2. Chitin and Chitosan—Sources, Properties and Extraction Methods 2.1. Occurrence and Structure of Chitin Chitin is a naturally occurring polysaccharide and, after cellulose, is the second most abundant biopolymer on Earth. Its widespread presence in crustacean exoskeletons, insect cuticles, and fungal cell walls results in an estimated biospheric production of approximately 1011 tons annually [25]. Chitin’s structure consists primarily of chains of N-acetyl-D-glucosamine units (see Figure 3).