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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 A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

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 animals but are present in most other
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
including
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
,
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
and
plants Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Docto ...

plants
and in most
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s (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 The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into ...

land plants
is composed of the polysaccharides
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
,
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

hemicellulose
s and
pectin Pectin (from grc, πηκτικός ', "congealed, curdled") is a structural acidic heteropolysaccharide contained in the primary and middle lamella The middle lamella is a layer that cements together the primary cell wall A cell wall is a ...

pectin
. Often, other polymers such as
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and Bark (botany), bark, because they l ...

lignin
,
suberin Suberin, cutin Cutin is one of two wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compo ...

suberin
or
cutin Cutin is one of two wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. Th ...
are anchored to or embedded in plant cell walls. Algae possess cell walls made of glycoproteins and polysaccharides such as
carrageenan Carrageenans or carrageenins ( , from Irish Irish most commonly refers to: * Someone or something of, from, or related to: ** Ireland, an island situated off the north-western coast of continental Europe ** Northern Ireland, a constituent unit of t ...
and
agar Agar ( or ), or agar-agar, is a jelly-like substance, obtained from red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of ...
that are absent from land plants. In bacteria, the cell wall is composed of
peptidoglycan Peptidoglycan or murein is a polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidia ...

peptidoglycan
. The cell walls of
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
have various compositions, and may be formed of
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
S-layer An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) i ...
s,
pseudopeptidoglycanPseudopeptidoglycan (also known as pseudomureinWhite, David. (1995) ''The Physiology and Biochemistry of Prokaryotes'', pages 6, 12-21. (Oxford: Oxford University Press). .) is a major cell wall component of some Archaea that differs from bacterial p ...
, or
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s. Fungi possess cell walls made of the
N-acetylglucosamine ''N''-Acetylglucosamine (GlcNAc) is an amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula RC(=O)NR′R″, where R, R', and R″ represent or ...

N-acetylglucosamine
polymer
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
. Unusually,
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grou ...

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 FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resources ...
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 Asmund Rudolphi (14 July 1771 – 29 November 1832) was a Swedish-born German naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in their natural environment, leaning mor ...

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 Hugo von Mohl FFRS HFRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged to be "eminently distin ...

Hugo von Mohl
(1853, 1858) advocated the idea that the cell wall grows by apposition.
Carl Nägeli Carl Wilhelm von Nägeli (26 or 27 March 1817 – 10 May 1891) was a Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other use ...
(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 Eduard Adolf Strasburger (1 February 1844 – 18 May 1912) was a Polish- German professor and one of the most famous botanist Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', ...
(1882, 1889), and the intussusception theory by
Julius Wiesner
Julius Wiesner
(1886). In 1930, Ernst Münch coined the term ''
apoplast 300px, The apoplastic and symplastic pathways Inside a plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert ligh ...
'' in order to separate the "living"
symplast Image:Apoplast and symplast pathways.svg, 300px, The apoplastic and symplastic pathways The symplast of a plant is the inner side of the plasma membrane in which water and low-molecular-weight Solution, solutes can freely diffusion, diffuse. Symplas ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
", 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 Morphogenesis (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...
. 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 Osmosis (, ) is the spontaneous net movement or diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration In chemistry Chemistry is the ...
environments by preventing osmotic lysis and helping to retain water. Their composition, properties, and form may change during the
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication In , DNA replication is the of pro ...

cell cycle
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 Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle tensi ...
. The apparent rigidity of primary plant tissues is enabled by cell walls, but is not due to the walls' stiffness. Hydraulic
turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane cell membrane vs. Prokaryotes A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In b ...
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 Seaweed, or macroalgae, refers to thousands of species of Macroscopic scale, macroscopic, Multicellular organism, multicellular, ocean, marine algae. The term includes some types of ''Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyt ...

seaweed
s that bend in
water current A current in a fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other w ...
s. As John Howland explains The apparent rigidity of the cell wall thus results from inflation of the cell contained within. This
inflation In economics, inflation refers to a general progressive increase in prices of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a r ...
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
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and Bark (botany), bark, because they l ...

