The Photosynthetic Life-giving Pigments

Pigments are colorful chemical compounds that reflect only certain wavelengths of visible light. This makes them appear “colorful”. Flowers, coral and even animal and human skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths.

There are three principle classes of pigments:

Chlorophyll is the most important chelate in nature. It is capable of channelling the energy of sunlight into chemical energy through the process of photosynthesis. Chlorophyll is a greenish pigment that contains 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.

Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their orange 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 that 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 (blue-green algae) and rhodophyta (red algae). Phycobilins are useful to the organisms that use them for soaking up light energy. Both phycocyanin 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. These pigments chemically bond to antibodies and as such, have been found to prevent tumorogenesis.

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 (blue-green algae), 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.


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