Keith Taylor, age 14, of Camden, N.J., for his question:
WHAT IS PIGMENT?
Pigment is coloring matter. In biology, pigment is any chemical molecule that reflects or transmits visible light. The color of a pigment depends on its selective absorption of certain wavelengths of light and its reflection of other wavelengths.
An example of this is found in the important plant pigment called chlorophyll. It absorbs light in the violet and the orange to red portions of the light spectrum, converting this light energy to chemical energy. It reflects light in the blue, green and yellow portions of the spectrum. Thus, chlorophyll appears green.
Chlorophyll and many other pigments act as catalysts, substances that accelerate or facilitate chemical reactions but are not used up in the reaction.
The carotenoids, a group of red, orange and yellow pigments that occur widely in living organisms, also contain many catalytic members. Some carotenoids are involved in the synthesis of vitamin A, important in vision and growth, and others act as accessory pigments in photosynthesis, transferring light energy that they absorb to chlorophyll for conversion to chemical energy.
Other pigments are important in providing concealing coloration in animals and plants. The function of such coloration is to deceive possible predators or prey. Many moths and butterflies so closely match the background patterns of the tree trunks on which they usually rest that they cannot be distinguished from the trees from only a few inches away.
Pigments also protect organisms by providing coloration that matches that of an inedible or otherwise undesirable organism.
Chemically, pigments fall into a number of large groups, but are often divided into two major groups. The first group comprises pigments that contain nitrogen. The second group is formed of pigments without nitrogen.
The group of pigments that contain nitrogen includes hemoglobins, chlorophylls, bile pigments and dark colored pigments called melanin, a chemical that s responsible for variations in the color of human skin. Melanin is found in many animal groups.
Related to melanin is the indigoids, of which the well known plant pigment indigo is an example. Riboflavin, which is also known as vitamin 8 12', is one of a number of pale yellow to green pigments that are produced by several plant groups.
Carotenoids are members of the group of pigments that are formed without nitrogen. Also in this group are the important plant pigments called flavonoids. In leaves, flavonoids selectively admit light wavelengths that are important to photosynthesis, while blocking out untraviolet light, which is destructive to cell nuclei and proteins.
Flavonoids also are important in flower color, particularly in red and blue pigments. Bright fall colors are produced by the conversion of colorless flavonoids, called flavonols, into colored forms, called anthocyanins.