Carotenoids (/kəˈrɒtɪnɔɪd/), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi.[1] Carotenoids give the characteristic color to pumpkins, carrots, corn, tomatoes, canaries, flamingos, salmon, lobster, shrimp, and daffodils.[1] Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these organisms. The only land dwelling arthropods known to produce carotenoids are aphids, and spider mites, which acquired the ability and genes from fungi.,[2][3][4] It is also produced by endosymbiotic bacteria in whiteflies.[5] Carotenoids from the diet are stored in the fatty tissues of animals,[1] and exclusively carnivorous animals obtain the compounds from animal fat. In the human diet, absorption of carotenoids is improved when consumed with fat in a meal.[6] Cooking carotenoid-containing vegetables in oil increases carotenoid bioavailability.[1][6]
There are over 1,100 known carotenoids [7] which can be further categorized into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons and contain no oxygen).[1] All are derivatives of tetraterpenes, meaning that they are produced from 8 isoprene molecules and contain 40 carbon atoms. In general, carotenoids absorb wavelengths ranging from 400 to 550 nanometers (violet to green light). This causes the compounds to be deeply colored yellow, orange, or red. Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species,[1] but many plant colors, especially reds and purples, are due to polyphenols.
Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they provide photoprotection via non-photochemical quenching.[8] Carotenoids that contain unsubstituted beta-ionone rings (including beta-carotene, alpha-carotene, beta-cryptoxanthin, and gamma-carotene) have vitamin A activity (meaning that they can be converted to retinol). In the eye, lutein, meso-zeaxanthin, and zeaxanthin are present as macular pigments whose importance in visual function, as of 2016, remains under clinical research.[1][9]
Biosynthesis
Structure and function
Properties
Carotenoids belong to the category of tetraterpenoids (i.e., they contain 40 carbon atoms, being built from four terpene units each containing 10 carbon atoms). Structurally, carotenoids take the form of a polyene hydrocarbon chain which is sometimes terminated by rings, and may or may not have additional oxygen atoms attached.
- Carotenoids with molecules containing oxygen, such as lutein and zeaxanthin, are known as xanthophylls.
- The unoxygenated (oxygen free) carotenoids such as α-carotene, β-carotene, and lycopene, are known as carotenes. Carotenes typically contain only carbon and hydrogen (i.e., are hydrocarbons), and are in the subclass of unsaturated hydrocarbons.
Their color, ranging from pale yellow through bright orange to deep red, is directly linked to their structure. Xanthophylls are often yellow, hence their class name. The double carbon-carbon bonds interact with each other in a process called conjugation, which allows electrons in the molecule to move freely across these areas of the molecule. As the number of conjugated double bonds increases, electrons associated with conjugated systems have more room to move, and require less energy to change states. This causes the range of energies of light absorbed by the molecule to decrease. As more wavelengths of light are absorbed from the longer end of the visible spectrum, the compounds acquire an increasingly red appearance.
Carotenoids are usually lipophilic due to the presence of long unsaturated aliphatic chains as in some fatty acids. The physiological absorption of these fat-soluble vitamins in humans and other organisms depends directly on the presence of fats and bile salts.[1]
Foods
Beta-carotene, found in pumpkins, sweet potato, carrots and winter squash, is responsible for their orange-yellow colors.[1] Dried carrots have the highest amount of carotene of any food per 100-gram serving, measured in retinol activity equivalents (provitamin A equivalents).[17] Vietnamese gac fruit contains the highest known concentration of the carotenoid lycopene.[18] Although green, kale, spinach, collard greens, and turnip greens contain substantial amounts of beta-carotene.[1] The diet of flamingos is rich in carotenoids, imparting the orange-colored feathers of birds
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