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How is chitin related to the exoskeletons of crabs, lobsters, and insects?

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A biochemical flowchart shows the metabolic pathways for carbohydrate utilization, including glycolysis, the pentose phosphate pathway, and the tricarboxylic acid (TCA) cycle. The diagram traces the conversion of glucose, fructose, and glycerol through various intermediates, leading to energy production, carbon dioxide release, and biosynthesis of amino acids and porphyrins. Key steps and branches are labeled, with colored boxes highlighting biosynthetic outputs. — Carbohydrate Pathways for the utilization of carbohydrates.

Chitin is a high-molecular-weight polysaccharide that serves as the primary structural component of the exoskeleton in arthropods, including insects, crabs, and lobsters.123

Biochemically, chitin is composed of repeating units of the amino sugar N-acetyl-d-glucosamine. It is often described as "animal cellulose" due to its structural similarity to that plant-based polysaccharide. In the arthropod integument, chitin fibers are embedded in a matrix of protein to form a tough [cuticle].132

The physical properties of these exoskeletons are further modified to suit the animal's needs:
* Sclerotization: The protein chains within the cuticle are cross-linked to form a hard, plastic-like material called sclerotin.2
* Mineralization: In many crustaceans, such as crabs and lobsters, the cuticle is further hardened by the incorporation of calcareous substances like aragonite or calcite.24
* Flexibility: In areas requiring movement, the exoskeleton is divided into segments connected by flexible membranes.24

Because the exoskeleton is a rigid structure, these animals must undergo molting—shedding their old cuticle to grow and secreting a new, larger one.24