Carbon assimilation- Mechanisms & diversity

This presentation intends to offer a bird's eye view of Carbon assimilation in plants along with diversity.

1. PLANT METABOLISM
Carbon Assimilation : Biology & Chemistry along with
diversity
By
N. Sannigrahi, Associate Professor,
Department Of Botany,
Nistarini College, Purulia (W.B) INDIA

2. WHAT IS CARBON ASSIMILATION?
Lightdependentreactionsassurestheproductionof
assimilatorypowersandreducingpowersrequiredforthe
reductionofatmosphericcarbonintocarbohydratemolecules
andthepathofcarboninphotosynthesisincludesthose
reactionswhichincorporatecarbonintomorereducedormore
energeticcompoundsbyanumberofenzymemediated
biochemicalreactions.Therearethreemajorandoneminor
pathwaysbywhichatmosphericCO2canbeassimilatedin
photosynthesis.ThefirstistheCalvincycleorC3cyclesince
theearlyproductinthispathwayisaC3compound3-
Phosphoglycericacid(PGA).Itisalsoknownasreductive
pentosephosphatepathwayorphotosyntheticcarbonreduction
cycle(PCR).ThesecondiscalledC4cyclebecausetheearly
productsareC4acids-malateoraspartate,whileterminal
stepsincludethereactionsofCalvincycle.

3. CARBON ASSIMILATION
Inthethirdgroup,muchoftheCO2isfixedintheprocess
knownasCrassulaceanAcidMetabolism(CAM),a
specializedpatternofphotosynthesisinwhichCO2is
absorbedandstoredatnightasmalicacidandreleasedduring
thedaybydecarboxylationinsidethetissueinwhichitisfixed
bytheCalvincycle.Thispermitstheeconomicuseofthe
waterasanexerciseofthewaterconservationbecausethe
stomatacanremainclosedduringthedaytimewhenthereis
almostnoCO2assimilationdirectlyfromtheair.Intimately
connectedwithdependentontheC3cycle,thereisanother
minorcyclenamedasC2photo-respiratorycarbonoxidation
cycleortheC2cycle.Anotherpathwayisexhibitedbyaquatic
algawhichpossesmechanismsbyactivelyacquiringinorganic
carbon(c1)suchasCO2andHCO3-fromtheexternal
mediumandareabletousethisC1toelevatetheCO2
concentrationaroundtheactivesiteofRUBISCOunder
extremesCO2limitingcondition.

4. CARBON ASSIMILATION -DIVERSITY
TheseplantsmaybecalledC-1plantsandthepathway
adoptedbythemcanbeconsideredasC1pathway.Justlike
C4plantswhere4-CcompounddonatesCO2byde-
carboxylation,1-CcompoundlikeHCO3-intheseC-1plants
donatesCO2forphotosyntheticfixation.
Fromtheabovedifferentbiochemicalvariantsof
photosyntheticassimilationofCO2,itispossibletomaketwo
importantgeneralconclusions.
1.TheC3cycleistheonlyknownsequenceofreactions
capableofphotosyntheticconversionofCO2carbohydrate.
OneoftheimportantcharacteristicsoftheC3cycleisitsauto
catalyticabilityofgeneratingCO2acceptormoleculeRuBP,
whiletheothercyclesdonothavethisproperty.Infact,the
otherpathwaysrequireoperationofseveralenzymesin
additiontothoseCalvincycle,whereasthereactionsofCalvin
cyclearecommontoallplants.

5. CARBON ASSIMILATION -DIVERSITY
TheCalvincycleregeneratesitsownbiochemicalcomponents
thatarenecessarytomaintaintheoperationofthecycle.The
rateoftheoperationofthecyclecanbeenhancedby
increasingtheconcentrationoftheinteremediates.Iftheleaves
orisolatedchloroplastsarekeptindarkareilluminatedCO2
fixationstartsonlyafteralagperiod,calledinductionperiod
andtherateofphotosynthesisincreaseswithtime.This
increaseispartlyduetotheactivationofenzymesbylightand
partlyduetotheconcentrationofintermediates.
2.OtherpathwaysliketheC4andCAMprovideamechanism
inplantsforconcentratingCO2atthesiteofRuBP
carboxylationandthusimprovetheCO2-absorbingcapacity
oftheplants.TheC2cycleisaninevitableconsequencesof
theC3cyclereactionandbothcyclesareintegratedtogether.

