Photosynthesis Lect 2

8/9/2019 Photosynthesis Lect 2

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Photosynthesis


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The Photochemical Reactions

Photosystem I

Photosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor


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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

A primary electron acceptor in the reactioncenter accepts an excited electron fromchlorophyll a


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How a

photosystemharvestslight

Thylakoid

Photon

Light-harvesting

complexes

Photosystem

Reaction

center

STROMA

Primary electronacceptor

e

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Thylakoidmembr

ane


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Red drop effect

quantum yield of photosynthesis (black curve) falls off drastically forfar-red light of wavelengths greater than 680 nm, indicating that far-redlight alone is inefficient in driving photosynthesis.

slight dip near 500 nm reflects the somewhat lower efficiency ofphotosynthesis using light absorbed by accessory pigments,carotenoids.


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Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Absorptionoflightby

chlorop

lastpigme

nts

400 500 600 700


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Enhancement effect

The rate of photosynthesis when red and far-red light are giventogether is greater than the sum of the rates when they are given apart.

The enhancement effect provided essential evidence in favor of theconcept that photosynthesis is carried out by two photochemicalsystems working in tandem but with slightly different wavelengthoptima.


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Far-red light is very effective in oxidizing the cytochrome fin thechloroplast.

If green light is also present, some of the cytochrome becomesreduced.

The 2 wavelengths have opposite effects antagonistic.

clear demonstration of the existence of 2 photochemical systems:one that reduces cytochrome and one that oxidizes it.

done with red alga, in which PS II is driven best by green light and

PS I is driven best by FR light.

Antagonistic Effects


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There are two types of photosystems in thethylakoid membrane

Photosystem II functions first (the numbers reflectorder of discovery) and is best at absorbing a

wavelength of 680 nm

Photosystem I is best at absorbing a wavelengthof 700 nm

The two photosystems work together to use lightenergy to generate ATP and NADPH


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The antenna complex is a transmembrane pigment protein, with three helical regionsthat cross the nonpolar part of the membrane.

Approximately 15 chlorophyll aand bmolecules are associated with the complex, aswell as several carotenoids. The positions of several of the chlorophylls are shown,and two of the carotenoids form an X in the middle of the complex.

In the membrane, the complex is trimeric and aggregates around the periphery of the

PSII reaction center complex. (After Khlbrandt et al. 1994)

Two-dimensional view of the structure of theLHCII antenna complex from higher plants


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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+H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e

Primaryacceptor

Photosystem I(PS I)

e

e

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light

The Photochemical Systems


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Noncyclic Electron Flow(Photophosphorylation)

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

Photophosphorylation process of making ATPfrom ADP and Pi using energy derived from light(photo).

Noncyclic electron flow, the primary pathway,

involves both photosystems and produces ATPand NADPH.


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1. Photosystem II

--a photon of lightstrikes a pigmentmolecule in a LHC

-- is relayed to otherpigment moleculesuntil it reaches one ofthe two P680 chl amolecules in the PSIIreaction center.

- -it excites one of theP680 chl aelectronsto a higher energystate.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

Energyofelectrons

O2


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2. Primary ElectronAcceptor

The electron iscaptured by theprimary electron

acceptor.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

Energyofelectrons

O2

e

e

+

2 H+H2O

O21/2


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3. Photolysis-- splitting of H2O into2 H and an O atom.-- supply of electrons

one 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

H2O CO2

Energyofelectrons

O2

e

e

+

2 H+H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP


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5.

LightP680

e


Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

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)

Light

5. Phosphorylation

Exergonic fall of electrons toa lower energy level provides

energy for ATP synthesis.


