Carotenoids are yellow, orange, or red pigments synthesized by many plants, fungi, and bacteria. In plants, carotenoids can occur in roots, stems, leaves, flowers, and fruits. Within a plant cell, carotenoids are found in the membranes of plastids, organelles surrounded by characteristic double membranes. Chloroplasts are the most important type of plastid and they synthesize and store carotenoids as well as perform photosynthesis. Two of the best known carotenoids are Beta-carotene and lycopene. Beta-carotene gives carrots, sweet potatoes, and other vegetables their orange color. Lycopene gives tomatoes their red color. When a human eats carrots or other foods containing carotenoids, the liver splits the carotenoid molecule in half to create two molecules of vitamin-A, an essential micro-nutrient.
Chemists have identified about 500 different, naturally occurring carotenoids. Each consists of a long hydrocarbon chain with a 6-carbon ionone ring at each end. All carotenoids consist of 40 carbon atoms and are synthesized from eight 5-carbon isoprene subunits connected head-to-tail. There are two general classes of carotenoids: carotenes and xanthophylls. Carotenes consist only of carbon and hydrogen atoms; beta-carotene is the most common carotene. Xanthophylls have one or more oxygen atoms; lutein is one of the most common xanthophylls.
Carotenoids have two important functions in plants. First, they can contribute to photosynthesis. They do this by transferring some of the light energy they absorb to chlorophylls, which then use this energy to drive photosynthesis. Second, they can protect plants which are over-exposed to sunlight. They do this by harmlessly dissipating excess light energy which they absorb as heat. In the absence of carotenoids, this excess light energy could destroy proteins, membranes, and other molecules. Some plant physiologists believe that carotenoids may have an additional function as regulators of certain developmental responses in plants.