Photosynthetic Pigments

A pigment is a generic term for a molecule that absorbs light and has a color. Plants contain many pigments, giving rise to the various colors we see. Flowers and fruits obviously contain a large number of organic molecules that absorb light. Leaves, stems and roots also contain a variety of pigments. Such pigment molecules include anthocyanins, flavanoids, flavines, quinones and cytochromes, just to name a few. However, none of these should be considered a photosynthetic pigment. Photosynthetic pigments are the only pigments that have the ability to absorb energy from sunlight and make it available to the photosynthetic apparatus. In land plants, there are two classes of these photosynthetic pigments, the chlorophylls and the carotenoids.

The ability of chlorophyll and carotenoid molecules to absorb the energy of light and use it effectively is related to their molecular structure and to their organization within the cell. You learned in a previous lesson (The Interaction of Light with Biological Molecules) that pigments absorb the energy from photons through systems of conjugated double bonds. Examine the molecular structure of representative chlorophyll and carotenoid molecules in Figures: Structure A and B.

Structure A: Molecular structure of lutein, a carotenoid.

Notice the linear system of conjugated double bonds in the carotenoid (lutein) and the zig-zag of conjugated double bonds in the large ring structure of the chlorophyll. These systems of conjugated double bonds are responsible for the ability to absorb light energy from photons.

Structure B: Molecular structure of chlorophyll.