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A cell wall is a structural layer surrounding some types of cells, just outside the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...

cell membrane
. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mechanism. Cell walls are absent in many eukaryotes, including animals, but are present in some other ones like
fungi
fungi
,
algae
algae
and
plants
plants
, and in most prokaryotes (except mollicute bacteria). A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters. The composition of cell walls varies between taxonomic group and species and may depend on cell type and developmental stage. The primary cell wall of
land plants
land plants
is composed of the polysaccharides
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
,
hemicellulose
hemicellulose
s and
pectin
pectin
. Often, other polymers such as
lignin
lignin
, suberin or cutin are anchored to or embedded in plant cell walls. Algae possess cell walls made of glycoproteins and polysaccharides such as carrageenan and agar that are absent from land plants. In bacteria, the cell wall is composed of peptidoglycan. The cell walls of
archaea
archaea
have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls made of the N-acetylglucosamine polymer
chitin
chitin
. Unusually,
diatom A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...

diatom
s have a cell wall composed of biogenic silica.


History

A plant cell wall was first observed and named (simply as a "wall") by
Robert Hooke Robert Hooke Fellow of the Royal Society, FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using ...
in 1665. 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. In 1804,
Karl Rudolphi
Karl Rudolphi
and J.H.F. Link proved that cells had independent cell walls. 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 by Eduard Strasburger (1882, 1889), and the intussusception theory by Julius Wiesner (1886). In 1930, Ernst Münch coined the term '' apoplast'' in order to separate the "living" symplast from the "dead" plant region, the latter of which included the cell wall. 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
extracellular matrix
", as used for animal cells, but others preferred the older term.


Properties

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. 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 osmotic lysis and helping to retain water. Their composition, properties, and form may change during the and depend on growth conditions.


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 s that bend in water currents. As John Howland explains The apparent rigidity of the cell wall thus results from inflation of the cell contained within. This inflation is a result of the . In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed by in xylem cell walls, or suberin in cork cell walls. These compounds are rigid and , making the secondary wall stiff. Both
wood Wood is a porous and fibrous structural tissue found in the Plant stem, stems and roots of trees and other woody plants. It is an organic materiala natural composite material, composite of cellulose fibers that are strong in tension and emb ...

wood
and bark cells of
tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondar ...
s have secondary walls. Other parts of plants such as the leaf stalk may acquire similar reinforcement to resist the strain of physical forces.


Permeability

The primary cell wall of most
plant cell Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids ...
s is freely permeable to small molecules including small
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
, with size exclusion estimated to be 30-60
kDa The dalton or unified atomic mass unit (symbols: Da or u) is a Non-SI units mentioned in the SI, non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an chemical bond, unbound neutral atom of carbon-12 in its ...
. The pH is an important factor governing the transport of molecules through cell walls.


Evolution

Cell walls evolved independently in many groups. The
photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s (so-called plant and algae) is one group with cellulose cell walls, where the cell wall is closely related to the evolution of
multicellularity A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
, terrestrialization and vascularization. The CesA cellulose synthase evolved in ''
Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's ...
'' and was part of
Archaeplastida The Archaeplastida (or kingdom Plant#Current definitions of Plantae, Plantae ''Sensu#Common qualifiers, sensu lato'' "in a broad sense"; pronounced Help:IPA/English, /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the autotrop ...
since
endosymbiosis An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
;
secondary endosymbiosis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolution Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These ...
events transferred it (with the
arabinogalactan Arabinogalactan, also known as galactoarabinan, larch arabinogalactan, and larch gum, is a biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Like other polymers, biopolymers consist of monomeric uni ...
proteins) further into
brown algae Brown algae (singular: alga), comprising the class (biology), class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the ...
and oomycetes. 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. Fungi use a chitin-glucan-protein cell wall. They share the 1,3-β-glucan synthesis pathway with plants, using homologous GT48 family 1,3-Beta-glucan synthases to perform the task, suggesting that such an enzyme is very ancient within the eukaryotes. Their glycoproteins are rich in
mannose Mannose is a sugar monomer of the hexose, aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several Congenital disorder of glycosylati ...
. The cell wall might have evolved to deter viral infections. Proteins embedded in cell walls are variable, contained in
tandem repeat Tandem repeats occur in DNA when a pattern of one or more nucleotides is repeated and the repetitions are directly adjacent to each other. Several protein domains also form tandem repeats within their amino acid primary structure, such as armadi ...
s subject to
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
. An alternative scenario is that fungi started with a -based cell wall and later acquired the GT-48 enzymes for the 1,3-β-glucans via
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...
. The pathway leading to 1,6-β-glucan synthesis is not sufficiently known in either case.


