photosynthesis: the light...

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Photosynthesis:Photosynthesis:The Light ReactionsThe Light Reactions

OVERALL REACTION OFOVERALL REACTION OFPHOTOSYNTHESISPHOTOSYNTHESIS

6CO6CO22 + 6H + 6H220 0 CC66HH1212OO66 + 6O + 6O22

Light,Light,ChlorophyllChlorophyll

In reality, photosynthesis adds one COIn reality, photosynthesis adds one CO22 at a time: at a time:

COCO22 + H + H22O + light energy O + light energy →→ CH CH22O + OO + O22

CHCH22O represents the general formula for a sugar.O represents the general formula for a sugar.

LIGHT REACTIONS LIGHT REACTIONS VS.VS. CALVIN CYCLECALVIN CYCLE

••THE LIGHT REACTIONS CONVERT SUNLIGHTTHE LIGHT REACTIONS CONVERT SUNLIGHTINTO CHEMICAL ENERGY (ATP AND NADPH).INTO CHEMICAL ENERGY (ATP AND NADPH).

•• THE CALVIN CYCLE FIXES COTHE CALVIN CYCLE FIXES CO22 MOLECULES MOLECULESAND REDUCES THEM TO SUGAR USING THEAND REDUCES THEM TO SUGAR USING THEATP AND NADPH FROM THE LIGHT REACTIONS.ATP AND NADPH FROM THE LIGHT REACTIONS.

UC Santa Cruz Biology BIO20B: Animal and Plant Physiology Dr. Lincoln Taiz

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The Light Reactions & Calvin CycleThe Light Reactions & Calvin CycleWork TogetherWork Together

LIGHT HASLIGHT HASCHARACTERISTICS OFCHARACTERISTICS OF

BOTH A PARTICLE AND ABOTH A PARTICLE AND AWAVEWAVE

A WAVE IS CHARACTERIZED BY AA WAVE IS CHARACTERIZED BY AWAVELENGTH AND A FREQUENCYWAVELENGTH AND A FREQUENCY

c = c = λνλν

Velocity lambda Velocity lambda nunu

(wavelength) (frequency) (wavelength) (frequency)

LIGHT IS ALSO A PARTICLE CALLED ALIGHT IS ALSO A PARTICLE CALLED APHOTONPHOTON

EACH PHOTON CONTAINS AN AMOUNT OF ENERGYCALLED A QUANTUM.

THE ENERGY OF LIGHT IS NOT CONTINUOUS, BUTEXISTS IN DISCREET PACKETS OR QUANTA WHICH AREPROPORTIONAL TO THE WAVELENGTH.

E = hν, where h is Planck’s constant

energy frequencyenergy frequency (ν = c/λ)


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The entire range of electromagnetic radiation is theThe entire range of electromagnetic radiation is theelectromagnetic spectrumelectromagnetic spectrum..

The most important segment for life is a narrow bandThe most important segment for life is a narrow bandbetween 380 to 750 nm, between 380 to 750 nm, visible lightvisible light..

TheThePlantPlantCellCell

CHLOROPLASTCHLOROPLAST

StromaStromalamellaelamellae

DoubleDoublemembranemembrane

Grana Grana stackstack((thylakoidsthylakoids))

StromaStroma

CHLOROPLASTCHLOROPLAST


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ENGELMANNENGELMANNEXPERIMENTEXPERIMENT

(1881)(1881)

FIRST CRUDEFIRST CRUDEACTIONACTIONSPECTRUM FORSPECTRUM FORPHOTOSYNTHESISPHOTOSYNTHESIS

Total PigmentTotal Pigment

DiscrepancyDiscrepancy

When light meets matter, it may be reflected,When light meets matter, it may be reflected,transmitted, or absorbed.transmitted, or absorbed.

Different pigments absorbDifferent pigments absorbphotons of differentphotons of differentwavelengths.wavelengths.

