Difference Between C3 and C4 Cycle

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5/2/2017 DifferenceBetweenC3andC4Cycle|Characteristics,Process,Function
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DifferenceBetweenC3andC4Cycle
April17,2017 •byLakna •6minread
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MainDifference–C3vsC4Cycle
C3andC4cyclearetwotypesofcyclicreactionsthatoccuras
thedark reaction ofthe photosynthesis. Photosynthesis is the
production of simple organic molecules, glucose from
inorganicmolecules,carbondioxideandwater,usingsunlight
astheenergysource.Duringphotosynthesis,thelightreaction
isfollowedbythedarkreaction.C3cycleisalsocalledCalvin
cycle,whereas theC4cycleiscalledHatchSlack cycle.The
main difference between C3 and C4 cycle is the first stable
compound produced by these reactions; the first stable
compound produced in the C3 cycle is a three carbon
compoundcalled3phosphoglycericacid(PGA)whereasthefirststablecompoundproducedintheC4cycleisa
fourcarboncompoundcalledoxaloaceticacid(OAA).
Thisarticleexplores,
1.WhatisC3Cycle
–Characteristics,Process,Function
2.WhatisC4Cycle
–Characteristics,Process,Function
3.WhatisthedifferencebetweenC3andC4Cycle
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WhatisC3Cycle
C3cycle is oneof the tworeaction pathways thatcan occur in the dark reaction of photosynthesis. It occurs inall
plants. Three steps are observed in C3 cycle. During the first step, carbon dioxide is fixed into ribulose 1,5
bisphosphate,forming an unstablesix carbon compound,which is thenhydrolyzed into threecarbon compound, 3
phosphoglycerate.Carbondioxidefixationiscatalyzedbytheenzyme,rubisco,whichisfoundinthestromalsurface
ofthethylakoidmembraneinthechloroplast.ThefixationofcarbondioxideistheratelimitingstepoftheC3cycle.
Duetothecatalyticimperfectionoftheenzyme,rubisco,theenzymereactswithmolecularoxygenbyaprocesscalled
photorespiration.Twomoleculesof3phosphoglycerateare formed by the first step of C3cycleperasinglecarbon
dioxide fixation. During the second step, one molecule of 3phosphoglycerate is reduced, forming three types of
hexose phosphates: fructose 6phosphate, glucose 6phosphate and glucose 1phosphate. Remaining 3
phosphoglycerateisrecycled,formingribulose1,5bisphosphate.TheC3cycleisshowninfigure1.
Figure1:C3Cycle
WhatisC4Cycle
C4cycleistheotherreactionpathwayoccurringinthedarkreactionofphotosynthesis.Theplants,growinginhotand
dryenvironmentslikesugarcane,corn,andcrabgrass,utilizeC4pathwayduringphotosynthesis.Thegasexchanging
stomataporesarekeptclose most of the day in these plants in order to reduce the excessive loss of moisture in dry
andhotconditions.Thereby,thecarbondioxideconcentrationinsideplantleavesisalsoreducedbytheprogressionof

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C3 cycle. When carbon dioxide concentration is low,
photorespiration is enhanced, reducing the efficiency of
photosynthesis. In order to increase the efficiency of
photosynthesis during dry and hot conditions, those C4
plantsconducttheC4cycle.
TwotypesofcellsareinvolvedintheC4Cycle:mesophyll
cellsandbundlesheath cells. Vasculartissue of the leafis
surroundedbybundlesheathcells.Thestructureoftheleaf
ofC4 plants is described by Kranz anatomy.Phosphoenol
pyruvatereactswithcarbondioxideinthemesophyllcells,
forming oxaloacetate, which is a four carbon compound.
The reaction is catalyzed by phosphoenol pyruvate
carboxylaseenzyme,whichisinsensitivetooxygen.Oxaloacetateisthenreducedtomalate,whichistransferredinto
bundlesheathcells.Inthebundlesheathcells,malateisdecarboxylatedbyremovingthecarbondioxide,enteringinto
theC3cycle.TheC4cycleisshowninfigure2.
Figure2:C4Cycle