6. CARBON ASSIMILATION
TheCalvincycle(C
3-cycle)orPCR-cyclecanbedivided
intothreestages:
(a)Car-boxylation,duringwhichatmosphericCO
2combines
with5-Cacceptormoleculeribulose1,5-bisphosphate(RuBP)
andconvertsitinto3-phosphoglycericacid(3-PGA);
(b)Reduction,whichconsumesATP+NADPH(produced
duringprimaryphotochemicalreaction)andconverts3-PGA
into3-phosphoglyceraldehyde(3PGAld)ortriosephosphate
(TRI-OSE-P);and
(c)FormationofhexosesugarandregenerationofRuBP
whichconsumesadditionalATP,sothatthecyclecontinues.
Theallthestepsofthecarbonassimilationartecatalyzedby
thedifferentenzymesalongwiththereducingpowersand
assimilatorypowersharvestedfromthelightdependent
biochemicalpathwaystakeplaceinthegranaregionsof
mesophylltissuesofthegreenplants.

7. CALVIN CYCLE

8. STEPS OF CARBON ASSIMILATION
CalvincyclewasoutlinedbyCalvinandhisCo-researchers
(Benson,etal.,1950,Bassham&Calvin,1957;Calvin&
Bassham,1962)Thiscyclehasbeenfoundinall
photosyntheticorganismssofarandthedetailsareasfollow.
Carboxylation:(i)TheCO
2isacceptedbyRibulose1,5-
bisphosphate(RuBP)alreadypresentinthecellsanda6-
carbonadditioncompoundisformedwhichisunstable.Itsoon
getshydrolyzedinto2moleculesof3-phosphoglycericacid
(3PGA).Boththesereactionstakeplaceinthepresenceof
ribulosebisphosphatecarboxylase(Rubisco).3-
Phosphoglycericacidisthefirststableproductofdarkreaction
ofphotosynthesis.
(b)Reduction:(ii)3-Phosphoglycericacidisreducedto3-
phosphoglyceraldehydebytheassimilatorypower(generated
inlightreaction)inthepresenceoftriosephosphate
dehydrogenase.

9. DARK REACTION
c)FormationofHexoseSugarandRegenerationofRuBP:
(iii)Someofthemoleculesof3-phosphoglyceraldehyde
isomerizesintodihydroxyaeetonephosphate,bothofwhich
thenuniteinthepresenceoftheenzymealdolasetoformfruc-
tose1,6-bisphophate.
iv)Fructose1,6-bisphosphateisconvertedintofructose6-
phosphateinthepresenceofphosphatase.
(v)Someofthefructose-6-phosphate(hexosesugar)istapped
offfromtheCalvincycleandisconvertedintoglucose,
sucrose,andstarch.Sucroseissynthesizedincytosolwhile
starchissynthesizedinchloroplast.
(vi)Someofthemoleculesof3-phosphoglyceraldehyde
producedinstep(ii)insteadofforminghexosesugars,are
divertedtoregenerateribulose1,5-bisphosphateinthesystem
asfollows:

10. CALVIN CYCLE

11. CALVIN CYCLE
vii)3-Phosphoglyceraldehydereactswithfructose-6-
phosphateinthepresenceofenzymetransketolasetoform
erythrose-4-phosphate(4-Catomssugar)andxylulose5-
phosphate(5-Catomssugar).
(viii)Erythrose-4-phosphatecombineswithdihydroxyaceotone
phosphateinthepresenceoftheenzymealdolasetoform
sedoheptulose1,7-bisphosphate(7-Catomssugar).
ix)Sedoheptulose1,7-bisphosphatelosesonephosphate
groupinthepresenceofphosphatasetoformsedoheptulose-7-
phosphate.
(x)Sedoheptulose-7 phosphatereactswith3-
phosphoglyceraldehydeinthepresenceoftransketolaseto
formxylulose-5-phosphateandribose-5-phosphate(both5-
carbonatomssugars).

