Carotenoids are a group of photosynthetic pigments used by plants, in addition to chlorophyll, as light-harvesting chemicals in photosynthesis. There are two main types of carotenoids: the oxygen-containing xanthophylls and the hydrocarbon carotenes.
Carotenoids are usually red, orange, or yellow, and include the familiar compound carotene, which gives carrots their color. Carotenoids are located in chloroplasts and in plastids, e.g., in many flowers and carrot roots, and also in the photosynthetic lamellae of cyanobacteria and some bacteria. In leaf chloroplasts the carotenoid colors are masked by chlorophyll until this is lost in the fall. Carotenoids also occur in some fungi. They increase in concentration in many ripening fruits, such as the tomato. The carotene of food is changed to vitamin A in the vertebrate liver.
Carotenoids 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 the energy in sunlight 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.