lignin
in
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
cell walls, or
suberin Suberin, cutin Cutin is one of two wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compo ...

suberin
in
cork Cork or CORK may refer to: Materials * Cork (material), an impermeable buoyant plant product ** Cork (plug), a cylindrical or conical object used to seal a container ***Wine cork Places Ireland * Cork (city) ** Metropolitan Cork, also known as G ...
cell walls. These compounds are rigid and
waterproof Waterproofing is the process of making an object or structure waterproof or water-resistant so that it remains relatively unaffected by water or resisting the ingress of water under specified conditions. Such items may be used in wet environme ...

waterproof
, making the secondary wall stiff. Both
wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. ...

wood
and
bark Bark may refer to: * Bark (botany), an outer layer of a woody plant * Bark (sound), a vocalization of some animals Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, entertainment, and media * ''Bark'' (Jefferson Airp ...
cells of
tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including only wood plants with se ...

tree
s have secondary walls. Other parts of plants such as the
leaf stalk In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anci ...
may acquire similar reinforcement to resist the strain of physical forces.


Permeability

The primary cell wall of most
plant cell Plant cells are eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molec ...

plant cell
s is freely permeable to small molecules including small proteins, with size exclusion estimated to be 30-60
kDa The dalton or unified atomic mass unit (symbols: Da or u) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ' ...
. 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 convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthetic
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
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 a unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a fe ...
, terrestrialization and vascularization. The CesA cellulose synthase evolved in ''
Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

Cyanobacteria
'' and was part of
Archaeplastida The Archaeplastida (or kingdom Plantae Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae a ...
since endosymbiosis;
secondary endosymbiosis Symbiogenesis, endosymbiotic theory, or serial endosymbiotic theory is the leading evolutionary theory of the origin of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ' ...
events transferred it (with the
arabinogalactanArabinogalactan is a biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Biopolymers consist of monomeric units that are Covalent_bond, covalently bonded to form larger molecules. There are three main cl ...

arabinogalactan
proteins) further into
brown algae The brown algae (singular: alga), comprising the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
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 synthase 1,3-Beta-glucan synthase is a glucosyltransferaseGlucosyltransferases are a type of glycosyltransferase Most glycosyltransferase enzymes form one of two folds: GT-A or GT-B Glycosyltransferases (GTFs, Gtfs) are enzyme Enzymes () are protei ...
s 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 Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosaccharides joine ...

mannose
. The cell wall might have evolved to deter viral infections. Proteins embedded in cell walls are variable, contained in
tandem repeat Tandem bicycle Tandem, or in tandem, is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction. The original use of the term in English English usually refers to: * Eng ...
s subject to homologous recombination. An alternative scenario is that fungi started with a
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
-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 A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
. 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 Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with ...

osmotic pressure
s of several times
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer A barometer is a scientific instrument that is used to measure air pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the ...
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 wallThe secondary cell wall is a structure found in many plant cells, located between the primary cell wall and the plasma membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, an ...
, 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 Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
, possess a secondary wall containing
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and Bark (botany), bark, because they l ...

lignin
, which strengthens and waterproofs the wall. *The
middle lamella The middle lamella is a layer that cements together the primary cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the ...
, a layer rich in
pectin Pectin (from grc, πηκτικός ', "congealed, curdled") is a structural acidic heteropolysaccharide contained in the primary and middle lamella The middle lamella is a layer that cements together the primary cell wall A cell wall is a ...

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 A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s are
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
,
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

hemicellulose
and
pectin Pectin (from grc, πηκτικός ', "congealed, curdled") is a structural acidic heteropolysaccharide contained in the primary and middle lamella The middle lamella is a layer that cements together the primary cell wall A cell wall is a ...

pectin
. The cellulose
microfibrilA 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 most ...