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6. Photosystem I-- 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)

NADPHATP

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)

e

e

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light


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7. 2nd ETC

-- 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)

e

e

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light


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8. NADPH

-- transfer of electrons from Fd to NADP+

by NADP+ reductase-- 2 electrons required for NADP+

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)

e

e

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light


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A mechanical analogyfor the light reactions ATP

Photosystem II

e

e

ee

Millmakes

ATP

e

e

e

Photosystem I

NADPH


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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 the

chemical energy of ATP

The spatial organization of chemiosmosis differs inchloroplasts and mitochondria


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MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthase

Key

H+ Diffusion

ADP + P

H+i

Higher [H+]

Lower [H+]


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The current model for the thylakoid membrane isbased on studies in several laboratories

Water is split by photosystem II on the side of themembrane facing the thylakoid space

The diffusion of H+ from the thylakoid space backto 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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STROMA(Low H+ concentration)

Light

Photosystem IICytochrome

complex

2 H+

Light

Photosystem I

NADP+

reductase

Fd

PcPq

H2OO2

+2 H+

1/2

2 H+

NADP+ + 2H+

+ H+NADPH

ToCalvin

cycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATP

ADP

+

PH+

i

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light


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Cyclic Electron Flow:A Second Photophosphorylation Sequence

Cyclic electron flow uses only PS I andproduces only ATP.

No NADPH is produced.

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


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Cyclic Electron Flow

Photosystem I

Photosystem II ATP

Pc

Fd

Cytochrome

complex

Pq

Primary

acceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor


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Regulation between noncyclic & cyclic electron flow

The concentration of NADPH may helpregulate which pathway, cyclic vs. noncyclic,electrons take through the light reactions.

-- If the chloroplasts runs low on ATP for the Calvin cycle,NADPH will begin to accumulate.

-- The rise in NADPH may stimulate a temporary shift fromnoncylic to cyclic electron flow until ATP supply catches

up with demand.


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Functions of the Light Reactions

The light reactions use solar power to generate ATP andNADPH, which provide chemical energy and reducingpower, respectively, to the sugar-making reactions of theCalvin cycle.

Whether ATP synthesis is driven by noncyclic or cyclicelectron flow, the actual mechanism is the same

(chemiosmosis

process that uses membranes to coupleredox reactions to ATP synthesis).


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Calvin Cycle

Th C l i l ATP d NADPH


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The Calvin cycle uses ATP and NADPHto convert CO2 to sugar

The cycle builds sugar from smaller molecules byusing ATP and the reducing power of electronscarried by NADPH

Carbon enters the cycle as CO2 and leaves as asugar named glyceraldehyde-3-phosphate (G3Por PGAL)

The Calvin cycle, like the citric acid cycle,regenerates its starting material after moleculesenter and leave the cycle


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The Calvin cycle (C3 pathway) has threephases:

1. Carbon fixation (catalyzed by rubisco)

2. Reduction

3. Regeneration of the CO2 acceptor (RuBP)

The function of the pathway is to produce a singlemolecule of glucose.


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Carbon Fixation:

- involves carboxylation: 6 CO2 combine with 6RuBP to produce 12 PGA.- RuBP carboxylase (rubisco) catalyzes the

merging of CO2 and RuBP.

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

3

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3 P P

Ribulose bisphosphate(RuBP)


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Reduction:

12 ATP and 12 NADPH are used toconvert 12 PGA to 12 PGAL or G3P.

-- ATP and NADH are incorporated intoPGAL, making PGAL very energy-rich.-- ADP, Pi, NADP+ are released and thenre-energized in noncyclic photo-

phosphorylation.

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

CO2


Rubisco

3 P P



6 P


6 ADP

CALVINCYCLE

3

P P


3

6 NADP+

6

6 NADPH

P i

6 P

1,3-Bisphosphoglycerate

P

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)

Output

Phase 2:Reduction

Glucose andother organiccompounds


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Regeneration:6 ATP are used

to convert 10PGAL to 6 RuBP.

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

CO2


Rubisco

3 P P



6 P


6 ADP

CALVINCYCLE

3

P P


3

6 NADP+

6

6 NADPH

P i

6 P

1,3-Bisphosphoglycerate

P

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)

Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

P5

G3P

Regenerating the 3 RuBP originallyused to combine with 3 CO2allowsthe cycle to repeat.


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Carbohydrate Synthesis

12 PGAL were created in Step 2 (Reduction), but only 10were used in Step 3 (Regeneration). What happened to theremaining 2?

These two remaining PGAL are used to build glucose (and

also other monosaccharides like fructose and maltose).


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Summary of Calvin Cycle

The cycle takes CO2 from the atmosphere and the energy

in ATP and NADPH to create a glucose molecule.

