Chlorophyll is a constituent pigment of plants, and many color changes in ripening fruit are due to a loss of chlorophyll unmasking other pigments. Light is necessary for chlorophyll synthesis in fruit. Chlorophyll is synthesized by a specialized pathway for tetrapyrrole rings. The starting point is the condensation of two molecules of gamma-aminolevulinic acid to form porphobilinogen. Four molecules of porphobilinogen condense to form a tetrapyrrole ring followed by subsequent modifications of the side chains, the incorporation of magnesium, and finally the attachment of the polyisoprene-derived long chain alcohol. Early steps in the biosynthetic sequence are similar to those in the biosynthesis of heme in animals; the process becomes plant specific at the insertion of magnesium.
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