12. CALVIN CYCLE
(xi)Xylulose-5-phosphateisconvertedintoanother5-C
atomssugarribulose-5-phosphateinthepresenceofthe
enzymephosphoketopentoseepimerase.
(xii)Ribose-5-phosphateisalsoconvertedintoribulose-5-
phosphate.Thereactioniscatalyzedbyphosphopentose
isomerase.
(xiii)Ribulose-5-phosphateisfinallyconvertedinto
ribulose1,5-bisphosphateinthepresenceof
phosphopentosekinaseandATP,thuscompletingtheCalvin
cycle.
Becausefirstvisibleproductofthiscycleis3-
phosphoglycericacidwhichisa3-Ccompound,Calvin
cycleisalsoknownasC
3-pathway.(Recentstudieswith
algalcells,leavesandisolatedchloroplastshaveshownthat
‘darkreactions’ofphotosynthesisarenotcompletely
independentoflight

13. NET REACTION OF CALVIN CYCLE

14. C4-PHOTOSYNTHETIC CYCLE
Amajornewpathwayforcarbonflowduringphotosynthesis
otherthanCalvincyclewasestablishedinsugarcane,maize,
sorghumandrelatedgrasses.InitialstudiesbyKaprilov(1960)
inRussiaonmaizeandbyKortschak,Hart&Burr(1965)in
Hawaiionsugarcanerevealedthatthemajorearlyproductsby
photosynthesizingleavesexposedtoRadioactivecarbon
sourcesandC4acidsmalateandaspartate.Hatch&slack
(1966)inAustraliaconfirmedtheearlierobservationand
proposedacyclicreactionmechanisminwhichaC3acidis
carboxylatedtoyieldaC4acidandsubsequentlydonatesone
carbonasCO2tothereductivephotosyntheticcycleorCalvin
cyclewherethesecondcarboxylationtakesplace.Thenew
pathwaywasoriginallynamedasC4dicarboxylicacid
pathway.ItisnowreferredasC4pathwayandtheplants
exercisedarecalledC4plants.

15. HATCH-SLACK CYCLE
Thus,C4plantscanbedefinedonthebasisofthefollowing
characters:
PrimaryinitialproductsofCO2fixationarethe4-carbon
dicarboxylicacidsOAA,malateandaspartate.Hence,the
namehasbeenderivedfrominitialcarbonfixation.
CO2fixationintoC4acidsoccursinthelightnotindarkness
likeCAMplants.
C₄carbonfixationortheHatch–Slackpathwayisoneofthree
knownphotosyntheticprocessesofcarbonfixationinplants.It
owesthenamestothediscoverybyMarshallDavidsonHatch
andCharlesRogerSlackthatsomeplants,whensuppliedwith
¹⁴CO2,incorporatethe¹⁴Clabelintofour-carbonmolecules
first.

16. HATCH-SLACK CYCLE
1.Hatch-SlackCycleoperatesinC
4plantsonly.
2.Hatch-SlackCyclehasafasterrateofCO
2fixation.
3.FixedCO
2isreleasedbackinbundlesheathcellswhereitis
finallyfixedandreducedbyCalvincycle.
4.TheprimaryacceptorofCO
2isPEP,a3-carboncompound.
5.ThefirststableproductisOAA,a4-carboncompound.
6.Hatch-SlackCyclecanoperateunderverylowCO
2
concentration.
7.FixationofonemoleculeofCO
2requires2ATPmolecules
inadditiontothatrequiredinC
3cycle.
8.OptimumtemperaturefortheoperationofC
4cycleis30to
45°C.

17. HATCH-SLACK CYCLE
9.Hatch-SlackCyclehasnosuchgain.
10.C4quantumyieldindependentofCo2concentrationand
temperature.
11.C4plantsaremostefficientandabundantinhot,dryand
highlighthabitats,
12.AllC4plantsarebasicallyC3asglucosesynthesisisdone
byCalvincycleavenue.
TheC3acidremainingafterC4acidde-carboxylationdiffuses
backintothemesophyllcellswhereitisconvertedtoPEPby
theenzymepyruvateorthophosphatedikinase,thus
regeneratingtheCO2acceptor.Thelaststepisthecritical
operationoftheprocess.

18. DIMORHISM OF CHLOROPLAST

19. GENERA HAVING BOTH C3 & C4
Serial
No.
Family Name Genus Name
1. Aizoaceae Mollugo
2. Amaranthaceae Aerva,Alternanthera
3. Boraginaceae Heliotropium
4. Chenopodiaceae Artplex, Brassica, Kochla, Suaeda
5. Asteraceae Flaveria,Pectis
6. Cyperaceae Cyperus, Scripus
7. Euphorbiaceae Chamaesyce, Euphorbia
8. Poaceae Alloteropis, Panicum
9. Nyctaginaceae Boerhaavia
10. Zygophyllaceae Kallstroemia, Zygophyllum

