Editing Cell wall (section)

===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.