Plant cell walls

The walls of plant cells must have sufficient tensile strength to withstand internal
osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a Solution (chemistry), solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a soluti ...
s of several times
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The Standard atmosphere (unit), standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equival ...
that result from the difference in solute concentration between the cell interior and external solutions. Plant cell walls vary from 0.1 to several µm in thickness.


Layers

Up to three strata or layers may be found in plant cell walls: *The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing. *The
secondary cell wall The secondary cell wall is a structure found in many plant cell Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing ...
, 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 , which strengthens and waterproofs the wall. *The middle lamella, a layer rich in 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 In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
s are
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
, and . The cellulose microfibrils 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 Xyloglucan is a hemicellulose that occurs in the primary cell wall of all vascular plants; however, all enzymes responsible for xyloglucan metabolism are found in Charophyceae algae.LEV Del Bem and M Vincentz (2010) Evolution of xyloglucan-related ...
. In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronarabinoxylan, another type of hemicellulose. Primary cell walls characteristically extend (grow) by a mechanism called acid growth, mediated by expansins, extracellular proteins activated by acidic conditions that modify the hydrogen bonds between and cellulose. 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 A plant cuticle is a protecting film covering the outermost skin layer (epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis ...
. Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many Repeat unit, repeating subunits. Due to t ...
s that make up
wood Wood is a porous and fibrous structural tissue found in the Plant stem, stems and roots of trees and other woody plants. It is an organic materiala natural composite material, composite of cellulose fibers that are strong in tension and emb ...

wood
(largely secondary cell walls) include: * cellulose, 35-50% *
xylan Xylan (; ) (CAS number: 9014-63-5) is a type of hemicellulose, a polysaccharide consisting mainly of xylose residues. It is found in plants, in the secondary cell walls of dicotyledon, dicots and all cell walls of Poaceae, grasses. Xylan is th ...
, 20-35%, a type of hemicellulose * , 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. Additionally, structural
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP),
arabinogalactan Arabinogalactan, also known as galactoarabinan, larch arabinogalactan, and larch gum, is a biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Like other polymers, biopolymers consist of monomeric uni ...
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
glycosylated Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not alw ...
, contain
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline (Carbon, C5Hydrogen, H9Oxygen, O3Nitrogen, N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank. Structure and discovery In 1902, Hermann Emil Fischer isolated hydroxy ...
(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 The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...
may contain cutin. The
Casparian strip The Casparian strip is a band-like thickening in the center of the root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant bo ...
in the
endodermis The endodermis is the central, innermost layer of Cortex (botany), cortex in land plants. It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobe, hydrophobic substances (Casparian strip) to restrict ap ...
roots and
cork Cork or CORK may refer to: Materials * Cork (material) Cork is an Permeability (earth sciences), impermeable buoyancy, buoyant material, the Cork cambium, phellem layer of bark (botany), bark tissue that is harvested for commercial use prima ...
cells of plant bark contain suberin. Both cutin and suberin are polyesters that function as permeability barriers to the movement of water. 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 In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
wall polymers. Secondary walls - especially in grasses - may also contain microscopic
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
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 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 image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
during
cytokinesis Cytokinesis () is the part of the cell division biological process, process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of Mitosis, nuclear d ...
, and the primary cell wall is then deposited inside the middle lamella. 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 Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and spermatozoon, sperm tail. Its mos ...
aligned at all angles. Cellulose microfibrils are produced at the plasma membrane by the 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. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. The cells are held together and share the gelatinous membrane called the ''middle lamella'', which contains
magnesium Magnesium is a chemical element with the Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group ...
and
calcium Calcium is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemica ...
pectates (salts of pectic acid). Cells interact though plasmodesmata, 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. 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. Cells with secondary cell walls can be rigid, as in the gritty
sclereid Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified cell wall, cellular walls that form small bundles of durable layers of tissue in most plants.Evert, Ray F; Eichhorn, Susan E. Esau's Plant Anatomy: Meristems, Cel ...
cells in
pear Pears are fruits produced and consumed around the world, growing on a tree and harvested in the Northern Hemisphere in late summer into October. The pear tree and shrub are a species of genus ''Pyrus'' , in the Family (biology), family Rosacea ...
and
quince The quince (; ''Cydonia oblonga'') is the sole member of the genus ''Cydonia'' in the Malinae subtribe (which also contains apples and pears, among other fruits) of the Rosaceae family (biology), family. It is a deciduous tree that bears hard ...
fruit. Cell to cell communication is possible through pits in the secondary cell wall that allow plasmodesmata to connect cells through the secondary cell walls.


