Chlorophyll a and chlorophyll b are the most abundant plant pigments. Chlorophylls b and a are found in higher plants and algae. Bacteriochlorophylls differ slightly in structure. Chlorophylls a and b are related to the protoporphyrin IX found in hemoglobin and myoglobin. However, the bound metal in the chlorophylls is Mg2+ rather than Fe2+. In Figure 17.7c, the accessory pigments -carotene and phycocyanin are also shown.
All of these molecules absorb light in the visible region of the spectrum because they have large conjugated double-bond systems. Because chlorophylls b and a absorb strongly in both the deep blue and red, the light that is not absorbed but reflected from chloroplasts is green, the color we associate with most growing plants. The other observed colors, such as the red, brown, or purple of algae and photosynthetic bacteria, are accounted for by differing amounts of accessory pigments. Loss of chlorophylls in autumn leaves allows the colors of the accessory pigments, as well as nonphotosynthetic pigments, to become evident. Some photosynthetic bacteria use pigments that absorb wavelengths up to about 1000 nm, in the near infrared.
1. Porphyrin and Chlorophyll Metabolism
2. Photosynthetic Pigments