20. THREE VARIANTS OF C4 PHOTOSYNTHESIS
Seria
l No.
Principal
C4 acids to
the BSC
Decarboxylating
enzymes
Variant
name
Principal C3
acids
returned to
MC
Examples
1. Malate NADP dependent
malic enzyme
(chloroplast)
NADP-MEPyruvate Maize, Crab
grass,
Sorghum
2. AspartateNAD dependent
malic enzyme(
Mitichondrial)
NAD-ME Alanine Millet
(Panicum
miliacium)
3. AspertatePEPcase (Cytosol)PEP-CK Alanine/Pyr
uvate
Guinea
grass(
Panicum
maximum)

21. HATCH-SLACK CYCLE

22. REACTIONS OF C4 CYCLE
1.FixationofCO2bythecarboxylationof
phosphoenolpyruvateinmesophyllcells(MC)toformOAAor
MalicorAsparticacid,
2.TransportoftheC4acidsintoBundleSheathCell(BSC)
3.DecarboxylationoftheC4acidswithintheBSCandthe
generationofCO2whichisthenreducedtocarbohydratevia
Calvincycle,
4.TransportoftheC3acids–pyruvateoralaninethatis
formedbythedecarboxylationstepbacktothemesophyllcells
andtheregenerationoftheCO2acceptorofPEP.
Theundesiredexpenditureof2ATPsperCO2fixationisan
unavoidablelossofthismechanismsandtheusualNADHHis
usedforthereductionofCO@bytheusualprocessofCalvin
cycle.

23. HATCH-SLACK CYCLE

24. MECHANISM
However,incategoryofC
4plants,naturehasevolveda
mechanismtoavoidoccurrenceofphotorespiration,whichis
thoughttobeaharmfulprocess.
C
4pathwayrequiresthepresenceoftwotypesof
photosyntheticcells,i.e.,mesophyllcellsandbundlesheath
cells.Thebundlesheathcellsarearrangedinawreathlike
manner.ThiskindofarrangementofcellsiscalledKranz
anatomy(Kranz:wreath).InKranzanatomy,themesophyll
andbundlesheathcellsareconnectedbyPlasmodesmataor
cytoplasmbridges.
TheC
4plantscontaindimorphicchloroplasts.Thechloroplasts
inmesophyllcellsaregranal,whereasinbundlesheathcells
theyareagranal.

25. MECHANISM
Thegranalchloroplastscontainthyllakoidswhicharestacked
toformgrana,asformedinC
3plants.However,inagranal
chloroplastsofbundlesheathcellsgranaareabsentand
thylakoidsarepresentonlyasstromalamellae.
Thepresenceoftwotypesofcells(granalandagranal)allows
occurrenceoflightandcarbon(dark)reactionsseparatelyin
eachtype.
Here,releaseofO
2takesplaceinonetype,whilefixationof
CO
2catalyzedbyRubiscoenzymeoccursinanothertypeof
cells.
InC
4plants(maize,sugarcane,etc.),lightreactionsoccurin
mesophyllcells,whereasCO
2assimilationtakesplacein
bundlesheathcells.Sucharrangementofcellsdoesnotallow
O
2releasedinmesophyllcellstoenterinbundle-sheathcells.

26. OVERALL PATHWAY

27. UNIQUENESS
Hence,Rubiscoenzyme,whichispresentonlyinbundle-
sheathcells,doesnotcomeintocontactwithO
2,andthus,
oxygenationofRuBPiscompletelyavoided.
InC
4plants,aCO
2concentratingmechanismispresentwhich
helpsinreducingtheoccurrenceofphotorespiration(i.e.,
oxygenationofinitialacceptorRuBP).ThistypeofCO
2
concentratingmechanismiscalledC
4pathway.
Themembersofthefamilieshavingthisphotosynthetic
pathwaysare-
Acanthaceae,Aizoaceae,Amaranthaceae,Boraginaceae,
Cappridaceae,Caryophyllaceae,Asteraceae,Cyperaceae,
Euphorbiaceae,Poaceae,Nyctaginaceae,Polygonaceae,
Portulacaceae,Scrophulariaceae,Zygophylaceaeetc.

