The term carotene is used for several related substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are responsible for the orange colour of the carrot for which it is named, and many other fruits and vegetables (for example, sweet potatoes and orange cantaloupe melon). Carotenes are also responsible for the orange colours in dry foliage. They also (in lower concentrations) impart the yellow colouration to milk-fat and butter. Omnivorous animal species which are poor converters of coloured dietary carotenoids to colourless retinoids, have yellowed-coloured body fat as a result of the carotenoid retention. The typical yellow-coloured fat of humans is a result of fat storage of carotenes from their diets. Carotenes contribute to photosynthesis by transmitting the light energy they absorb from chlorophyll. They also protect plant tissues by helping to absorb the energy from singlet oxygen, an excited form of the oxygen molecule O2 which is formed during photosynthesis. Chemically, carotene is a terpene, synthesized biochemically from eight isoprene units. It comes in two primary forms designated by characters from the Greek alphabet: alpha-carotene (a-carotene) and beta-carotene (ÃŸ-carotene). Gamma, delta, epsilon, and zeta (?, d, e, and ?-carotene) also exist. Since they are hydrocarbons, and therefore contain no oxygen, carotenes are fat-soluble and insoluble in water (in contrast with other carotenoids, such as xanthophylls, which are slightly less chemically hydrophobic). ÃŸ-Carotene is composed of two retinyl groups, and is broken down in the mucosa of the small intestine by beta-carotene 15,15'-monooxygenase to retinal, a form of vitamin A. ÃŸ-Carotene can be stored in the liver and body fat and converted to retinal as needed, thus making it a form of vitamin A for humans and some other mammals. The carotenes a-carotene and ?-carotene, due to their single retinyl group (beta-ionone ring), also have some vitamin A activity (though less than ÃŸ-carotene), as does the xanthophyll carotenoid ÃŸ-cryptoxanthin. All other carotenoids, including lycopene, have no beta-ring and thus no vitamin A activity (although they may have antioxidant activity and thus biological activity in other ways). Animal species differ greatly in their ability to convert retinyl (beta-ionone) containing carotenoids to retinals. Carnivores in general are poor converters of dietary ionine-containg carotenoids, and pure carnivores such as cats and ferets lack beta-carotene 15,15'-monooxygenase and cannot convert any carotenoids to retinals at all (resulting in carotenes not being a form of vitamin A for these species).