Lightdependentphaseofphotosynthesisinvolvestheabsorptionoflightbythephotosystems, excitationandflowofelectronsthroughanelectrontransportchain,chemiosmoticsynthesisof ATP,andreductionofNADPtoNADPH.Theflowofexcitedelectronsthroughanelectron transport
chain during light reaction is of two different types Le., non-cyclic and
cyclic. In non-cyclicelectronflow,theexcitedelectronsafterleavinga particularphotosystemdonotcomeback;theseelectronsafterlosingtheirenergyare incorporatedintoanothermolecule.Ontheotherhand,incyclicelectronflow,theexcited electronsafterleavingaparticularphotosystemfinallycomebacktotheirphotosystem again.ThemostimportanteventinlightreactionistheproductionofATP. ThisproductionofATPduringlightreactioniscalledphotophosphorylationand themechanismiscalledchemiosmosis.Therearetwotypesofphotophosphorylation.
WhenjusttwophotonsstriketheantennacomplexofPS-II,thetwoelectronsbecomeexcited andbegin
tomove along theatomsofdifferentpigmentswithinphotosystem.Ultimately,theabsorbedenergyreachesthereactioncentreofPS-II(P680)andcausesitstwoelectronstobeexcited.
TheseexcitedelectronsarecapturedbytheprimaryelectronacceptorofPS-llandleavetwo"electronholesinthephotosystembehindmakingchlorophyllastrongoxidizingagent.
Photolysisofwater
The electron holes of photosystem must be filled so
that in the presence of water splittingenzyme
reactions can proceed. When water reacts with oxidized state of chlorophyll inphotosystem,itbreaksupinto2Hions,2eand%0.Sincethisbreakdownoccursinthepresence of
sunlight therefore, it is termed as photolysis of water. The electrons released
from water areusedtofillthe"electronholes"ofPS-II.
Thecytochromecomplexisnotonlyanelectroncarrierbutitalsoworksasprotonpump.Theelectronflowthroughthecytochromecomplexstimulatesittopumptheprotonsfromstromato the thylakoid inner space. In this way
the energy of flowing electrons is transformed into agradientofprotons(H)inthethylakoidinnerspace.Theprotongradientactivatesanenzymeinthylakoid membrane called ATP
synthase which not only moves the protons back into thestroma but also catalyzes a reaction in which ADP and Pi are
combined to form ATP(photophosphorylation).Thiswholemechanismwhichinvolvesflowofelectron,pumpingof protonsandgenerationofATPbythylakoidmembranesiscalledchemiosmosis.ThisATP, generatedbylightreactionswillprovidechemicalenergyforthesynthesisofsugarduringCalvincycle.Theenergizedelectronsafterlosingtheirenergy,movefromcytochromecomplex otheplastocyanin(PC)andfinallyincorporatedintothePS-I
TheprimaryelectronacceptorofphotosystemIpassesthephotoexcitedelectronstoasecondelectron transportchain: Theelectronsare accepted by ferredoxin(Fd).Itisanironcontainingprotein.AnenzymecalledNADPreductase(flavoproteinenzyme)transferstheelectronsfromFd
to NADP NADP combines with electrons and hydrogen ions to form NADPH (reduced).
TheNADPHwillprovidereducingpowerforthesynthesisDOAR ofsugarintheCalvincycle.
The path of electron transport through the two photosystems
during non-cyclicphotophosphorylationisknownasZ-Schemeduetoitsconceptualzigzagshape.
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