# Chitin

Chitin (C8H13O5N)n (/ˈkaɪtɪn/ KY-tin), a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans (e.g., crabs, lobsters and shrimps) and insects, the radulae of molluscs, cephalopod beaks, and the scales of fish and lissamphibians.[1] The structure of chitin is comparable to another polysaccharide - cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved useful for several medicinal, industrial and biotechnological purposes.

Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in β-(1→4)-linkage.
Haworth projection of the chitin molecule.
A close-up of the wing of a leafhopper; the wing is composed of chitin.

## Etymology

The English word "chitin" comes from the French word chitine, which was derived in 1821 from the Greek word χιτών (chiton), meaning covering.[2]

A similar word, "chiton", refers to a marine animal with a protective shell.

## Chemistry, physical properties and biological function

Chemical configurations of the different monosaccharides (glucose and N-acetylglucosamine) and polysaccharides (chitin and cellulose) presented in Haworth projection

The structure of chitin was determined by Albert Hofmann in 1929.[3]

Chitin is a modified polysaccharide 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 amine group. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.

A cicada emerges from its chitinous larval exoskeleton.

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 molluscs, 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.[4] Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar (mainly chitin) to the stiff, light elytron of a beetle (containing a large proportion of sclerotin).[5]

In butterfly wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic crystals that produce various iridescent colors serving phenotypic signaling and communication for mating and foraging.[6] The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in biomimicry.[6] Scarab beetles in the genus Cyphochilus 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 nanometres, which serve to scatter light. The multiple scattering of light is thought to play a role in the unusual whiteness of the scales.[7][8] In addition, some social wasps, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper.[9]

Chitosan is produced commercially by deacetylation of chitin; chitosan is soluble in water, while chitin is not.[10]

Nanofibrils have been made using chitin and chitosan.[11]

## Health effects

Chitin-producing organisms like protozoa, fungi, arthropods, and nematodes are often pathogens in other species.[12]

### Humans and other mammals

Humans and other mammals have chitinase and chitinase-like proteins that can degrade chitin; they also possess several immune receptors that can recognize chitin and its degradation products in a pathogen-associated molecular pattern, initiating an immune response.[12]

Chitin is sensed mostly in the lungs or gastrointestinal tract where it can activate the innate immune system through eosinophils or macrophages, as well as an adaptive immune response through T helper cells.[12] Keratinocytes in skin can also react to chitin or chitin fragments.[12] According to in vitro studies, chitin is sensed by receptors, such as FIBCD1, KLRB1, REG3G, Toll-like receptor 2, CLEC7A, and mannose receptors.[12][13]

The immune response can sometimes clear the chitin and its associated organism, but sometimes the immune response is pathological and becomes an allergy;[14] allergy to house dust mites is thought to be driven by a response to chitin.[13]

### Plants

Plants also have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein.[12] The first chitin receptor was cloned in 2006.[15] When the receptors are activated by chitin, genes related to plant defense are expressed, and jasmonate hormones are activated, which in turn activate systematic defenses.[16] Commensal fungi have ways to interact with the host immune response that as of 2016 were not well understood.[15]

Some pathogens produce chitin-binding proteins that mask the chitin they shed from these receptors.[16][17] Zymoseptoria tritici is an example of a fungal pathogen that has such blocking proteins; it is a major pest in wheat crops.[18]

## Fossil record

Chitin was probably present in the exoskeletons of Cambrian arthropods such as trilobites. The oldest preserved chitin dates to the Oligocene, about 25 million years ago, consisting of a scorpion encased in amber.[19]

## Uses

### Agriculture

Chitin is a good inducer of plant defense mechanisms for controlling diseases.[20] It has also been assessed as a fertilizer that can improve overall crop yields.[21]

### Industrial

Chitin is used in industry in many processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods.[22] Processes to size and strengthen paper employ chitin and chitosan.[23][24]

## 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.[14][18] Chitin and chitosan have been explored as a vaccine adjuvant due to its ability to stimulate an immune response.[12]

