halobacterium,and somemethanogens.InHalobacterium,theproteinsin 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 positivesodiumionsthatneutralizethe charge.[43]Consequently,Halobacteriumthrives only under conditions with highsalinity.

In other Archaea, such asMethanomicrobiumandDesulfurococcus,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]

Other cell coverings

Manyprotistsandbacteriaproduce other cell surface structures apart from cell walls, external (extracellular matrix) or internal.[44][45][46]Manyalgaehave a sheath or envelope ofmucilageoutside the cell made ofexopolysaccharides.Diatomsbuild afrustulefromsilicaextracted from the surrounding water;radiolarians,foraminiferans,testate amoebaeandsilicoflagellatesalso produce a skeleton fromminerals,calledtestin some groups. Manygreen algae,such asHalimedaand theDasycladales,and somered algae,theCorallinales,encase their cells in asecretedskeleton ofcalcium carbonate.In each case, the wall is rigid and essentiallyinorganic.It is the non-living component of cell. Somegolden algae,ciliatesandchoanoflagellatesproduces a shell-like protective outer covering calledlorica.Somedinoflagellateshave athecaofcelluloseplates, andcoccolithophoridshavecoccoliths.

Anextracellular matrix(ECM) is also present inmetazoans.Itscompositionvaries between cells, butcollagensare the mostabundantprotein in the ECM.[47][48]

See also

References

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