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Lecture 16 Oct 7, 2005Photosynthesis I. Light Reactions

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Lecture OutlineLecture Outline

1. Importance of Photosynthesis to all life on earth- primary producer, generates oxygen, ancient

2. What needs to be accomplished in photosynthesis3. Structure of the chloroplast – 3 functional spaces4. How light energy is harvested

– antenna complex, pigments, light spectrum - splitting of water, excitation of e-

5. What work is done with capture light energy- “light” reactions - noncyclic e- transport - ATP and NADPH + H+

- cyclic electron transport – primarily ATP, limit O26. How ATP and NADPH + H+ power anabolic pathways

- “dark” reactions – the Calvin Cycle

3Figure 10.1

PhotoPhotoautoautotrophstrophs Make their own “food” by light

HeterHeterotrophsotrophs Obtain “food” from “other” sources

∆G < 0

Light energy

HeatMotion

PhotoAuto

trophs Heterotrophs

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PhotosynthesisPhotosynthesis is the ultimate energy sourceis the ultimate energy sourcefor almost all life on earthfor almost all life on earth

SolarEnergyInput

HerbivoreBiomassProduction

(Reflection/Heat)

(HeatMotion)

CarnivoreOmnivoreBiomass

(Heat)

PlantBiomassProduction

NetEnergy

AbsorbedAnd

utilized

Eat producers

NetEnergyutilized Net utilized

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PhotosynthesisPhotosynthesis

HH22OO COCO22

OO22

CC66HH1212OO66CarbohydrateCarbohydrate

LightLightOxidizedCarbonInput

ReducedCarbonOutput

WasteProduct

Basis for Basis for HeterotrophHeterotroph RespirationRespiration

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Photosynthesis is a remarkably similar process at the molecular/cell biology levelin a wide diversity of organisms

Evolutionarily Related Process, oran Evolutionarily Conserved Process

“ancient”

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(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 µm

40 µm(d) Cyanobacteria

1.5 µm(e) Pruple sulfurbacteria

Figure 10.2

Cyanobacteria“blue-green algae”

Prokaryotes

single cellsstick together as mats(but no cooperation)

EuglenaEuglenaChlamydamonasChlamydamonas

PhotosyntheicProtists

(Eukaryotes)

single cellaquatic

PlantsPlantsAn entireAn entireKingdomKingdom

Non-VascularPlants

true algaebryophytes-liverworts-mosses

Vascular PlantsFerns

Gymnosperms-conifers

Angiosperms-monocots- dicots

PhotosyntheticOrganisms

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Photosynthesis –is comprised of TWO Distinct Processes

which occur simultaneously (in most photosynthetic organisms)

Energy Capture Processes

Energy Utilization ProcessesMake Carbohydrate Make Carbohydrate NEEDNEED ATP and NADPHATP and NADPH

NOTE: ONLY OCCURIN THE PRESENCE OF

AN ENERGY SOURCE

useuse light to light to MakeMake ATPATP, , NADPHNADPHOO22 gasgas made as bymade as by--productproduct

“Light”Reactions

“Dark” ReactionsCalvin Cycle

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H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP +

ADP

+ P

O2

ATP

Figure 10.5

Light Light ReactionsReactions

(energy (energy capture)capture)

“Dark Reactions”Calvin Cycle

(energy utilization)

InterdependentInterdependent

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Structures all Photosynthetic Eukaryotes have in common

The organelle called the Chloroplast

This organelle is the SITE of photosynthesiswhere ALL photosynthetic reactions occur

Blue green algae (cyanobacteria)do not have internal membranes (they are prokaryotes!)