Fungal cell walls

There are several groups of organisms that have been called "fungi". Some of these groups (
Oomycete Oomycota forms a distinct phylogeny, phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are mycelia, filamentous and heterotrophic, and can reproduce both Sexual reproduction, sexually and Asexual reproductio ...
and
Myxogastria Myxogastria/Myxogastrea (myxogastrids, International Code of Zoological Nomenclature, ICZN) or Myxomycetes (International Code of Nomenclature for algae, fungi, and plants, ICN), is a Class (biology), class of slime molds that contains 5 o ...
) 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 of and other
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
s. True fungi do not have
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
in their cell walls.


True fungi

In fungi, the cell wall is the outer-most layer, external to the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
. The fungal cell wall is a matrix of three main components: * :
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many Repeat unit, repeating subunits. Due to t ...
s consisting mainly of unbranched chains of β-(1,4)-linked-
N-Acetylglucosamine ''N''-Acetylglucosamine (GlcNAc) is an amide derivative of the monosaccharide glucose. It is a secondary amide between glucosamine and acetic acid. It is significant in several biological systems. It is part of a biopolymer in the bacterial ...
in the
Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
and
Basidiomycota Basidiomycota () is one of two large division (mycology), divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. Mor ...
, or poly-β-(1,4)-linked-
N-Acetylglucosamine ''N''-Acetylglucosamine (GlcNAc) is an amide derivative of the monosaccharide glucose. It is a secondary amide between glucosamine and acetic acid. It is significant in several biological systems. It is part of a biopolymer in the bacterial ...
(
chitosan Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked Glucosamine, D-glucosamine (deacetylated unit) and N-Acetylglucosamine, ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shell ...
) in the
Zygomycota Zygomycota, or zygote fungi, is a former phylum, division or phylum of the kingdom Fungi. The members are now part of two Phylum, phyla: the Mucoromycota and Zoopagomycotina, Zoopagomycota. Approximately 1060 species are known. They are mostly t ...
. Both and
chitosan Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked Glucosamine, D-glucosamine (deacetylated unit) and N-Acetylglucosamine, ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shell ...
are synthesized and extruded at the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
. *
glucan A glucan is a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidi ...
s: glucose
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many Repeat unit, repeating subunits. Due to t ...
s that function to cross-link or
chitosan Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked Glucosamine, D-glucosamine (deacetylated unit) and N-Acetylglucosamine, ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shell ...
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. *
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s: 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 are
glycosylated Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not alw ...
and contain
mannose Mannose is a sugar monomer of the hexose, aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several Congenital disorder of glycosylati ...
, thus these proteins are called mannoproteins or mannans.


Other eukaryotic cell walls


Algae

Like plants, algae have cell walls. Algal cell walls contain either
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
s (such as cellulose (a
glucan A glucan is a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidi ...
)) or a variety of
glycoprotein Glycoproteins are proteins which contain oligosaccharide chains Covalent bond, covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a translation (genetics), cotranslational or posttranslational modifica ...
s (
Volvocales Chlamydomonadales, also known as Volvocales, are an order (biology), order of flagellated or pseudociliated green algae, specifically of the Chlorophyceae.See the National Center for Biotechnology Information, NCBI]webpage on Chlamydomonadales Da ...
) or both. The inclusion of additional
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
s in algal cells walls is used as a feature for algal taxonomy (biology), taxonomy. * Mannans: They form microfibrils in the cell walls of a number of marine
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
including those from the
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
, ''
Codium ''Codium'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological ...
'', '' Dasycladus'', and '' Acetabularia'' as well as in the walls of some
red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...
, like '' Porphyra'' and '' Bangia''. *
Xylan Xylan (; ) (CAS number: 9014-63-5) is a type of hemicellulose, a polysaccharide consisting mainly of xylose residues. It is found in plants, in the secondary cell walls of dicotyledon, dicots and all cell walls of Poaceae, grasses. Xylan is th ...
s: *
Alginic acid Alginic acid, also called algin, is a naturally occurring, edible polysaccharide found in brown algae. It is hydrophilic and forms a viscous natural gum, gum when hydrated. With metals such as sodium and calcium, its salts are known as alginates ...
: It is a common polysaccharide in the cell walls of
brown algae Brown algae (singular: alga), comprising the class (biology), class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the ...
. * Sulfonated polysaccharides: They occur in the cell walls of most algae; those common in red algae include agarose, carrageenan, porphyran, furcelleran and funoran. Other compounds that may accumulate in algal cell walls include
sporopollenin 270px, SEM image of pollen grains Sporopollenin is one of the most chemically inert biological polymers. It is a major component of the tough outer (exine) walls of plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes ...
and
calcium ions Calcium ions (Ca2+) contribute to the physiology and biochemistry of organisms' cell (biology), cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in ...
. The group of known as the
diatom A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...