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 Xyloglucan is a hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller ...
. 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 growthAcid growth refers to the ability of plant cells and plant cell walls to elongate or expand quickly at low (acidic) pH. The cell wall needs to be modified in order to maintain the turgor pressure. This modification is controlled by plant hormones lik ...
, mediated by
expansinExpansin refers to a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem ...
s, extracellular proteins activated by acidic conditions that modify the hydrogen bonds between
pectin Pectin (from grc, πηκτικός ', "congealed, curdled") is a structural acidic heteropolysaccharide contained in the primary and middle lamella The middle lamella is a layer that cements together the primary cell wall A cell wall is a ...

pectin
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 Cutin is one of two wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. Th ...
and
wax Waxes are a diverse class of organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prope ...
, forming a permeability barrier known as the
plant cuticle A plant cuticle is a protecting film covering the epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates ...
. Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s that make up
wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. ...

wood
(largely secondary cell walls) include: * cellulose, 35-50% *
xylan Xylan (; ) (CAS number A CAS Registry Number, also referred to as CASRN or CAS Number, is a unique numerical identifier An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of ...

xylan
, 20-35%, a type of hemicellulose *
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and Bark (botany), bark, because they l ...

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. Additionally, structural
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP),
arabinogalactanArabinogalactan is a biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Biopolymers consist of monomeric units that are Covalent_bond, covalently bonded to form larger molecules. There are three main cl ...

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
glycosylated Glycosylation (see also chemical glycosylationA chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosacchar ...

glycosylated
, contain
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5H9 O3 N), is an amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, side chain ...

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 The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
may contain
cutin Cutin is one of two wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. Th ...
. The
Casparian strip In plant anatomy, the Casparian strip (named after Robert Caspary) is a band of cell wall material deposited in the radial and transverse walls of the endodermis, and is chemically different from the rest of the cell wall - the cell wall being made ...
in the
endodermis The endodermis is the central, innermost layer of cortex in land plants The Embryophyta () or land plants are the most familiar group of green plants that form vegetation on earth. Embryophyta is a clade A clade (; from grc, , ''klados'' ...
roots and
cork Cork or CORK may refer to: Materials * Cork (material), an impermeable buoyant plant product ** Cork (plug), a cylindrical or conical object used to seal a container ***Wine cork Places Ireland * Cork (city) ** Metropolitan Cork, also known as G ...
cells of plant bark contain
suberin Suberin, cutin Cutin is one of two wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compo ...

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 A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * Chin ...
wall polymers. Secondary walls - especially in grasses - may also contain microscopic
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

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, 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 Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there ar ...

cytokinesis
, 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 Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
microfibrils 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 upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
and
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
pectate Pectic acid, also known as polygalacturonic acid, is a water-insoluble, Transparency (optics), transparent gelatinous acid existing in over-ripe fruit and some vegetables. It is a product of pectin degradation in plants, and is produced via the int ...
s (salts of ). 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 The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primar ...
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 In biology, a species is the basic unit of biological ...

pear
and
quince The quince (; ''Cydonia oblonga'') is the sole member of the genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of namin ...

quince
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 or oomycetes () form a distinct phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mole ...
and
Myxogastria Myxogastria/Myxogastrea (myxogastrids, ICZN The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention Convention may refer to: * Convention (norm), a custom or tradition, a standard of presentation or conduct ...
) 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
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
and other
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s. True fungi do not have
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

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 A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
. The fungal cell wall is a matrix of three main components: *
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
:
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s consisting mainly of unbranched chains of β-(1,4)-linked-
N-Acetylglucosamine ''N''-Acetylglucosamine (GlcNAc) is an amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula RC(=O)NR′R″, where R, R', and R″ represent or ...

N-Acetylglucosamine
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 ...

Ascomycota
and
Basidiomycota Basidiomycota () is one of two large divisions Division or divider may refer to: Mathematics *Division (mathematics) Division is one of the four basic operations of arithmetic, the ways that numbers are combined to make new numbers. The othe ...