A leaf looks greenA leaf looks greenbecause chlorophyll,because chlorophyll,the dominant pigment,the dominant pigment,absorbs red and blueabsorbs red and bluelight, while transmittinglight, while transmittingand reflecting greenand reflecting greenlight.light.

A A spectrophotometerspectrophotometer measures the ability of a pigment to measures the ability of a pigment toabsorb various wavelengths of light.absorb various wavelengths of light.

It beams narrow wavelengths of light through a solutionIt beams narrow wavelengths of light through a solutioncontainingcontaininga pigment anda pigment andmeasures themeasures thefraction of lightfraction of lighttransmitted attransmitted ateach wavelength.each wavelength.

An An absorptionabsorptionspectrumspectrum plots a plots apigmentpigment’’s lights lightabsorption versusabsorption versuswavelength.wavelength.


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The light reaction can perform work with thoseThe light reaction can perform work with thosewavelengths of light that are absorbed.wavelengths of light that are absorbed.

In theIn the thylakoid thylakoid are several pigments that differ in theirare several pigments that differ in theirabsorption spectrum.absorption spectrum.

Chlorophyll Chlorophyll aa,, the thedominant pigment,dominant pigment,absorbs best in the redabsorbs best in the redand blue wavelengths,and blue wavelengths,and least in the green.and least in the green.

Other pigmentsOther pigmentswith differentwith differentstructures havestructures havedifferentdifferentabsorptionabsorptionspectra.spectra.

Photons are absorbedPhotons are absorbedby clusters of pigmentby clusters of pigmentmolecules in themolecules in thethylakoid thylakoid membranes.membranes.

The energy of theThe energy of thephoton isphoton isconverted to theconverted to thepotential energypotential energyof an electronof an electronraised from itsraised from itsground state to anground state to anexcited state.excited state.

•• Excited electrons are unstable.Excited electrons are unstable.

•• Generally, they drop to their ground state in a billionthGenerally, they drop to their ground state in a billionthof a second, releasing heat energy.of a second, releasing heat energy.

•• Some pigments, including chlorophyll, release a photonSome pigments, including chlorophyll, release a photonof light, in a process called fluorescence, as well as heatof light, in a process called fluorescence, as well as heat

In theIn the thylakoid thylakoid membrane, chlorophyll is organizedmembrane, chlorophyll is organizedalong with proteins and smaller organic molecules intoalong with proteins and smaller organic molecules intophotosystemsphotosystems..

AA photosystem photosystem acts like a light-gathering acts like a light-gathering ““antennaantennacomplexcomplex”” consisting of a few hundred chlorophyll consisting of a few hundred chlorophyll a,a,chlorophyll chlorophyll b,b,and carotenoidand carotenoidmolecules.molecules.


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•• When any antenna molecule absorbs a photon, theWhen any antenna molecule absorbs a photon, theenergy (but not the electron) is transmitted fromenergy (but not the electron) is transmitted frommolecule to molecule until it reaches a particularmolecule to molecule until it reaches a particularchlorophyll chlorophyll aa molecule, called the molecule, called the reaction centerreaction center..

•• At the reaction center is aAt the reaction center is a primary electron acceptorprimary electron acceptorwhich removes an excited electron from the reactionwhich removes an excited electron from the reactioncenter chlorophyll a. center chlorophyll a. This starts the light reactions.This starts the light reactions.

EachEach photosystem photosystem(consisting of a reaction-(consisting of a reaction-center chlorophyll andcenter chlorophyll andprimary electronprimary electronacceptor surrounded byacceptor surrounded byan antenna complex)an antenna complex)functions in thefunctions in thechloroplast as a light-chloroplast as a light-harvesting unit.harvesting unit.

The Z-Scheme: The two The Z-Scheme: The two photosystems photosystems are arranged according toare arranged according totheir their redox redox potentials (reducing power).potentials (reducing power).

There are two types ofThere are two types of photosystems photosystems..

