Chapter10Chapter10

PhotosynthesisPhotosynthesis

BIOL223

PhotosynthesisandEnergy

• Photosynthesis

• Makingfoodfromlightenergy

• Photoautotrophs

• UseCO2andwatertomakesugars

• Madelifepossibleasweknowit

• Providescarbohydratesforall

higherlevelsoffoodchain

• PrimaryproducEon

• Liberatesatmosphericoxygen

• O2wasteproduct

PhotosynthesisandEnergy

• Carbondioxideconvertedtosugar

• ProcesscalledcarbonfixaEon

• SeriesofredoxreacEons

• Water(hydrogenatoms)loseselectrons

• Oxidized(OIL)

• Carbondioxidegainselectrons

• Reduced(RIG)

PhotosynthesisandEnergy

• SummaryequaEonforphotosynthesis

CO2+2H2O+photons→(CH2O)n+H2O+O2

carbondioxide+water+lightenergy→carbohydrate+oxygen+water

PhotosynthesisandEnergy

• SomewhatoppositeofcellularrespiraEon

Photosynthesis

CO2+2H2O+photons→(CH2O)n+H2O+O2

carbondioxide+water+lightenergy→carbohydrate+oxygen+water

CellularRespiraEon

(CH2O)6

C6H12O6+O2→CO2+H2O+energy

PhotosynthesisandEnergy• Energytrappedbyphotosynthesis

• approx100terawaSs

• 6xyearlyglobalpowerconsumpEonofhumancivilizaEon

• convertaround100,000,000,000(billion)tonsofcarbonintobiomassper

year

ThreeTypesofPhotosynthesizers

Domain: EukaryotaKingdom: Plantae

Domain: EukaryotaKingdom: Protista

Domain: ProkaryotaKingdom: Eubacteria

TheComponentsofPhotosynthesis• Chloroplasts

• Organellesinplantsandalgae,wherephotosynthesistakesplace

• Lightcapturedhere

• Existingreatabundanceinthemesophyllcellsofplantleaves

• Doublemembranebound

• likemitochondria

• Containthylakoids

• Stackedasgrana

• Intheliquidfilledstroma

Photosynthesis• Capturedenergycomesmostlyfromblueandredwavelengthsofvisiblesunlight

• Absorbedbypigmentsinthethylakoids

• Chlorophylla

• Accessorypigments

• Whyplantslookgreen

low energyhigh energylong wavelengthshort wavelength

gammarays

visiblelight

x-rays ultra-violet infrared microwaves radiowaves

SiteofPhotosynthesis

petiole

blade

epidermis

epidermis

stomata

cell wall

nucleus

vacuole

chloroplast

thylakoids

outer membrane

stroma

inner membranegranum

thylakoid

thylakoid compartment

mesophyllcells

Energy from sunlight is

absorbed by pigments in the

thylakoid membrane.

LeafThe 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 sectionIn 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 cellA 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.

ChloroplastEach chloroplast has an outermembrane 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 sometimesstack on one another to create. . .

A GranumElectrons 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

StagesofPhotosynthesis

• Twoprimarystages

• LightreacEons

• Stripselectronsfromwater

• oxidaEon

• Booststheseelectronstohigherenergylevels

• MakesATPandNADPH

• Calvincycle

• Makescarbohydratesfromhighenergyelectronsand

atmosphericCO2

TheLightReacEons

• Electronsderivedfromwater

• energeEcallyboostedby

photons

• Electronsphysicallytransferred

• passedalongthroughaseriesof

electroncarriers

• NADP+⇒NADPH

• carriesthemtosecondstage

StagesofPhotosynthesis

• Twoprimarystages(conEnued)

• LightreacEons

• Stripselectronsfromwater

• Booststheseelectronstohigherenergylevels

• Calvincycle

• LightindependentreacEons

• Makescarbohydratesfromhighenergyelectronsand

atmosphericCO2

• reducEon

TheCalvinCycle

• ElectronscarriedbyNADPH

• combinedwithcarbondioxide

• formshigh‐energysugar

• Glyceraldehyde3‐Phosphate(G3P)

• combinedintocomplexcarbohydrates

• poweredbyATPfromlightreacEons

• Occursinthestromaofthechloroplast

• FirstdescribedbyMelvinCalvin

LightReacEons• Photosynthesisworksthroughtwo

molecularcomplexes

• PhotosystemsIIandI

• Onthylakoidmembranes

• composedpartlyof

antennaemolecules

• chlorophyllandsome

accessorymolecules

• absorband

transmitsolar

energy

LightExcitaEon• pigmentabsorbslight

• goesfromagroundstatetoexcitedstate

• whichisunstable

• excitedelectronsfallbacktothegroundstate

• photonsaregivenoff

• fluorescence

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

Ener

gy o

f ele

ctro

n

e–

Chlorophyllmolecule

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

• pheophyEn

• Receivelightenergy

• transformittochemicalenergy

• splitswaterandstripselectronsoffliberatedhydrogenatoms

• alsoenergizesstrippedelectrons

• Includesprimaryelectronacceptors

• Receivethesehighenergyelectrons

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Thyl

akoi

d m

embr

ane

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

ElectronFlow

Pigmentmolecules

Light

P680

e–2

1

Photosystem II(PS II)

Primaryacceptor

• twopossibleroutesforelectronflow

• cyclicandlinear

• Linearelectronflow

• primarypathway

• involvesbothphotosystems

• producesATPandNADPHusinglightenergy

• photonhitspigmentmolecule

• energypassedamongpigmentmolecules

• unElitexcitesP680

• excitedelectronfromP680

• transferredtotheprimaryelectronacceptor

LinearElectronFlow(PSI)

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

Photosystem II(PS II)

• P680+(missinganelectron)

• verystrongoxidizingagent

• H2Oissplitbyenzymes

• electronstransferredfrom

hydrogenatomstoP680+

• reducingittoP680

• O2isreleased

LinearElectronFlow(PSII)

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 II(PS II)

• Electronspasseddownelectrontransportchain

• fromprimaryelectron

acceptorofPSIItoPSI

• Energyreleasedbyfall

• drivesthecreaEonofaprotongradient

• intothylakoidspace

• DiffusionofH+

• acrossmembranedrives

ATPsynthase