Chitin and chitosan are under development as scaffolds in studies of how tissue grows and how wounds heal, and in efforts to invent better bandages, surgical thread, and materials for allotransplantation.[10][25] Sutures made of chitin have been explored for many years, but as of 2015, none were on the market; their lack of elasticity and problems making thread have prevented commercial development.[26]

In 2014, a method for using chitosan as a reproducible form of biodegradable plastic was introduced.[27] Chitin nanofibers are extracted from crustacean waste and mushrooms for possible development of products in tissue engineering, medicine, and industry.[28]

## References

1. ^ Tang, WJ; Fernandez, JG; Sohn, JJ; Amemiya, CT (2015). "Chitin is endogenously produced in vertebrates". Curr Biol. 25: 897–900. doi:10.1016/j.cub.2015.01.058. PMC 4382437. PMID 25772447.
2. ^ Auguste Odier (presented: 1821 ; published: 1823) "Mémoire sur la composition chimique des parties cornées des insectes" (Memoir on the chemical composition of the horny parts of insects), Mémoires de la Société d'Histoire Naturelle de Paris, 1 : 29-42. From page 35: "… la Chitine (c'est ainsi que je nomme cette substance de chiton, χιτον, enveloppe) …" (… chitine (it is thus that I name this substance from chiton, χιτον, covering) …)
3. ^ Hofmann hydrolyzed chitin using a crude preparation of the enzyme chitinase, which he obtained from the snail Helix pomatia. See:
• A. Hofmann (1929) "Über den enzymatischen Abbau des Chitins und Chitosans" (On the enzymatic degradation of chitin and chitosan), Ph.D. thesis, University of Zurich (Zurich, Switzerland).
• P. Karrer and A. Hofmann (1929) "Polysaccharide XXXIX. Über den enzymatischen Abbau von Chitin and Chitosan I," Helvetica Chimica Acta, 12 (1) : 616-637.
• Nathaniel S. Finney and Jay S. Siegel (2008) "In Memorian: Albert Hofmann (1906-2008)," Chimia, 62 (5) : 444-447 ; see page 444. Available on-line at: University of Zurich
4. ^ Campbell, N. A. (1996) Biology (4th edition) Benjamin Cummings, New Work. p.69 ISBN 0-8053-1957-3
5. ^ Gilbert, Lawrence I. (2009). Insect development : morphogenesis, molting and metamorphosis. Amsterdam Boston: Elsevier/Academic Press. ISBN 978-0-12-375136-2.
6. ^ a b Saranathan V, Osuji CO, Mochrie SG, Noh H, Narayanan S, Sandy A, Dufresne ER, Prum RO (2010). "Structure, function, and self-assembly of single network gyroid (I4132) photonic crystals in butterfly wing scales". Proc Natl Acad Sci U S A. 107 (26): 11676–81. doi:10.1073/pnas.0909616107. PMC 2900708. PMID 20547870.
7. ^ Dasi Espuig M (16 August 2014). "Beetles' whiteness understood". BBC News: Science and Environment. Retrieved 15 November 2014.
8. ^ Burresi, Matteo; Cortese, Lorenzo; Pattelli, Lorenzo; Kolle, Mathias; Vukusic, Peter; Wiersma, Diederik S.; Steiner, Ullrich; Vignolini, Silvia (2014). "Bright-white beetle scales optimise multiple scattering of light". Scientific Reports. 4: 6075. doi:10.1038/srep06075. PMC 4133710. PMID 25123449.
9. ^ Kudô, K. Nest materials and some chemical characteristics of nests of a New World swarm-founding polistine wasp, (Hymenoptera Vespidae). Ethology, ecology & evolution 13.4 Oct 2001: 351-360. Dipartimento di biologia animale e genetica, Università di Firenze. 16 Oct 2014.
10. ^ a b Bedian, L; Villalba-Rodríguez, AM; Hernández-Vargas, G; Parra-Saldivar, R; Iqbal, HM (May 2017). "Bio-based materials with novel characteristics for tissue engineering applications - A review". International journal of biological macromolecules. 98: 837–846. doi:10.1016/j.ijbiomac.2017.02.048. PMID 28223133.
