lecture 17 oct 10, 2005 photosynthesis ii. calvin cycle
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Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle. Lecture Outline The Calvin Cycle fixes carbon makes reduced carbon compounds Reactions of the Calvin Cycle – anabolic pathway input of NADPH + H + , input of ATP Regulation of the Calvin Cycle - PowerPoint PPT PresentationTRANSCRIPT
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Lecture OutlineLecture Outline
1. The Calvin Cycle fixes carbonmakes reduced carbon compounds
2. Reactions of the Calvin Cycle – anabolic pathwayinput of NADPH + H+, input of ATP
3. Regulation of the Calvin Cycle4. The problem with oxygen – Photorespiration5. Tricks some plants use to limit photorespiration
- C4 anatomy, C4 metabolism – division of labor- CAM plants, the difference is night and day
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The purpose of the Carbon-fixation (Calvin Cycle) Reactions
CO2 + NADPH + H+ + ATP
C6H12O6 + NADP+ + ADP + Pi
carbohydrate
Note: synthesis of carbohydrate from CO2 is favorable onlybecause coupled to very favorable reactions
NADPH to NADP+
andATP to ADP + Pi
energy released is greater than it costs to make carbohydrate
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The Calvin cycle has three phasesThe Calvin cycle has three phases– Carbon fixationCarbon fixation– Reduction (energy input, reducing equiv Reduction (energy input, reducing equiv
input)input)
– Regeneration of the CORegeneration of the CO22 acceptor acceptor
(energy input – “priming (energy input – “priming step”)step”)
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Overview of Carbon-Fixation Reactions
CO2
5 Carbon Sugarwith 2 phosphates on it
3 Carbon Aldehyde with one phosphate on it
R-C-OH=O
R-C-H Higher EnergyCompound
=O
Red
uct
ion
+ 3 Carbon Acidwith one phosphate on it
2
ATPATP
ADP + Pi
NADPHNADPH
NADP+ATPATP
ADP + Pi
5 Carbon Sugarwith one phosphate on it
20/24
6 Carbon Sugar - Glucosewith NO phosphate on it
4/24
1224
2412
2
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Carried out by the enzyme “rubisco”(ribulose 1,5 bisphosphate carboxylase oxygenase)
Carbon FixationCarbon Fixation
Do need to know this enzymeKey regulatory enzyme
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Acid -COOHAcid -COOH
AldehydeAldehyde -C=O-C=O HH
PrimingPrimingStepStep
Input ofenergy
RegenerateRegenerateWhat started What started
withwith
CarbonCarbonFixationFixation
ReductionReductionRubiscoRubisco
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pH 8 / pH7
Regulation of Rubisco1st Enzyme in Calvin Cycle
Substrate/ProductSubstrate/Product availability
Allosterically regulated by NADPHNADPH and ATPATP
Very Narrow pH optimumNarrow pH optimum
Enzyme must be in reduced form
pH 8 or above, inactive at 7
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Integration of Light-Dependent and
Light-Independent Reactions
They generally occur AT THE SAME TIME
O2
CO2H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acidsFatty acids
Starch(storage)
Sucrose (export)
G3P
Photosystem IIElectron transport chain
Photosystem I
Chloroplast
Figure 10.21
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PhotoRespiration - “ the OXYGEN PROBLEM”
OxygenOxygen is a competing substrate for the 1st enzyme in the C3 cycle (Rubisco)
OO22
5 Carbon Sugarwith 2 phosphates on it
3 Carbon AcidAcidwith one phosphate on it
+22 Carbon AcidAcidwith one phosphate on it
Energy WastedWith NOsynthesisof glucose
COCO22
5 Carbon Sugarwith 2 phosphates on it
3 Carbon AcidAcidwith one phosphate on it+3 Carbon AcidAcidwith one phosphate on it
Net increasein materialmake glucose with“extra”
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Some Plants Deal with this problem by a
DIVISION OF LABOR BETWEEN CELLS DIVISION OF LABOR BETWEEN CELLS CC44 Plants Plants
MesophyllMesophyll cells perform “usual” cells perform “usual” noncyclic Light-Dependent Reactionsnoncyclic Light-Dependent Reactions
make make oxygenoxygen, , ATPATP and and NADPHNADPHDoDo NOTNOT perform the C perform the C33 (Calvin cycle) reactions (Calvin cycle) reactions
Bundle SheathBundle Sheath cells perform “ cells perform “UNUNusual” usual” cyclic Light-Dependent Reactionscyclic Light-Dependent Reactionsa a lot of ATPlot of ATP but but
very little NADPH very little NADPH and and very little Overy little O22
PERFORM the usual C3 (Calvin Cycle) reactions
