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Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
in nature (behind only cellulose); an estimated 1 billion tons of chitin are produced each year in the biosphere. It is a primary component of
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 ...
s in fungi (especially basidiomycetes and filamentous fungi), the exoskeletons of arthropods such as crustaceans and insects, the
radula The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...
e, cephalopod beaks and
gladii ''Gladius'' () is a Latin word meaning "sword" (of any type), but in its narrow sense it refers to the sword of ancient Roman foot soldiers. Early ancient Roman swords were similar to those of the Greeks, called '' xiphe'' (plural; singular ''xi ...
of
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
s and in some nematodes and diatoms. It is also synthesised by at least some fish and
lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...
ns. Commercially, chitin is extracted from the shells of crabs, shrimps, shellfishes and lobsters, which are major by-products of the seafood industry. The structure of chitin is comparable to cellulose, forming crystalline nanofibrils or whiskers. It is functionally comparable to the protein keratin. Chitin has proved useful for several medicinal, industrial and biotechnological purposes.


Etymology

The English word "chitin" comes from the
French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with Franc ...
word ''chitine'', which was derived in 1821 from the Greek word χιτών (''khitōn'') meaning covering. A similar word, "
chiton Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail s ...
", refers to a marine animal with a protective shell.


Chemistry, physical properties and biological function

The structure of chitin was determined by Albert Hofmann in 1929. Hofmann hydrolyzed chitin using a crude preparation of the enzyme chitinase, which he obtained from the snail ''Helix pomatia''. Chitin is a modified
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
that contains nitrogen; it is synthesized from units of ''N''-acetyl-D-glucosamine (to be precise, 2-(acetylamino)-2-deoxy-D-glucose). These units form covalent β-(1→4)-linkages (like the linkages between glucose units forming cellulose). Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer replaced with an
acetyl In organic chemistry, acetyl is a functional group with the chemical formula and the structure . It is sometimes represented by the symbol Ac (not to be confused with the element actinium). In IUPAC nomenclature, acetyl is called ethanoyl, ...
amine group. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength. In its pure, unmodified form, chitin is translucent, pliable, resilient, and quite tough. In most arthropods, however, it is often modified, occurring largely as a component of composite materials, such as in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the shells of crustaceans and
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
s, chitin produces a much stronger composite. This composite material is much harder and stiffer than pure chitin, and is tougher and less brittle than pure calcium carbonate.Campbell, N. A. (1996) ''Biology'' (4th edition) Benjamin Cummings, New Work. p.69 Another difference between pure and composite forms can be seen by comparing the flexible body wall of a
caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Sym ...
(mainly chitin) to the stiff, light elytron of a beetle (containing a large proportion of sclerotin). In butterfly wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic crystals that produce various
iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
colors serving phenotypic signaling and communication for mating and foraging. The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in
biomimicry Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
. Scarab beetles in the genus ''
Cyphochilus ''Cyphochilus'' is a genus of beetles with unusually bright white Scale (anatomy), scales that cover the whole exoskeleton. ''Cyphochilus'' inhabit Southeast Asia. Etymology Probably the same etymology as the former ''Cyphochilus (plant), Cyp ...
'' also utilize chitin to form extremely thin scales (five to fifteen micrometres thick) that diffusely reflect white light. These scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of
nanometre 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
s, which serve to scatter light. The
multiple scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
of light is thought to play a role in the unusual whiteness of the scales. In addition, some social wasps, such as ''
Protopolybia chartergoides ''Protopolybia chartergoides, ''also known as ''Pseudochartergus chartergoides,'' is a species of wasp within the genus ''Protopolybia''. It is a social wasp found in southern Central America and northern South America. Taxonomy and phylogeny ...
'', orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper. Chitosan is produced commercially by
deacetylation : In organic chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply ''acetates''. Deacetylation is the opposit ...
of chitin; chitosan is soluble in water, while chitin is not. Nanofibrils have been made using chitin and chitosan.


Humans and other mammals

Humans and other mammals have chitinase and chitinase-like proteins that can degrade chitin; they also possess several
immune receptor An immune receptor (or immunologic receptor) is a receptor, usually on a cell membrane, which binds to a substance (for example, a cytokine) and causes a response in the immune system. Types The main receptors in the immune system are pattern rec ...
s that can recognize chitin and its degradation products, initiating an immune response. Chitin is sensed mostly in the lungs or
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
where it can activate the
innate immune system The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the ...
through eosinophils or
macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...
s, as well as an
adaptive immune response The adaptive immune system, also known as the acquired immune system, is a subsystem of the immune system that is composed of specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system ...
through
T helper The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are consider ...
cells. Keratinocytes in skin can also react to chitin or chitin fragments.