but they themselves resemble chloroplasts

The extensively folded plasma membrane of cyanobacterialays the same role

as thylakoid membrane in chloroplasts

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Chloroplast

Mesophyll Cell

5 µm

Outermembrane

Intermembranespace

Innermembrane

ThylakoidThylakoidGranumGranumStromaStroma

1 µm

VeinLeaf cross section

Figure 10.3

Mesophyll

CO2 O2Stomata

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5

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ChloroplastsChloroplasts

-Contain their own DNA-Contain bacterial-like ribosomes-Believed derived from prokaryotic ancestor

cyanobacterium = blue-green alga -Double membrane organelle

defines three three functional spacesfunctional spaces

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StromaThylakoid

Space

Thylakoid Membrane

Intermembrane Space(transports things in and out of the chloroplast, but not central

to photosynthesis itself

Inner Inner ChlorplastChlorplastMembraneMembrane

OuterChlorplastMembrane

3 Central Players

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H+ThylakoidSpace

Thylakoid Membrane - Site of Light Harvestingis where ATP and NADPH are made

Stroma

Stroma - is where all thecarbon fixation reactions

take place

ThylakoidThylakoid SpaceSpace -- is the is the transient energy storage transient energy storage

shed for Hshed for H++ ionsionsgeneratedgenerated

in thein thelight light

reactionsreactions

pH5.5pH 8.5

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ThylakoidThylakoid MembraneMembrane – Light Harvesting Complex

PhotosystemPhotosystem IIII-- Antenna ComplexAntenna Complex-- WaterWater--Splitting ComplexSplitting Complex-- Reaction CenterReaction Center

“Excitation Complex”“Excitation Complex”

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Photon

ThylakoidThylakoid

PhotosystemPhotosystem IIIISTROMASTROMA

Thyl

akoi

dTh

ylak

oid

mem

bran

em

embr

ane

Transferof energy

Pigmentmolecules

THYLAKOID SPACETHYLAKOID SPACE(INTERIOR OF THYLAKOID)(INTERIOR OF THYLAKOID)

Figure 10.12

AntennaAntennaLightLight--harvestingharvesting

complexescomplexes

Primary electionacceptorReactionReaction

centercenter

Specialchlorophyll amolecules

e–

e-

H2O – O2

WaterWaterSplittingSplittingComplexComplex

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Photosystem Antenna Complex- chlorophyll & accessory pigments

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The Antenna Complexproteins which hold PIGMENTS

Pigments:

Chlorophylls - absorb all but greens

Xanthophylls - absorb all but yellows

Carotenoids - absorb all but orange/reds

Phycocyanin - absorb all but blue-green

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Reflected light Reflected light -- the colors we seethe colors we see

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7

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The electromagnetic spectrumThe electromagnetic spectrumthe higher the energy, the shorter the wavelength

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelengthLower energy

Figure 10.6

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Absorption Spectra of Antenna PigmentsAbsorption Spectra of Antenna PigmentsA

bsor

ptio

n of

ligh

t by

Abs

orpt

ion

of li

ght b

ych

loro

plas

t pig

men

tsch

loro

plas

t pig

men

ts

Chlorophyll Chlorophyll aa

Wavelength of light (nm)

Chlorophyll Chlorophyll bb

CarotenoidsCarotenoids

Figure 10.9

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Excitation of Chlorophyll by LightExcitation of Chlorophyll by Light

C

CH

CH2

CC

CC

CCNNC

H3C

C

CC

C C

C

CC

N

CC

C

C NMgH

H3C

H

C CH2 CH3

H

CH3C

HHCH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll ain chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Figure 10.10

Excitedstate

Ene

rgy

of e

lect

ion

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

HeatHeat

PhotonPhoton(fluorescence)(fluorescence)

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Isolated chlorophyllchlorophyll when illuminated– will fluoresce redfluoresce red, giving off light and heat

Blue lightBlue lightabsorbedabsorbed

e-

Red lightRed lightEmittedEmitted

With HeatWith Heat

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Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

Reaction Center Chlorophyll

electron boostedto high energy level

Capture

xxxx

need replacementelectron

e- transferred to an

electron transportchain

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Water splitting complex(a protein in thylakoid membrane)

HO

H

HO

H O=O

H+

H+

H+H+

e-

e-

e-

e-

Discard this, yuk

These e- go to replace electron

lost by chlorophyll

We’ll save H+ in the thylakoid space

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Stroma

Thylakoid Space

ThylakoidMembrane

H+

e-

HOH2

O=O(a gas)

