Definitions

chitin

chitin

[kahy-tin]
chitin, main constituent of the shells of arthropods. Chitin, a polysaccharide (see carbohydrate) analogous in chemical structure to cellulose, consists of units of a glucose derivative (N-acetyl-d-glucosamine) joined to form a long, unbranched chain. Like cellulose, chitin contributes strength and protection to the organism. In arthropods the chitinous shell, or exoskeleton, covers the surface of the body, does not grow, and is periodically cast off (molted). After the old shell is shed, a new, larger shell is secreted by the epidermis, providing room for future growth. The chitin is rigid except between some body segments and joints where it is thin and allows movement of adjacent parts. Chitin is also found in the cell walls of some fungi.

White, horny substance found in the external skeleton of crabs, lobsters, and many insects; in internal structures of some other invertebrates; and in some fungi, algae, and yeasts. It is a polysaccharide, the monomer unit being glucosamine. It is used industrially in purifying wastewater, thickening and stabilizing foods and pharmaceuticals, and sizing and strengthening paper, and as a wound-healing agent, an ion-exchange resin, a membrane for industrial separations, and a binder for dyes, fabrics, and adhesives.

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Chitin (C8H13O5N)n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and it is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans (like the crab, lobster and shrimp) and the insects, including ants, beetles and butterflies, the radula of mollusks and the beaks of the cephalopods, including squid and octopuses. Chitin has also proven useful for several medical and industrial purposes. Chitin is a biological substance which may be compared to the polysaccharide cellulose and to the protein keratin. Although keratin is a protein, and not a carbohydrate like chitin, both keratin and chitin have similar structural functions.

Chemistry, physical properties and biological function

Chitin is a polysaccharide; it is synthesized from units of N-acetylglucosamine (more completely, N-acetyl-D-glucos-2-amine). These units form covalent β-1,4 linkages (similar to the linkages between glucose units forming cellulose). Chitin may therefore be described as cellulose with one hydroxyl group on each monomer substituted with an acetylamine group. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.

In its unmodified form, chitin is translucent, pliable, resilient and quite tough. In arthropods, however, it is often modified, becoming embedded in a hardened proteinaceous matrix, which forms much of the exoskeleton. In its pure form it is leathery, but when encrusted in calcium carbonate it becomes much harder. The difference between the unmodified and modified forms can be seen by comparing the body wall of a caterpillar (unmodified) to a beetle (modified).

Chitin is one of many naturally occurring polymers. Its breakdown may be catalyzed by enzymes called chitinases, secreted by microorganisms such as bacteria and fungi, and produced by some plants. Some of these microorganisms have receptors to simple sugars from the decomposition of chitin. If chitin is detected, they then produce enzymes to digest it by cleaving the glycosidic bonds in order to convert it to simple sugars and ammonia.

Chemically, chitin is closely related to chitosan (a more water-soluble derivative of chitin). It is also closely related to cellulose in that it is a long unbranched chain of glucose derivatives. Both materials contribute structure and strength, protecting the organism.

Etymology

The English word "chitin" comes from the French word "chitine", which first appeared in 1836. These words were derived from the Latin word "chitōn", meaning mollusk. That is either influenced by, or related to the Greek word khitōn, meaning "tunic" or "frock", the Central Semitic word "*kittan", the Akkadian words "kitû" or "kita’um", meaning flax or linen, and the Sumerian word "gada" or "gida".

A similar word, "chiton", refers to a marine animal with a protective shell (also known as a "sea cradle").

Uses

Industrial

Chitin is used industrially in many processes. It is used in water purification, and as an additive to thicken and stabilize foods and pharmaceuticals. It also acts as a binder in dyes, fabrics, and adhesives. Industrial separation membranes and ion-exchange resins can be made from chitin. Processes to size and strengthen paper employ chitin.

Medicine

Chitin's properties as a flexible and strong material make it favorable as surgical thread. Its biodegradibility means it wears away with time as the wound heals. Moreover, chitin has some unusual properties that accelerate healing of wounds in humans.

Occupations associated with high environmental chitin levels, such as shellfish processors, are prone to high incidences of asthma. Recent studies have suggested that chitin may play a role in a possible pathway in human allergic disease. Specifically, mice treated with chitin develop an allergic response, characterized by a build-up of interleukin-4 expressing innate immune cells. Treatment with a chitinase enzyme abolishes the response.

Agriculture

Most recent studies point out that chitin is a good inductor for defense mechanisms in plants. It was recently tested as a fertilizer that can help plants develop healthy immune responses, and have a much better yield and life expectancy.

References

  • Martín-Gil FJ, Leal JA, Gómez-Miranda B, Martín-Gil J, Prieto A, Ramos-Sánchez MC. "Low temperature thermal behaviour of chitins and chitin-glucans". Thermochim. Acta, 1992, vol. 211, pp. 241-254

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