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Chapter10

Photosynthesis

BIOL223

PhotosynthesisandEnergy•  Photosynthesis

•  Makingfoodfromlightenergy

•  Photoautotrophs

•  UseCO2andwatertomakesugars

•  Madelifepossibleasweknowit

•  Providescarbohydratesforallhigherlevelsoffoodchain

•  PrimaryproducEon

•  Liberatesatmosphericoxygen

•  O2wasteproduct

PhotosynthesisandEnergy

•  Carbondioxideconvertedtosugar

•  ProcesscalledcarbonfixaEon

•  SeriesofredoxreacEons

•  Water(hydrogenatoms)loseselectrons

•  Oxidized(OIL)

•  Carbondioxidegainselectrons

•  Reduced(RIG)

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PhotosynthesisandEnergy

•  SummaryequaEonforphotosynthesis

CO2+2H2O+photons→(CH2O)n+H2O+O2

carbondioxide+water+lightenergy→carbohydrate+water+oxygen

PhotosynthesisandEnergy

•  SomewhatoppositeofcellularrespiraEon

Photosynthesis

CO2+2H2O+photons→(CH2O)n+H2O+O2

carbondioxide+water+lightenergy→carbohydrate+oxygen+water

CellularRespiraEon (CH2O)6

C6H12O6+O2→CO2+H2O+energy

ThreeTypesofPhotosynthesizers

Domain: EukaryotaKingdom: Plantae

Domain: EukaryotaKingdom: Protista

Domain: ProkaryotaKingdom: Eubacteria

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TheComponentsofPhotosynthesis•  Chloroplasts

•  Organellesinplantsandalgae,wherephotosynthesistakesplace

•  Lightcapturedhere

•  Existingreatabundanceinthemesophyllcellsofplantleaves

•  Doublemembranebound

•  likemitochondria

•  Containthylakoids

•  Stackedasgrana

•  Intheliquidfilledstroma

Photosynthesis•  Capturedenergycomesmostlyfromblueandredwavelengthsofvisiblesunlight

•  Absorbedbypigmentsinthethylakoids

•  Chlorophylla

•  Accessorypigments

•  Whyplantslookgreen

low energy high energy long wavelength short wavelength

gamma rays

visible light

x-rays ultra- violet infrared microwaves radiowaves

SiteofPhotosynthesis

petiole

blade

epidermis

epidermis stomata

cell wall

nucleus

vacuole

chloroplast

thylakoids

outer membrane

stroma inner membrane

granum

thylakoid thylakoid compartment

mesophyll cells

Energy from sunlight is

absorbed by pigments in the

thylakoid membrane.

Leaf The primary site of photosynthesis in plants, leaves have a two-part structure: a petiole (or stalk) and a blade (normally thought of as the leaf).

Leaf cross section In cross section, leaves have a sandwich- like structure, with epidermal layers at top and bottom and mesophyll cells in between. Most photosynthesis is performed within mesophyll cells. Leaf epidermis is pocked with a large number of microscopic openings, called stomata, that allow carbon dioxide to pass in and water vapor to pass out.

Mesophyll cell A single mesophyll cell within a leaf contains all the component parts of plant cells in general, including the organelles—called chloroplasts—that are the actual sites of photosynthesis.

Chloroplast Each chloroplast has an outer membrane at its periphery; then an inner membrane; then a liquid material, called the stroma, that has immersed within it a network of membranes, the thylakoids. These thylakoids sometimes stack on one another to create. . .

A Granum Electrons used in photosynthesis will come from water contained in the thylakoid compartment, and all the steps of photosynthesis will take place either within the thylakoid membrane, or in the stroma that surrounds the thylakoids.

1.

2.

3.

4.

5.

thylakoid membrane

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StagesofPhotosynthesis

•  Twoprimarystages

•  LightreacEons

•  Stripselectronsfromwater

•  oxidaEon

•  Booststheseelectronstohigherenergylevels

•  MakesATPandNADPH

•  Calvincycle

•  MakescarbohydratesfromhighenergyelectronsandatmosphericCO2

TheLightReacEons

•  Electronsderivedfromwater

•  energeEcallyboostedby

photons

•  Electronsphysicallytransferred

•  passedalongthroughaseriesof

electroncarriers

•  NADP+⇒NADPH

•  carriesthemtosecondstage

StagesofPhotosynthesis

•  Twoprimarystages(conEnued)

•  LightreacEons

•  Stripselectronsfromwater

•  Booststheseelectronstohigherenergylevels

•  Calvincycle

•  LightindependentreacEons

•  MakescarbohydratesfromhighenergyelectronsandatmosphericCO2

•  reducEon

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TheCalvinCycle

•  ElectronscarriedbyNADPH

•  combinedwithcarbondioxide

•  formshigh-energysugar

•  Glyceraldehyde3-Phosphate(G3P)

