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Lecture 17 Oct 10, 2005Photosynthesis II. Calvin Cycle

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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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DARK REACTIONS energy utilization

The Calvin Cycle

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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

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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