Photosystem Photosystem II has a reaction center has a reaction centerchlorophyll, P700, that has anchlorophyll, P700, that has anabsorption peak at 700nm.absorption peak at 700nm.

The Z-Scheme:The Z-Scheme:

Photosystem Photosystem IIII has a reaction has a reactioncenter with a peak at 680nm.center with a peak at 680nm.

The Z-SchemeThe Z-Scheme


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Plastoquinone Plastoquinone is a mobileis a mobileelectron carrier in theelectron carrier in themembranemembrane


Plastocyanin Plastocyanin is a copper-containingis a copper-containingprotein that acts as a mobile electronprotein that acts as a mobile electroncarrier on the surface of the membrane.carrier on the surface of the membrane.


The light reactions use the solar energy of photonsThe light reactions use the solar energy of photonsabsorbed by bothabsorbed by bothphotosystem photosystem I andI andphotosystem photosystem II toII toprovide chemicalprovide chemicalenergy in the formenergy in the formof ATP and reducingof ATP and reducingpower in the formpower in the formof the electronsof the electronscarried by NADPH.carried by NADPH.

During the light reactions,During the light reactions,there are two possible routesthere are two possible routesfor electron flow: cyclic andfor electron flow: cyclic and

noncyclicnoncyclic..


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Noncyclic Noncyclic electron flow, the predominantelectron flow, the predominantroute, produces both ATP and NADPH.route, produces both ATP and NADPH.

1. When1. When photosystem photosystem II absorbs light,II absorbs light,an excited electron is captured by thean excited electron is captured by theprimary electron acceptor, leaving theprimary electron acceptor, leaving thereaction center oxidized.reaction center oxidized.

An enzyme extracts electrons fromAn enzyme extracts electrons fromwater and supplies them to the oxidizedwater and supplies them to the oxidizedreaction center.reaction center.

Noncyclic Noncyclic Electron FlowElectron Flow

This reaction splits water into twoThis reaction splits water into twohydrogen ions and an oxygen atomhydrogen ions and an oxygen atomwhich combines with another towhich combines with another toform Oform O22..

Noncyclic Noncyclic Electron FlowElectron Flow Photoexcited Photoexcited electrons pass alongelectrons pass alongan electron transport chain beforean electron transport chain beforeending up at an oxidizedending up at an oxidizedphotosystem photosystem I reaction center.I reaction center.


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As these electrons pass along theAs these electrons pass along thetransport chain, their energy istransport chain, their energy isharnessed to produce ATP.harnessed to produce ATP.

The electrons from PSII fill an electronThe electrons from PSII fill an electron““holehole”” in an oxidized P700 center in an oxidized P700 centercreated when photons excite electronscreated when photons excite electronson theon the photosystem photosystem I complex.I complex.

Ultimately, these electrons areUltimately, these electrons arepassed from the transport chainpassed from the transport chain((Ferredoxin Ferredoxin and NADPand NADP++ Reductase) Reductase)to NADPto NADP++, creating NADPH., creating NADPH.

Under certain conditions,Under certain conditions, photoexcited photoexcitedelectrons fromelectrons from photosystem photosystem I can take anI can take analternative pathway, alternative pathway, cyclic electron flowcyclic electron flow..

Cyclic Electron FlowCyclic Electron Flow


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Excited electrons fromExcited electrons from photosystem photosystem I canI canbe transferred back to the be transferred back to the CytochromeCytochromecomplex.complex.

Cyclic Electron FlowCyclic Electron FlowThis process generates ATP, but notThis process generates ATP, but notNADPH.NADPH.

Noncyclic Noncyclic electron flow produces ATP and NADPH inelectron flow produces ATP and NADPH inroughly equal quantities.roughly equal quantities.

However, the Calvin cycle consumes more ATP thanHowever, the Calvin cycle consumes more ATP thanNADPH.NADPH.

