Carotenoids are a group of organic compounds found in both plants and animals. Plants and algae are main producers of carotenoids and use them in key roles of absorption of light energy for use in photosynthesis and protection of their tissues from photo damage. Animals can store carotenoids obtained from diet in their fatty tissues. The reason behind the well-known fact “carrot is good for eyes” is one of carotenoids named β-carotene which can have vitamin A activity (it can be converted to vitamin A) where vitamin A is needed for eyesight (and also immune function, skin and cellular health, growth and development).
There are over 1100 known carotenoids and all are derivatives of tetraterpenes where terpenes are hydrocarbons of biological origin having carbon skeletons formally derived from isoprene [CH2=C(CH3)CH=CH2] and tetraterpenes are terpenes with 40 carbon atoms. Carotenoids are formally derived from the acyclic parent, lycopene (also called Ψ, Ψ –carotene) by hydrogenation, dehydrogenation, cyclization, oxidation or combination of these processes. Carotenoids include two main groups of carotenes (which are unsaturated hydrocarbons and contain no oxygen) and xanthophylls (which contain oxygen) beside certain compounds that arise from rearrangement of the skeleton of lycopene or by loss of part of this structure.
Carotenoids generally absorb wavelengths in the range from 400 nm to 550 nm (from violet to green light) causing them to be yellow, orange or red. Class names of carotene and xanthophyll reflect this issue, the term carotene is from the Latin carota “carrot” because carotenes are responsible for the orange color of the carrot and the term xanthophyll is from Greek xanthos “yellow” (and phyllon “leaf”) because xanthophylls are often yellow (and they formed the yellow band in early chromatography of leaf pigments). Carotenoids give the characteristic color to carrots, corn, canaries, daffodils, egg yolks, rutabagas, buttercups, bananas, saffron and many others. They also (in lower concentrations) impart the yellow coloration to milk-fat and butter. The typical yellow-coloured fat of humans and chickens is a result of fat storage of carotenes from their diets. The orange and yellow colors of leaves in autumn are also because of carotenoids.
Famous examples of carotenoids include: β-Carotene, a deeply colored red-orange pigment abundant in plants and fruits; Zeaxanthin, one of the most common carotenoid alcohols found in nature, the pigment that gives paprika (made from bell peppers), corn, saffron, wolfberries, and many other plants and microbes their characteristic color; Lycopene, a bright red carotene and carotenoid pigment, found in fruits and vegetables like tomatoes, red carrots, watermelons, gac, and papayas.