Plants

Plants also have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein. The first chitin receptor was cloned in 2006. When the receptors are activated by chitin, genes related to plant defense are expressed, and
jasmonate Jasmonate (JA) and its derivatives are lipid-based plant hormones that regulate a wide range of processes in plants, ranging from growth and photosynthesis to reproductive development. In particular, JAs are critical for plant defense against herb ...
hormones are activated, which in turn activate systematic defenses. Commensal fungi have ways to interact with the host immune response that, , were not well understood. Some pathogens produce chitin-binding proteins that mask the chitin they shed from these receptors. ''
Zymoseptoria tritici ''Zymoseptoria tritici'', synonyms ''Septoria tritici'', ''Mycosphaerella graminicola'', is a species of filamentous fungus, an ascomycete in the family ''Mycosphaerellaceae''. It is a wheat plant pathogen causing septoria leaf blotch that is d ...
'' is an example of a fungal pathogen that has such blocking proteins; it is a major pest in wheat crops.


Fossil record

Chitin was probably present in the exoskeletons of
Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
arthropods such as trilobites. The oldest preserved chitin dates to the
Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
, about , consisting of a scorpion encased in amber.


Uses


Agriculture

Chitin is a good inducer of plant
defense mechanisms In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an Unconscious mind, unconscious psychological operation that functions to protect a person from anxiety-producing thoughts and feelings related to interna ...
for controlling
diseases A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that ar ...
. It has potential for use as a soil fertilizer or conditioner to improve fertility and plant resilience that may enhance crop yields.


Industrial

Chitin is used in many industrial processes. Examples of the potential uses of chemically modified chitin in
food processing Food processing is the transformation of agricultural products into food, or of one form of food into other forms. Food processing includes many forms of processing foods, from grinding grain to make raw flour to home cooking to complex industr ...
include the formation of edible films and as an additive to thicken and stabilize foods and food emulsions. Processes to size and strengthen paper employ chitin and chitosan.


Research

How chitin interacts with the immune system of plants and animals has been an active area of research, including the identity of key receptors with which chitin interacts, whether the size of chitin particles is relevant to the kind of immune response triggered, and mechanisms by which immune systems respond. Chitin is deacetylated chemically or enzymatically to produce chitosan, a highly biocompatible polymer which has found a wide range of applications in the biomedical industry. Chitin and chitosan have been explored as a vaccine adjuvant due to its ability to stimulate an immune response. Chitin and chitosan are under development as
scaffolds Scaffolding, also called scaffold or staging, is a temporary structure used to support a work crew and materials to aid in the construction, maintenance and repair of buildings, bridges and all other man-made structures. Scaffolds are widely used ...
in studies of how tissue grows and how wounds heal, and in efforts to invent better bandages,
surgical thread A surgical suture, also known as a stitch or stitches, is a medical device used to hold body tissues together and approximate wound edges after an injury or surgery. Application generally involves using a needle with an attached length of threa ...
, and materials for allotransplantation. Sutures made of chitin have been explored for many years, but , none were on the market; their lack of elasticity and problems making thread have prevented commercial development. In 2014, a method for using chitosan as a reproducible form of
biodegradable Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
plastic was introduced. Chitin nanofibers are extracted from crustacean waste and mushrooms for possible development of products in tissue engineering, drug delivery and medicine. In 2020, chitin was proposed for use in building structures, tools, and other solid objects from a composite material of chitin combined with Martian regolith. In this scenario, the biopolymers in the chitin act as the binder for the regolith
aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...
to form a concrete-like composite material. The authors believe that waste materials from food production (e.g. scales from fish, exoskeletons from crustaceans and insects, etc.) could be put to use as feedstock for manufacturing processes.


See also

* Biopesticide * Chitosan *
Chitobiose Chitobioses are a group of related disaccharides of β-1,4-linked glucosamine units. The term ''chitobiose'' is sometimes used to refer to different members of the group, depending on the method by which it was first isolated, resulting in some ...
* Lorica * Sporopollenin * Tectin


References

{{Authority control Acetamides Biomolecules Biopesticides Polysaccharides