6 CO2 + 18 ATP + 12 NADPH + H+

18 ADP + 18Pi + 12 NADP+ + 1 glucose

Alternative mechanisms of carbon fixation


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Alternative mechanisms of carbon fixationhave evolved in hot, arid climates

Dehydration is a problem for plants, sometimesrequiring tradeoffs with other metabolic processes,especially photosynthesis.

On hot, dry days, plants close stomata, whichconserves water but also limits photosynthesis.

The closing of stomata reduces access to CO2 andcauses O

2

to build up.

These conditions favor a seemingly wastefulprocess called photorespiration.


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An important property of rubisco is its ability tocatalyze both the carboxylation and theoxygenation of RuBP. Oxygenation is the primary

reaction in a process known as photorespiration.

In photorespiration, rubisco adds O2 to theCalvin cycle instead of CO2.

Photorespiration consumes O2 and organic fueland releases CO2, without producing ATP orsugar.

Photosynthetic CO2 Fixation and PhotorespiratoryOxygenation Are Competing Reactions


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The flow of carbon in the leaf is determined by the balancebetween two mutually opposing cycles. The Calvin cycle is capable of independent operation in the presenceof adequate substrates generated by photosynthetic electrontransport. The C2 oxidative photosynthetic carbon cycle (photorespiration)requires continued operation of the Calvin cycle to regenerate its

starting material, ribulose-1,5-bisphosphate.


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Photorespiration may be an evolutionary baggage - a

metabolic relic - because rubisco first evolved at a timewhen the atmosphere had far less O2 and more CO2.

In many plants, photorespiration is a problem because ona hot, dry day it can drain as much as 50% of the carbon

fixed by the Calvin cycle.

As CO2 becomes scarce in the leaf air spaces, rubiscoadds O2 to the Calvin Cycle instead of CO2.

A two-carbon compound (phosphoglycolate) is formed inthe chloroplast.

Peroxisomes and mitochondria rearrange and split the

compound, releasing CO2.

Photorespiration: An Evolutionary Relic?


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C4 Plants

special add-on feature to C3 pathway.

C4 plants minimize the cost of photorespiration byincorporating CO2 into four-carbon compounds inmesophyll cells.

These four-carbon compounds are exported tobundle-sheath cells, where they release CO2 thatis then used in the Calvin cycle.

C4 plants: rice, wheat, soybeans sugarcane, corn,members of the grass family

C3

plants: rice, wheat, soybeans


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Photosynthetic

cells of C4 plantleaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

C4 leaf anatomy

StomaBundle-sheathcell

Pyruvate (3 C)

CO2

Sugar

Vasculartissue

CALVINCYCLE

PEP (3 C)

ATP

ADP

Malate (4 C)

Oxaloacetate (4 C)

The C4 pathway

CO2PEP carboxylase

Mesophyllcell

CAM Pl t


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CAM Plants

another special add-on feature to C3 pathway.

CAM plants open their stomata at night,incorporating CO2 into organic acids.

Stomata close during the day, and CO2 is releasedfrom organic acids and used in the Calvin cycle.

succulent plants, many cacti, pineapple

CAM P th


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CAM Pathway

The physiology of this pathway is almost identical to C4

photosynthesis, with the following changes:

PEP carboxylase still fixes CO2 to OAA, as in C4. Instead ofmalate, however, OAA is converted to malic acid.(a minor difference)

Malic acid is shuttled to the vacuoleof the cell, not movedout of the cell to bundle sheath cells as in regular C4.

During the night, PEP carboxylase is active and malic acid

accumulates in the cells vacuole.

During the day, malic acid is shuttled out of the vacuoleand converted back to OAA (requiring 1 ATP to ADP),releasing CO2. The CO2 is now fixed by rubisco, and theCalvin cycle proceeds.


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Bundle-sheathcell

Mesophyllcell Organic acid

C4CO2

CO2

CALVINCYCLE

Sugarcane Pineapple

Organic acidsrelease CO2 toCalvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Organic acid

CAM

CO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night

SUMMARY


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SUMMARY

Light

CO2H2O

Light reactions Calvin cycle

NADP+

RuBP

G3PATP

Photosystem IIElectron transport

chainPhotosystem I

O2

Chloroplast

NADPH

ADP

+ P i

3-Phosphoglycerate

Starch(storage)

Amino acidsFatty acids

Sucrose (export)

Th I t f Ph t th i A R i


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The Importance of Photosynthesis:A Review

The energy entering chloroplasts as sunlight getsstored as chemical energy in organic compounds.

