Ligand

The porphyrin ring is a chelate that plays several different roles in the chemistry of living things.

Bound to magnesium, it forms chlorophyll, the green pigment which is central to photosynthesis in plants. Bound to iron, it forms cytochrome molecules, which assist in the transfer of energy throughout living cells. Hemoglobin, which gives blood its red color and carries oxygen to body cells, is also an iron-porphyrin molecule. Iron can form six bonds to ligands, and the porphyrin ring uses only four bonds. Of the remaining two, one holds a protein molecule, and the other holds the oxygen molecule which will be delivered to the cells.