A cell wall is a structural layer surrounding some types of
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
s, 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 interior of all cells from the outside environment ( ...
. 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
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
,
algae and
plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
, and in most
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
s (except
mollicute
Mollicutes is a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin ''mollis'' (meaning "soft" or "pliable"), and ''cutis'' (meaning "skin"). Individuals are very small, typically only 0 ...
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 plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as sist ...
is composed of the polysaccharides
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
,
hemicellulose
A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls.Scheller HV, Ulvskov Hemicelluloses.// Annu Rev ...
s and
pectin. Often, other polymers such as
lignin,
suberin
Suberin, cutin and lignins are complex, higher plant epidermis and periderm cell-wall macromolecules, forming a protective barrier. Suberin, a complex polyester biopolymer, is lipophilic, and composed of long chain fatty acids called suberin aci ...
or
cutin
Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, which assist in cuticle st ...
are anchored to or embedded in plant cell walls. Algae possess cell walls made of
glycoproteins
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosy ...
and
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 linkages. This carbohydrate can react with w ...
such as
carrageenan
Carrageenans or carrageenins ( ; ) are a family of natural linear sulfated polysaccharides that are extracted from red edible seaweeds. Carrageenans are widely used in the food industry, for their gelling, thickening, and stabilizing properties. T ...
and
agar that are absent from land plants. In bacteria, the cell wall is composed of
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...
. The cell walls of
archaea have various compositions, and may be formed of glycoprotein
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) ...
s,
pseudopeptidoglycan
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 ...
, or polysaccharides. Fungi possess cell walls made of the
N-acetylglucosamine polymer
chitin. Unusually,
diatoms have a cell wall composed of
biogenic silica
Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. For example, microscopic particles of silica called phytoliths can be found in grasses and other plant ...
.
History
A plant cell wall was first observed and named (simply as a "wall") by
Robert Hooke 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, who is credited with being the "father of helminthology".
Life
Rudolphi was born in Stockholm to German parents. He was awarded his PhD in 1793 an ...
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 H FRSE (8 April 1805 – 1 April 1872) was a German botanist from Stuttgart. He was the first person to use the word " protoplasm".
Life
He was a son of the Württemberg statesman Benjamin Ferdinand von Mohl (1766–18 ...
(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 botanist. He studied cell division and pollination but became known as the man who discouraged Gregor Mendel from further work on genetics. He rejected natural selection ...
(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
Dr. Julius Ritter von Wiesner (20 January 1838 – 9 October 1916) was a professor of botany at the University of Vienna, a specialist in the physiology and anatomy of plants.
In 1870 he became a professor at the forestry academy of Mariabru ...
(1886).
In 1930,
Ernst Münch
Ernst Münch (26 November 1876 – 9 October 1946) was a German plant physiologist who proposed the Pressure Flow Hypothesis in 1930.
He studied in Aschaffenburg, and then in Munich with Robert Hartig
Robert Hartig (born: Heinrich Julius ...
coined the term ''
apoplast
Inside a plant, the apoplast can mean the space outside of cell membranes, where material can diffuse freely; that is, the extracellular spaces.
''Apoplast '' can also refer especially to the continuum of cell walls of adjacent cells; fluid and ...
'' in order to separate the "living"
symplast
The symplast of a plant is the inner side of a cell membrane in which water and low-molecular-weight solutes can freely diffuse. Symplast cells have more than one nucleus.
''Symplast'' could also refer to the connection of the inner contents (c ...
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, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
", as used for animal cells,
but others preferred the older term.
Properties
![Eukaryota cell strucutre](https://upload.wikimedia.org/wikipedia/commons/7/7c/Eukaryota_cell_strucutre.PNG)
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 ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...
.
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 of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region o ...
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) and some of its organelles, and sub ...
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 materials t ...
. 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 against the cell wall.
It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibriu ...
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 seaweeds that bend in water current
A current in a fluid is the magnitude and direction of flow within that fluid, such as a liquid or a gas.
Types of fluid currents include:
* Air current
* Boundary current
* Current (stream), a current in a river or stream
* Longshore current
* ...