Basidiomycota
, or poly-β-(1,4)-linked-
N-Acetylglucosamine ''N''-Acetylglucosamine (GlcNAc) is an amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula RC(=O)NR′R″, where R, R', and R″ represent or ...

N-Acetylglucosamine
(
chitosan Chitosan is a linear polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as ca ...

chitosan
) in the
Zygomycota Zygomycota, or zygote fungi, is a former division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication *Division algorithm, a method for computing the result of mathematical division Military *D ...
. Both
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
and
chitosan Chitosan is a linear polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as ca ...

chitosan
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 A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
. *
glucan A glucan is a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as catalyst, whi ...
s: glucose
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s that function to cross-link
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
or
chitosan Chitosan is a linear polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as ca ...

chitosan
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 biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
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 (see also chemical glycosylationA chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosacchar ...

glycosylated
and contain
mannose Mannose is a sugar Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosaccharides joine ...

mannose
, 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 carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s (such as cellulose (a
glucan A glucan is a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as catalyst, whi ...
)) or a variety of
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
s (
Volvocales Chlamydomonadales, also known as Volvocales, are an order of flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like Appendage, appendages called fla ...

Volvocales
) or both. The inclusion of additional
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s in algal cells walls is used as a feature for algal
taxonomy Taxonomy is the practice and science of categorization Categorization is the human ability and activity of recognizing shared features or similarities between the elements of the experience Experience refers to conscious , an English Par ...
. * Mannans: They form microfibrils in the cell walls of a number of
marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
including those from the
genera Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
, ''
Codium ''Codium'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...
'', '' Dasycladus'', and ''Acetabularia'' as well as in the walls of some red algae, like ''Porphyra'' and ''Bangia''. * Xylans: * Alginic acid: It is a common polysaccharide in the cell walls of
brown algae The brown algae (singular: alga), comprising the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
. * Sulfonated polysaccharides: They occur in the cell walls of most algae; those common in red algae include
agar Agar ( or ), or agar-agar, is a jelly-like substance, obtained from red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of ...
ose,
carrageenan Carrageenans or carrageenins ( , from Irish Irish most commonly refers to: * Someone or something of, from, or related to: ** Ireland, an island situated off the north-western coast of continental Europe ** Northern Ireland, a constituent unit of t ...
, porphyran, furcelleran and funoran. Other compounds that may accumulate in algal cell walls include sporopollenin and calcium, calcium ions. The group of
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
known as the
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grou ...

diatom
s Biosynthesis, synthesize their cell walls (also known as frustules or valves) from silicon dioxide, silicic 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 and possibly an explanation for higher growth rates in diatoms. In brown algae, phlorotannins may be a constituent of the cell walls.


Water molds

The group
Oomycete Oomycota or oomycetes () form a distinct phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mole ...
s, also known as water molds, are saprotrophic Plant pathology, plant pathogens like fungi. Until recently they were widely believed to be fungi, but organelle, structural and molecular biology, molecular evidence has led to their reclassification as heterokonts, related to autotrophic
brown algae The brown algae (singular: alga), comprising the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
and
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grou ...

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 found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as catalyst, whi ...
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
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5H9 O3 N), is an amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, side chain ...

hydroxyproline
, which is not found in fungal cell walls.


Slime molds

The dictyostelids are another group formerly classified among the fungi. They are slime molds that feed as unicellular amoebae, but aggregate into a reproductive stalk and sporangium under certain conditions. Cells of the reproductive stalk, as well as the spores formed at the apex, possess a
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
wall. The spore wall has three layers, the middle one composed primarily of cellulose, while the innermost is sensitive to cellulase and pronase.


