Chitin is a polysaccharide used as armor or cell walls for fungi and arthropods. It is also used for teeth and beaks. Fungi evolved chitin as a protective strategy and continued to occupy a niche as decomposers. Arthropods’ exoskeletons are made of chitin, which allows for rapid growth through moulting. Most arthropods’ exoskeletons consist of chitin embedded in a protein matrix or combined with calcium carbonate.
Chitin is a long-chain polysaccharide that serves as the armor or cell wall for fungi and arthropods, including all crustaceans and insects. Less famous, it is used for the radula (teeth) of molluscs and the beak of cephalopods such as squid and octopuses. Chitin is a stronger version of keratin, fibrous structural proteins used by reptiles, birds, amphibians and mammals. In some animals such as crustaceans, the shell can be a combination of this substance and keratin. Its molecular formula is (C8H13O5N)n.
About a billion years ago, chitin got its evolutionary start when the first single-celled fungi split off from other single-celled organisms that eventually became animals. This adaptive strategy has made fungi among the most rigid of early eukaryotes, but also the most protected. Fungi continued to occupy a niche as decomposers, and at one point after the Late Permian mass extinction, they were the dominant life form on Earth. Most people are unaware that fungi are actually more closely related to animals than to plants.
Another of chitin’s great successes is its central location in relation to the body plan of arthropods. The exoskeletons of arthropods are made of this substance. Instead of growing gradually, like most other animals, arthropods grow in rapid stages. When an arthropod outgrows its exoskeleton, it sheds it in a process called moulting. It has less than an hour to grow until its new underlying exoskeleton hardens and it becomes unable to grow further. This moulting process can be repeated dozens of times until the animal reaches its maximum size and eventually dies. Since arthropods are among the most successful animal groups in the world, with millions of unique species, we can say that chitin is a very useful building material for animal bodies.
The exoskeletons of most arthropods are not actually pure chitin, but consist of this material embedded in a hard protein matrix. The difference between modified and unmodified chitin can be seen by looking at caterpillars (unmodified) and beetles (modified). The beetle’s exoskeleton is more durable. In other arthropods, such as some molluscs and crustaceans, the substance is combined with calcium carbonate to create a much stronger shell.
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