Color is quantum physics
Molecules have color when their conjugated π electron systems have a HOMO-LUMO energy gap corresponding to visible light photons (400–700 nm). The longer the conjugated system, the smaller the gap, and the longer the wavelength absorbed. Carotenoids are long conjugated systems. β-Carotene has 11 conjugated double bonds, absorbs blue-violet light (~450 nm), and reflects the complement: orange. Lycopene absorbs further toward red → red.
β-Carotene: provitamin A and antioxidant
β-Carotene (C₄₀H₅₆) is symmetric: two identical β-ionone rings connected by 11 conjugated double bonds. The intestinal enzyme β-carotene-15,15'-dioxygenase cleaves it symmetrically into two retinal (vitamin A aldehyde) molecules. The conjugated system also makes it an excellent singlet oxygen quencher — absorbing the energy of ¹O₂ and dissipating it as heat without degradation.
Lycopene: the red of tomatoes
Lycopene (C₄₀H₅₆) — same formula as β-carotene but no terminal rings — has a fully linear chain maximizing conjugation. Absorbs further toward red → tomatoes are red. No provitamin A activity (requires β-ionone ring). Bioavailability from cooked tomato with oil is 3–4 times higher than raw tomato because cooking breaks cell matrix and promotes cis-isomerization (better micellar solubility).
Lutein and zeaxanthin: the carotenoids of the eye
The only carotenoids present in the human retina, accumulating in the macula lutea at concentrations >1,000× plasma levels via specific transporters. They filter high-energy blue light (400–470 nm) and capture radicals from phototransduction. The AREDS2 trial showed lutein/zeaxanthin supplementation reduces risk of advanced macular degeneration by 26% in high-risk individuals.
Why flamingos are pink
Astaxanthin (with ketone and hydroxyl groups on the terminal rings) is synthesized by microalgae and biomagnified through the food chain: microalgae → zooplankton → fish → salmon/flamingos. Flamingos are born white — they acquire their pink color exclusively from dietary astaxanthin. Farmed salmon without sufficient krill receives synthetic astaxanthin added to feed; otherwise its flesh would be pale gray.