Chitin

Chitin is a polysaccharide which is found widely in nature where it functions in a manner similar to collagen in chordates. It forms the tough fibrous exoskeletons of insects, crustaceans and other arthropods, and, in addition to its presence in some fungi it occurs in at least one alga. The structure of chitin is similar to that of cellulose but with glucose replaced with N-acetyl-D-glucosaminyl units linked β-D-(1-4) in a linear chain. It is normally produced from the shells of lobster, crab or shrimp.