diatom
s synthesize their cell walls (also known as frustules or valves) from
silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
. 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 and possibly an explanation for higher growth rates in diatoms. In brown algae,
phlorotannin Phlorotannins are a type of tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and Precipitation (chemistry), precipitate proteins and various other organic compounds including amino acids and ...
s may be a constituent of the cell walls.


Water molds

The group
Oomycete Oomycota forms a distinct phylogeny, phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are mycelia, filamentous and heterotrophic, and can reproduce both Sexual reproduction, sexually and Asexual reproductio ...
s, also known as water molds, are
saprotroph Saprotrophic nutrition or lysotrophic nutrition is a process of Chemotroph#Chemoheterotroph, chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most o ...
ic plant pathogens like fungi. Until recently they were widely believed to be fungi, but
structural A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
and
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
evidence has led to their reclassification as
heterokont Heterokonts are a group of protists A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common ...
s, related to
autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
ic
brown algae Brown algae (singular: alga), comprising the class (biology), class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the ...
and
diatom A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...

diatom
s. Unlike fungi, oomycetes typically possess cell walls of cellulose and
glucan A glucan is a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidi ...
s rather than chitin, although some genera (such as '' Achlya'' and '' Saprolegnia'') do have chitin in their walls. The fraction of cellulose in the walls is no more than 4 to 20%, far less than the fraction of glucans. Oomycete cell walls also contain the
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline (Carbon, C5Hydrogen, H9Oxygen, O3Nitrogen, N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank. Structure and discovery In 1902, Hermann Emil Fischer isolated hydroxy ...
, which is not found in fungal cell walls.


Slime molds

The
dictyostelid The dictyostelids (Dictyostelia/Dictyostelea, International Code of Zoological Nomenclature, ICZN, or Dictyosteliomycetes, ICBN) are a group of cellular slime molds, or social amoebae. Multicellular behavior When food (normally bacteria) is rea ...
s are another group formerly classified among the fungi. They are
slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mul ...
s that feed as unicellular
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
e, but aggregate into a reproductive stalk and
sporangium A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many other l ...
under certain conditions. Cells of the reproductive stalk, as well as the
spore In biology, a spore is a unit of sexual reproduction, sexual or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of ...
s formed at the apex, possess a
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
wall. The spore wall has three layers, the middle one composed primarily of cellulose, while the innermost is sensitive to
cellulase Cellulase (EC 3.2.1.4; systematic name 4-β-D-glucan 4-glucanohydrolase) is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysaccharide ...
and
pronase Pronase is a commercially available mixture of protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins int ...
.


