In leaves, the carotenoids are usually masked by the chlorophylls. In the autumn, as the quantity of chlorophyll in the leaf declines, the carotenoids become visible and produce the yellows and reds of autumn foliage. The figure below shows the structure of beta-carotene , one of the most abundant carotenoids. Note again the system of alternating single and double bonds that in this molecule runs along the hydrocarbon chain that connects the two benzene rings. As in chlorophyll, the electrons of the double bonds actually migrate though the chain, and also make this molecule an efficient absorber of light.
Many animals use ingested beta-carotene as a precursor for the synthesis of vitamin A .