Chlorophylls

Most land plants have two forms of chlorophyll (Chl), designated as Chl a and Chl b. They differ in that Chl a has a methyl (the red -CH₃) group on the perimeter of its large ring (called a tetrapyrrole) which is oxidized to form a formyl (-CH=O) group in Chl b. This difference permits these two pigment molecules to absorb slightly different wavelengths of light. Notice in this diagram the conjugated double bond system that weaves around the tetrapyrrole; this electron cloud is the component of the molecule responsible for absorption of photons. Because the electron cloud on the tetrapyrrole ring can be polarized in two different orientations, there are actually two different excited singlet states to which the electrons can be elevated. 

The lowest excited singlet state requires absorption of the energy in a red photon (640 to 700 nm). The second excited singlet state is at a higher energy level from the ground state and requires the absorption of the energy in a blue photon (430 to 475 nm) because photons with shorter wavelengths have more energy. Thus, chlorophyll absorbs photons from both the red and blue regions of the visible spectrum because it has two excited singlet states. When we see light reflected by, or transmitted through leaves, we perceive them to be green. This is because chlorophylls are the major leaf pigments and they do not absorb green photons (about 500 to 600nm). 

Also notice that the chlorophyll structure includes a long hydrophobic chain that is attached to the tetrapyrrole ring by an ester bond. This ’tail’ is a phytyl group and makes the entire chlorophyll molecule insoluble in water. This insolubility keeps the chlorophyll pigments localized within the chloroplast (an intracellular organelle) photosynthetic membranes. Organization into specialized membranes within the chloroplast is essential to the safe production of photosynthetic energy.

Chlorophyll is synthesized in the chloroplast. The first step converts the amino acid glutamatic acid into delta-amino levulinic acid. From here, a number of steps construct the four cyclic rings that make up the tetrapyrroles. The pathway also includes the requirement for light in the photochemical reaction that produces protoporphyrin IX. Elements of this chemical pathway (many steps are omitted) are shown in Figure: Chlorophyll Pathway. 

This is an important pathway because it produces tetrapyrrole molecules in addition to chlorophyll. The pathway is identical up to the point of Protoporphyrin IX, which is produced in the chloroplast. Enzymes in the chloroplast can then insert a Mg²⁺ into the center of the tetrapyrrole and initiate biosynthesis of chlorophyll, or can insert Fe²⁺ into the tetrapyrrole to form heme. Protoporphyrin IX is also exported from the chloroplast to the mitochondrion, where it is used to produce a high abundance of cytochromes in that organelle. This pathway is also important because heme serves as a substrate in the production of phytochrome, which is a light sensing molecule essential for normal photomorphogenesis. Thus, products of the tetrapyrrole pathway are involved in photosynthesis (chlorophylls and cytochromes in the chloroplast), respiration (cytochromes in the mitochondrion) and development (phytochrome).