chitin

thumb|250px|right|Structure of the chitin molecule, showing two of the N-Acetylglucosamine|N-acetylglucosamine units that repeat to form long chains in β-(1→4)-linkage. thumb|250px|right|Haworth projection of the chitin molecule. thumb|A close-up of the wing of a leafhopper; the wing is composed of chitin. thumb|right|A cicada emerges from its nymphal exoskeleton; the shed exoskeleton is mostly modified chitin ([[sclerotin) but the wings and much of the adult body are still unsclerotized chitin at this stage]]

Chitin (C8H13O5N)n ( ) is a long-chain polymer of N-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide 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 walls in fungi (especially filamentous and mushroom-forming fungi), the exoskeletons of arthropods such as crustaceans and insects, the radulae, cephalopod beaks and gladii of molluscs and in some nematodes and diatoms. It is also synthesised by at least some fish and lissamphibians. Commercially, chitin is extracted from the shells of crabs, shrimps, shellfish 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.