LinearElectronFlow(PSI)

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Photosystem II(PS II)

• InPSI(likePSII)

• transferredlight

energyexcitesP700

• losesanelectron

toacceptor

• P700+

• acceptselectron

fromPSII

• viatheelectron

transportchain

LinearElectronFlow(PSI)

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

Electron transport chain

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–e–

6

Photosystem II(PS II)

• Eachelectron

• fallsdownelectrontransportchain

• fromprimaryelectronacceptorofPSI

• toferredoxin(Fd)

• electronsthentransferredtoNADP+

• reducedtoNADPH

• goestoCalvincycle

CyclicElectronFlow

• Cyclicelectronflow

• usesonlyphotosystemIandproducesATP

• butnotNADPH

• nooxygenreleased

• generatessurplusATP

• meeKngdemandforCalvincycle

• Someorganismssuchaspurplesulfurbacteria

• havePSIbutnotPSII

• thoughttohaveevolvedbeforelinearelectronflow

• mayprotectcellsfromlight‐induceddamage

Fig.10‐15

ATPPhotosystem IIPhotosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+

WhatMakestheLightReacKonsSoImportant?

• TwoacKonsofgreatconsequencetakeplaceinthelightreacKons

1. Waterissplit,yieldingbothelectronsandoxygen.

• TheelectronsmovethroughthelightreacKons.

• Theoxygeniswhatorganismssuchasourselvesbreathein.

2. Theelectronsthatarederivedfromthewater

• andthengivenanenergyboostbythesun’srays

• aretransferredtoadifferentmolecule:theiniKalelectron

acceptor

• Thisisthemeansthroughwhichthesun’senergyistransferred

intothelivingworld

ImportanceofLightReacKons

• Also…

• fallofelectronsthroughtheelectrontransportchainbetween

photosystemsIIandI

• yieldsenergythatproducesATP

• usedtopowerthesecondstageofphotosynthesis

LightReacKons

PLAY Light Reactions

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

Light NADP+

reductaseNADP+ + H+

NADPH

+2 H+

H2O O2

e–e–

1/21

2

3

TheCalvinCycle

• Carbondioxidefromtheatmosphere

• Combinedwithasugar

• RuBP(Ribulose‐1,5‐bisphosphate)

• RuBisCo(Ribulose‐1,5‐bisphosphatecarboxylase/oxygenase)

• EnzymethatcatalyzesRuBPandCO2

• resulKngcompound

• energizedwithaddiKonofelectrons

• suppliedbylightreacKons

• Makes3‐carbonsugars

• Latercombinedintocarbohydrates

TheCalvinCycle

• G3P

• Glyceraldehyde3‐phosphate

• High‐energysugar

• TheresultoftheCalvincycle

• Finalproductofphotosynthesis

• Canbeusedforenergyorplantstructure

TheCalvinCycle

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

InputCO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P61,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

TheCalvinCycle

• AllthesestepsarepoweredbyATPproducedinthelightreacKons

Summary

PhotorespiraKonandtheC4Pathway• Inplants

• theenzymerubiscofrequentlybindswithoxygenratherthanwithcarbondioxide

• aprocesscalledphotorespiraKonthatundercutsphotosynthesis.

PhotorespiraKon

• problemincreasesastemperaturerises

• becauseasplantsclosetheirstomatatokeepinmoisture

• theyalsokeepoutCO2

• increasinglikelihoodthatrubiscowillbindwithoxygen

• Rubisco‐oxygenthenmetabolized

• Releasingcarbondioxide

• EvoluKonaryrelic?

• AtmosphericO2lowwhenrubiscoevolved

• MaybeprotecKve

• Againstdamagefrombuildupoflightrxnproducts

PhotorespiraKon

• Somewarm‐climateplants

• HaveevolvedameansofdealingwithphotorespiraKon

• C4photosynthesis

• MostplantsC3

C4Photosynthesis• PEPcarboxylase

• enzymethatbindswithcarbondioxidebutnotwithoxygen

• Formsa4‐carboncompound(HenceC4)

• OxaloaceKcacid

• FromcombinaKonofCO2witha3‐carbonmolecule

phosphoenolpyruvate(PEP)

• Occursinthemesophyllcells

• OxaloaceKcacid

• thenshucledtobundlesheathcellstoCalvincycle

• whereCO2isreleasedtobindwithrubisco

C4PhotosynthesisC4 leaf anatomy

Mesophyll cellPhotosyntheticcells of C4plant leaf

Bundle-sheathcell

Vein(vascular tissue)

Stoma

The C4 pathway

Mesophyllcell CO2PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)ADP

ATP

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

CAMPlants

• CrassulaceanAcidMetabolism

• Dry‐weatherplantssuchascacK(Crassulacea)employanother

formofphotosynthesis

• CAMphotosynthesis

CAMPhotosynthesis

• CAMphotosynthesis

• stomataopenonlyatnight

• lednginandfixingcarbondioxide

• Carbondioxideis“banked”unKlsunrise

• whenphotonssupplyenergyforCalvincycle.

Photosynthesis

Photosynthesis Song

Youshouldnowbeableto:

1. Describethestructureofachloroplast

2. DescribetherelaKonshipbetweenanacKonspectrumandanabsorpKonspectrum

3. Tracethemovementofelectronsinlinearelectronflow

4. Tracethemovementofelectronsincyclicelectronflow

5. DescribethesimilariKesanddifferencesbetweenoxidaKvephosphorylaKoninmitochondriaandphotophosphorylaKoninchloroplasts

6. DescribetheroleofATPandNADPHintheCalvincycle

7. DescribethemajorconsequencesofphotorespiraKon

8. DescribetwoimportantphotosyntheKcadaptaKonsthatminimizephotorespiraKon

5. DescribethesimilariKesanddifferencesbetweenoxidaKvephosphorylaKoninmitochondriaandphotophosphorylaKoninchloroplasts

6. DescribetheroleofATPandNADPHintheCalvincycle

7. DescribethemajorconsequencesofphotorespiraKon

8. DescribetwoimportantphotosyntheKcadaptaKonsthatminimizephotorespiraKon