Cyclic electron flow allows the chloroplast to generateCyclic electron flow allows the chloroplast to generateenough surplus ATP to satisfy the higher demand forenough surplus ATP to satisfy the higher demand forATP in the Calvin cycle.ATP in the Calvin cycle.

Chemiosmoticmechanism of ATPproduction in thechloroplast


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Both chloroplasts and mitochondria generateBoth chloroplasts and mitochondria generateATP by the same ATP by the same chemiosmotic chemiosmotic mechanism.mechanism.

An electron transport chain pumps protons acrossAn electron transport chain pumps protons acrossa membrane as electrons are passed along a seriesa membrane as electrons are passed along a seriesof more electronegative carriers.of more electronegative carriers.

This builds the proton-motive force in the form ofThis builds the proton-motive force in the form ofan Han H++ gradient across the membrane. gradient across the membrane.

ATPATP synthase synthase molecules harness the proton-molecules harness the proton-motive force to generate ATP as Hmotive force to generate ATP as H++ diffuses back diffuses backacross the membraneacross the membrane..


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When illuminated, the pH in theWhen illuminated, the pH in thethylakoid thylakoid space drops to about 5 andspace drops to about 5 andthe pH in thethe pH in the stroma stroma increases toincreases toabout 8, aabout 8, a thousandfold thousandfold different in Hdifferent in H++

concentration.concentration.

The light-reaction The light-reaction ““machinerymachinery””produces ATP and NADPH onproduces ATP and NADPH onthethe stroma stroma side of theside of the thylakoid thylakoid..

The proton gradient isThe proton gradient isformed by both theformed by both thesplitting of water andsplitting of water andelectron transport.electron transport.

PHOTOSYNTHESIS:PHOTOSYNTHESIS:THE CARBONTHE CARBONREACTIONSREACTIONS

THE LIGHT AND CARBON REACTIONS OFTHE LIGHT AND CARBON REACTIONS OFPHOTOSYNTHESISPHOTOSYNTHESIS


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Rubisco

(Triose-phosphate)

The Calvin CycleThe Calvin Cycle

RibuloseRibulose-1,5--1,5-bisphosphatebisphosphatecarboxylasecarboxylase//oxygenaseoxygenase

““RUBISCORUBISCO””RUBISCO makes up about 40% of the totalRUBISCO makes up about 40% of the totalprotein of the leaf.protein of the leaf.

RUBISCO the most abundant protein onRUBISCO the most abundant protein onearth.earth.

The Calvin cycle has three phases.The Calvin cycle has three phases.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In the carbon fixation phase, eachIn the carbon fixation phase, eachCOCO22 molecule is attached to a five- molecule is attached to a five-carbon sugar,carbon sugar, ribulose bisphosphate ribulose bisphosphate((RuBPRuBP).).

This is catalyzed byThis is catalyzed by RuBP carboxylaseRuBP carboxylaseoror RubiscoRubisco..

The six-carbon intermediate splits inThe six-carbon intermediate splits inhalf to form two molecules of 3-half to form two molecules of 3-phosphoglycerate phosphoglycerate per COper CO22..


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During the Reduction Phase, eachDuring the Reduction Phase, each

3-3-phosphoglycerate phosphoglycerate first receivesfirst receivesanother phosphate group from ATP toanother phosphate group from ATP toform 1,3form 1,3 bisphosphoglycerate bisphosphoglycerate..

Then, a pair of electrons from NADPHThen, a pair of electrons from NADPHreduces each 1,3reduces each 1,3 bisphosphoglycerate bisphosphoglycerate totoGlyceraldehyde Glyceraldehyde 3-P (G3P or 3-P (G3P or triosetriosephosphate).phosphate).

The electrons from NADPH reduce aThe electrons from NADPH reduce acarboxyl group to a carbonyl group tocarboxyl group to a carbonyl group tomake the three carbon sugar.make the three carbon sugar.