28. CRASSULACEAN ACID METABOLISM

29. CAM –WHAT IS & WHERE ?
Thistypeofmetabolism,referstoamechanismof
photosynthesis,thatis,differentfromC
3andC
4pathways.
Crassulaceanacidmetabolism(CAM)isfoundonlyin
succulentsandotherxerophytesorplantsthatgrowindry
conditions.Inthistypeofmetabolism,CO
2istakenupbythe
leavesongreenstemsthroughstomatawhichremainopen
duringnight.However,duringdaytime,stomatainsuchplants
remainclosedtoconservemoisture.TheCO
2takenupby
succulentplantsinnightisfixedinthesimilarwayasittakes
placeinC
4plantstoformmalicacid,whichisbeingstoredin
vacuole.Hence,malicacidformedduringnightisusedduring
daytimeasasourceofCO
2forphotosynthesistoproceed
throughC
3pathway.Crassulaceanmetabolismisakindof
adaptationfoundincertainsucculentplantssuchaspineapple
toproceedphotosynthesiswithoutmuchlossofwater,which
generallyoccursinplantswithC
3andC
4pathways.

30. CAM CYCLES

31. SIGNIFICANCE OF CAM
SignificanceofCAMCycle.CrassulaceanAcidMetabolism
orCAMcycle.Itisoneofthecarbonpathwaysidentifiedin
succulentplantsgrowinginsemi-aridorXericcondition.This
wasfirstobservedinCrassulaceaefamilyplantslike
Bryophyllum,Sedum,Kalanchoeandisthereasonbehind
thenameofthiscycle.
Itiswelladaptedtohot,dryenvironments,
UptakeofCO2atnightwhenCO2ismostlyreadilyavailable
invernalplants,
Tradeoffdesiccationorstarvation,
CO2acquisitionatnightprovidescompetitivedamage,
HighenergycostsandlowCO2assimilationratesresultinlow
productivity.

32. MECHANISM OF CAM
ItisinterestingtoknowthatintheplantspossessingCalvin
cycle,theenzymeRuBPcarboxylasecaninitiatethereversal
ofphotosyntheticreactions.Thisprocessoccurswhenthereis
lowCO
2,concentrationbuthighO
2,concentration.Atmid-
day,whentemperatureandCO
2contentarehigh,theaffinity
ofRuBPcarboxylaseincreasesforO
2butdecreasesforCO
2.
Thus,itconvertsRuBPto3-carboncompound(PGA)anda2-
carboncompound(phosphoglycolate).Thephosphoglycolate
isconvertedrapidlytoglycolateintheperoxisomes.
Glycolateisfurtherconvertedtoglycine,serine,CO
2andNH
3
withoutthegenerationofATPorNADPH.Thusnetresultis
oxidationoforganicfoodsynthesizedduringphotosynthesis.
Thisprocessiscalledphotorespirationorglycolatepathwayas
itoccursathighrateinthepresenceoflight.Asalready
mentionedthatphotorespirationisalosstothenetproductivity
ofgreenplantshavingCalvincycle.

33. MECHANISM OF CAM
ThegreenplantshavingCalvincycleareC
3plants.
Overcomingphoto-respiratorylossposesachallengetoplants
growinginthetropics.Photorespirationoccursduetofactthat
theactivesiteofenzymeRubisco(ribulosebisphosphate
carboxylaseoxygenase)issameforbothcarboxylationand
oxygenation.TheoxygenationofRuBP(ribulose
bisphosphate)inthepresenceofO
2isfirstreactionof
photorespirationthatleadstotheformationofonemoleculeof
phosphoglycolate,atwo-carboncompoundandonemolecule
ofPGA.WherePGAisusedinCalvincycle,and
phosphoglycolateisdephosphorylatedtoformglycolateinthe
chloroplast.

34. C2 CYCLE

35. MECHANISM
Fromchloroplast,glycolateisdiffusedtoperoxisomewhereit
isoxidisedtoinglyoxylate.Hereglyoxylateisusedtoform
aminoacid,glycine.Now,glycineentersmitochondriawhere
twoglycinemolecules(4carbons)giverisetoonemoleculeof
serine(3carbons)andonemoleculeofCO
2(onecarbon).
Now,sereneistakenupbyperoxisome,andthroughaseries
ofreactionsisbeingconvertedintoglycerate.
Thisglycerateleavestheperoxisomeandentersthe
chloroplast,whereitisphosphorylatedtoformPGA.Now
PGAmoleculeenterstheCalvincycletomakecarbohydrates,
butoneCO
2moleculereleasedinmitochondriaduring
photorespirationhastobere-fixed.Thismeans,75percentof
thecarbonlostbytheoxygenationofRuBPisrecoveredand
25percentislostasreleaseofonemoleculeofCO
2.
Photorespirationisalsoknownasphotosyntheticcarbon
oxidationcycle.