•  combinedintocomplexcarbohydrates

•  poweredbyATPfromlightreacEons

•  Occursinthestromaofthechloroplast

•  FirstdescribedbyMelvinCalvin

LightReacEons•  Photosynthesisworksthroughtwo

molecularcomplexes

•  PhotosystemsIIandI

•  Onthylakoidmembranes

•  composedpartlyof

antennaemolecules

•  chlorophyllandsomeaccessorymolecules

•  absorbandtransmitsolar

energy

LightExcitaEon•  pigmentabsorbslight

•  goesfromagroundstatetoexcitedstate

•  whichisunstable

•  excitedelectronsfallbacktothegroundstate

•  photonsaregivenoff

•  fluorescence

(a) Excitation of isolated chlorophyll molecule

Heat

Excited state

(b) Fluorescence

Photon Ground state

Photon (fluorescence)

Ener

gy o

f ele

ctro

n

e–

Chlorophyll molecule

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PhotosystemStructure•  Photosystemantennaemolecules

•  Afewhundredchlorophyllaandsomeaccessorymolecules(pigments)

•  Pigments

•  Carotene-anorangepigment

•  Xanthophyll-ayellowpigment

•  PhaeophyEna[1]-agray-brownpigment

•  PhaeophyEnb[1]-ayellow-brownpigment

•  Chlorophylla-ablue-greenpigment

•  Chlorophyllb-ayellow-greenpigment

•  transmitsolarenergytoreacEoncenters

PhotosystemStructure•  ReacEoncentermolecules

•  Pairofmodifiedchlorophyllamolecules

•  andPhaeophyEn

•  Receivelightenergy

•  transformittochemicalenergy

•  splitswaterandstripselectronsoffliberatedhydrogenatoms

•  alsoenergizesstrippedelectrons

•  Includesprimaryelectronacceptors

•  Receivethesehighenergyelectrons

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

STROMA

e–

Pigment molecules

Photon

Transfer of energy

Special pair of chlorophyll a molecules

Thyl

akoi

d m

embr

ane

Photosystem

Primary electron acceptor

Reaction-center complex

Light-harvesting complexes

ElectronFlow

Pigment molecules

Light

P680

e– 2

1

Photosystem II (PS II)

Primary acceptor

•  twopossibleroutesforelectronflow

•  cyclicandlinear

•  Linearelectronflow

•  primarypathway

•  involvesbothphotosystems

•  producesATPandNADPHusinglightenergy

•  photonhitspigmentmolecule

•  energypassedamongpigmentmolecules

•  unElitexcitesP680

•  excitedelectronfromP680

•  Passedtoprimaryelectronacceptor-phaeophyEn

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LinearElectronFlow(PSI)

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e– e–

2 H+

O2

+ 3

H2O

1/2

Photosystem II (PS II)

•  P680+(missinganelectron)

•  verystrongoxidizingagent

•  H2Oissplitbyenzymes

•  electronstransferredfromhydrogenatomstoP680+

•  reducingittoP680

•  O2isreleased

LinearElectronFlow(PSII)

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e– e–

2 H+

O2

+ 3

H2O

1/2

4

Pq

Pc

Cytochrome complex

Electron transport chain

5

ATP

Photosystem II (PS II)

•  Electronspasseddownelectrontransportchain

•  fromprimaryelectronacceptor(phaeophyEn)ofPSIItoPSI

•  Energyreleasedbyfall

•  drivesthecreaEonofaprotongradient

•  intothylakoidspace

•  DiffusionofH+

•  acrossmembranedrivesATPsynthase

LinearElectronFlow(PSI)

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e– e–

2 H+

O2

+ 3

H2O

1/2

4

Pq

Pc

Cytochrome complex

Electron transport chain

5

ATP

Photosystem I (PS I)

Light

Primary acceptor

e–

P700

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Photosystem II (PS II)

•  InPSI(likePSII)

•  transferredlight

energyexcitesP700

•  losesanelectron

toacceptor

•  P700+

•  acceptselectron

fromPSII

•  viatheelectron

transportchain

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LinearElectronFlow(PSI)

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e– e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome complex

Electron transport chain

5

ATP

Photosystem I (PS I)

Light

Primary acceptor

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

•  meeEngdemandforCalvincycle

•  Someorganismssuchaspurplesulfurbacteria

•  havePSIbutnotPSII

•  thoughttohaveevolvedbeforelinearelectronflow

•  mayprotectcellsfromlight-induceddamage

Fig.10-15

ATP Photosystem II

Photosystem I

Primary acceptor

Pq

Cytochrome complex

Fd

Pc

Primary acceptor

Fd

NADP+ reductase

NADPH

NADP+ + H+

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WhatMakestheLightReacEonsSoImportant?

•  TwoacEonsofgreatconsequencetakeplaceinthelightreacEons

1.  Waterissplit,yieldingbothelectronsandoxygen.

•  TheelectronsmovethroughthelightreacEons.

•  Theoxygeniswhatorganismssuchasourselvesbreathein.

