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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
Photosystem II(PS II)
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
Photosystem II(PS II)
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
Photosystem II(PS II)
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
Photosystem II(PS II)
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
Photosystem II(PS II)
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
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
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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
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
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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
(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)
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
(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)
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
Vessel Sieve tube Source cell
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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
Sink cell(storageroot)
H2O
Vessel Sieve tube Source cell
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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
Sink cell(storageroot)
H2O
Vessel Sieve tube Source cell
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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
Sink cell(storageroot)
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