36. SIGNIFICANCE
Photorespirationisarespiratoryprocessinmanyhigherplants.
Thisisalsoknownastheoxidativephotosynthetic,or
C
2photosynthesisorcarboncycle.SometimesinC
3plants,
RuBisCObindstooxygenmoleculesandthereactiondeviates
fromtheregularmetabolicpathway.ThecombinationofRuBP
andoxygenmoleculesleadstotheformationofonemolecule
ofphosphoglycerateandphosphoglycolate.Thispathwayis
calledphotorespiration.Duringphotorespiration,nosugaror
ATPmoleculesaresynthesized,butjustCO
2isreleasedatthe
expenseofATPandthewholeprocessisfutile.
Itseemsprobablethatphotorespirationservestoprotectthe
photochemicalapparatusfromlightdamagebythedissipation
ofphotochemicalenergywhichconcomitantCO2assimilation
byconsuminglightgeneratedreductant.

37. COMPARSION AMONG C3, C4 & CAM
C3plants C4-Plants CAM-Plants
Plants operate Calvin cycle
only inall green cells
Plants operate C4 cycles in
MC in addition to C3 cycle
in BSC
Plants operate only C3
cycle in MC for carbon
assimilation.
Only CO2 acceptor is
RuBP
Two CO2 acceptors-PEP &
RuBP
Same as C4
The first stable product is
PGA (C3 acid).
The firststable product is
C4 compound
The initial fixation product
is C4 compound.
Kranz anatomy of
dimorphism of chloroplast
absent.
Dimorphism of
chloroplasts in the name of
Kranz anatomy is observed
NoKranz anatomy.
There is no concentrating
device, fixation and
assimilation of C takes
place through Calvin cycle
in the day. No
decarbxylation mechanism.
There is initial CO2
concentratingmechanisms
with the involvement of BC
& BSC
Night acidification
followed by light
decarboxylation is found
Photorespiration is
prominent.
Photorespiration can be
detected due to PEPcase
Photorespiration can not be
detected

38. C1-PHOTOSYNTHETIC CYCLE
AnotheroperationofaCO2concentratingmechanism(CCM)
hasbeendiscoveredrecentlyandisexhibitedbyaquaticalgae
wherethedissolvedinorganiccarbon(C1)istransportedinto
thecellacrosstheplasmamembraneintheformofeitherCO2
orHCO3(-)butstoredthereasHCO3(-)ions.Its
decarboxylationi.etheformationofCO2andH2Obythe
enzymecarbonicanhydrase(CA)leadstoenrichmentwith
CO2aroundtheactivesiteofrubiscoundertheextremeCO2
limitingcondition.ThisresultsinenhancedaffinityforCO2
andimprovedphotosyntheticefficiency.
TherearefourmajorcomponentsofCO2concentrating
mechanisms(CCM)inanorganismsshowingC1
photosynthesisasfollows:
1.Mechanismtotransportinorganiccarbon(C1)intocelland
chloroplastsituatedintheplasmamembrane,

39. C1-PHOTOSYNTHETIC CYCLE
2.AnenergysupplysystemtodriveCO2&HCO3(-)across
plasmamembranewhichislinkedwithplasma-membrane
boundETSandATPasedrivenprotonpump,
3.ACO2leakagecontroldevicetoreducetheeffluxofCO2
outofthecelltothesurroundingmedium.ExcessCO2maybe
maintainedasHCO3(-)poolinsidethecell.Forthisreason,a
microenvironmentdevelopedinalgaeintheformof
carboxysomesincyanobacteriaandpyrenoidsinchloroplast
ofgreenalgae.BothRubiscoandCA(carbonicdehydrase)
remaininthesestructures.Thisenzymedrivenpathwayis
associatedwiththecarbonassimilationmechanisms.
4.SubmergedAquaticMicrophytes(SAM)showmixed
pathways.TheSAMplantsexhibitalltheknowntypesof
CCM.