11. ^ Jeffryes, C; Agathos, SN; Rorrer, G (June 2015). "Biogenic nanomaterials from photosynthetic microorganisms". Current Opinion in Biotechnology. 33: 23–31. doi:10.1016/j.copbio.2014.10.005. PMID 25445544.
12. Elieh Ali Komi, D; Sharma, L; Dela Cruz, CS (1 March 2017). "Chitin and Its Effects on Inflammatory and Immune Responses". Clinical Reviews in Allergy & Immunology. doi:10.1007/s12016-017-8600-0. PMC 5680136. PMID 28251581.
13. ^ a b Gour, N; Lajoie, S (September 2016). "Epithelial Cell Regulation of Allergic Diseases". Current Allergy and Asthma Reports. 16 (9): 65. doi:10.1007/s11882-016-0640-7. PMID 27534656.
14. ^ a b Gómez-Casado, C; Díaz-Perales, A (October 2016). "Allergen-Associated Immunomodulators: Modifying Allergy Outcome". Archivum Immunologiae et Therapiae Experimentalis. 64 (5): 339–47. doi:10.1007/s00005-016-0401-2. PMID 27178664.
15. ^ a b Sánchez-Vallet, A; Mesters, JR; Thomma, BP (March 2015). "The battle for chitin recognition in plant-microbe interactions". FEMS Microbiology Reviews. 39 (2): 171–83. doi:10.1093/femsre/fuu003. ISSN 0168-6445. PMID 25725011.
16. ^ a b Sharp, Russell G. (21 November 2013). "A Review of the Applications of Chitin and Its Derivatives in Agriculture to Modify Plant-Microbial Interactions and Improve Crop Yields". Agronomy. 3 (4): 757–793. doi:10.3390/agronomy3040757.
17. ^ Rovenich, H; Zuccaro, A; Thomma, BP (December 2016). "Convergent evolution of filamentous microbes towards evasion of glycan-triggered immunity". The New Phytologist. 212 (4): 896–901. doi:10.1111/nph.14064. PMID 27329426.
18. ^ a b Kettles, GJ; Kanyuka, K (15 April 2016). "Dissecting the Molecular Interactions between Wheat and the Fungal Pathogen Zymoseptoria tritici". Frontiers in Plant Science. 7: 508. doi:10.3389/fpls.2016.00508. PMC 4832604. PMID 27148331.
19. ^ Briggs, DEG (29 January 1999). "Molecular taphonomy of animal and plant cuticles: selective preservation and diagenesis". Philosophical Transactions of the Royal Society B: Biological Sciences. 354 (1379): 7–17. doi:10.1098/rstb.1999.0356. PMC 1692454.
20. ^ El Hadrami, A; Adam, L. R.; El Hadrami, I; Daayf, F (2010). "Chitosan in Plant Protection". Marine Drugs. 8 (4): 968–987. doi:10.3390/md8040968. PMC 2866471. PMID 20479963.
21. ^ Chitosan#Agricultural .26 Horticultural use
22. ^ Shahidi, F.; Arachchi, J.K.V.; Jeon, Y.-J. (1999). "Food applications of chitin and chitosans". Trends in Food Science & Technology. 10: 37–51. doi:10.1016/s0924-2244(99)00017-5.
23. ^ Hosokawa J, Nishiyama M, Yoshihara K, Kubo T (1990). "Biodegradable film derived from chitosan & homogenized cellulose". Ind. Eng. Chem. Res. 44: 646–650.
24. ^ Gaellstedt M, Brottman A, Hedenqvist MS (2005). "Packaging related properties of protein and chitosan coated paper". Packaging Technology and Science. 18: 160–170.
25. ^ Cheung, R. C.; Ng, T. B.; Wong, J. H.; Chan, W. Y. (2015). "Chitosan: An Update on Potential Biomedical and Pharmaceutical Applications". Marine Drugs. 13 (8): 5156–5186. doi:10.3390/md13085156. PMC 4557018. PMID 26287217.
26. ^ Ducheyne, Paul; Healy, Kevin; Hutmacher, Dietmar E.; Grainger, David W.; Kirkpatrick, C. James, eds. (2011). Comprehensive biomaterials. Amsterdam: Elsevier. p. 230. ISBN 9780080552941.
27. ^ "Harvard researchers develop bioplastic made from shrimp shells". Fox News. 16 May 2014. Retrieved 24 May 2014.
28. ^ Ifuku, Shinsuke (2014). "Chitin and Chitosan Nanofibers: Preparation and Chemical Modifications". Molecules. 19 (11): 18367–80. doi:10.3390/molecules191118367. PMID 25393598.
Amino sugar

