collagen
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Collagen molecule.
Collagen is a fibrous protein that is the principal constituent of white fibrous connective tissue; it occurs in tendons, skin, bone, cartilage, and ligaments. It is relatively inelastic (not flexible) but has a high tensile strength (ability to resist forces of stretching). Animal hide is chiefly collagen, converted by tanning to leather. When collagen is boiled it yields gelatin and glue.