Fig. 10.17.2

To obtain one net G3P, we require 3 COTo obtain one net G3P, we require 3 CO22

(3C) and three(3C) and three RuBP RuBP (15C).(15C).

After fixation and reduction we produceAfter fixation and reduction we producesix six molecules of G3P (18C).molecules of G3P (18C).

One of these six G3P (3C) represents a netOne of these six G3P (3C) represents a netgain of carbohydrate (one G3P).gain of carbohydrate (one G3P).

One G3P can exit the cycle and be usedOne G3P can exit the cycle and be usedby the plant cell.by the plant cell.

The other five G3Ps (15C) remain in theThe other five G3Ps (15C) remain in thecycle to regenerate three cycle to regenerate three RuBPs RuBPs (the five(the fivecarbon acceptor).carbon acceptor).

ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATEONLY ONE-SIXTH OF THE TRIOSE PHOSPHATEIS USED FOR SUGAR OR STARCH - THEIS USED FOR SUGAR OR STARCH - THE

REMAINDER IS USED FOR REMAINDER IS USED FOR REGENERATIONREGENERATION OF OFFIVE-CARBON ACCEPTERSFIVE-CARBON ACCEPTERS

6ATP, 6NADPH6ATP, 6NADPH

3 CO2 3 CO2 →→ 6 PGA 6 PGA →→ 6 6 TrioseTriose-P-P

1Triose-P1Triose-P5 Triose-P5 Triose-P

3 RUBP3 RUBP

REGENERATIONREGENERATION3ATP3ATP

Carboxylation Reduction

Net Gain


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In the Regeneration Phase, theIn the Regeneration Phase, theformation of the 5 carbon COformation of the 5 carbon CO22

acceptor (acceptor (RuBPRuBP), these five G3P), these five G3Pmolecules are rearranged to form 3molecules are rearranged to form 3RuBPsRuBPs..

To do this, the cycle must spend threeTo do this, the cycle must spend threemore molecules of ATP (one permore molecules of ATP (one perRuBPRuBP) to complete the cycle and) to complete the cycle andprepare for the next.prepare for the next.


Fig. 10.17.3

BOOK KEEPING: HOW MUCHBOOK KEEPING: HOW MUCHENERGY IS USED IN THEENERGY IS USED IN THE

CALVIN CYCLE?CALVIN CYCLE?

For the net synthesis of one G3PFor the net synthesis of one G3Pmolecule, the Calvin recyclemolecule, the Calvin recycleconsumes nine ATP and sixconsumes nine ATP and sixNADPH.NADPH.

It It ““costscosts”” three ATP and two NADPH three ATP and two NADPHper COper CO22..

The G3P from the Calvin cycle is theThe G3P from the Calvin cycle is thestarting material for metabolicstarting material for metabolicpathways that synthesize otherpathways that synthesize otherorganic compounds, includingorganic compounds, includingglucose and other carbohydrates.glucose and other carbohydrates.


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Fig. 10.17.3

In photosynthesis, the energy that enters the chloroplastsIn photosynthesis, the energy that enters the chloroplastsas sunlight becomes stored as chemical energy inas sunlight becomes stored as chemical energy inorganicorganiccompounds.compounds.

SUMMARYSUMMARY


Fig. 10.20

PHOTORESPIRATION:PHOTORESPIRATION:Major cause of inefficiency ofMajor cause of inefficiency of

the Calvin Cyclethe Calvin Cycle

PHOTORESPIRATIONPHOTORESPIRATION

RUBISCO can also react with O2 :

RUBP + O2 → 3-PGA + 2-phosphoglycolate

In the presence of equal amounts of COIn the presence of equal amounts of CO22 and O and O22,,RUBISCO reacts with CORUBISCO reacts with CO22 about 80 times faster than about 80 times faster thanwith Owith O22..

However, the concentration of OHowever, the concentration of O22 in an aqueous in an aqueoussolution is about 24 times higher than thesolution is about 24 times higher than theconcentration of COconcentration of CO22..

