photosynthesis lect 1

8/9/2019 Photosynthesis Lect 1

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Photosynthesis


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OBJECTIVES & SCOPE

Part I: Energy Conservation in Photosynthesis andthe Organization of the Chloroplast

Topics will include:

photosynthesis as a redox process the photosynthetic electron transport chain, its organization

in the thylakoid membrane, and its role in generatingreducing potential and ATP

the dynamic nature of the thylakoid membrane, showinghow changes in the organization of light-harvestingapparatus influence the absorption and distribution of lightenergy

the roles of pigments, e.g., chlorophylls, carotenoids


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OBJECTIVES

Part II: Carbon Metabolism & Interrelated VariationsTopics will include: the path of carbon, energetics, and regulation of the photosynthetic

carbon reduction cycle, or C3 metabolism photorespiration and how limitations are imposed on carbon

assimilation in C2 plants by the photosynthetic carbon oxidation cycle the biochemistry and ecological implications of a specialized CO2

concentration mechanism the C4 syndrome - exhibited by manytropical and subtropical plants

the biology of crassulacean acid metabolism (CAM), in which CO2

uptake and carbon reduction are separated in time as means forimproving water economy translocation and distribution of photoassimilates some of the factors involved in regulating allocation of carbon between

the synthesis of starch (for storage) in the chloroplast and sucrose (forexport) in the cytosol


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PART I

Energy Conservation in Photosynthesisand the Organization of the Chloroplast


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Autotrophs are self-feeders; they sustain themselves

without eating anything derived from other organisms.(auto, self; trophos, feed)

Autotrophs are the producers of the biosphere, producingorganic molecules from CO

2

and other inorganicmolecules.

Almost all plants are photoautotrophs, using the energy ofsunlight to make organic molecules from water and carbon

dioxide. Heterotrophs are the biospheres consumers. Unable to

make their own food, they live on organic materialsproduced by other organisms. (hetero, other)

Autotrophs vs. Heterotrophs


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Photosynthesisoccurs in plants,algae, certain other

protists, and someprokaryotes

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 m

1.5 m

40 m


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Photosynthesis can be defined as the light-drivensynthesis of carbohydrate.

6 CO2 + 12 H2O + Light energy

C6H12O6 + 6 O2 + 6 H2O


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6 CO2 + 12 H2O + Light energyC6H12O6 + 6 O2 + 6 H2O

A. Photosynthesis is a redox reaction.

B. CO2 is reduced to a carbohydrate.

C. Water is oxidized (to oxygen).

D. Water supplies the electrons for the reduction;water is cleaved in the process yieldingoxygen as a by-product.


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6 CO2 + 12 H2O + Light energyC6H12O6 + 6 O2 + 6 H2O

E. Light provides the energy for the reaction.

F. Photosynthesis is an energy conversion processthat converts light energy to chemical energy(carbohydrate).

In a broad sense, it is an example of the 1st Lawof Thermodynamics - energy cannot be creatednor destroyed, but it can be changed from oneform to another.


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Photosynthesis as a redox reaction.

Some definitions:(a) Reduction - gain of electrons.(b) Oxidation - loss of electrons.(c) helpful mnemonics to remember:

"oil rig" - oxidation is loss, reduction is gain, or"Leo says grrrr" - loss equals oxidation, gainreduction.

(d) Redox reaction - reaction in which onecomponent is oxidized and the other is reduced.Electrons must come from somewhere and gosomewhere.


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How can you tell if a molecule has beenoxidized or reduced?

(1) look for a change in valence (i.e., Fe2+

Fe3+

represents an oxidation because an electron waslost, increasing the total positive charge);

(2) In many biological redox reactions, oxidation isusually accompanied by a loss of protons (H+)and reduction is accompanied by a gain ofprotons; and

(3) look for a increase/decrease in the number ofoxygen atoms (oxidation/reduction).


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The reduction sequence of carbon:

carbon dioxide (most oxidized form of carbon)

carboxyl (organic acid) carbonyl (aldehydes,

ketones) hydroxyl (alcohols) methyl

methane (most reduced form of carbon).

