Editing Cell wall (section)

==Properties==
{{more citations needed section|date=November 2017}}
[[File:Eukaryota cell strucutre.PNG|thumb|right|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 and [[morphogenesis]].<ref name="bid">{{cite journal | vauthors = Bidhendi AJ, Geitmann A | title = Relating the mechanics of the primary plant cell wall to morphogenesis | journal = Journal of Experimental Botany | volume = 67 | issue = 2 | pages = 449–61 | date = January 2016 | pmid = 26689854 | doi = 10.1093/jxb/erv535 | url = https://academic.oup.com/jxb/article/67/2/449/2884961 | doi-access = free }}</ref> 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 stable [[osmotic]] environments by preventing [[Cytolysis|osmotic lysis]] and helping to retain water. Their composition, properties, and form may change during the [[cell cycle]] and depend on growth conditions.<ref name=bid/>

===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 considerable [[tensile strength]]. The apparent rigidity of primary plant tissues is enabled by cell walls, but is not due to the walls' stiffness. Hydraulic [[turgor 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 in [[seaweed]]s that bend in [[Ocean current|water current]]s. As John Howland explains

{{Blockquote|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.<ref name="Howland 2000">{{cite book| last = Howland | first = John L. | name-list-style = vanc | year = 2000 | title = The Surprising Archaea: Discovering Another Domain of Life | pages = 69–71 | publisher = Oxford University Press | location = Oxford | isbn = 978-0-19-511183-5}}</ref>}}

The apparent rigidity of the cell wall thus results from inflation of the cell contained within. This [[turgor pressure|inflation]] is a result of the [[osmosis|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 by [[lignin]] in [[xylem]] cell walls, or [[suberin]] in [[cork cambium|cork]] cell walls. These compounds are [[Structural rigidity|rigid]] and [[waterproof]], making the secondary wall stiff. Both [[wood]] and [[Bark (botany)|bark]] cells of [[tree]]s have secondary walls. Other parts of plants such as the [[petiole (botany)|leaf stalk]] may acquire similar reinforcement to resist the strain of physical forces.

===Permeability===
The primary cell wall of most [[plant cell]]s is freely permeable to small molecules including small [[Protein|proteins]], with size exclusion estimated to be 30-60 [[Atomic mass unit|kDa]].<ref>{{cite book |author1=Harvey Lodish |author2=Arnold Berk |author3=Chris A. Kaiser |author4=Monty Krieger |author5=Matthew P. Scott |author6=Anthony Bretscher |author7=Hidde Ploegh |author8=Paul Matsudaira |title=Loose-leaf Version for Molecular Cell Biology |url=https://books.google.com/books?id=0bUVAAAAQBAJ |date=1 September 2012|publisher=W. H. Freeman|isbn=978-1-4641-2746-5}}</ref> The pH is an important factor governing the transport of molecules through cell walls.<ref>{{cite book | first = C. Michael | last = Hogan | date = 2010 | chapter-url = http://www.eoearth.org/article/Abiotic_factor?topic=49461 | chapter = Abiotic factor | title = Encyclopedia of Earth | veditors = Monosson E, Cleveland C | publisher = National Council for Science and the Environment  | location = Washington DC | archive-url = https://web.archive.org/web/20130608071757/http://www.eoearth.org/article/Abiotic_factor?topic=49461 | archive-date = 2013-06-08 }}</ref>