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 is an increase in the general price level 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 reduct ...
is a result of the passive uptake of water.
In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed by lignin in xylem
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
cell walls, or suberin
Suberin, cutin and lignins are complex, higher plant epidermis and periderm cell-wall macromolecules, forming a protective barrier. Suberin, a complex polyester biopolymer, is lipophilic, and composed of long chain fatty acids called suberin aci ...
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 ...
, making the secondary wall stiff. Both wood
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
and bark cells of tree
In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
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 cells 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 non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...
. The pH is an important factor governing the transport of molecules through cell walls.
Evolution
Cell walls evolved independently in many groups.
The photosynthetic eukaryotes (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, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
, terrestrialization and vascularization. The CesA cellulose synthase evolved in '' Cyanobacteria'' and was part of Archaeplastida
The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor gr ...
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 evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possi ...
events transferred it (with the arabinogalactan
Arabinogalactan, also known as galactoarabinan, larch arabinogalactan, and larch gum, is a biopolymer consisting of arabinose and galactose monosaccharides. Two classes of arabinogalactans are found in nature: plant arabinogalactan and microbial a ...
proteins) further into brown algae
Brown algae (singular: alga), comprising the 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 temperate and p ...
and oomycetes
Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the result ...
. 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 glucosyltransferase enzyme involved in the generation of beta-glucan in fungi. It serves as a pharmacological target for antifungal drugs such as caspofungin, anidulafungin, and micafungin, deemed 1,3-Beta-glucan s ...
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 monomer of the 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 disorders of glycosylation ...
. 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 armadil ...
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 cellular organisms but may ...
. An alternative scenario is that fungi started with a 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 and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
. 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 pressures 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 (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
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, 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 in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
, possess a secondary wall containing lignin, which strengthens and waterproofs the wall.
*The middle lamella
The middle lamella is a layer that cements together the primary cell walls of two adjoining plant cells. It is the first formed layer to be deposited at the time of cytokinesis
Cytokinesis () is the part of the cell division process during ...
, a layer rich in pectins. 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 m ...
s are cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
, hemicellulose
A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls.Scheller HV, Ulvskov Hemicelluloses.// Annu Rev ...
and pectin. The cellulose microfibril 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 sperm tail. Its most frequently ...
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 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-relate ...
. 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 Acid 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 ...
, mediated by expansin Expansins are 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 fun ...
s, extracellular proteins activated by acidic conditions that modify the hydrogen bonds between 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 waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, which assist in cuticle st ...
and wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...
, forming a permeability barrier known as the plant cuticle.
Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s that make up wood
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
(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 dicots and all cell walls of grasses. Xylan is the third most abundan ...
, 20-35%, a type of hemicellulose
* lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.
Additionally, structural protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
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 consisting of arabinose and galactose monosaccharides. Two classes of arabinogalactans are found in nature: plant arabinogalactan and microbial a ...
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 ...
, contain hydroxyproline
(2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 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 hydroxyproline from hydrolyzed gelati ...
(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 may contain cutin
Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, which assist in cuticle st ...
. The Casparian strip
The Casparian strip is a band-like thickening in the center of the root endodermis (radial and tangential walls of endodermal cells) of vascular plants (Pteridophytes and Spermatophytes). The composition of the region is mainly suberin, lignin and ...
in the endodermis
The endodermis is the central, innermost layer of cortex in land plants. It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobic substances (Casparian strip) to restrict apoplastic flow of water to th ...
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 and lignins are complex, higher plant epidermis and periderm cell-wall macromolecules, forming a protective barrier. Suberin, a complex polyester biopolymer, is lipophilic, and composed of long chain fatty acids called suberin aci ...
. 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 ...
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
Lamella (plural lamellae) means a small plate or flake in Latin, and in English may refer to:
Biology
* Lamella (mycology), a papery rib beneath a mushroom cap
* Lamella (botany)
* Lamella (surface anatomy), a plate-like structure in an animal
...
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 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 nuclear division in mitosis and mei ...