40. FACTORS AFFECTING CARBON REDUCTION
“Whenaprocessisconditionedastoitsrapiditybyanumber
ofseparatefactors,therateoftheprocessislimitedbythe
paceoftheslowestfactor.”-thisisknownasBlackman'slawof
limitingfactors.Inadditiontothese,thecardinalvaluesalso
playanimportantrolefortheregulationofphotosynthesis
process.Therearelotoffactorshavingdirectandindirect
impactuponthephotosynthesis.Theseareclassifiedas
InternalandExternalfactors.
INTERNAL:Chlorophyll,Protoplasmicfactor,Leafanatomy,
UltrastructureofChloroplast
EXTERNAL:Sunlight(Quality,Intensity,Duration),
Temperature,Water,Oxygen.

41. CARDINAL VALUES
TheoryofthreecardinalpointswasgivenbySachsin1860.
Accordingtothisconcept,thereisminimum,optimumand
maximumforeachfactor.Foreveryfactor,thereisaminimum
valuewhenthismechanismsofphotosynthesisstarts,an
optimumvalueshowinghighestrateandamaximumvalue,
abovethelimitthereactionsbecomefailstotakeplace.
LawofLimitingFactor:
Themostadvocatedtheorythatstatestheregulationof
biochemicalreactionsinpresenceofnumberofvariablesis
thelawoflimitingfactorsgivenbyBlackmanin1905.When
severalfactorsaffectanybiochemicalprocess,thenthislaw
comesintoeffect.Thisstatesthat:ifachemicalprocessis
affectedbymorethanonefactor,thenitsratewillbe
determinedbythefactorwhichisnearesttoitsminimalvalue.

42. FACTORS OF PHOTOSYNTHESIS
WhenCO
2,lightandotherfactorsarenotlimiting,therateof
photosynthesisincreaseswithariseintemperature,overa
rangefrom6°Ctoabout37°C.Abovethistemperature,there
isanabruptfallintherateandthetissuediesat43°C.High
temperaturescausetheinactivationofenzymesandtherefore
affecttheenzymaticallycontrolled‘dark’reactionsof
photosynthesis.
Theoptimumtemperatureforthemaximumfallsbetween20-
30°C.Above25-30°Cthemaximumrateisnotmaintainedas
thetimefactorbeginstooperateandtheoptimumtemperature
isreducedfrom37°Cto30°C.Givenotherfactorsarelimiting,
therateofphotosynthesisfollowsVantHoffsrulebetween
6°C-30°Cto35°Ci.e.,itdoubleswitheachincreaseof10°C.
ThereasonbeingthatallthereactionsoftheCalvincycleare
temperaturedependentandtherateofdiffusionofCO
2tothe
chloroplastsisacceleratedbyhightemperature.

43. CARBON-DI-OXIDE
Nearly0.032%byvolumeofcarbondioxideispresentinthe
atmosphereandatthislowlevelitactsasalimitingfactor.
Underlaboratoryconditionswhenlightandtemperatureare
notlimitingfactors,increaseinCO
2concentrationinthe
atmospherefrom0.03%to0.3-1%raisesrateof
photosynthesis.
WiththefurtherincreaseintheconcentrationofCO
2
progressivelytherateofcarbonassimilationincreasesslightly
andthenitbecomesindependentofCO
2concentration.
Thereafter,itremainsconstantoverawiderangeofCO
2
concentrations.Plantsvaryintheirabilitytoutilizehigh
concentrationsofCO
2.Intomatoes,highconcentrationof
CO
2,abovethephysiologicalrange,exertsharmfulinfluence
causingleafsenescence.Duringtheearlyperiodoftheearth,
theconcentrationofCO
2intheatmospherewasashighas
20%.

44. LIGHT
Thephotosyntheticallyactiveregionofthespectrumoflight
isatwavelengthsfrom400-700nm.Greenlight(550nm)
playsanimportantroleinphotosynthesis.Lightsupplies
energyfortheprocess.
Lightvariesinintensity,qualityandduration.Abriefaccount
onthesethreeaspectsisgivenasfollows:
WhenCO
2andtemperaturearenotlimitingandlight
intensitiesarelow,therateofphotosynthesisincreaseswithan
increaseinitsintensity.Atapointsaturationmaybereached,
whenfurtherincreaseinlightintensityfailstoinduceincrease
inphotosynthesis.Optimumorsaturationintensitiesmayvary
withdifferentplantspeciese.g.,C
4andC
3.C
3plantsbecome
saturatedatlevelsconsiderablylowerthanfullsunlightbutC
4
plantsareusuallynotsaturatedatfullsunlight.

