Biomedical Engineering Reference
In-Depth Information
FIGURE 22.6
(See.color.insert.).Structure.of.antenna.protein.system.LH1-RC.complex.surrounded.by.LH2.
electron-accepting.role.is.played.by.one.of.the.two.quinines.that.are.present.in.one.of.the.
subunits.
22.3.2 Photosynthetic Reaction Center
Like. their. antenna. counterparts,. photosynthetic. reaction. centers. (RCs). are. pigment-pro-
tein.complexes.that.are.another.subunit.present.in.photosynthetic.units.of.all.green.plants,.
as.well.as.in.many.bacteria.and.algae,.where.the.key.step.of.solar.energy.transformation.
takes.place..The.molecular.excitations.resulting.from.the.sun's.irradiation.are.followed.by.
a.number.of.electron.transfer.reactions.inalized.in.a.stable.charge-separated.radical.pair..
Conceptually,.the.reaction.centers.can.be.seen.as.the.point.of.transition.where.the.photon-
generated. charge. carriers. are. utilized. to. spur. chemical. reactions. required. for. biological.
metabolism.
RC. proteins. bind. a. number. of. photoactive. pigments,. bacteriochlorophyll. (BChl). mol-
ecules.in.the.anoxygenic.bacteria.and.chlorophylls.(Chls).in.oxygen-evolving.organisms,.
and.pheophytins.(chlorophyll-like.molecules.without.the.Mg.cation),.which.are.responsible.
for.light.absorption.and.electron.excitation.catalyzing.transfer.of.the.electron.within.the.
chromophore.molecules. 7 .Reaction.centers.of.bacteria.are.better.understood.for.their.less.
complicated.structure.(e.g.,.purple.bacteria)..The.more.sophisticated.RCs.of.green.plants,.
photosystems.I.and.II.(PS.I.and.PS.II),.with.a.very.complex.arrangement.of.multisubunit.
protein,.are.still.under.study..Giardi.and.Pace 8 .described.two.main.groups.of.the.photo-
synthetic.reaction.centers:.(1).PS.II,.containing.pheophytin.as.the.primary.electron.accep-
tor. and. quinone. as. the. secondary. acceptor,. and. (2). PS. I,. with. sulfur. clusters. serving. as.
early.acceptors..Both.RC.types.occur.in.eukarya.and.bacteria. 11
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