Hence, theHence, the carboxylationcarboxylation reaction outruns thereaction outruns theoxygenationoxygenation reaction by only about reaction by only about 3 to 1.3 to 1.


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EVOLUTIONARYEVOLUTIONARYADAPTATIONS TO MINIMIZEADAPTATIONS TO MINIMIZE

PHOTORESPIRATIONPHOTORESPIRATION

NO FORM OF RUBISCO HASNO FORM OF RUBISCO HASEVER EVOLVED THAT DOES NOTEVER EVOLVED THAT DOES NOT

REACT WITH OXYGENREACT WITH OXYGEN

OXYGENATION IS APPARENTLYOXYGENATION IS APPARENTLYAN INHERENT PROPERTY OFAN INHERENT PROPERTY OF

THE ENZYMETHE ENZYME

COCO22-CONCENTRATION-CONCENTRATIONMECHANISMSMECHANISMS

CC44 PHOTOSYNTHESIS - PHOTOSYNTHESIS - SpatialSpatial separation separationof COof CO22-fixation via PEP -fixation via PEP Carboxylase Carboxylase and theand theCalvin Cycle; Calvin Cycle; Kranz Kranz leaf anatomy.)leaf anatomy.)

CRASSULACEAN ACID METABOLISM (CAM)CRASSULACEAN ACID METABOLISM (CAM)- - Temporal Temporal separation of COseparation of CO22-fixation via PEP-fixation via PEPCarboxylase Carboxylase and the Calvin Cycle; stomataand the Calvin Cycle; stomataopen at night.open at night.

COCO2 2 PUMPS ON PLASMA MEMBRANE - OnlyPUMPS ON PLASMA MEMBRANE - Onlyin algae and in algae and cyanobacteriacyanobacteria


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BASIC LEAF ANATOMY

CC33 vs vs CC44 Leaf Anatomy Leaf Anatomy

Typical CTypical C33 Leaf Leaf SUGAR CANE, A C4 MONOCOT KRANZ LEAFKRANZ LEAF

ANATOMYANATOMY

Large chloroplasts inbundle sheath

Radialarrangement ofmesophyll cells


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FLAVERIA, A C4 DICOTKRANZ LEAFKRANZ LEAF

ANATOMYANATOMY

Large chloroplasts inbundle sheath

Plasmodesmata (poresbetween cells)

CRASSULACEAN ACIDCRASSULACEAN ACIDMETABOLISM (CAM)METABOLISM (CAM)

((NOT RESTRICTED TO CRASSULACEAE;NOT RESTRICTED TO CRASSULACEAE;FOUND IN NUMEROUS DESERTFOUND IN NUMEROUS DESERTSUCCULENTS, SUCH AS CACTI,SUCCULENTS, SUCH AS CACTI,

EUPHORBIAS, ETC.)EUPHORBIAS, ETC.)

STOMATAL OPENING AND COSTOMATAL OPENING AND CO22 FIXATION FIXATIONOCCUR AT NIGHTOCCUR AT NIGHT

Openingstomates only atnight preventswater loss due totranspiration.


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Stomata areclosed during theday.

Calvin Cycleoperates duringthe day.


Fig. 10.19


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PLANT FORM &PLANT FORM &FUNCTIONFUNCTION

ColeusColeus

Plants have twobasic organsystems: shootand root…

(Adapted forphotosynthesisand sexualreproduction)

(Nonphotosynthetic;adapted foranchorage and waterand nutrientabsorption)

•• Although all angiosperms have a number of featuresAlthough all angiosperms have a number of featuresin common, two plants groups, the monocots andin common, two plants groups, the monocots anddicotsdicots, differ in many anatomical details., differ in many anatomical details.


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• Modified shoots with diverse functions haveevolved in many plants.

