| MadSci Network: Molecular Biology |
Chlorophyll and Heme are pigments composed of a modified porphyrin ring coordinating a metal ion (Magnesium for Chlorophyll, Iron for Heme). Chlorophyll probably evolved from heme, which is also found in cytochromes.
Cytochromes are in almost every form of life on the planet; they are used by organisms to transport energy - usually in the form of electrons. Cytochrome c is found in all mitochondria (the power plants of cells) and in many types of bacteria. It is an essential part of oxidative phosphorylation - the process that coverts food energy into ATP, which cells can use for energy. The porphyrin ring of Cytochrome c coordinates an iron ion, and is the same as the heme in hemoglobin. Its job is to transmit two high-energy electrons to a lower state, ultimately reducing oxygen to water.
Chlorophyll performs almost the same function as Cytochrome c, absorbing light to produce two high-energy electrons, which are transmitted through an electron transfer chain to produce useful energy for the cell. Hemoglobin's function is also related: it carries oxygen from the lungs to the tissues by using heme to bind two electrons which happen to be attached to the oxygen molecule. So chlorophyll and hemoglobin represent new ways for organisms to use old materials.
As to swapping the metal ions between chlorophyll and heme, switching ions would destroy the activities of both molecules. For hemoglobin to bind oxygen, it requires an ion which can coordinate with five other atoms, and still have room to bind oxygen. A feat impossible for Magnesium. A similar problem would occur with chlorophyll containing an iron ion, with very different chemical properties from magnesium.