In organic chemistry, an amino sugar (or more technically a 2-amino-2-deoxysugar) is a sugar molecule in which a hydroxyl group has been replaced with an amine group. More than 60 amino sugars are known, with one of the most abundant being N-Acetyl-d-glucosamine, which is the main component of chitin.

Derivatives of amine containing sugars, such as N-acetylglucosamine and sialic acid, whose nitrogens are part of more complex functional groups rather than formally being amines, are also considered amino sugars. Aminoglycosides are a class of antimicrobial compounds that inhibit bacterial protein synthesis. These compounds are conjugates of amino sugars and aminocyclitols.

Arthropod cuticle

The cuticle forms the major part of the integument of the Arthropoda. It includes most of the material of the exoskeleton of the insects, Crustacea, Arachnida, and Myriapoda.

Arthropod exoskeleton

Arthropods are covered with a tough, resilient integument or exoskeleton of chitin. Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. This happens in parts of the body where there is a need for rigidity or elasticity. Typically the mineral crystals, mainly calcium carbonate, are deposited among the chitin and protein molecules in a process called biomineralization. The crystals and fibres interpenetrate and reinforce each other, the minerals supplying the hardness and resistance to compression, while the chitin supplies the tensile strength. Biomineralization occurs mainly in crustaceans; in insects and Arachnids the main reinforcing materials are various proteins hardened by linking the fibres in processes called sclerotisation and the hardened proteins are called sclerotin.

In either case, in contrast to the carapace of a tortoise or the cranium of a vertebrate, the exoskeleton has little ability to grow or change its form once it has matured. Except in special cases, whenever the animal needs to grow, it moults, shedding the old skin after growing a new skin from beneath.

Benzoylurea

Benzoylureas are chemical derivatives of N-benzoyl-N′-phenylurea (benzoylurea). They are best known for their use as insecticides. They act as insect growth regulators by inhibiting synthesis of chitin in the insect's body.

One of the more commonly used benzoylurea pesticides is diflubenzuron. Others include chlorfluazuron, flufenoxuron, hexaflumuron, and triflumuron. Lufenuron is the active compound in flea control medication for pet dogs and cats.

3-(Iodoacetamido)-benzoylurea (3-IAABU) is one of several benzoylurea compounds which have been investigated as potential anticancer agents.

Buprofezin

Buprofezin is an insecticide used for control of insect pests such as mealybugs, leafhoppers and whitefly on vegetable crops. It is a growth regulator, acting as an inhibitor of chitin synthesis. It is banned in some countries due to its negative environmental impacts, being especially toxic to aquatic organisms as well as non-target insects, though is of low toxicity to humans and other mammals.

Carbohydrate

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 Cm(H2O)n (where m may be different from n). This formula holds true for monosaccharides. Some exceptions exist; for example, deoxyribose, a sugar component of DNA, has the empirical formula C5H10O4. The carbohydrates are technically hydrates of carbon; structurally it is more accurate to view them as aldoses and ketoses.