Prokaryotic cell walls


Bacterial cell walls

Around the outside of the cell membrane is the
bacterial cell wall A cell wall is a structural layer surrounding some types of cell (biology), cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as ...
. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
chains cross-linked by unusual
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptide ...
s containing D-
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s. Bacterial cell walls are different from the cell walls of
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s and which are made of
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
and , respectively. 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, although
L-form bacteria L-form bacteria, also known as L-phase bacteria, L-phase variants or cell wall-deficient (CWD) bacteria, are growth forms derived from different bacteria. They lack cell walls. Peptidoglycan (murein) is absent. Two types of L-forms are distinguish ...
can be produced in the laboratory that lack a cell wall. The antibiotic
penicillin Penicillins (P, PCN or PEN) are a group of beta-lactam antibiotic, β-lactam antibiotics originally obtained from ''Penicillium'' Mold (fungus), moulds, principally ''Penicillium chrysogenum, P. chrysogenum'' and ''Penicillium rubens, P. ru ...
is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse. The
lysozyme Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
enzyme can also damage bacterial cell walls. There are broadly speaking two different types of cell wall in bacteria, called
gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacte ...
and
gram-negative Gram-negative bacteria are bacteria that do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidogly ...
. The names originate from the reaction of cells to the
Gram stain In microbiology and bacteriology, Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish bac ...
, a test long-employed for the classification of bacterial species. Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and
teichoic acid Teichoic acids (''cf.'' Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, ...
s. In contrast, gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing
lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the Bacterial ...
s and
lipoprotein A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...
s. Most bacteria have the gram-negative cell wall and only the
Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive bacteria, gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom ...
and
Actinomycetota The ''Actinomycetota'' (synonym ''Actinobacteria'') are a phylum of mostly Gram-positive bacteria. They can be terrestrial animal, terrestrial or aquatic animal, aquatic. They are of great economic importance to humans because agriculture and fo ...
(previously known as the low G+C and high G+C gram-positive bacteria, respectively) have the alternative gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility, for instance
vancomycin Vancomycin is a glycopeptide antibiotic medication used to treat a number of bacterial infections. It is recommended intravenously as a treatment for complicated skin infections, sepsis, bloodstream infections, endocarditis, bone and joint infe ...
can kill only gram-positive bacteria and is ineffective against gram-negative pathogens, such as ''
Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...
'' or ''
Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common Bacterial capsule, encapsulated, Gram-negative bacteria, gram-negative, Aerobic organism, aerobic–facultative anaerobe, facultatively anaerobic, Bacillus (shape), rod-shaped bacteria, bacterium that can ca ...
''.


Archaeal cell walls

Although not truly unique, the cell walls of
Archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
are unusual. Whereas peptidoglycan is a standard component of all bacterial cell walls, all archaeal cell walls lack peptidoglycan, though some
methanogen Methanogens are microorganisms that produce methane as a Metabolism, metabolic byproduct in Hypoxia (environmental), hypoxic conditions. They are Prokaryote, prokaryotic and belong to the Domain (biology), domain Archaea. All known methanogens are ...
s have a cell wall made of a similar polymer called pseudopeptidoglycan. There are four types of cell wall currently known among the Archaea. One type of archaeal cell wall is that composed of pseudopeptidoglycan (also called
pseudomurein Pseudopeptidoglycan (also known as pseudomurein;White, David. (1995) ''The Physiology and Biochemistry of Prokaryotes'', pages 6, 12-21. (Oxford: Oxford University Press). . PPG hereafter) is a major cell wall component of some Archaea that differs ...
). This type of wall is found in some
methanogen Methanogens are microorganisms that produce methane as a Metabolism, metabolic byproduct in Hypoxia (environmental), hypoxic conditions. They are Prokaryote, prokaryotic and belong to the Domain (biology), domain Archaea. All known methanogens are ...
s, such as '' Methanobacterium'' and '' Methanothermus''. While the overall structure of archaeal ''pseudo''peptidoglycan superficially resembles that of bacterial peptidoglycan, there are a number of significant chemical differences. Like the peptidoglycan found in bacterial cell walls, pseudopeptidoglycan consists of
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many Repeat unit, repeating subunits. Due to t ...
chains of
glycan The terms glycans and polysaccharides are defined by IUPAC as synonyms meaning "compounds consisting of a large number of monosaccharides linked glycosidically". However, in practice the term glycan may also be used to refer to the carbohydrate p ...
cross-linked by short
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptide ...
connections. However, unlike peptidoglycan, the sugar
N-acetylmuramic acid ''N''-Acetylmuramic acid (NAM or MurNAc) is an organic compound with the chemical formula . It is a monomer of peptidoglycan in most bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisti ...
is replaced by N-acetyltalosaminuronic acid, and the two sugars are bonded with a ''β'',1-3 glycosidic linkage instead of ''β'',1-4. Additionally, the cross-linking peptides are L-amino acids rather than D-amino acids as they are in bacteria. A second type of archaeal cell wall is found in ''
Methanosarcina ''Methanosarcina'' is a genus of Euryarchaeota, euryarchaeote archaea that produce methane. These single-celled organisms are known as Anaerobic organism, anaerobic methanogens that produce methane using all three metabolic pathways for methano ...
'' and '' Halococcus''. This type of cell wall is composed entirely of a thick layer of
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
s, which may be
sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
d in the case of ''Halococcus''. Structure in this type of wall is complex and not fully investigated. A third type of wall among the
Archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
consists of
glycoprotein Glycoproteins are proteins which contain oligosaccharide chains Covalent bond, covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a translation (genetics), cotranslational or posttranslational modifica ...
, and occurs in the
hyperthermophile A hyperthermophile is an organism that thrives in extremely hot environments—from 60 °C (140 °F) upwards. An optimal temperature for the existence of hyperthermophiles is often above 80 °C (176 °F). Hyperthermophiles are often within the doma ...
s, ''
Halobacterium ''Halobacterium'' (common abbreviation ''Hbt.'') is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, vi ...
'', and some
methanogen Methanogens are microorganisms that produce methane as a Metabolism, metabolic byproduct in Hypoxia (environmental), hypoxic conditions. They are Prokaryote, prokaryotic and belong to the Domain (biology), domain Archaea. All known methanogens are ...
s. In ''Halobacterium'', the
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s in the wall have a high content of
acid An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid A Lewis acid (named for the American p ...
ic
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s, giving the wall an overall negative charge. The result is an unstable structure that is stabilized by the presence of large quantities of positive
sodium Sodium is a chemical element with the Symbol (chemistry), symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the ...
ion An ion () is an atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has n ...
s that neutralize the charge. Consequently, ''Halobacterium'' thrives only under conditions with high
salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
. In other Archaea, such as '' Methanomicrobium'' and '' Desulfurococcus'', the wall may be composed only of surface-layer
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s, known as an '' S-layer''. S-layers are common in bacteria, where they serve as either the sole cell-wall component or an outer layer in conjunction with
polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
. Most Archaea are Gram-negative, though at least one Gram-positive member is known.


