BISC 101 Lecture 23: Photosynthesis

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BISC101–Lecture23–Photosynthesis:PartII
LightProperties
● ThelightreactionsconvertsolarenergytochemicalenergyofATPandNADPH
● ThylakoidsconvertlightenergyintothechemicalenergyofATPandNADPH
● Light:Aformofelectromagneticenergy
o Likeotherformsofelectromagneticenergy,lighttravelsinrhythmicwaves
● Wavelength:Thedistancebetweencrestsofelectromagneticwaves
o Wavelengthsofelectromagneticradiationrangefromlessthanananometer
(gammarays)tomorethanakilometer(radiowaves)
● Themostimportantsegmentforlifeisanarrowbandbetween380to750nm
o Thebandofvisiblelight
PhotosyntheticPigments:TheLightReceptors
● Pigments:Substancesthatabsorbvisiblelight
o Differentpigmentsabsorbdifferentwavelengths
o Wavelengthsthatarenotabsorbedarereflectedortransmitted
▪ Ex:Leavesappeargreenbecausechlorophyllreflectsandtransmits
greenlight
● Spectrophotometer:Measuresapigment’sabilitytoabsorbvariouswavelengths
o Thismachinesendslightthroughpigmentsandmeasuresthefractionoflight
transmittedateachwavelength
1. Whitelightisseparatedintocolors(wavelengths)byaprism
2. Onebyone,thedifferentcolorsoflightarepassedthroughthesample
3. Thetransmittedlightstrikesaphotoelectrictube,whichconvertslightenergyto
electricity
4. Electriccurrentismeasuredbyagalvanometer
o Themeterindicatesfractionoflighttransmittedthroughthesample,from
whichwecandeterminetheamountoflightabsorbed
● Absorption
Spectrum:Agraphplottingapigment’slightabsorptionversus
wavelength
● Action
Spectrum:
Plotstherateofphotosynthesisversuswavelength
o Theresultingactionspectrumresemblestheabsorptionspectrumfor
chlorophylla
o Theresultdoesnotmatchexactlyduetotheabsorptionoflightbyaccessory
pigmentslikechlorophyllb
andcarotenoids
● Importantfunctionofcarotenoidsisphotoprotection
● Photoprotection:
o Carotenoidsabsorbanddissipateexcessivelightenergythatwoulddamage
chlorophyllorinteractwithoxygen
o Interactionswithoxygenwillformreactiveoxidativemoleculesthatare
dangeroustothecell
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ExcitationofChlorophyllbyLight
● Whenapigmentabsorbslight,itgoesfromagroundstatetoanexcitedstate
o Thisisunstable
● Whenexcitedelectronsfallbacktothegroundstate,lightisgivenoffintheformof
anafterglow
o Fluorescence:Afterglowoflightgivenoffbyelectronsfallingfromexcitement
toground
● Ifilluminated,anisolatedsolutionofchlorophyllwillfluoresce,givingofflightand
heat
Photosystems
● Photosystems:Reactioncentercomplexesassociatedwithlightharvesting
complexes
● Photostems:Complexesonthethylakoidmembrane
o Chlorophyllisorganizedwithotherorganicmoleculesandproteinsinto
photostems
● Eachphotosystemcomprisesareactioncentercomplexsurroundedbyseveral
lightharvestingcomplexes
● LightHarvestingComplexes:Pigmentmoleculesboundtoproteins
o Ex:Chlorophylla
,b
,andcarotenoids
● ReactionCenterComplex:Associationofproteinsholdinga“specialpair”of
chlorophylla
moleculesandaprimaryelectronacceptor
● Whenaphotonstrikesapigmentmoleculeinthelightharvestingcomplex,the
energyisthenpassedfrompigmentmoleculetopigmentmoleculeuntilitreaches
thespecialpairof“chlorophylla
”moleculesinthereactioncentercomplex
o Theelectronsfromthese“chlorophylla
”moleculesbecomeexcitedand
eventuallyistransferredtotheprimaryelectronacceptorinthe
reactioncentercomplex
● The2chlorophylla
moleculesare“special
”becausetheyareabletousethe
energyfromlighttoboostoneoftheirelectronstoahigherenergylevelandto
transfertheexcitedelectrontotheprimaryelectronacceptor
o Thisisthefirststepinlightreaction
● Therearetwotypesofphotosystemsinthethylakoidmembrane:
o PhotosystemII(PSII)functionsfirst(thenumbersreflectorderofdiscovery)
andisbestatabsorbingawavelengthof680nm
▪ Thereactioncenterchlorophylla
ofPSIIiscalledp680
o PhotosystemI(PSI)isbestatabsorbingawavelengthof700nm
▪ Thereactioncenterchlorophylla
ofPSIiscalledp700
ElectronFlow
● Duringthelightreactions,therearetwopossibleroutesforelectronflow
1. CyclicRoute
2. LinearRoute
o Linearelectronflow,theprimarypathway,involvesbothphotosystemswhile
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