Photosynthetic pigment
A photosynthetic pigment is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis.
Plants
Green plants have five closely-related photosynthetic pigments (in order of increasing polarity):
- Carotene - an orange pigment
- Xanthophyll - a yellow pigment
- Chlorophyll a - a blue-green pigment
- Chlorophyll b - a yellow-green pigment
- Phaeophytin - a gray pigment
Chlorophyll a is the most common of the five, present in every plant that performs photosynthesis. The reason that there are so many pigments is that each absorbs light more efficiently in a different part of the spectrum. Chlorophyll absorbs well at a wavelength of about 400-450 nm and at 650-700 nm; chlorophyll b at 450-500 nm. Xanthophyll absorbs well at 400-530 nm. However, none of the pigments absorbs well in the green-yellow region, which is responsible for the abundant green we see in nature.
Bacteria
Like plants, the cyanobacteria use water as an electron donor for photosynthesis and therefore liberate oxygen; they also use chlorophyll as a pigment. In addition, most cyanobacteria use phycobilin to capture light energy and pass it on to the chlorophylls. (Some cyanobacteria, the prochlorophytes, use chlorophyll b instead of phycobilin.) It is thought that the chloroplasts in plants and algae all evolved from cyanobacteria.
Several other groups of bacteria use the bacteriochlorophyll pigments (similar to the chlorophylls) for photosynthesis. Unlike the cyanobacteria, these bacteria do not produce oxygen; they typically use hydrogen sulfide rather than water as the electron donor.
Recently, a very different pigment has been found in some marine γ- proteobacteria: proteorhodopsin. It is similar to and probably originated from bacteriorhodopsin (see below under archaea).
Algae
Green algae, red algae and glaucophytes all use chlorophylls. Red algae and glaucophytes also use phycobilin, but green algae do not.
Archaea
Photosynthesis in archaea is quite different from the systems in other domains of life. Photosynthetic archaea (the halobacteria) use the pigment bacteriorhodopsin which acts directly as a proton pump when exposed to light.