<P> A light - harvesting complex has a complex of subunit proteins that may be part of a larger supercomplex of a photosystem, the functional unit in photosynthesis . It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone . Light - harvesting complexes are found in a wide variety among the different photosynthetic species . The complexes consist of proteins and photosynthetic pigments and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment, toward the reaction center using Förster resonance energy transfer . </P> <P> Absorption of a photon by a molecule takes place leading to electronic excitation when the energy of the captured photon matches that of an electronic transition . The fate of such excitation can be a return to the ground state or another electronic state of the same molecule . When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another . This process is called resonance energy transfer, and the rate depends strongly on the distance between the energy donor and energy acceptor molecules . Light - harvesting complexes have their pigments specifically positioned to optimize these rates . </P> <P> Chlorophylls and carotenoids are important in light - harvesting complexes present in plants . Chlorophyll b is almost identical to chlorophyll a except it has a formyl group in place of a methyl group . This small difference makes chlorophyll b absorb light with wavelengths between 400 and 500 nm more efficiently . Carotenoids are long linear organic molecules that have alternating single and double bonds along their length . Such molecules are called polyenes . Two examples of carotenoids are lycopene and β - carotene . These molecules also absorb light most efficiently in the 400--500 nm range . Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in fruits and flowers . </P>

What is the advantage of having more than one pigment molecule in photosystem
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