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tetrapyrroles A group of pigments that are present in most living organisms. Most of those found in plants are light-sensitive. (Those in animals serve a variety of functions.) The tetrapyrrole molecule consists of 4 joined pyrrole rings, a pyrrole ring consisting of 4 CH units and 1 NH unit. Tetrapyrroles may be linear (i.e. the pyrrole rings are joined side by side) or cyclic.