The term is most common in biochemistry, where it is a synonym of 'saccharide', a group that includes sugars, starch, and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates, are commonly referred to as sugars. The word saccharide comes from the Greek word σάκχαρον (sákkharon), meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix -ose, as in the monosaccharides fructose (fruit sugar) and glucose (starch sugar) and the disaccharides sucrose (cane or beet sugar) and lactose (milk sugar).

Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the storage of energy (e.g. starch and glycogen) and as structural components (e.g. cellulose in plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an important component of coenzymes (e.g. ATP, FAD and NAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.They are found in a wide variety of natural and processed foods. Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from sugarcane or sugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, many fruits, and some vegetables. Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes.

Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, and are metabolized by these bacteria to yield short-chain fatty acids.

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 mechanism. Cell walls are present in most prokaryotes (except mycoplasma bacteria), in algae, plants and fungi but rarely in other eukaryotes including animals. 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 species and may depend on cell type and developmental stage. The primary cell wall of land plants is composed of the polysaccharides cellulose, hemicelluloses and pectin. Often, other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls. Algae possess cell walls made of glycoproteins and polysaccharides such as carrageenan and agar that are absent from land plants. In bacteria, the cell wall is composed of peptidoglycan. The cell walls of archaea have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls made of the N-acetylglucosamine polymer chitin. Unusually, diatoms have a cell wall composed of biogenic silica.

Chetan, Iran

Not to be confused with Chitin, an organic compound.

Chetan (Persian: چتن‎; also known as Chasan, Chetīn, Chītan, Chīten, Chitin, and Chittin) is a village in Panjak-e Rastaq Rural District, Kojur District, Nowshahr County, Mazandaran Province, Iran. At the 2006 census, its population was 568, in 134 families.

Chitin synthase

In enzymology, a chitin synthase (EC 2.4.1.16) is an enzyme that catalyzes the chemical reaction

UDP-N-acetyl-D-glucosamine + [1,4-(N-acetyl-beta-D-glucosaminyl)]n ${\displaystyle \rightleftharpoons }$ UDP + [1,4-(N-acetyl-beta-D-glucosaminyl)]n+1

Thus, the two substrates of this enzyme are UDP-N-acetyl-D-glucosamine and [[[1,4-(N-acetyl-beta-D-glucosaminyl)]n]], whereas its two products are UDP and [[[1,4-(N-acetyl-beta-D-glucosaminyl)]n+1]].

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-N-acetyl-D-glucosamine:chitin 4-beta-N-acetylglucosaminyl-transferase. Other names in common use include chitin-UDP N-acetylglucosaminyltransferase, chitin-uridine diphosphate acetylglucosaminyltransferase, chitin synthetase, and trans-N-acetylglucosaminosylase. This enzyme participates in aminosugars metabolism.

Chitinase

Chitinases (chitodextrinase, 1,4-beta-poly-N-acetylglucosaminidase, poly-beta-glucosaminidase, beta-1,4-poly-N-acetyl glucosamidinase, poly[1,4-(N-acetyl-beta-D-glucosaminide)] glycanohydrolase, (1->4)-2-acetamido-2-deoxy-beta-D-glucan glycanohydrolase) are hydrolytic enzymes that break down glycosidic bonds in chitin.As chitin is a component of the cell walls of fungi and exoskeletal elements of some animals (including worms and arthropods), chitinases are generally found in organisms that either need to reshape their own chitin or dissolve and digest the chitin of fungi or animals.

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 crustaceans with an alkaline substance, like sodium hydroxide.

Chitosan has a number of commercial and possible biomedical uses. It can be used in agriculture as a seed treatment and biopesticide, helping plants to fight off fungal infections. In winemaking, it can be used as a fining agent, also helping to prevent spoilage. In industry, it can be used in a self-healing polyurethane paint coating. In medicine, it is useful in bandages to reduce bleeding and as an antibacterial agent; it can also be used to help deliver drugs through the skin.

More controversially, chitosan has been asserted to have use in limiting fat absorption, which would make it useful for dieting, but there is evidence against this.

