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Carotenoids are organic pigments naturally occurring in plants and some other photosynthetic organisms like algae, some types of fungus and some bacteria.

Carotenoids are characterized by a large (35-40 carbon atoms) polyene chain, sometimes terminated by rings. Carotenoids where some of the double bonds have been oxidized are know as xantophylls, the un-oxidized carotenoids are known as carotenes

Their colour, ranging from pale yellow, through bright orange, to deep red, is directly linked to their structure: The double carbon-carbon bonds interact with each other in a process called conjugation. As the number of double bonds increases, the wavelength of the absorbed light increases, giving the compound an increasingly red appearance.

In photosynthetic organisms, carotenoids play a vital role in the photosynthetic reaction centre. They either participate in the energy-transfer process, or protect the reaction center from auto-oxidation. In non-photosynthetic organisms, carotenoids have been linked to oxidation-preventing mechanisms.

Animals are incapable of synthesizing carotenoids, and must obtain them through their diet.

Probably the most well-known carotenoid is the one that gives this group its name, carotene, found in carrots and responsible for their bright orange colour. The pink colour of flamingos and salmon, and the red colouring of lobsters are also caused by carotenoids.

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