Tetrapyrroles are a class of chemical compounds whose molecules contain four pyrrole rings held together by direct covalent bonds or by one-carbon bridges ( or units), in either a linear or a cyclic fashion. A pyrrole ring in a molecule is a five-atom ring where four of the ring atoms are carbon and one is nitrogen. In cyclic tetrapyrroles, lone electron pairs on nitrogen atoms facing the center of the macrocycle ring can bond or chelate with a metal ion such as iron,cobalt, or magnesium.