Sugar made in the chloroplasts supplies chemicalenergy and carbon skeletons to synthesize theorganic molecules of cells.

In addition to food production, photosynthesis

produces the oxygen in our atmosphere.

SUMMARY


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SUMMARY

LIGHT REACTIONS:

carried out by molecules in the thylakoid membranes

convert light energy to the chemical energy of ATP andNADPH

split H2O and release O2 to the atmosphere

CALVIN CYCLE REACTIONS:

take place in the stroma

use ATP and NADPH to convert CO2 to the sugar G3P

return ADP, inorganic phosphate, and NADP+ to the lightreactions

Phloem transports the products of photosynthesis and


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Phloem transports the products of photosynthesis andother organic nutrients

Translocation occurs through sieve-tube elements, or sieve-tubemembers. End walls between them are called sieve plates.

Companion cell, non-conducting cell alongside each sieve tubeelement, and connected to it by plasmodesmata

Mo ement from S gar So rces to S gar Sinks


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Movement from Sugar Sources to Sugar Sinks

Phloem sap is an aqueous solution that is mostly

sucrose.

It travels from a sugar source to a sugar sink.

A sugar source is an organ that is a net producerof sugar, such as mature leaves.

A sugar sink is an organ that is a net consumer or

storer of sugar, such as a tuber or bulb, growingroots, shoot tips, stems, fruits.


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Sugar must be loaded into sieve-tube membersbefore being exposed to sinks (phloem loading).

In many plant species, sugar moves by symplastic

and apoplastic pathways.


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Loading of Sucrose into Phloem

Mesophyll cell

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Companion(transfer) cell

Mesophyll cellBundle-sheath cell

Phloemparenchyma cell

Sieve-tubemember

Protonpump

Low H+ concentration

Sucrose

High H+ concentrationCotransporter

Key

Apoplast

Symplast


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In many plants, phloem loading requires active

transport.

Proton pumping and cotransport of sucrose andH+ enable the cells to accumulate sucrose.

Pressure Flow: The Mechanism of Translocation


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in Angiosperms

In studying angiosperms, researchers haveconcluded that sap moves through a sieve tube bybulk flow driven by positive pressure, known aspressure flow.

Vessel Sieve tube Source cell


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1.Loading of sugar into thesieve tube at the source

reduces water potentialinside the sieve-tubeelements.

This causes the tube totake up water by osmosis.

Vessel(xylem)

Sieve tube(phloem)

Sucrose

Source cell(leaf)

H2O

H2O

Sucrose

Sink cell(storageroot)

H2O



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2.The uptake of water

generates a positivepressure that forces thesap to flow along thetube.

Vessel(xylem)

Sieve tube(phloem)

Sucrose

Source cell(leaf)

H2O

H2O

Sucrose


H2O



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3.The pressure is relievedby the unloading ofsugar and theconsequent loss of waterat the sink.

Vessel(xylem)

Sieve tube(phloem)

Sucrose

Source cell(leaf)

H2O

H2O

Sucrose


H2O



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4.

In leaf-to-roottranslocation, xylemrecycles water fromsink to source.

Vessel(xylem)

Sieve tube(phloem)

Sucrose

Source cell(leaf)

H2O

H2O

Sucrose


H2O


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The pressure flow hypothesis explains why

phloem sap always flows from source to sink.

Experiments have built a strong case for pressureflow as the mechanism of translocation in

angiosperms.

Experiment by Rogers and Peel


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p y g

Sap

droplet

Aphid feeding Stylet in sieve-tubemember (LM)

Stylet

Severed styletexuding sap

Sap droplet

Sieve-tubemember

25 m

What if there are more sinks than source?


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What if there are more sinks than source?

Sinks vary in energy demands and capacity tounload sugars. In some plants, there are moresinks than can be supported by sources.

Self-thinning removing sinks, e.g., plants mayabort some flowers, young fruits, or seeds.

results to larger but fewer fruits

Documents

Photosynthesis Lect 2