, 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 with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
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 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 2 of the periodic ta ...
and calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
pectate
Pectic acid, also known as polygalacturonic acid, is a water-insoluble, 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 interaction between pecti ...
s (salts of pectic acid
Pectic acid, also known as polygalacturonic acid, is a water-insoluble, 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 interaction between pec ...
). Cells interact though plasmodesma
Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and spe ...
ta, which are inter-connecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall.
In some plants and cell types, after a maximum size or point in development has been reached, a ''secondary wall'' is constructed between the plasma membrane and primary wall. 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 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, Cells, and T ...
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 Rosaceae, bearing the po ...
and 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 forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...
and Myxogastria
Myxogastria/Myxogastrea (myxogastrids, ICZN) or Myxomycetes ( ICN), is a class of slime molds that contains 5 orders, 14 families, 62 genera, and 888 species. They are colloquially known as the ''plasmodial'' or ''acellular ...
) 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 and other polysaccharides. True fungi do not have cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
in their cell walls.
True fungi
In fungi, the cell wall is the outer-most layer, external to the plasma membrane. The fungal cell wall is a matrix of three main components:
* chitin: polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s consisting mainly of unbranched chains of β-(1,4)-linked- N-Acetylglucosamine in the Ascomycota and Basidiomycota, or poly-β-(1,4)-linked- N-Acetylglucosamine (chitosan
Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked D-glucosamine (deacetylated unit) and ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shells of shrimp and other crustacean ...
) in the Zygomycota
Zygomycota, or zygote fungi, is a former division or phylum of the kingdom Fungi. The members are now part of two phyla: the Mucoromycota and Zoopagomycota. Approximately 1060 species are known. They are mostly terrestrial in habitat, living ...
. Both chitin and chitosan
Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked D-glucosamine (deacetylated unit) and ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shells of shrimp and other crustacean ...
are synthesized and extruded at the plasma membrane.
* glucans: glucose polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s that function to cross-link chitin or chitosan
Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked D-glucosamine (deacetylated unit) and ''N''-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shells of shrimp and other crustacean ...
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 residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
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 ...
and contain mannose
Mannose is a sugar monomer of the 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 disorders of glycosylation ...
, thus these proteins are called mannoproteins or mannans.
Other eukaryotic cell walls
Algae
Like plants, algae have cell walls. Algal cell walls contain either polysaccharides (such as cellulose (a glucan)) or a variety of glycoproteins (Volvocales
Chlamydomonadales, also known as Volvocales, are an order of flagellated or pseudociliated green algae, specifically of the Chlorophyceae.See the NCBIbr>webpage on Chlamydomonadales Data extracted from the Chlamydomonadales can form planar or ...
) or both. The inclusion of additional polysaccharides in algal cells walls is used as a feature for algal taxonomy
Taxonomy is the practice and science of categorization or classification.
A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
.
* Mannans: They form microfibrils in the cell walls of a number of marine green algae including those from the genera, ''Codium
''Codium'' is a genus of seaweed in the Chlorophyta of the order Bryopsidales. Paul Silva was an expert on the genus ''Codium'' taxonomy at the University of California, Berkeley. There are about 50 species worldwide.
Description
The genus h ...
'', '' Dasycladus'', and ''Acetabularia
''Acetabularia'' is a genus of green algae in the family Polyphysaceae, Typically found in subtropical waters, ''Acetabularia'' is a single-celled organism, but gigantic in size and complex in form, making it an excellent model organism for stu ...
'' as well as in the walls of some red algae, like ''Porphyra
''Porphyra'' is a genus of coldwater seaweeds that grow in cold, shallow seawater. More specifically, it belongs to red algae phylum of laver species (from which comes laverbread), comprising approximately 70 species.Brodie, J.A. and Irvine, ...
'' and ''Bangia
''Bangia'' is an extant genus of division Rhodophyta that grows in marine or freshwater habitats. ''Bangia'' has small thalli with rapid growth and high reproductive output, and exhibits behavior characteristic of r-selected species. The plants ...
''.
* 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 dicots and all cell walls of grasses. Xylan is the third most abundan ...
