Chitin

Chitin Definition

Chitin is a large, structural polysaccharide derived from modified glucose chains. In insects, fungi, and certain invertebrates and fish, chitin is found in their exoskeletons, cell walls, and hard structures. In terms of abundance, chitin is second only to cellulose. 

Chitin is synthesized by organisms in the biosphere every year in excess of 1 billion tons. Insect wings are formed when calcium carbonate combines with this molecule, or clam shells are formed when calcium carbonate is combined with this molecule.

Unlike cellulose, vertebrates cannot digest chitin on their own. Animals that eat insects often have symbiotic bacteria and protozoa that break down the fibrous chitin into glucose molecules. Because chitin is a biodegradable molecule that dissolves over time, it is used in a number of industrial applications, such as surgical thread and dye binders.

Function of Chitin

Chitin is a structural polymer, similar to cellulose and keratin. Polymers made from monosaccharides form strong fibers. It is possible for fibers to form weak bonds between each other when they are secreted inside or outside of cells in an organized manner. The whole structure becomes stronger as a result. 

Keratin is a fibrous protein, while chitin and cellulose are made from glucose monomers. Because they are only observed in certain groups of organisms, structural polymers emerged early in the evolution of life. There is only cellulose in plants, keratin in animals, and chitin in arthropods, mollusks, and fungi. During the evolution of life, chitin and cellulose both evolved early on, while keratin appeared long after plants and fungi had broken off from each other.

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

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.

Related Biology Terms

• Keratin – A structural polymer seen in animals made of proteins.
• Cellulose – A structural polymer seen in plants made of glucose, like chitin.
• Homopolysaccharide – Polymers of sugars that are made from the same type sugar.
• Heteropolysaccharide – Sugar polymers that consists of monomers of different types.

FAQ’s