Carotenoids

Carotenoids are yellow-to-orange pigments, chemically classed as tetraterpenes (almost all built from 40 carbon atoms), and they absorb light strongly in the blue-to-violet part of the visible spectrum - a region chlorophyll absorbs less efficiently. Because they sit alongside chlorophyll in the thylakoid membrane and physically intercept excess light energy and reactive byproducts before they can damage chlorophyll, carotenoids are often called shield or protective pigments; at the same time, like every other accessory pigment, they also absorb light and pass the captured energy on to chlorophyll, widening the usable light spectrum. Carotenoids are responsible for several familiar colour changes in plants - the colour shift as fruit ripens, the colours of many flowers, and the yellow/orange colours leaves take on as they senesce in autumn. Carotenoids split into two sub-groups. Carotenes are orange, red, yellow or brownish hydrocarbon (purely lipid, no oxygen) pigments, mostly with the formula C40H56; beta-carotene is the most abundant carotene in plants and is notable as the biological precursor of vitamin A, while lycopene is the specific red carotene pigment responsible for the colour of tomato, red pepper and rose-hip fruits. Xanthophylls are also tetraterpenes but, unlike carotenes, contain oxygen (formu …