Hatch-Slack Pathway (C4 Cycle)

Hatch-Slack Pathway or C4 Cycle

1. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) 1
Hatch-SlackPathway(C4Cycle)
Itisalternatetypeofcarbonfixationhaveevolvedthatminimize
photorespirationandoptimizetheCalvincycle.
ItisalsoknownasC4Cycle.
ThiscyclewasdiscoveredbyHatchandSlack,inAustralia(1966).
InitiallyCO2isfixedbyfourcarbonacid.
Theseplantsareadaptedtoaridclimateconditionandabletogivehigher
photosyntheticoutputevenathightemperatureandhighlightintensity.
Itispresentinbothmonocotsanddicots.
TheyareparticularlycommoninGramineae(corn,sugarcane),
Chenopodiaceae(Atriplex)andCyperaceaefamilies.
InC4plantstherearetwodistincttypesofphotosyntheticcells:
a.Bundlesheathcells
b.Mesophyllcells
ThebasicC4cyclecompletesinfivestages:
1.FixationofCO2bythecarboxylationofphosphoenolpyruvateinthe
mesophyllcelltoformaC4acid(Oxaloacetaticacid).
2.OxaloacetatethenconvertsintootherC4acidssuchasmalateoraspartate.
3.TransportoftheC4acidstothebundlesheathcells.
4.DecarboxylationoftheC4acidswithinthebundlesheathcellsand
generationofCO2,whichenterintheC3Cycle.
5.TransportoftheC3acidthatisformedbythedecarboxylationstepbackto
themesophyllcell.

2. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) 2
TheC4cycletransportsdifferentC4acidsfrommesophylltobundlesheath
cells,employsdifferentenzymestodecarboxylatetheC4acidsinthebundle
sheathcellsandtransportsdiferrentC3acidsfrombundlesheathcellsto
mesophyllcells.
Basedonthesedifferences,therearethreedifferenttypesofC4cycles-
i.NADP+-malicenzymetype(NADP-ME)
ii.NAD+-malicenzymetype(NAD-ME)
iii.PEPCarboxykinasetype(PEPCK)
NADP+-malicenzymetypedecarboxylatingenzymetransportC4acid(Malate)
frommesophylltobundleofsheathcellsandC3acid(pyruvate)transportedfrom
bundlesheathcelltomesophyll.

3. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) 3
NAD+-malicenzymetypedecarboxylatingenzymetransportC4acid
(Aspartate)frommesophylltobundleofsheathcellsandC3acid(Alanine)
transportedfrombundlesheathcelltomesophyll.
PEPCarboxykinasetypedecarboxylatingenzymetransportC4acid
(Aspartate/Malate)frommesophylltobundleofsheathcellsandC3acid
(Phosphoenolpyruvate)transportedfrombundlesheathcelltomesophyll.