pigment(redirected from exogenous pigment)
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pigment,substance that imparts color to other materials. In paint, the pigment is a powdered substance which, when mixed in the liquid vehicle, imparts color to a painted surface. The pigments used in paints are nearly all metallic compounds, but organic compounds are also used (see lakelake,
in dyeing, an insoluble pigment formed by the reaction between an organic dye and a mordant. The color of a lake depends upon the mordant as well as the dye used. Generally, lakes are not as colorfast as many inorganic dyes, but their colors are more brilliant.
..... Click the link for more information. ). Most black pigments are organic, e.g., bone black (animal black or charcoal) and lampblack. Some of the metallic pigments occur naturally. The brilliant and beautiful coloring of the rock and soil in some parts of the United States, especially in the Grand Canyon of the Colorado River, the Painted Desert of Arizona, and Bryce and Zion canyons of Utah, is largely produced by such compounds, chiefly oxides. Yellow ocher, sienna, and umber are oxides of iron. Litharge is a yellow oxide of lead. Red lead is also an oxide of this metal. Lead chromate, or chrome yellow, is an important yellow pigment. White lead, or basic lead carbonate, is a pigment long in use; it is rendered more durable by mixture with zinc oxide. Cadmium yellow is a sulfide of cadmium. Ultramarine is an important blue pigment, as is Prussian blue (ferric ferrocyanide). Green pigment is produced by mixing Prussian blue and chrome yellow. Vermilion (mercuric sulfide) is red. Pigments occur in plant and animal bodies. The bright colors of plants, for example, are the result of the presence of such substances as chlorophyll (green) and xanthophyll (yellow), both of which are also found in some animals. Among others are carotene, the yellow of carrots and certain other vegetables, and anthocyanin, which imparts blue, red, and purple to flowers. Blood receives its color from the hemoglobin in the red corpuscles. Coloration of human skin is caused by the presence of pigments (see pigmentationpigmentation,
name for the coloring matter found in certain plant and animal cells and for the color produced thereby. Pigmentation occurs in nearly all living organisms.
..... Click the link for more information. ).
(in biology), a coloring substance that is incorporated into body tissues. Pigments are colored by intramolecular chromophores, atomic groups that selectively absorb light in the visible part of the solar spectrum. Pigments play an important and varied role in the life processes of organisms, especially photobiologic processes.
Occurrence. The most widespread pigments, porphyrins and carotenoids, are found in the majority of plants and animals. Porphyrins are included in the chlorophyll molecules of green plants, in bacteriochlorophylls of photosynthesizing bacteria, and in the respiratory pigments of animals, for example, hemoglobin, myoglobin, and chlorocruorin. Cytochromes, which, like hemoglobin, contain the iron-porphyrin complex heme, are very common. Carotenoids, which are unsaturated isoprenoid hydrocarbons, and their oxidized derivatives xanthophylls are yellow, orange, or red pigments and are present in green plants, algae, fungi, and bacteria. Blue-green and red algae contain auxiliary photosynthetic pigments, called phycobilins, whose nonprotein portion consists of a chain of four pyrrole rings; the blue phycobilin is phycocyanin, and the red—phycoerythrin. Structurally similar to the phycobilins is phytochrome, which is found in plants, and the animal-bile pigments, which are formed during the decomposition of hemoglobin. The large group of plant pigments called flavonoids encompasses substances that differ in chemical structure, color, and occurrence, for example, an-thocyanins and flavones. Flavonoids give color to flowers, fruits, and leaves.
The eyes of animals contain visual pigment, which is structurally complex. Other common pigments of plant and animal tissues are the respiratory chromogens, which are derivatives of quinone, and the melanins, which are found in the skin, fur, and hairs of animals. Fungal and bacterial pigments display a wide variety of chemical structures. Pigments that are identical or close in chemical structure may be found in groups of organisms that are phylogenetically distant from each other.
Pigments are usually contained in various cell structures and, less commonly, dissolved in body fluids. For example, chlorophyll is concentrated in chloroplasts, carotenoids in chromo-plasts and chloroplasts, hemoglobin in red blood cells, and flavonoids in the cell sap of plants. Pigments in combination with proteins and lipids are a structural part of biological membranes. Many animal and plant species have specialized pigment cells, or chromatophores.
Biological role. The pigment system links metabolic processes to light from the environment. One of the most important functions of pigments in plants is participation in photosynthesis. The absorption of light by pigments also plays a role in the growth, development, and movement of plants. The most important function of pigments in animals is participation in visual processes. Hemoglobin and other pigments in the blood transport oxygen from the respiratory organs to the tissues. Cytochromes, respiratory chromogens, and certain other pigments act as enzymes in tissue respiration. Carotenoids and flavonoids in plants and melanins in animals protect the organism against the harmful effects of the sun’s ultraviolet light. Pigments account for coloration, an important feature of an organism’s adaptation to the environment. Coloration in plants serves to attract insect pollinators and birds who spread seeds; in animals it helps to protect against enemies and acts as camouflage while the animal stalks prey.
Until the second half of the 19th century, plant and animal pigments, such as alizarin, indigo, and carmine, were widely used as dyes. Some pigments are used in the food-processing industry and medicine, for example, riboflavin, carotene, and antibiotic pigments.
A. A. KRASNOVSKII
In man. Disruption of any stage of pigment metabolism in man results in the accumulation of various metabolic products and in the development of certain diseases. A distinction is made between hereditary and acquired disturbances; the former are caused by hereditary defects in pigment synthesis and in the synthesis of the chemical precursors of pigments in the liver— red blood cells. Acquired disturbances in pigment metabolism can follow prolonged feverish periods, deficiencies of the vitamins folic and pantothenic acids, and liver diseases, including hepatitis, hepatic tumors, and obstructions of the biliary tract. They can also result from poisoning, Addison’s disease, or some blood diseases. Diseases of pigment metabolism occur at all ages with varying frequencies. The hereditary forms are generally seen in children. Three main groups of disturbances of pigment metabolism are recognized: hemoglobinopathies, hyperbilirubinemias, and porphyrias.
IU. A. KNIAZEV
REFERENCESTsvet, M. S. Khromofilly v rastitel’nom i zhivotnom mire. Warsaw, 1910.
Timiriazev, K. A. Solntse, zhizn’ i khlorofill (vol. 1 of Izbr. soch.). Moscow, 1948.
Prosser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. Chapters 8 and 19. (Translated from English.)
Biokhimiia rastenii. Moscow, 1968. Chapters 24, 26, and 28. (Translated from English.)
Konev, S. V., and I. D. Volotovskii. Vvedenie v molekuliarnuiu fotobiologiiu. Minsk, 1971.
Lemberg, R., and J. W. Legge. Hematin Compounds and Bile Pigments. New York-London, 1949.
Chemistry and Biochemistry of Plant Pigments. London-New York, 1965.
Photobiology of Microorganisms. London, 1970.
A finely divided material which contributes to optical and other properties of paint, finishes, and coatings. Pigments are insoluble in the coating material, whereas dyes dissolve in and color the coating. Pigments are mechanically mixed with the coating and are deposited when the coating dries. Their physical properties generally are not changed by incorporation in and deposition from the vehicle. Pigments may be classified according to composition (inorganic or organic) or by source (natural or synthetic). However, the most useful classification is by color (white, transparent, or colored) and by function. Special pigments include anticorrosive, metallic, and luminous pigments.