Prokaryotic cell walls


Bacterial cell walls

Around the outside of the cell membrane is the Bacterial cell structure#Cell wall, bacterial cell wall. Bacterial cell walls are made of
peptidoglycan Peptidoglycan or murein is a polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidia ...

peptidoglycan
(also called murein), which is made from
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
chains cross-linked by unusual peptides containing D-amino acids. Bacterial cell walls are different from the cell walls of plants and fungi which are made of
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
and
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
, 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 can be produced in the laboratory that lack a cell wall. The antibiotic penicillin is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse. The lysozyme enzyme can also damage bacterial cell walls. There are broadly speaking two different types of cell wall in bacteria, called gram-positive and gram-negative. The names originate from the reaction of cells to the Gram stain, 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 acids. 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 lipopolysaccharides and lipoproteins. Most bacteria have the gram-negative cell wall and only the Firmicutes and Actinobacteria (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 can kill only gram-positive bacteria and is ineffective against gram-negative pathogens, such as ''Haemophilus influenzae'' or ''Pseudomonas aeruginosa''.


Archaeal cell walls

Although not truly unique, the cell walls of Archaea are unusual. Whereas
peptidoglycan Peptidoglycan or murein is a polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidia ...

peptidoglycan
is a standard component of all bacterial cell walls, all archaeal cell walls lack
peptidoglycan Peptidoglycan or murein is a polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidia ...

peptidoglycan
, though some methanogens have a cell wall made of a similar polymer called
pseudopeptidoglycanPseudopeptidoglycan (also known as pseudomureinWhite, David. (1995) ''The Physiology and Biochemistry of Prokaryotes'', pages 6, 12-21. (Oxford: Oxford University Press). .) is a major cell wall component of some Archaea that differs from bacterial p ...
. There are four types of cell wall currently known among the Archaea. One type of archaeal cell wall is that composed of
pseudopeptidoglycanPseudopeptidoglycan (also known as pseudomureinWhite, David. (1995) ''The Physiology and Biochemistry of Prokaryotes'', pages 6, 12-21. (Oxford: Oxford University Press). .) is a major cell wall component of some Archaea that differs from bacterial p ...
(also called pseudomurein). This type of wall is found in some methanogens, 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 ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
chains of glycan cross-linked by short peptide connections. However, unlike peptidoglycan, the sugar N-Acetylmuramic acid, N-acetylmuramic acid is replaced by N-Acetyltalosaminuronic acid, 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'' and ''Halococcus''. This type of cell wall is composed entirely of a thick layer of
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s, which may be sulfated 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 consists of
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
, and occurs in the hyperthermophiles, ''Halobacterium'', and some methanogens. In ''Halobacterium'', the
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s in the wall have a high content of acidic amino acids, 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 ions that Neutralization (chemistry), neutralize the charge. Consequently, ''Halobacterium'' thrives only under conditions with high salinity. In other Archaea, such as ''Methanomicrobium'' and ''Desulfurococcus'', the wall may be composed only of surface-layer
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s, known as an ''
S-layer An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) i ...
''. S-layers are common in bacteria, where they serve as either the sole cell-wall component or an outer layer in conjunction with polysaccharides. Most Archaea are Gram-negative, though at least one Gram-positive member is known.


Other cell coverings

Many protists and bacteria produce other cell surface structures apart from cell walls, external (
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
) or internal. Many
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
have a sheath or envelope of mucilage outside the cell made of exopolysaccharides. Diatoms build a frustule from
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

silica
extracted from the surrounding water; radiolarians, foraminiferans, testate amoebae and silicoflagellates also produce a skeleton from minerals, called test (biology), test in some groups. Many
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
, such as ''Halimeda'' and the Dasycladales, and some red algae, the Corallinales, encase their cells in a secreted skeleton of calcium carbonate. In each case, the wall is rigid and essentially inorganic. It is the non-living component of cell. Some golden algae, ciliates and choanoflagellates produces a shell-like protective outer covering called lorica (biology), lorica. Some dinoflagellates have a theca of
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
plates, and coccolithophorids have coccoliths. An
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
(ECM) is also present in metazoans. Its Chemical composition, composition varies between cells, but collagens are the most abundance (chemistry), abundant protein in the ECM.


See also

* Extracellular matrix * Bacterial cell structure * Plant cell


References


External links


Cell wall ultrastructure


{{DEFAULTSORT:Cell Wall Plant physiology Organelles