Chitin

Ephraim Cohen

The Hebrew University oj

Chitin is a globally abundant biopolymer, second only to cellulose and possibly lignin in terms of biomass. Owing to extensive hydrolytic activity mainly by soil and marine chitinolytic microorganisms, chitin is not accumulated in the biosphere, because it is similar to cellulose and unlike lignin. Chitin, which is absent from plants and vertebrates, is present to a small or large extent in most invertebrates, notably in cuticles of arthropods, in primary septum and scar buds of yeast, and in cell walls of most filamentous fungi. Chemically detectable chitin has been verified in 25-million-year-old insect fossils. Chitin is almost invariably covalently or noncovalently associated with other structural molecules in contact with the external environment; examples include carbohydrate polymers in fungi and the cuticular proteins that comprise up to 50% by weight of arthropod cuticles. The chitoprotein supramolecular matrix occurs in peritrophic membranes of insects and in the arthropod exoskeleton, where the rigid chitin microfibrils contribute greatly to its mechanical strength.

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