Colorful Plant Pigments

Chapter 4 introduces the major families of plant pigments. Compounds that are composed of molecules with long sequences of single bonds alternating with double bonds (i.e. conjugated double bonds) provide most of the colors. Metal ions bonding to organic molecules can further enhance absorption of light. Different functional groups or changes in pH can affect the colors of pigments. The molecular structures of pigments determine whether the compounds are water-soluble or fat-soluble—a large number of OH groups, especially when bonded to sugars in glycosides, increases water-solubility. The solubility of pigments determines where they are stored in plant cells. Plant pigments can be grouped in a few families of related compounds: a comparison of different chlorophylls is followed by the terpenoid structures of carotenoid pigments. The large family of flavonoid pigments is characterized by typical three-ring structures with phenolic groups. White and yellow flavone pigments, blue and purple anthocyanins, and brown tannins have flavonoid structures. Betalains, a smaller group of purple pigments and unrelated to flavonoids, are only found in select plant families. The chapter concludes with a description of plant scents and pigments that combine to attract specific pollinators and a discussion of plant pigments in fall coloration.