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Any of a class of yellow, orange, red, and purple pigments that are widely distributed in nature. Carotenoids are generally fat-soluble unless they are complexed with proteins. In plants, carotenoids are usually located in quantity in the grana of chloroplasts in the form of carotenoprotein complexes. Carotenoprotein complexes give blue, green, purple, red, or other colors to crustaceans, echinoderms, nudibranch mollusks, and other invertebrate animals. Some coral coelenterates exhibit purple, pink, orange, or other colors due to carotenoids in the calcareous skeletal material. Cooked or denatured lobster, crab, and shrimp show the modified colors of their carotenoproteins.
The general structure of carotenoids is that of aliphatic and aliphatic-alicyclic polyenes, with a few aromatic-type polyenes. Most carotenoid pigments are tetraterpenes with a 40-carbon (C40) skeleton. More than 300 carotenoids of known structure are recognized, and the number is still on the rise.
There are several biochemical functions in which the role of carotenoids is well understood. These include carotenoids in the photosynthetic apparatus of green plants, algae, and photosynthetic bacteria, where carotenoids function as a blue light-harvesting pigment (antenna or accessory pigment) for photosynthesis. Thus carotenoids make it possible for photosynthetic organisms more fully to utilize the solar energy in the visible spectral region. See Chlorophyll, Photosynthesis
Another function of carotenoids is to protect biological systems such as the photosynthetic apparatus from photodynamic damage. This is done by quenching the powerful photodynamic oxidizing agent, singlet oxygen, produced as an undesirable by-product of the exposure of pigmented organisms to light.
Perhaps the most important industrial application of carotenoids is in safe coloration of foods, as exemplified in the coloring and fortification of margarine and poultry feedstuff.