Chitin is a large, structural polysaccharide made from chains of modified glucose. Chitin is found in the exoskeletons of insects, the cell walls of fungi, and certain hard structures in invertebrates and fish. In terms of abundance, chitin is second to only cellulose. In the biosphere, over 1 billion tons of chitin are synthesized each year by organisms. This extremely versatile molecule can form solid structures on its own as in insect wings, or can combine with other components like calcium carbonate to make even stronger substances like the shell of a clam.
Like cellulose, no vertebrate animals can digest chitin on their own. Animals that eat a diet of insects often have symbiotic bacteria and protozoa which can break down the fibrous chitin into the glucose molecules that compose it. However, because chitin is a biodegradable molecule that dissolves over time, it is used in a number of industrial applications, such as surgical thread and binders for dyes and glues.
Function of Chitin
Chitin, like cellulose and keratin, is a structural polymer. Made from smaller monomers, or monosaccharides, structural polymers form strong fibers. When secreted inside or outside of cells in an organized way, the fibers form weak bonds between each other. This adds strength to the entire structure. Chitin and cellulose are both made from glucose monomers, while keratin is a fibrous protein. The various structural polymers arose early in the evolution of life, because they are seen only in certain groups. Cellulose is exclusive to plants, keratin to animals, and chitin to the arthropods, mollusks and fungi. Chitin and cellulose both evolved early-on in the history of life, while keratin arose in certain animals long after plants and fungi had branched off from the other eukaryotes.
Structure of Chitin
Chitin is made up of modified glucose monosaccharides. Glucose exists as a ring of carbon and oxygen molecules. Bonds between glucose molecules are known as glycosidic bonds. The oxygens that typically form hydroxyl groups bonded to the carbon ring can also form a bond with another carbon instead of a hydrogen. In this way, monosaccharides can be linked together in long chains. Chitin is formed by a series of glycosidic bonds between substituted glucose molecules.
Chitin is different from cellulose because of the substitution that occurs on the glucose molecule. Instead of a hydroxyl group (OH), the glucose molecules in chitin have an amyl group attached that consists of carbon and nitrogen. Nitrogen is an electrically positive molecule, while the oxygen double bonded to the group is electrically negative. This produces a dipole in the molecule, which increases the hydrogen bonds that can formed between these molecules and the molecules around them. When combined in a matrix with various compounds and other chitin molecules, the resulting structure can be very hard because of all the weak interactions between nearby molecules.
Examples of Chitin as a Biological Compound
Chitin in Arthropods
One of the most diverse groups of animals in the world are the Arthropods. Arthropods are invertebrate animals which have a segmented body plan and hard exoskeleton made of chitin and various proteins. The combination of a protected body plan that exists in variable segments is extremely successful in many different ecosystems. Arthropods exists everywhere, from the bottom of the ocean to highest places organisms inhabit. Arthropods also vary in size from microscopic mites that live at the base of hairs to giant crabs and insects that can be meters long. The exoskeletons of all of these creatures consists of chitin deposited along with structural proteins. Mixed with different proteins, chitin also makes the wings of many insects as a more flexible material. The adaptability of chitin to be molded into these different forms has allowed the arthropods to be evolve into millions of different forms.
Chitin in Fungi
In fungi, chitin is used to create a cell wall. Much like cellulose in plants, the chitin is deposited extracellularly with proteins and other molecules. This forms a rigid cell wall between cells, which help the organisms retain their shape. Much like in plant cells, water can be retained in the cells to create water pressure against the cell wall. This is known as turgor pressure and adds to the strength of each cell. Fungi are able to push through multiple layers of leaf litter as they grow, which can weigh several pounds. This comes in part from the strength of chitin as a structural fiber.
Chitin in Mollusks
Chitin is seen in a range of other forms in the mollusks. Chitin is used in both lower mollusks and the more derived cephalopods. In mollusks such as snails, chitin is a part of the radulae, an organ that looks like a spiked tongue. The mollusks use the radulae to scrape algae and other food from the hard surfaces it grows on. The cephalopods also use chitin, but to form a beak which can be used to bite through the hard shells of their prey items. Ironically, most of the prey items are arthropods, and their shells are also made from chitin.