Carotenoids are produced by all photosynthetic organisms and are primarily used as accessory pigments to chlorophyll in the light-harvesting part of photosynthesis.
They are highly unsaturated with conjugated double bonds, which enables carotenoids to absorb light of various wavelengths. At the same time, the terminal groups regulate the polarity and properties within lipid membranes.
Most carotenoids are tetraterpenoids, regular  isoprenoids. Several modifications to these structures exist: including cyclization, varying degrees of saturation or unsaturation, and other functional groups.  Carotenes typically contain only carbon and hydrogen, i.e., they are hydrocarbons. Prominent members include α-carotene, β-carotene, and lycopene, are known as carotenes. Carotenoids containing oxygen include lutein and zeaxanthin. They are known as xanthophylls.  Their color, ranging from pale yellow through bright orange to deep red, is directly related to their structure, especially the length of the conjugation. Xanthophylls are often yellow, giving their class name.
Carotenoids also participate in different types of cell signaling. They are able to signal the production of abscisic acid, which regulates plant growth, seed dormancy, embryo maturation and germination, cell division and elongation, floral growth, and stress responses.