Other uses of chitosan that have been researched include use as a soluble dietary fiber.

Chytridiomycota

Chytridiomycota are a division of zoosporic organisms in the kingdom Fungi, informally known as chytrids. The name is derived from the Greek χυτρίδιον chytridium, meaning "little pot", describing the structure containing unreleased zoospores. Chytrids are one of the early diverging fungal lineages, and their membership in kingdom Fungi is demonstrated with chitin cell walls, a posterior whiplash flagellum, absorptive nutrition, use of glycogen as an energy storage compound, and synthesis of lysine by the α-amino adipic acid (AAA) pathway.Chytrids are saprobic, degrading refractory materials such as chitin and keratin, and sometimes act as parasites. There has been a significant increase in the research of chytrids since the discovery of Batrachochytrium dendrobatidis, the causal agent of chytridiomycosis.

Insect growth regulator

An insect growth regulator (IGR) is a substance (chemical) that inhibits the life cycle of an insect. IGRs are typically used as insecticides to control populations of harmful insect pests such as cockroaches and fleas.

Limpet

Limpets are aquatic snails with a shell that is broadly conical in shape and a strong, muscular foot.

Although all limpets are members of the class Gastropoda, limpets are polyphyletic, meaning that the various groups that are called "limpets" have descended independently from different ancestral gastropods. This general category of conical shell is technically known as "patelliform", meaning dish-shaped. Some species of limpet live in fresh water, but these are the exception. All members of the large and ancient marine clade Patellogastropoda are limpets, and within that clade the family Patellidae in particular are often called the "true limpets".

Other groups, not in the same family, are also called limpets of one type or another because of the similar shapes of their shells. Examples include the Fissurellidae, the "keyhole limpet" family, which is part of the clade Vetigastropoda (many other members of the Vetigastropoda do not have the morphology of limpets) and the Siphonariidae, the "false limpets", which use a siphon to pump water over their gills.

Lufenuron

Lufenuron is the active ingredient in the veterinary flea control medication Program, and one of the two active ingredients in the flea, heartworm, ringworm and anthelmintic medicine milbemycin oxime/lufenuron (Sentinel).

Lufenuron is stored in the animal's body fat and transferred to adult fleas through the host's blood when they feed. Adult fleas transfer it to their growing eggs through their blood, and to hatched larvae feeding on their excrement. It does not kill adult fleas.Lufenuron, a benzoylurea pesticide, inhibits the production of chitin in insects. Without chitin, a larval flea will never develop a hard outer shell (exoskeleton). With its inner organs exposed to air, the insect dies from dehydration soon after hatching or molting (shedding its old, smaller shell).Lufenuron is also used to fight fungal infections, since fungus cell walls are about one third chitin.Lufenuron is also sold as an agricultural pesticide for use against lepidopterans, eriophid mites, and western flower thrips. It is an effective antifungal in plants.

Natural fiber

Natural fibers or natural fibres (see spelling differences) are fibres that are produced by plants, animals, and geological processes. They can be used as a component of composite materials, where the orientation of fibers impacts the properties. Natural fibers can also be matted into sheets to make products such as paper, felt or fabric.The earliest evidence of humans using fibers is the discovery of wool and dyed flax fibers found in a prehistoric cave in the Republic of Georgia that date back to 36,000 BP. Natural fibers can be used for high-tech applications, such as composite parts for automobiles. Compared to composites reinforced with glass fibers, composites with natural fibers have advantages such as lower density, better thermal insulation, and reduced skin irritation. Further, unlike glass fibers, natural fibers can be broken down by bacteria once they are no longer in use.

Natural fibers are good sweat absorbents and can be found in a variety of textures. Cotton fibers made from the cotton plant, for example, produce fabrics that are light in weight, soft in texture, and which can be made in various sizes and colors. Clothes made of natural fibers such as cotton are often preferred over clothing made of synthetic fibers by people living in hot and humid climates.

