Chitin is a major constituent of the exoskeleton, or external skeleton, of many arthropods such as insects, spiders, and crustaceans. Exoskeletons made of this durable and firm compound support and protect the delicate soft tissues of these animals, which lack an internal skeleton. Chitin is a polysaccharide , a type of carbohydrate that has a basic structure of a repeating chain of sugar molecules. Chitin is analogous in structure to cellulose, the compound that provides structural support to plant tissues. In addition to being found in arthropod exoskeletons, chitin is also found in the cell walls of some species of fungi.
Chitin does not work alone in forming exoskeletons. It is associated with a number of proteins, including an elastic, rubberlike substance called resilin. The identity and nature of these proteins determines whether the exoskeleton will be rigid, like a beetle's shell, or soft and flexible like the joints of a crab leg. Chitin also associates with nonprotein compounds, such as the calcium carbonate that is part of the shells of crustaceans such as crabs, lobsters, and shrimp.
Animals that wear their skeletons on the outside are relatively inflexible because of their armor rigidity. Arthropods can bend their limbs or the segments of their body only at the joints, where the exoskeleton is thinner. Therefore, it is important that the composition and character of the exoskeleton complement the anatomy it covers and the overall ecology of the organism.
Chitin confers a number of protective benefits to animals with exoskeletons. As well as defining the basic shape of the animal, the tough shell that encases arthropods protects the wearer from dessication , or dehydration. This particular function is essential to terrestrial arthropods, which may perish if too much water is lost from their blood and body tissues. Shells also provide effective protection against some predators.
Chitinous exoskeletons must be molted, or shed, as the animal grows because the rigid shell does not expand with the rest of the body. After the old shell is cast off, a new, larger exoskeleton is secreted by glands in the epidermis. Newly molted individuals are particularly vulnerable to attack because they have little protection while they wait for their new shells to harden.
Exoskeletons would be impractical for larger animals because chitin is not strong enough to protect and support them. Land-dwelling invertebrates, who do not benefit from the buoyant support of water, are limited in size because as an exoskeleton gets larger, it becomes thicker and heavier. These animals would not be able to move very well under the weight of this protective armor.
see also Bone; Keratin.
Judy P. Sheen
Anderson, D. T. Atlas of Invertebrate Anatomy. Portland: International. Specialized Bood Service, 1997.
Chitin is a polymer, a repeating arrangement of a chemical structure. Chitin is found in the supporting structures of many organisms. Of relevance to microbiology, chitin is present in fungal species such as mushrooms, where it can comprise from 5% to 20% of the weight of the organism.
The backbone of chitin is a six-member carbon ring that has side groups attached to some of the carbon atoms. This structure is very similar to that of cellulose. One of the side groups of chitin is known as acetamide, whereas cellulose has hydroxy (OH) side groups.
Chitin is a noteworthy biological feature because it is constructed solely from materials that are naturally available. In contrast, most polymers are man-made and are comprised of constituents that must be artificially manufactured.
The purpose of chitin is to provide support for the organism. The degree of support depends on the amount and the thickness of chitin that is present. In fungi such as mushrooms, chitin confers stability and rigidity, yet allows some flexibility. This allows the mushrooms to stand and still be flexible enough to sway without snapping.
The role of chitin as a support structure is analogous to the peptidoglycan supportive layer that is a feature of Gram-positive and Gram-negative bacteria . The think peptidoglycan layer in Gram-positive bacteria provides a rigid and robust support. The peptidoglycan layer in Gram-negative bacteria that is only one molecule thick does not provide the same degree of structural support. Other mechanical elements of the Gramnegative cell wall are necessary to shore up the structure.
In the ocean, where many creatures contain chitin, seadwelling bacteria called Vibrio furnisii have evolved a sensory system that detects discarded chitin. The bacteria are able to break down the polymer and use the sugar molecules as metabolic fuel.
See also Fungi
chi·tin / ˈkītn/ • n. Biochem. a fibrous substance consisting of polysaccharides and forming the major constituent in the exoskeleton of arthropods and the cell walls of fungi. DERIVATIVES: chi·tin·ous / ˈkītn-əs/ adj.