Note: In a redox reaction, each step requires the

addition (or removal) of two electrons and twoprotons for reduction (oxidation). Two steps alsorequire the addition/removal of water.


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Photosynthesis as a Redox Process

Photosynthesis is a redox process

-- water is oxidized

-- carbon dioxide is reduced


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Biological redox reactions may require electrondonors and/or acceptors.

These include:

(1) NAD+; (2) NADP+; and (3) FAD;

--- coenzymes (organic compounds, other than

the substrate, required by an enzyme for activity):

NAD(P)+ (ox) + 2e- + 2H+ NAD(P)H (red) + H+

FAD (ox) + 2e- + 2H+ FADH2 (red)


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Tracking Atoms Through Photosynthesis:Scientific Inquiry

CO2 + 2H2O [CH2O] + O2 + H2O

Using a heavy oxygen isotope as label 18O, C.B. van

Niel (1930s) was able to show that the oxygen producedduring photosynthesis originates in water, not in carbondioxide.

About 20 yrs. later:

Experiment 1: CO2 + 2H2O [CH2O] + O2 + H2OExperiment 2: CO2 + 2H2O [CH2O] + O2 + H2O


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Tracking Atoms through Photosynthesis

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2


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The Splitting of Water

Chloroplasts split water into hydrogen and oxygen,incorporating the electrons of hydrogen into sugarmolecules.


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Chloroplasts: The Sites of Photosynthesis in Plants

Leaves are the major locations of photosynthesis

Their green color is from chlorophyll, the greenpigment within chloroplasts

Chlorophyll absorbs light energy, which drives thesynthesis of organic molecules in the chloroplast

Through microscopic pores called stomata, CO2

enters the leaf and O2 exits


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Chloroplasts are found mainly in cells of the

mesophyll, the interior tissue of the leaf

A typical mesophyll cell has 30-40 chloroplasts

The chlorophyll is in the membranes of thylakoids(connected sacs in the chloroplast); thylakoidsmay be stacked in columns called grana

Chloroplasts also contain stroma, a dense fluid


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Leaf cross sectionVein

Mesophyll

Stomata CO2 O2

Mesophyll cellChloroplast

5 m

Outermembrane

IntermembranespaceInnermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 m


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Chemistry - Chloroplasts contain:

1. DNA - circular loop; 120-160 kilobases that

code for about 120 proteins

2. RNA

3. ribosomes

4. proteins - some are coded by the nucleargenome, others by the chloroplastic genome.

For example, rubisco, an important enzyme,has 2 different subunits, one from each source.

The nuclear genes are essential for chloroplast

function.


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5. pigments - make up about 7% of the

chloroplast.

These are molecules with a color that absorblight.

Two major groups of pigments in higherplants, chlorophylls and carotenoids/xanthophylls.

These occur in the thylakoids because theyare highly hydrophobic (fat soluble).


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The Two Stages of Photosynthesis:A Preview


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The Two Stages of Photosynthesis:A Preview

Photosynthesis consists of the light reactions (the

photopart) and Calvin cycle (the synthesispart)

The light reactions (in the thylakoids) split water,release O2, produce ATP, and form NADPH

The Calvin cycle (in the stroma) forms sugar fromCO2, using ATP and NADPH

The Calvin cycle begins with carbon fixation,incorporating CO2 into organic molecules


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H2O

LIGHTREACTIONS

Chloroplast

Light


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H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2


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H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)


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The light reactions convert solar energy to thechemical energy of ATP and NADPH

Chloroplasts are solar-powered chemical factories

Their thylakoids transform light energy into thechemical energy of ATP and NADPH


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The Nature of Sunlight

Light has characteristics of both a wave and a particle.

Light is a form of electromagnetic energy, also calledelectromagnetic radiation.

Like other electromagnetic energy, light travels in rhythmic

waves.

Wavelength = distance between crests of waves.

Wavelength determines the type of electromagnetic

energy.

Light also behaves as though it consists of discreteparticles, called photons. Each photoncontains anamount of energy called quantum.