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CC44 leaf anatomy leaf anatomy and the and the CC44 pathway pathway
CO2
MesophyllMesophyll cellcell
BundleBundle--sheathsheath
cellcell
VeinVein(vascular tissue)(vascular tissue)
Photosyntheticcells of C4 plantleaf
Stomata
Mesophyllcell
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C) PEP (3 C)
Malate (4 C)
ADP
ATP
Bundle-Sheathcell
CO2
Pyruate (3 C)
CALVINCYCLE
Sugar
Vasculartissue
Figure 10.19
CO2
MesophyllMesophyllProduce NADPH and ATPPEP carboxylase insensitive to O2
Malate brings across reducing equivalents
NADPHNADPHusedused
NADPHNADPHregeneratedregenerated
CyclicCyclice- flowe- flow
Little OLittle O22
Malate (4 C)Oxaloacetate (4 C)
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CO2
CH3
C=OC=O \ O-Pi
“ “PEP”PEP”
OH /C=OCH2
C=OC=O + Pi
\ OHOxaloacetateOxaloacetate 4C acid4C acid
OH / C=O CH2
H-H-C-O-H-H C=O \ OH malatemalate 4C acid4C acid CH3
C=OC=O \ O-Hpyruvatepyruvate
ATPATP
ADPADP
NADPHNADPH++HH++
NADPNADP++
PEP
carboxylase
“carries”carries”Reducing Reducing
equivalentsequivalents
Mesophyll Cell C4 MetabolismMesophyll Cell C4 Metabolism
Needs ATP and NAPDPH + H+
Non-cyclic electron flowNon-cyclic electron flow
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OH /C=OCH2
C=OC=O \ OHOxaloacetateOxaloacetate 4C acid4C acid
OH / C=O CH2
H-H-C-O-H-H C=O \ OH malatemalate 4C acid4C acid
NADPHNADPH++HH++
NADPNADP++
“carries”carries”Reducing Reducing
equivalentsequivalents
Bundle Sheath - Cell C4 MetabolismBundle Sheath - Cell C4 Metabolism
Needs ATPATP Only
Cyclic electron flowCyclic electron flowVery low OVery low O22
CO2
CH3
C=OC=O \ O-Hpyruvatepyruvate
ATATPP
CalvinCalvinCycleCycle
glucoseglucose
ATATPP
NADPNADPHH
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Mesophyll cells provide a means for bundle sheathMesophyll cells provide a means for bundle sheathcells to acquire NADPH + Hcells to acquire NADPH + H++ reducing power reducing power
Mesophyll cells provide carbon dioxide to bundle sheathMesophyll cells provide carbon dioxide to bundle sheathcells at higher concentration than in aircells at higher concentration than in air
Bundle Sheath cells not making oxygen, so very little Bundle Sheath cells not making oxygen, so very little competitor with Ccompetitor with C33 reactions reactions
Costs more energyCosts more energy to do business this way… to do business this way… but has the but has the advantage when COadvantage when CO22 is limiting is limiting (when stomates are closed - like on hot days)(when stomates are closed - like on hot days)
Who cares as long as the sun is shining ?Who cares as long as the sun is shining ?ATP is not limitingATP is not limiting
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CAM PlantsCAM PlantsCacti, pineapple
– Open their stomata only at night, too hot during day – survive very adverse (dry) conditionsNIGHTNIGHT
Perform PEP carboxylase reaction at night (CO2 assimilation) accumulate malate to high concentration in central vacuole use sugar oxidation/catabolism to power (NADH and ATP)
carbon fixation
DAYDAYPerform “light” reactions during the day
mostly cyclic e- flow to produce ATP (low O2)decarboxylate malate to yield CO2 and NADPH + H+
perform C3 reactions (Calvin Cycle) to produce sugars and starch
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Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.
(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.
(b)
PineappleSugarcane
Bundle-sheath cell
Mesophyll Cell
Organic acid
CALVINCYCLE
Sugar
CO2 CO2
Organic acid
CALVINCYCLE
Sugar
C4 CAM
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Night
Day
1
2 Organic acidsrelease CO2 toCalvin cycle
Figure 10.20
SunlightSunlightPowersPowers
BothBothphasesphases
SugarSugarOxidationOxidationPowersPowers
OneOnePhasePhase
GenerallyGenerallySlowSlow
growinggrowing
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SummarySummary
1. Photosynthetic “light” reactions produce ATP and reducing potential NADPH + H+
2. Dark reactions use ATPATP and reducing potential to synthesize carbohydrates- powers reductionreduction of 3-carbon acidacid
to 3-carbon aldehydealdehyde - - powers regeneration of starting material
5-carbon di-phosphate (priming step for CO2 fixation)
3. Rubisco enzyme regulated tightlytightly by allosteric modulators pH, and reducing status of stroma
4. OO22 interferes with carbon fixation by Rubisco enzyme
5. Metabolic “tricks” to avoid photorespiration- C4 metabolismC4 metabolism - CAM metabolismCAM metabolism