2.  Theelectronsthatarederivedfromthewater

•  andthengivenanenergyboostbythesun’srays

•  aretransferredtoadifferentmolecule:theiniEalelectron

acceptor

•  Thisisthemeansthroughwhichthesun’senergyistransferred

intothelivingworld

LightReacEons

PLAY Light Reactions

Light

Fd

Cytochrome complex

ADP +

i H+

ATP P

ATP synthase

To Calvin Cycle

STROMA (low H+ concentration)

Thylakoid membrane

THYLAKOID SPACE (high H+ concentration)

STROMA (low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

Light NADP+ reductase

NADP+ + H+

NADPH

+2 H+

H2O O2

e– e– 1/2 1

2

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TheCalvinCycle

•  Carbondioxidefromtheatmosphere

•  Combinedwithasugar

•  RuBP(Ribulose-1,5-bisphosphate)

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

•  EnzymethatcatalyzesRuBPandCO2

•  resulEngcompound

•  energizedwithaddiEonofelectrons

•  suppliedbylightreacEons

•  Makes3-carbonsugars

•  Latercombinedintocarbohydrates

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TheCalvinCycle

•  G3P

•  Glyceraldehyde3-phosphate

•  High-energysugar

•  TheresultoftheCalvincycle

•  Finalproductofphotosynthesis

•  Canbeusedforenergyorplantstructure

TheCalvinCycle

Ribulose bisphosphate (RuBP)

3-Phosphoglycerate

Short-lived intermediate

Phase 1: Carbon fixation

(Entering one at a time)

Rubisco

Input CO2

P

3 6

3

3

P

P P P

ATP 6

6 ADP

P P 6 1,3-Bisphosphoglycerate

6

P

P 6

6 6 NADP+

NADPH

i

Phase 2: Reduction

Glyceraldehyde-3-phosphate (G3P)

1 P Output G3P

(a sugar)

Glucose and other organic compounds

Calvin Cycle

3

3 ADP

ATP

5 P

Phase 3: Regeneration of the CO2 acceptor (RuBP)

G3P

TheCalvinCycle

•  AllthesestepsarepoweredbyATPproducedinthelightreacEons

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Summary

PhotorespiraEonandtheC4Pathway•  Inplants

•  theenzymerubiscofrequentlybindswithoxygenratherthanwithcarbondioxide

•  aprocesscalledphotorespiraEonthatundercutsphotosynthesis.

PhotorespiraEon

•  problemincreasesastemperaturerises

•  becauseasplantsclosetheirstomatatokeepinmoisture

•  theyalsokeepoutCO2

•  increasinglikelihoodthatrubiscowillbindwithoxygen

•  Rubisco-oxygenthenmetabolized

•  Releasingcarbondioxide

•  EvoluEonaryrelic?

•  AtmosphericO2lowwhenrubiscoevolved

•  MaybeprotecEve

•  Againstdamagefrombuildupoflightrxnproducts

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PhotorespiraEon

•  Somewarm-climateplants

•  HaveevolvedameansofdealingwithphotorespiraEon

•  C4photosynthesis

•  MostplantsC3

C4Photosynthesis•  PEPcarboxylase

•  enzymethatbindswithcarbondioxidebutnotwithoxygen

•  Formsa4-carboncompound(HenceC4)

•  OxaloaceEcacid

•  FromcombinaEonofCO2witha3-carbonmoleculephosphoenolpyruvate(PEP)

•  Occursinthemesophyllcells

•  OxaloaceEcacid

•  thenshuiledtobundlesheathcellstoCalvincycle

•  whereCO2isreleasedtobindwithrubisco

C4PhotosynthesisC4 leaf anatomy

Mesophyll cell Photosynthetic cells of C4 plant leaf

Bundle- sheath cell

Vein (vascular tissue)

Stoma

The C4 pathway

Mesophyll cell CO2 PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C) ADP

ATP

Pyruvate (3C)

CO2 Bundle- sheath cell

Calvin Cycle

Sugar

Vascular tissue

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CAMPlants

•  CrassulaceanAcidMetabolism

•  Dry-weatherplantssuchascacE(Crassulacea)employanotherformofphotosynthesis

•  CAMphotosynthesis

CAMPhotosynthesis

•  CAMphotosynthesis

•  stomataopenonlyatnight

•  lejnginandfixingcarbondioxide

•  Carbondioxideis“banked”unElsunrise

•  whenphotonssupplyenergyforCalvincycle.

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Youshouldnowbeableto:

1.  Describethestructureofachloroplast

2.  DescribetherelaEonshipbetweenanacEonspectrumandanabsorpEonspectrum

3.  Tracethemovementofelectronsinlinearelectronflow

4.  Tracethemovementofelectronsincyclicelectronflow

5.  DescribethesimilariEesanddifferencesbetweenoxidaEvephosphorylaEoninmitochondriaandphotophosphorylaEoninchloroplasts

6.  DescribetheroleofATPandNADPHintheCalvincycle

7.  DescribethemajorconsequencesofphotorespiraEon

8.  DescribetwoimportantphotosyntheEcadaptaEonsthatminimizephotorespiraEon

5.  DescribethesimilariEesanddifferencesbetweenoxidaEvephosphorylaEoninmitochondriaandphotophosphorylaEoninchloroplasts

6.  DescribetheroleofATPandNADPHintheCalvincycle

7.  DescribethemajorconsequencesofphotorespiraEon

8.  DescribetwoimportantphotosyntheEcadaptaEonsthatminimizephotorespiraEon