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 gum when hydrated. With metals such as sodium and calcium, its salts are known as alginates. Its colou ...
: It is a common polysaccharide in the cell walls of brown algae
Brown algae (singular: alga), comprising the 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 temperate and p ...
.
* Sulfonated polysaccharides: They occur in the cell walls of most algae; those common in red algae include agarose, carrageenan
Carrageenans or carrageenins ( ; ) are a family of natural linear sulfated polysaccharides that are extracted from red edible seaweeds. Carrageenans are widely used in the food industry, for their gelling, thickening, and stabilizing properties. T ...
, porphyra
''Porphyra'' is a genus of coldwater seaweeds that grow in cold, shallow seawater. More specifically, it belongs to red algae phylum of laver species (from which comes laverbread), comprising approximately 70 species.Brodie, J.A. and Irvine, ...
n, 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 spores and pollen grains. It is chemically very stable and is usually well ...
and calcium ions
Calcium ions (Ca2+) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contractio ...
.
The group of algae known as the diatoms synthesize their cell walls (also known as frustules
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 ...
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 alga
Brown algae (singular: alga), comprising the 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 temperate an ...
e, phlorotannin
Phlorotannins are a type of tannins found in brown algae such as kelps and rockweeds or sargassacean species, and in a lower amount also in some red algae. Contrary to hydrolysable or condensed tannins, these compounds are oligomers of phloro ...
s may be a constituent of the cell walls.
Water molds
The group Oomycete
Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...
s, also known as water molds, are saprotroph
Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...
ic plant pathogens
Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...
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 ...
and molecular evidence has led to their reclassification as heterokont
Heterokonts are a group of protists (formally referred to as Heterokonta, Heterokontae or Heterokontophyta). The group is a major line of eukaryotes. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which a ...
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 Wo ...
ic brown algae
Brown algae (singular: alga), comprising the 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 temperate and p ...
and diatoms. Unlike fungi, oomycetes typically possess cell walls of cellulose and glucans rather than chitin, although some genera (such as '' Achlya'' and ''Saprolegnia
''Saprolegnia'' is a genus of water moulds often called cotton moulds because of the characteristic white or grey fibrous patches they form. Current taxonomy puts ''Saprolegnia'' as a genus of the heterokonts in the order Saprolegniales.
Habits ...
'') 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 compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
hydroxyproline
(2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 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 hydroxyproline from hydrolyzed gelati ...
, which is not found in fungal cell walls.
Slime molds
The dictyostelid
The dictyostelids (Dictyostelia/Dictyostelea, ICZN, or Dictyosteliomycetes, ICBN) are a group of cellular slime molds, or social amoebae.
Multicellular behavior
When food (normally bacteria) is readily available dictyostelids behave as individu ...
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 or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...
e, 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 with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
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 polysacchar ...
and pronase Pronase is a commercially available mixture of proteases isolated from the extracellular fluid of ''Streptomyces griseus''. Activity extends to both denatured and native proteins leading to complete or nearly complete digestion into individual amino ...
.
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 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 a filtering mech ...
. Bacterial cell walls are made of peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...
(also called murein), which is made from polysaccharide 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 tetrapeptides.
...
s containing D-amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s. Bacterial cell walls are different from the cell walls of plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s and fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
which are made of cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
and 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
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 distingui ...
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
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 bact ...
and gram-negative
Gram-negative bacteria are bacteria that do not retain the 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 peptidoglycan cell wa ...
. 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 b ...
, 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 τεῖχος, ''teīkhos'', "wall", to be specific a fortification wall, as opposed to τοῖχος, ''toīkhos'', a regular wall) are bacterial copolymers of glycerol phosphate or ribitol phosphate and carbohydr ...
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 outer ...
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, su ...
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 cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earl ...
and Actinomycetota
The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to s ...
(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, bloodstream infections, endocarditis, bone and joint infections, ...
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, non-motile, coccobacillary, facultatively anaerobic, capnophilic pathogenic bacterium of the family Pasteurellaceae. The bacter ...
'' or ''Pseudomonas aeruginosa
''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aerug ...
''.