––StolonsStolons, such as the “runners” of strawberry plants,grow on the surface and enable a plant to colonizelarge areas asexually when a parent plant gives riseto many smaller offspring.

–These shoots, which include stolons, rhizomes,tubers, and bulbs, are often mistaken for roots.

– Rhizomes, like those of ginger, are horizontalstems that grow underground.

–Tubers, including potatoes, are the swollen ends ofrhizomes specialized for food storage.–Bulbs, such as onions, are vertical, undergroundshoots consisting mostly of the swollen bases of leavesthat store food.

•• Plant taxonomists use leaf shape, spatialPlant taxonomists use leaf shape, spatialarrangement of leaves, and the pattern ofarrangement of leaves, and the pattern ofveins to help identify and classify plants.veins to help identify and classify plants.––For example, simple leaves have a single,For example, simple leaves have a single,undivided blade, while compound leaves haveundivided blade, while compound leaves haveseveral leaflets attached to the petiole.several leaflets attached to the petiole.––A compound leaf has a bud where its petioleA compound leaf has a bud where its petioleattaches to the stem, not at the base of the leaflets.attaches to the stem, not at the base of the leaflets.

• Some plants have leaves that have becomeadapted by evolution for other functions.–These include tendrils to cling to supports, spines ofcacti for defense, leaves modified for water storage,and brightly colored leaves that attract pollinators.


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•• The first organ to emerge from the germinating seed is theThe first organ to emerge from the germinating seed is theradicleradicle, the embryonic root., the embryonic root.

––Next, the shoot tip must break through the soil surface.Next, the shoot tip must break through the soil surface.

––In garden beans and many otherIn garden beans and many other dicots dicots, a hook forms in the, a hook forms in thehypocotylhypocotyl, and growth pushes it aboveground., and growth pushes it aboveground.

––Stimulated by light, theStimulated by light, the hypocotyl hypocotyl straightens, raising thestraightens, raising thecotyledons and cotyledons and epicotyl epicotyl above the surface.above the surface.

SeedSeedGerminationGermination

•• Peas, though in the same family as beans, have a differentPeas, though in the same family as beans, have a differentstyle of germinating.style of germinating.

––A hook forms in theA hook forms in the epicotyl epicotyl rather than therather than the hypocotyl hypocotyl, and, andthe shoot tip is lifted gently out of the soil by elongation of thethe shoot tip is lifted gently out of the soil by elongation of theepicotyl epicotyl and straightening the hook.and straightening the hook.

––Pea cotyledons, unlike those of beans, remain behindPea cotyledons, unlike those of beans, remain behindunderground.underground.

•• Corn and other grasses, which are monocots, use a differentCorn and other grasses, which are monocots, use a differentmethod for breaking ground when they germinate.method for breaking ground when they germinate.

––TheThe coleoptile coleoptile pushes upward through the soil and into thepushes upward through the soil and into theair.air.

––The shoot tip then grows straight up through the protectiveThe shoot tip then grows straight up through the protectivesheath provided by the tubularsheath provided by the tubular coleoptile coleoptile..

• Each organ of a planthas three tissue systems:the dermal, vascular,and ground tissuesystems.

3. Plant organs are composed of three tissuesystems: dermal vascular, and ground

–Each system is continuousthroughout the plant body.


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•• The The dermal tissuedermal tissue, or , or epidermisepidermis, is generally, is generallya single layer of tightly packed cells thata single layer of tightly packed cells thatcovers and protects all young parts of thecovers and protects all young parts of theplant.plant.

••The epidermis of leaves and most stemsThe epidermis of leaves and most stemssecretes a secretes a waxy coatingwaxy coating, the , the cuticlecuticle, that retards, that retardsthe evaporation of water from the surface.the evaporation of water from the surface.

• Vascular tissue - continuous throughout the plant,is involved in the transport of materials betweenroots and shoots.

–Xylem conveys water and dissolved minerals upwardfrom roots into the shoots.

