Chlorophyll

Chlorophyll a is the primary photosynthetic pigment - it alone sits at the reaction centre and is directly responsible for the photochemistry of converting light into chemical energy, while every other pigment (chlorophyll b, c, d, e, the carotenoids, and the phycobilins) is an accessory pigment that absorbs light at wavelengths chlorophyll a captures poorly and passes that energy along to it. Structurally, a chlorophyll molecule has a distinctive tadpole-like shape, made of a hydrophilic 'head' and a lipophilic 'tail.' The head is a magnesium-porphyrin ring system, built from four pyrrole rings (each pyrrole ring itself made of four carbon atoms and one nitrogen atom) linked together into a closed ring by carbon (methine) bridges, with a magnesium ion coordinated at the very centre; different side groups attached around this porphyrin ring are what distinguish the different chlorophyll types from one another. The tail is a 20-carbon phytol alcohol, attached to carbon 7 of pyrrole ring IV through a long propionic-acid ester linkage; being lipophilic, this tail anchors the whole chlorophyll molecule into the lipid thylakoid membrane. Chlorophyll a is biosynthesised starting from succinic acid, an intermediate of the Krebs cycle, which is first activated by the addition of coenzyme A and then reacted with the simple amino acid glycine; this reaction sequence eventually produces chlorophyll a, and its completion requires an adequate supply of magnesium, iron, copper, zinc, manganese, potassium and nitrogen - a shortage of any one of these minerals interrupts chlorophyll synthesis and produces chlorosis (yellowing), the same deficiency symptom covered under mineral nutrition. Comparing chlorophyll a with its relatives highlights exactly how small a chemical change separates one pigment from another: chlorophyll b differs from chlorophyll a only by carrying a -CHO (aldehyde) group in place of chlorophyll a's -CH3 (methyl) group at the 3rd carbon of pyrrole ring II; chlorophyll c differs from chlorophyll a by lacking the phytol tail entirely; chlorophyll d differs from chlorophyll a by carrying an -O-CHO group in place of a -CH=CH2 group at the 2nd carbon of pyrrole ring I; and pheophytin, closely related to chlorophyll a, is structurally identical to it except that it has completely lost its central mag …