Other cell coverings

Many
protist A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukary ...
s and
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
produce other cell surface structures apart from cell walls, external () or internal. Many have a sheath or envelope of
mucilage Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of m ...
outside the cell made of exopolysaccharides.
Diatom A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
s build a
frustule A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on d ...
from
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
extracted from the surrounding water;
radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell (biology), cell into the inner and outer portions of endoplasm and Ecto ...
s,
foraminiferan Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm ...
s,
testate amoebae Testate amoebae (formerly thecamoebians, Testacea or Thecamoeba) are a polyphyletic group of unicellular amoeboid protists, which differ from naked amoebae in the presence of a test (biology), test that partially encloses the cell, with an apertur ...
and silicoflagellates also produce a skeleton from
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s, called
test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
in some groups. Many
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
, such as ''
Halimeda ''Halimeda'' is a genus of green macroalgae. The algal body (thallus) is composed of calcified green segments. Calcium carbonate is deposited in its tissues, making it inedible to most herbivores. However one species, ''Halimeda tuna'', was descri ...
'' and the
Dasycladales Dasycladales is an order of large unicellular green algae in the class Ulvophyceae. It contains two families, the Dasycladaceae and the Polyphysaceae. These single celled algae are from 2 mm to 200 mm long. They live on substrat ...
, and some
red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...
, the
Corallinales Coralline algae are red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic ...
, encase their cells in a
secreted 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that ...
skeleton of
calcium carbonate Calcium carbonate is a chemical compound with the Chemical formula, formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisti ...
. In each case, the wall is rigid and essentially
inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
. It is the non-living component of cell. Some golden algae,
ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
s and
choanoflagellate The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconne ...
s produces a shell-like protective outer covering called lorica. Some
dinoflagellate The dinoflagellates (Greek language, Greek δῖνος ''dinos'' "whirling" and Latin language, Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually consid ...
s have a
theca In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), c ...
of
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...

cellulose
plates, and coccolithophorids have
coccolith Coccoliths are individual plates or scales of calcium carbonate Calcium carbonate is a chemical compound with the Chemical formula, formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite (most ...
s. An (ECM) is also present in
metazoans Animals are multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellula ...
. Its composition varies between cells, but
collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
s are the most abundant protein in the ECM.


See also

*
Extracellular matrix In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cel ...
*
Bacterial cell structure The bacterium, despite its simplicity, contains a well-developed cell structure which is responsible for some of its unique biological Biology is the scientific study of life. It is a natural science with a broad scope but has several uni ...
*
Plant cell Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids ...


References


External links


Cell wall ultrastructure


{{DEFAULTSORT:Cell Wall Plant physiology Organelles