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===Photophysics===
The length of the multiple [[Conjugated system|conjugated double bonds]] determines their color and photophysics.<ref name=":2">{{Cite journal|last=Vershinin|first=Alexander|date=1999-01-01|title=Biological functions of carotenoids - diversity and evolution|journal=BioFactors|language=en|volume=10|issue=2–3|pages=99–104|doi=10.1002/biof.5520100203|pmid=10609869|s2cid=24408277|issn=1872-8081}}</ref><ref>{{cite journal |doi=10.1021/cr020674n |title=Ultrafast Dynamics of Carotenoid Excited States−From Solution to Natural and Artificial Systems |year=2004 |last1=Polívka |first1=Tomáš |last2=Sundström |first2=Villy |journal=Chemical Reviews |volume=104 |issue=4 |pages=2021–2072 |pmid=15080720 }}</ref> After absorbing a photon, the carotenoid transfers its excited electron to [[chlorophyll]] for use in photosynthesis.<ref name=":2" /> Upon absorption of light, carotenoids transfer excitation energy to and from [[chlorophyll]]. The singlet-singlet energy transfer is a lower energy state transfer and is used during photosynthesis.<ref name="Cogdell-1978"/> The triplet-triplet transfer is a higher energy state and is essential in photoprotection.<ref name="Cogdell-1978"/> Light produces damaging species during photosynthesis, with the most damaging being [[reactive oxygen species]] (ROS).<ref>{{Cite journal |last=Aizpuru |first=Aitor |last2=González-Sánchez |first2=Armando |date=2024-07-20 |title=Traditional and new trend strategies to enhance pigment contents in microalgae |url=https://link.springer.com/article/10.1007/s11274-024-04070-3 |journal=World Journal of Microbiology and Biotechnology |language=en |volume=40 |issue=9 |pages=272 |doi=10.1007/s11274-024-04070-3 |issn=1573-0972 |pmc=PMC11271434 |pmid=39030303}}</ref> As these high energy ROS are produced in the chlorophyll the energy is transferred to the carotenoid’s polyene tail and undergoes a series of reactions in which electrons are moved between the carotenoid bonds in order to find the most balanced (lowest energy) state for the carotenoid.<ref name=":2" />
Carotenoids defend plants against [[singlet oxygen]], by both energy transfer and by chemical reactions. They also protect plants by quenching triplet chlorophyll.<ref>{{cite journal |doi=10.1104/pp.111.182394 |title=Chemical Quenching of Singlet Oxygen by Carotenoids in Plants |year=2012 |last1=Ramel |first1=Fanny |last2=Birtic |first2=Simona |last3=Cuiné |first3=Stéphan |last4=Triantaphylidès |first4=Christian |last5=Ravanat |first5=Jean-Luc |last6=Havaux |first6=Michel |journal=Plant Physiology |volume=158 |issue=3 |pages=1267–1278 |pmid=22234998 |pmc=3291260 }}</ref> By protecting lipids from free-radical damage, which generate charged [[Lipid peroxidation|lipid peroxides]] and other oxidised derivatives, carotenoids support crystalline architecture and hydrophobicity of lipoproteins and cellular lipid structures, hence oxygen solubility and its diffusion therein.<ref>{{Cite book|url=https://www.worldcat.org/oclc/148650411|title=Carotenoids: physical, chemical, and biological functions and properties|date=2010|publisher=CRC Press|author=John Thomas Landrum|isbn=978-1-4200-5230-5|location=Boca Raton|oclc=148650411}}</ref>
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