<P> Chlorophyll a contains a magnesium ion encased in a large ring structure known as a chlorin . The chlorin ring is a heterocyclic compound derived from pyrrole . Four nitrogen atoms from the chlorin surround and bind the magnesium atom . The magnesium center uniquely defines the structure as a chlorophyll molecule . The porphyrin ring of bacteriochlorophyll is saturated, and lacking alternation of double and single bonds causing variation in absorption of light . </P> <P> Side chains are attached to the chlorin ring of the various chlorophyll molecules . Different side chains characterize each type of chlorophyll molecule, and alters the absorption spectrum of light . For instance, the only difference between chlorophyll a and chlorophyll b is that chlorophyll b has an aldehyde instead of a methyl group at the C - 7 position . </P> <P> Chlorophyll a has a long hydrophobic tail, which anchors the molecule to other hydrophobic proteins in the thylakoid membrane of the chloroplast . Once detached from the porphyrin ring, this long hydrocarbon tail becomes the precursor of two biomarkers, pristane and phytane, which are important in the study of geochemistry and the determination of petroleum sources . </P> <P> Chlorophyll a biosynthetic pathway utilizes a variety of enzymes . Genes code for the enzymes on the Mg - tetrapyrroles of both bacteriochlorophyll a and chlorophyll a . It begins with glutamic acid, which is transformed into a 5 - aminolevulinic acid (ALA). Two molecules of ALA are then reduced to porphobilinogen (PBG), and four molecules of PBG are then coupled, forming protoporphyrin IX . When forming protoporphyrin, Mg - chelatase acts as a catalyst for the insertion of Mg into the chlorophyll a structure . The pathway then uses either a light - dependent process, driven by the enzyme protochlorophyllide oxidoreductase . Protochlorophyllide is a precursor to the production of a chlorophyll a molecule, or a light - independent process driven by other enzymes, to form a cyclic ring, and the reduction of another ring in the structure . Attachment of the phytol tail completes the process of chlorophyll biosynthesis . </P>

4 pyrrole rings in head of chlorophyll are linked by