Editing Cell wall (section)

==Plant cell walls==
The walls of plant cells must have sufficient tensile strength to withstand internal [[osmotic pressure]]s of several times [[atmospheric pressure]] that result from the difference in solute concentration between the cell interior and external solutions.<ref name=Romaniuk/> Plant cell walls vary from 0.1 to several μm in thickness.<ref>{{Cite book | title = Biology | url = https://archive.org/details/essentialbiology00camp_0 | url-access = registration | last1 = Campbell | last2 = Reece | last3 = Urry | last4 = Cain | last5 = Wasserman | last6 = Minorsky | last7 = Jackson | first1 = Neil A. | first2 = Jane B. | first3 = Lisa A. | first4 = Michael L. | first5 = Steven A. | first6 = Peter V. | first7 = Robert B. | name-list-style = vanc | edition = 8th | isbn = 978-0-8053-6844-4 | pages = [https://archive.org/details/essentialbiology00camp_0/page/118 118] | year = 2008 | publisher = Pearson Benjamin Cummings }}</ref>

===Layers===

[[File:Plant Cell Wall.png|thumb|Cell wall in multicellular plants – its different layers and their placement with respect to protoplasm (highly diagrammatic)]]

[[File:Plant cell wall diagram-en.svg|thumb|right|Molecular structure of the primary cell wall in plants]]
Up to three strata or layers may be found in plant cell walls:<ref>{{cite book | first1 = Bob B | last1 = Buchanan | first2 = Wilhelm | last2 = Gruissem | first3 = Russell L | last3 = Jones | name-list-style = vanc | title = Biochemistry & molecular biology of plants | edition = 1st | publisher = American society of plant physiology | year = 2000 | isbn = 978-0-943088-39-6 | url-access = registration | url = https://archive.org/details/biochemistrymole00buch }}</ref>

*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. Some cells, such as the conducting cells in [[xylem]], possess a secondary wall containing [[lignin]], which strengthens and waterproofs the wall.
*The '''[[middle lamella]]''', a layer rich in [[pectin]]s. This outermost layer forms the interface between adjacent plant cells and glues them together.

===Composition===
In the primary (growing) plant cell wall, the major [[carbohydrate]]s 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]].<ref name="Fry1989">{{cite journal |last1=Fry |first1=Stephen C. | name-list-style = vanc |title=The Structure and Functions of Xyloglucan |journal=Journal of Experimental Botany |volume=40 |issue=1 |year=1989 |pages=1–11 |doi=10.1093/jxb/40.1.1}}</ref> 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 called [[acid growth]], mediated by [[expansin]]s, extracellular proteins activated by acidic conditions that modify the hydrogen bonds between [[pectin]] and cellulose.<ref>{{cite journal | vauthors = Braidwood L, Breuer C, Sugimoto K | title = My body is a cage: mechanisms and modulation of plant cell growth | journal = The New Phytologist | volume = 201 | issue = 2 | pages = 388–402 | date = January 2014 | pmid = 24033322 | doi = 10.1111/nph.12473 | doi-access = free | bibcode = 2014NewPh.201..388B }}</ref> This functions to increase cell wall extensibility. The outer part of the primary cell wall of the plant epidermis is usually impregnated with [[cutin]] and [[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 [[polymer]]s that make up [[wood]] (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.

[[File:Allium-Mitose10-DM100x BL28.jpg|thumb|Photomicrograph of onion root cells, showing the centrifugal development of new cell walls (phragmoplast)]]
Additionally, structural [[protein]]s (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 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]] may contain [[cutin]]. The [[Casparian strip]] in the [[endodermis]] roots and [[cork (material)|cork]] cells of plant bark contain [[suberin]]. Both cutin and suberin are polyesters that function as permeability barriers to the movement of water.<ref>{{cite journal | vauthors = Moire L, Schmutz A, Buchala A, Yan B, Stark RE, Ryser U | title = Glycerol is a suberin monomer. New experimental evidence for an old hypothesis | journal = Plant Physiology | volume = 119 | issue = 3 | pages = 1137–46 | date = March 1999 | pmid = 10069853 | pmc = 32096 | doi = 10.1104/pp.119.3.1137 }}</ref> 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 and [[cross-link]] wall polymers.

Secondary walls - especially in grasses - may also contain microscopic [[silica]] 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, [[Tropaeolum majus|nasturtium]]<ref name=reid>{{cite book | vauthors = Reid J | chapter-url = https://books.google.com/books?id=NmKF0hxhpdMC&pg=PA228 | chapter = Carbohydrate metabolism:structural carbohydrates | veditors = Dey PM, Harborne JB |  title = Plant Biochemistry | publisher = Academic Press | date = 1997 | pages = 205–236 | isbn = 978-0-12-214674-9 }}</ref>{{rp|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===
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.{{clarify|reason=how can this be, since the middle lamella is defined as the pectic material between the primary cell walls?|date=September 2016}} 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. Cellulose microfibrils are produced at the plasma membrane by the [[cellulose synthase (UDP-forming)|cellulose 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.<ref>{{cite journal | vauthors = Jarvis MC | title = Cellulose biosynthesis: counting the chains | journal = Plant Physiology | volume = 163 | issue = 4 | pages = 1485–6 | date = December 2013 | pmid = 24296786 | pmc = 3850196 | doi = 10.1104/pp.113.231092 }}</ref> 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 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 plasma membrane and primary wall.<ref>{{Cite book| title=Biology| url=https://archive.org/details/essentialbiology00camp_0| url-access=registration|last1=Campbell |last2=Reece |last3=Urry|last4=Cain|last5=Wasserman|last6=Minorsky|last7=Jackson|first1=Neil A.|first2=Jane B.|first3=Lisa A. |first4=Michael L. |first5=Steven A.|first6=Peter V.|first7=Robert B. | name-list-style = vanc |edition=8th|isbn=978-0-8053-6844-4 |pages = [https://archive.org/details/essentialbiology00camp_0/page/119 119] |year=2008| publisher=Pearson Benjamin Cummings}}</ref> 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.<ref name=Abeysekera>{{cite journal | vauthors = Abeysekera RM, Willison JH |title=A spiral helicoid in a plant cell wall |journal=Cell Biology International Reports |volume=11 |issue=2 |date=1987 |pages=75–79 |doi=10.1016/0309-1651(87)90106-8 |doi-broken-date=24 March 2025 }}</ref> Cells with secondary cell walls can be rigid, as in the gritty [[sclereid]] cells in [[pear]] and [[quince]] fruit. Cell to cell communication is possible through [[pit (botany)|pits]] in the secondary cell wall that allow plasmodesmata to connect cells through the secondary cell walls.