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The electromagnetic spectrum is the entire range

of electromagnetic energy, or radiation.

Visible light consists of colors we can see,including wavelengths that drive photosynthesis.

Electromagnetic SpectrumElectromagnetic Spectrum


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Visible light

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

105 nm 103 nm 1 nm 103 nm 106 nm1 m

(109 nm) 103 m

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy

Shorter wavelength

Higher energy

Electromagnetic Spectrum


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Chloroplast

Light

Reflectedlight

Absorbedlight

Transmittedlight

Granum


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A spectrophotometer measures a pigments abilityto absorb various wavelengths

This machine sends light through pigments and

measures the fraction of light transmitted at eachwavelength


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White

light

Refracting

prism

Chlorophyll

solution

Photoelectric

tube

Galvanometer

The high transmittance(low absorption)

reading indicates thatchlorophyll absorbsvery little green light.

Greenlight

Slit moves topass light

of selectedwavelength

0 100


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White

light

Refracting

prism

Chlorophyll

solution

Photoelectric

tube

The low transmittance(high absorption)

reading indicates thatchlorophyll absorbsmost blue light.

Blue

light

Slit moves topass light

of selectedwavelength

0 100


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An absorption spectrum is a graph plotting a

pigments light absorption versus wavelength

The absorption spectrum of chlorophyll asuggests that violet-blue and red light work best

for photosynthesis

An action spectrum profiles the relativeeffectiveness of different wavelengths of radiation

in driving a process


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Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Absorp

tionofligh

tby

chloroplastpigme

nts

400 500 600 700


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Action spectrum

Rateo

fphoto-

synthesis

(measured

byO2

release)


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The action spectrum of photosynthesis was first

demonstrated in 1883 by Thomas Engelmann

In his experiment, he exposed different segmentsof a filamentous alga to different wavelengths

Areas receiving wavelengths favorable tophotosynthesis produced excess O2

He used aerobic bacteria clustered along the algaas a measure of O

2production


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Chlorophyll ais the main photosynthetic pigment

Accessory pigments, such as chlorophyll b,broaden the spectrum used for photosynthesis

Accessory pigments called carotenoids absorband dissipate excessive light that would otherwisedamage chlorophyll (i.e., photoprotection).


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Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Absorp

tionofligh

tby

chloroplastpigments

400 500 600 700

C in chlorophyll a


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CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:light-absorbinghead of

molecule; notemagnesium atomat center

Hydrocarbon tail:interacts with

hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown


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Excitation of Chlorophyll by Light

When a pigment absorbs light, it goes from aground state to an excited state, which is unstable

When excited electrons fall back to the ground

state, photons are given off, an afterglow calledfluorescence

If illuminated, an isolated solution of chlorophyllwill fluoresce, giving off light and heat


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Excited

state

Heat

Photon(fluorescence)

Groundstate

Chlorophyllmolecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

e


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A Photosystem: A Reaction Center Associated

with Light-Harvesting Complexes

A photosystem consists of a reaction centersurrounded by light-harvesting complexes

The light-harvesting complexes (pigmentmolecules bound to proteins) funnel the energy ofphotons to the reaction center


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A primary electron acceptor in the reaction center

accepts an excited electron from chlorophyll a

Solar-powered transfer of an electron from achlorophyll amolecule to the primary electron

acceptor is the first step of the light reactions


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How a

photosystemharvestslight

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reaction

center

STROMA

Primary electron

acceptor

e

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Thylakoidmemb

rane


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LightP680

e

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

Energyofelectrons

O2

e

e

+

2 H+H

2

O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e

Primaryacceptor

Photosystem I(PS I)

ee

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light


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Noncyclic Electron Flow

During the light reactions, there are two possibleroutes for electron flow: cyclic and noncyclic

Noncyclic electron flow, the primary pathway,involves both photosystems and produces ATPand NADPH

H2O CO2


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1.--a photon of lightstrikes a pigmentmolecule in a LHC

-- is relayed to otherpigment molecules until

it reaches one of the 2P680 chl a molecules inthe PSII reaction center.