Polysaccharide

Polysaccharides () are polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic linkages, and on hydrolysis give the constituent monosaccharides or oligosaccharides. They range in structure from linear to highly branched. Examples include storage polysaccharides such as starch and glycogen, and structural polysaccharides such as cellulose and chitin.

Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure, these macromolecules can have distinct properties from their monosaccharide building blocks. They may be amorphous or even insoluble in water. When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present they are called heteropolysaccharides or heteroglycans.Natural saccharides are generally of simple carbohydrates called monosaccharides with general formula (CH2O)n where n is three or more. Examples of monosaccharides are glucose, fructose, and glyceraldehyde. Polysaccharides, meanwhile, have a general formula of Cx(H2O)y where x is usually a large number between 200 and 2500. When the repeating units in the polymer backbone are six-carbon monosaccharides, as is often the case, the general formula simplifies to (C6H10O5)n, where typically 40≤n≤3000.

As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides contain three to ten monosaccharide units; but the precise cutoff varies somewhat according to convention. Polysaccharides are an important class of biological polymers. Their function in living organisms is usually either structure- or storage-related. Starch (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar glucose polymer is the more densely branched glycogen, sometimes called "animal starch". Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of moving animals.

Cellulose and chitin are examples of structural polysaccharides. Cellulose is used in the cell walls of plants and other organisms, and is said to be the most abundant organic molecule on Earth. It has many uses such as a significant role in the paper and textile industries, and is used as a feedstock for the production of rayon (via the viscose process), cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure, but has nitrogen-containing side branches, increasing its strength. It is found in arthropod exoskeletons and in the cell walls of some fungi. It also has multiple uses, including surgical threads. Polysaccharides also include callose or laminarin, chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan and galactomannan.

Schizophora

The Schizophora are a section of true flies containing 78 families, which are collectively referred to as muscoids, although technically the term "muscoid" should be limited to flies in the superfamily Muscoidea; this is an example of informal, historical usage persisting in the vernacular. The section is divided into two subsections, the Acalyptratae and Calyptratae, which are commonly referred to as acalyptrate muscoids and calyptrate muscoids, respectively.

The defining feature of the Schizophora is the presence of a special structure used to help the emerging adult fly break free of the puparium; this structure is an inflatable membranous sac called the ptilinum that protrudes from the face, above the antennae. The inflation of the ptilinum (using fluid hemolymph rather than air) creates pressure along the line of weakness in the puparium, which then bursts open along the seam to allow the adult to escape. When the adult emerges, the fluid is withdrawn, the ptilinum collapses, and the membrane retracts entirely back inside the head. The large, inverted, "U"-shaped suture in the face through which it came, however, is still quite visible, and the name "Schizophora" ("split-bearers") is derived from this ptilinal or frontal suture. The term was first used by Eduard Becher.

In contrast to eggs of other arthropods, most insect eggs are drought-resistant, because inside the maternal chorion, two additional membranes develop from embryonic tissue, the amnion and the serosa. This serosa secretes a cuticle rich in chitin that protects the embryo against desiccation. In the Schizophora, however, the serosa does not develop, but these flies lay their eggs in damp places, such as rotting organic matter.

Squid

Squid are cephalopods in the superorder Decapodiformes with elongated bodies, large eyes, eight arms and two tentacles. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.

Squid diverged from other cephalopods during the Jurassic and occupy a similar role to teleost fish as open water predators of similar size and behaviour. They play an important role in the open water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates, with groups of Humboldt squid having been observed hunting cooperatively. They are preyed on by sharks, other fish, sea birds, seals and cetaceans, particularly sperm whales.

Squid can change colour for camouflage and signalling. Some species are bioluminescent, using their light for counter-illumination camouflage, while many species can eject a cloud of ink to distract predators.

Squid are used for human consumption with commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific and elsewhere. They are used in cuisines around the world, often known as "calamari". Squid have featured in literature since classical times, especially in tales of giant squid and sea monsters.

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