Photosynthetic Pigments
Photosynthetic Pigments
Pigments are colorful compounds.
Pigments are chemical compounds which reflect only certain  wavelengths of visible light.  This makes them appear "colorful".  Flowers,  corals, and even animal skin contain pigments which give them their colors.  More important than their reflection of light is the ability of pigments  to absorb certain wavelengths.
Because they interact with light to absorb only certain wavelengths,  pigments are useful to plants and other autotrophs --organisms  which make their own food using photosynthesis.   In plants , algae,  and cyanobacteria ,  pigments are the means  by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments,  each of a different color, to capture more of the sun's energy.
There are three basic classes of pigments.
Chlorophylls are greenish pigments which contain a porphyrin ring.  This is a stable ring-shaped molecule around which  electrons are free to migrate.  Because the electrons move freely, the ring has  the potential to gain or lose electrons easily, and thus the potential to provide  energized electrons to other molecules.  This is the fundamental process by  which chlorophyll "captures" the energy of sunlight.
There are several kinds of chlorophyll, the most important being  chlorophyll "a".  This is the molecule which makes photosynthesis possible,  by passing its energized electrons on to molecules which will manufacture  sugars.  All plants, algae, and cyanobacteria which photosynthesize contain  chlorophyll "a".  A second kind of chlorophyll is chlorophyll "b", which occurs only in "green algae" and in the plants .   A third form of chlorophyll which is common is (not  surprisingly) called chlorophyll "c", and is found only in the photosynthetic  members of the Chromista as well as the dinoflagellates .   The differences between the chlorophylls of these major groups was one of  the first clues that they were not as closely related as previously thought.
Carotenoids are usually red, orange, or yellow pigments, and  include the familiar compound carotene, which gives carrots their color.   These compounds are composed of two small six-carbon rings connected by a  "chain" of carbon atoms.  As a result, they do not dissolve in water, and must  be attached to membranes within the cell.  Carotenoids cannot transfer sunlight  energy directly to the photosynthetic pathway, but must pass their absorbed  energy to chlorophyll.  For this reason, they are called accessory pigments.  One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae as well as the diatoms .
Phycobilins are water-soluble pigments, and are therefore found in  the cytoplasm, or in the stroma of the chloroplast.  They occur only in Cyanobacteria and Rhodophyta .
The picture at the right shows the two classes of phycobilins which may be  extracted from these "algae".  The vial on the left contains the bluish pigment phycocyanin, which gives the Cyanobacteria their name.  The vial on the  right contains the reddish pigment phycoerythrin, which gives the  red algae their common name.
Phycobilins are not only useful to the organisms which use them for  soaking up light energy; they have also found use as research tools.  Both  pycocyanin and phycoerythrin fluoresce at a particular wavelength.   That is, when they are exposed to strong light, they absorb the light energy,  and release it by emitting light of a very narrow range of wavelengths.  The  light produced by this fluorescence is so distinctive and reliable, that  phycobilins may be used as chemical "tags".  The pigments are chemically  bonded to antibodies, which are then put into a solution of cells.  When the  solution is sprayed as a stream of fine droplets past a laser and computer  sensor, a machine can identify whether the cells in the droplets have been  "tagged" by the antibodies.  This has found extensive use in cancer research,  for "tagging" tumor cells.