45. OXYGEN
OxygenhasbeenshowntoinhibitphotosynthesisinC
3plants
whileC
4plantsshowlittleeffect.ItissuggestedthatC
4plants
havephotorespirationandhighO
2stimulatesit.Therateof
photosynthesisincreasesby30-50%whentheconcentrationof
oxygeninairisreducedfrom20%to0.5%andCO
2,lightand
temperaturearenotthelimitingfactors.
Oxygenisinhibitorytophotosynthesisbecauseitwouldfavor
amorerapidrespiratoryrateutilizingcommonintermediates,
thusreducingphotosynthesis.Secondly,oxygenmaycompete
withCO
2andhydrogenbecomesreducedinplaceofCO
2.
Thirdly,O
2destroystheexcited(triplet)stateofchlorophyll
andthusinhibitsphotosynthesis.
ItmaybestatedthatdirecteffectofO
2onphotosynthesis
remainstobeunderstood.
Thedifferentfactorsplayaverycrucialroleinthisregard.

46. WATER
Waterisanessentialrawmaterialincarbonassimilation.Less
than1%ofthewaterabsorbedbyaplantisusedin
photosynthesis.Thedecreaseinwatercontentsofthesoilfrom
fieldcapacitytothepermanentwiltingpointresultsinthe
decreasedphotosynthesis.
Theinhibitoryeffectisprimarilyattributedtoincreased
dehydrationofprotoplasmandalsostomatalclosure.The
removalofwaterfromtheprotoplasmalsoaffectsitscolloidal
state,impairsenzymaticefficiency,inhibitsvitalprocesseslike
respiration,photosynthesisetc.Dehydrationmayevendamage
themicromolecularstructureofthechloroplasts.
Itisalsoassumedthatprimaryfactorofdehydrationin
retardingphotosynthesisisduetostomatalclosurewhich
reducesCO
2absorption.Waterdeficiencymaycausedryingof
thecellwallsofmesophyllcells,reducingtheirpermeabilityto
CO
2.Waterdeficiencymayaccumulatesugarsandthus
increaserespirationanddecreasephotosynthesis.

47. CHLOROPHYLL & CHEMICALS
Therateofphotosynthesisintwovarietiesofbarleyhaving
normalgreenleavesandyellowleaveswasstudied.CO
2,light
andtemperaturewerenotlimitingfactors.Therateof
assimilationperunitareaofleafsurfaceinthetwovarieties
wasthesameeventhoughthegreen-leavedvarietycontained
tentimesmorechlorophyllthantheyellowone.Clearly,the
chlorophyllinthegreenleavesissurplus.Leaveshavinghigh
chlorophyllcontentdonotphotosynthesizerapidlysincethey
lacktheenzymesorco-enzymestousetheproductsofthe
lightreactionstoreduceavailableCO
2.
CompoundslikeHCN,H
2S,etc.whenpresenteveninsmall
quantities,depresstherateofphotosynthesisbyinhibiting
enzymes.Inadditionchloroform,etheretc.,alsostop
photosynthesisandtheeffectisreversibleatlow
concentrations.However,athighconcentrationsthecellsdie

48. CONCLUSION
Thus,photosynthesisistheuniquepropertyofplantsinvolving
theenergytransductionbythelightenergyconvertedinto
chemicalenergynecessaryforthedifferentvitalfunctionsof
theorganisms.Thestarchproducedbytheplantsconverted
intodifferentotherbiochemicalingredientsasreflectedbythe
yieldofthedifferenttypeoftheplants.Thebiologyand
chemistryofphotosynthesisanddiversityofthisunique
mechanismsaregreatconcernofthescientists.TheC3.C4,
C2andC1alongwithCAMarethedifferentdiversemodesof
carbonassimilationofplantscorrespondingtothedifferent
ecologicalconditionsasavenuesforadaptations.The
consumersdependuponautotrophyandthehumanbeings
drinkaglassofsunlightindirectlyasmeansofenergyand
transduction.Still,photosynthesisisthemagicofrealitytothe
plantbiologists,

49. THANKS FOR YOUR JOURNEY
Acknowledgement:
1.Googleforimages
2.Differentwebpagesforcontentandenrichment,
3.PlantPhysiology-Mukherji&Ghosh
AppliedPlantPhysiology-ArupKumarMitra
AtextbookofBotany-Hait,Bhattacharya&Ghosh
PlantPhysiology-Devlin
Disclaimer:Thispresentationhasbeenpreparedforonline
freestudymaterialsforacademicdomainwithoutany
financialinterest.