Archaeal cell walls
Although not truly unique, the cell walls of Archaea are unusual. Whereas peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...
is a standard component of all bacterial cell walls, all archaeal cell walls lack peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...
, though some methanogens have a cell wall made of a similar polymer called pseudopeptidoglycan
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 ...
. There are four types of cell wall currently known among the Archaea.
One type of archaeal cell wall is that composed of pseudopeptidoglycan
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 ...
(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 methanogens, such as ''Methanobacterium
In taxonomy, ''Methanobacterium'' is a genus of the Methanobacteriaceae family of Archaea.See the NCBIbr>webpage on Methanobacterium Data extracted from the Despite the name, this genus belongs not to the bacterial domain but the archaeal doma ...
'' 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-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
chains of glycan 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 tetrapeptides.
...
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 bacterial cell walls, which is built from alternating units of ''N''-acetylglucosamine (GlcNAc) and ''N''-acet ...
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 acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s 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 euryarchaeote archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as ...
'' and '' Halococcus''. This type of cell wall is composed entirely of a thick layer of polysaccharides, which may be sulfate
The sulfate or sulphate ion is a 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 salts of sulfuric acid and many ...
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 consists of glycoprotein, 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'', and some methanogens. In ''Halobacterium'', the protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s in the wall have a high content of acidic amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
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 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 of the periodic table. Its only stable ...
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s that neutralize the charge. Consequently, ''Halobacterium'' thrives only under conditions with high salinity.
In other Archaea, such as '' Methanomicrobium'' and ''Desulfurococcus
In taxonomy, ''Desulfurococcus'' is a genus of the Desulfurococcaceae.See the NCBI
The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes ...
'', 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 residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
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) ...
''. 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 linkages. This carbohydrate can react with w ...
. 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 cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s and bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
produce other cell surface structures apart from cell walls, external (extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
) or internal. Many algae 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
Extracellular polymeric substances (EPSs) are natural polymers of high molecular weight secreted by microorganisms into their environment. EPSs establish the functional and structural integrity of biofilms, and are considered the fundamental comp ...
. Diatoms 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 ...
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 ...
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 into the inner and outer portions of endoplasm and ectoplasm. The elab ...
s, foraminiferan
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an ...
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 that partially encloses the cell, with an aperture from which the ...
and silicoflagellate
Dictyochales (Silicoflagellates, or Dictyochophyceae ''sensu stricto'') are a small group of unicellular heterokont algae, found in marine environments.
Characteristics
In one stage of their life cycle, they produce a siliceous skeleton, comp ...
s 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, such as '' Halimeda'' 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 substrates ...
, and some red algae, the Corallinales, encase their cells in a secreted 440px
Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...
skeleton of calcium carbonate. 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 chemist ...
. It is the non-living component of cell. Some golden algae
The Chrysophyceae, usually called chrysophytes, chrysomonads, golden-brown algae or golden algae are a large group of algae, found mostly in freshwater. Golden algae is also commonly used to refer to a single species, '' Prymnesium parvum'', whic ...
, ciliate
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...
s and choanoflagellates produces a shell-like protective outer covering called lorica. Some dinoflagellates have a theca
In biology, a theca (plural thecae) is a sheath or a covering.
Botany
In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a comm ...
of cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
plates, and coccolithophorid
Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingd ...
s have coccolith
Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as '' Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere' ...
s.
An extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
(ECM) is also present in metazoans
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
. Its composition
Composition or Compositions may refer to:
Arts and literature
*Composition (dance), practice and teaching of choreography
*Composition (language), in literature and rhetoric, producing a work in spoken tradition and written discourse, to include v ...
varies between cells, but collagens are the most abundant protein in the ECM.
See also
* Extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
* Bacterial cell structure
The bacterium, despite its simplicity, contains a well-developed cell structure which is responsible for some of its unique biological structures and pathogenicity. Many structural features are unique to bacteria and are not found among archaea o ...
* Plant cell
References
External links
Cell wall ultrastructure
{{DEFAULTSORT:Cell Wall
Plant physiology
Organelles