H+

H+

H+

e-e-

e-

HO

HHO-

H+

HO- HO-

HO-

e-

e-

PSII

e-

PSI

e-

NADP+ NADPH

HH++

ATPaseATPaseH+

H+H+

H+ H+H+

An “HAn “H++ pump”pump”

ADP+ Pi

ATP

pH 5.5pH 5.5

pH 8.5pH 8.5

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Key Players in the light reactions

a. photosystem II: captures light energy “boost” e- to a higher energy level, splits water into H+ e- and O2

b. Electron transport H+ pump: lets e-

“fall” to lower energy level, uses energy to form H+ gradient

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c. another photosystem: photosystem I: captures light

energy re-“boosts” e- to a higher energy level – forms NADPH + H+

*makes reducing equivalents*

d. ATP synthase (H+ ATPase): usesH+ gradient to power ATP synthesis

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• Produces NADPH, ATP, and oxygen

Figure 10.13 Photosystem II(PS II)

Photosystem-I(PS I)

ATP

NADPH

NADP+

ADP

CALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

Pq

Cytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

ElectronTransportchain

Electron transport chain

P700

Light

NADPH

NADP+

+ 2 H+

+ H+

1

5

7

2

3

4

6

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Photosystem II-Light Energy used to

Form H+ gradient(ATP Synthesis)

Photosystem I-Light Energy used

to make reducingequivalents

(NADPH + H+)

NonNon--CyclicCyclicElectron Electron

FlowFlow

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NonNon--Cyclic Electron FlowCyclic Electron Flow

MillmakesATP

ATP

e–

e–e–

e–

e–

Phot

on

PhotosystemPhotosystem IIII PhotosystemPhotosystem II

e–

e–

NADPH

Phot

onFigure 10.14

Photosystem I-Light Energy canalso be used to make H+ gradient)

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cyclic electron flowcyclic electron flow– photosystem I is used primarily– Primarily ATP is produced– Little O2 produced

Primaryacceptor

Pq

Fd

Cytochromecomplex

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

ATPFigure 10.15

Photosystem II Photosystem I

NADP+

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Cyclic e- flow

PhotosystemPhotosystemIIIIPhotosystemPhotosystem

II

ElectronElectronTransportTransportHH++ gradientgradient(ATP (ATP synthsynth))

NADPNADP++

ReductaseReductase

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LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2OO2

1

1⁄2

2

Photosystem ILight

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

Figure 10.17

LightDependentReactionsProduce

NADPHNADPHAndATPATP

To powerThe CalvinCycle

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Next Time:Next Time:

the DARK Sidethe DARK Side

the Light the Light inindependent reactionsdependent reactions

The Calvin CycleThe Calvin Cycle

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Summary Summary

1.1. Photosynthesis ultimate source of energy for life Photosynthesis ultimate source of energy for life On earthOn earth

2.2. Ancient Process Ancient Process –– highly conservedhighly conserved3.3. ThylakoidThylakoid membrane, membrane, ThylakoidThylakoid Space, Space, StromaStroma4.4. Photosynthetic light reactionsPhotosynthetic light reactions

--capture energy from sunlight capture energy from sunlight –– light harvesting pigmentslight harvesting pigments--use energy to use energy to “split” water“split” water--use energy to boost electron to high energy level use energy to boost electron to high energy level (PS II)(PS II)--electron transport lets electron fall to low energyelectron transport lets electron fall to low energy

state, energy used to make state, energy used to make HH++ gradientgradient ((ATPATP))--electron reelectron re--boosted by light absorption to highboosted by light absorption to high

energy state energy state (PS I)(PS I)-- high energy electron used to reduce high energy electron used to reduce NADPNADP++ to to

NADPH + HNADPH + H++

5.5. Can vary relative amount of ATP/NADPH madeCan vary relative amount of ATP/NADPH madeby cyclic electron flowby cyclic electron flow