- -it excites one of theP680 electrons to ahigher energy state.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

2

Energyofelectrons

O2

H2O CO2


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2.The electron iscaptured by theprimary electronacceptor.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

2

Energyofelectrons

O2

e

e

+

2 H+H

2O

O21/2

3.H2O CO2


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-- splitting of H2O into2 H and an O atom.-- supply of electronsone by one to P680,each replacing an e-

lost to the primary e-acceptor.-- O atom combineswith another O atom,forming O2.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

Energyofelectrons

O2

e

e

+

2 H+H

2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

H2O CO2


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4.passing ofphotoexcited e-from the primarye-acceptor of PSII to PS I via ETC(Pq, ctytochromecomplex, Pc)

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

Energyofelectrons

O2

e

e

+

2 H+H

2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

H2O CO2 5


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5.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

Energyofelectrons

O2

e

e

+

2 H+H

2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e

Primaryacceptor

Photosystem I(PS I)

Light

5.Exergonic fall of electrons to alower energy level provides energyfor ATP synthesis.


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A mechanical analogy forthe light reactions

ATP

Photosystem II

e

e

ee

Millmakes

ATP

e

e

e

Photosystem I

NADPH

6.transfer of light energy via a LHC to PS I


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-- transfer of light energy via a LHC to PS I,exciting an e- of one of the 2 P700 chl amolecules.-- capture of photoexcited e- by PS Is primary e-acceptor, creating an e- hole in P700.

-- filling the hole by an e- that reaches thebottom of the ETC from PS II.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

Energyofelectrons

O2

e

e

+

2 H+H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e

Primaryacceptor

Photosystem I(PS I)

ee

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light


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7.-- passing of photoexcited e-s from PS Isprimary e- acceptor down a second ETCthrough the protein ferredoxin (Fd).

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

Energyofelectrons

O2

e

e

+

2 H+H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e

Primaryacceptor

Photosystem I(PS I)

ee

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light

H O


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8.-- transfer of electrons from Fd toNADP+ by NADP+ reductase-- 2 electrons required for its reduction

to NADPH

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

Energyofelectrons

O2

e

e

+

2 H+H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e

Primaryacceptor

Photosystem I(PS I)

ee

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light


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Cyclic Electron Flow

Cyclic electron flow uses only photosystem I andproduces only ATP.

Cyclic electron flow generates surplus ATP,satisfying the higher demand in the Calvin cycle.

No NADPH is produced.



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Photosystem IPhotosystem II ATP

Pc

Fd

Cytochrome

complex

Pq

Primary

acceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor


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Functions of the Light Reactions


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The light reactions use solar power to generateATP and NADPH, which provide chemicalenergy and reducing power, respectively, to thesugar-making reactions of the Calvin cycle.

Whether ATP synthesis is driven by noncyclic orcyclic electron flow, the actual mechanism is thesame [chemiosmosis process that uses

membrane electrochemical potential gradients(i.e., proton gradients) to couple the synthesis ofATP].



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Proton motive force is the energized state of amembrane created by a proton gradient andusually formed through the action of an ETC.

Specifically, the ATP required for carbonreduction and other metabolic activities of thechloroplasts is synthesized byphotophosphorylation in accordance with

chemiosmotic mechanism.


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A Comparison of Chemiosmosis in Chloroplastsand Mitochondria

Chloroplasts and mitochondria generate ATP bychemiosmosis, but use different sources of energy

Mitochondria transfer chemical energy from foodto ATP; chloroplasts transform light energy into thechemical energy of ATP

The spatial organization of chemiosmosis differs inchloroplasts and mitochondria

Mitochondrion Chloroplast


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MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthaseKey

H

+

Diffusion

ADP + P

H+i

Higher [H+]

Lower [H+]


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In the current model for the thylakoid membrane,water is split by photosystem II on the side of themembrane facing the thylakoid space

The diffusion of H+ from the thylakoid space back

to the stroma powers ATP synthase ATP and NADPH are produced on the side facing

the stroma, where the Calvin cycle takes place


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photosynthesis lect 1

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