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DifferenceBetweenC3andC4Cycle
FirstStableCompound
C3Cycle:ThefirststablecompoundproducedintheC3cycleisa threecarboncompoundcalled3phosphoglyceric
acid.
C4Cycle:ThefirststablecompoundproducedintheC4cycleisafourcarboncompoundcalledoxaloaceticacid.
FirstObservation
C3Cycle:C3cyclewasfirstobservedbyMelvinCalvin.
C4Cycle:C4cyclewasfirstobservedbyHatchandSlack.
AlternativeNames
C3Cycle:C3cycleiscalledCalvincycle.
C4Cycle:C4cycleiscalledHatchSlackcycle.
Presence
C3Cycle:C3cycleisfoundinallplants.
C4Cycle:C4cycleisfoundonlyinC4plantslikesorghumandmaize.
PrimaryCarbonDioxideAcceptor
C3Cycle:Theprimarycarbondioxideacceptorisafivecarboncompound,Ribulosebiphosphate(RUBP).
C4Cycle:Theprimarycarbondioxideacceptorisathreecarboncompound,phosphoenolpyruvicacid(PEP).
CarboxylaseEnzyme
C3Cycle:ThecarboxylaseenzymeisRubiscoinC3plants.
C4Cycle:ThecarboxylaseenzymesarePEPcarboxylaseandRubisco.
CarbonFixation
C3Cycle:AsinglecarbonfixationoccursinC3cycle.
C4Cycle:DoublecarbonfixationsoccurinC4cycle.

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EfficiencyinCarbonFixation
C3Cycle:CarbonfixationislessefficientandslowinC3cycle.
C4Cycle:CarbonfixationismoreefficientandfastinC4cycle.
RequirementsoftheCarbonFixation
C3Cycle:Thefixationofasinglecarbonmoleculerequires3ATPand2NADH.
C4Cycle:Thefixationofasinglecarbonfixationrequires5ATPand3NADH.
TypesofChloroplasts
C3Cycle:GranularchloroplastsareinvolvedintheC3cycle.
C4Cycle:GranularandagranularchloroplastsareinvolvedintheC4cycle.
KranzAnatomyinLeaves
C3Cycle:KranzanatomyisabsentinleavesoftheC3plants.
C4Cycle:KranzanatomyispresentinleavesoftheC4plants.
Cells
C3Cycle:C3cycleiscarriedoutbymesophyllcells.
C4Cycle:C4cycleiscarriedoutbybothmesophyllcellsandbundlesheathcells.
OptimumTemperature
C3Cycle:TheoptimumtemperatureoftheC3cycleis2025degreesofCelsius.
C4Cycle:TheoptimumtemperatureoftheC4cycleis3045degreesofCelsius.
AtVeryLowCarbonDioxideConcentrations
C3Cycle:C3cycleisunabletoproceedatverylowcarbondioxideconcentrations.
C4Cycle:C4cyclecanproceedatverylowcarbondioxideconcentrations.
EffectofOxygen

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C3Cycle:C3cycleisinhibitedbyoxygen.
C4Cycle:Noinhibitionofc4cycleisobservedwithC4cycle.
Sunlight
C3Cycle:C3cyclecanbesaturatedwithsunlight.
C4Cycle:C4cycledoesnotsaturatewithsunlight.
Photorespiration
C3Cycle:ConsiderableamountofphotorespirationisobservedinC3cycle.
C4Cycle:NegligibleamountofphotorespirationisobservedinC4cycle.
Conclusion
C3andC4cyclearethetwotypesofdarkreactionsthatoccurduringphotosynthesis.C3cycleoccursinallplantsat
2025degreesCelsius,whereastheC4 cycle occurs only in C4plantsat3045degreesCelsius. During C3 cycle, a
single carbon fixation event is observed whereas, during C4 cycle, two carbon fixation events are observed.
Photorespiration occurs during C3 cycle but negligible amounts of photorespiration occur during C4 cycle. The
efficiencyofC3cycle islowwhencomparedtotheefficiencyofC4cycle.ThemaindifferencebetweenC3andC4
cycleisthenumberofcarbonsinthefirststablecompoundproducedineachcycle.
Reference:
1. Berg, Jeremy M. “The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water.” Biochemistry. 5th
edition.U.S.NationalLibraryofMedicine,01Jan.1970.Web.16Apr.2017.
2. Lodish, Harvey. “CO2 Metabolism during Photosynthesis.” Molecular Cell Biology. 4th edition. U.S. National
LibraryofMedicine,01Jan.1970.Web.16
ImageCourtesy:
1.“Calvincycle4″ByMikeJones–Ownwork(CCBYSA3.0)viaCommonsWikimedia
2.“HatchSlackpathway2″ByHatchSlackpathway.svg:*HatchSlackpathway.png:Adenosinederivativeswork:
Jamousederivativework:Adenosine(talk)–HatchSlackpathway.svg(CCBYSA2.5)viaCommonsWikimedia
0

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AbouttheAuthor:Lakna
Lakna,agraduateinMolecularBiology&Biochemistry,isaMolecularBiologistandhasa
broadandkeeninterestinthediscoveryofnaturerelatedthings
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