–Phloem transports sugars made in mature leaves to theroots and to nonphotosynthetic parts of the shoot system.

• Ground tissue is the filler tissue that isneither dermal tissue nor vascular tissue.

–In dicot stems, ground tissue is divided into pith,internal to vascular tissue, and cortex, external tothe vascular tissue.

–The functions of ground tissue includephotosynthesis, storage, and support.

Parenchyma

4. Plant tissues are composed of three basiccell types:

Collenchyma

Sclerenchyma


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• Mature parenchyma cells have primary wallsthat are relatively thin and flexible, and mostlack secondary walls.

–Parenchyma cells are often depicted as “typical”plant cells because they generally are the leastspecialized.

• Parenchyma cells perform most of themetabolic functions of the plant, synthesizingand storing various organic products.

–For example, photosynthesis occurs within thechloroplasts of parenchyma cells in the leaf.

–Some cells inthe stems androots haveplastids calledamyloplasts thatstore starch.

ChlorenchymaChlorenchyma

(Elodea leaf)

Aerenchyma

(Common insubmerged rootsand shoots.)


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• The protoplasts of neighboring cells, includingparenchyma, are connected by plasmodesmata,cytoplasmic channels that pass through pores inthe walls.

–The endoplasmicreticulum iscontinuous throughthe plasmodesmata.

• Collenchyma cells have thicker primary wallsthan parenchyma cells, though the walls areunevenly thickened.

–Grouped into strands or cylinders, collenchyma cells helpsupport young parts of the plant shoot.

–Young cells and petioles often have a cylinder ofcollenchyma just below their surface, providing supportwithout restraining growth.

–Functioning collenchyma cells are living and flexible andelongate with the stems and leaves they support.

• Sclerenchyma cells also function as supportingelements of the plant, with thick secondary wallsusually strengthened by lignin.

–They are much more rigidthan collenchyma cells.

–Unlike parenchyma cells,they cannot elongate andoccur in plant regions thathave stopped lengthening.

Fibers


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•• Most Most sclerenchyma sclerenchyma cells are dead atcells are dead atfunctional maturity, but they produce rigidfunctional maturity, but they produce rigidsecondary cells walls before the protoplastsecondary cells walls before the protoplastdies.dies.

•• Vessel elements andVessel elements and tracheids tracheids in the xylemin the xylemareare sclerenchyma sclerenchyma cells that function for bothcells that function for bothsupport and transport.support and transport.

•Two other sclerenchyma cells, fibers andsclereids, are specialized entirely in support.

–Fibers are long, slender and tapered, and usuallyoccur in groups.

•Those from hemp fibers are used for making rope andthose from flax for weaving into linen.

–Sclereids, shorter than fibers and irregular inshape, impart the hardness to nutshells and seedcoats and the gritty texture to pear fruits.

SclereidsFibers

•• The water conducting elements ofThe water conducting elements ofxylem, thexylem, the tracheids tracheids and and vessel elementsvessel elements,,are elongated cells that are dead atare elongated cells that are dead atfunctional maturityfunctional maturity, when these cells are, when these cells arefully specialized for their function.fully specialized for their function.

––The thickened cell walls form a nonlivingThe thickened cell walls form a nonlivingconduit through which water can flow.conduit through which water can flow.

••Vessel elements stack to form Vessel elements stack to form vesselsvessels..


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Macerated Xylem Vessel Elements


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Tracheids in pine

• Both tracheids and vessels have secondary wallsinterrupted by pits, thinner regions where onlyprimary walls are present.

•Tracheids are long, thin cells with tapered ends.–Water moves from cell to cell mainly through pits.–Because their secondary walls are hardened with lignin,tracheids function in support as well as transport..

•Vessel elements are generally wider, shorter,thinner walled, and less tapered than tracheids.

–Vessel elements are aligned end to end, forming longmicropipes, xylem vessels.–The ends are perforated, enabling water to flow freely

The Phloem

• In the phloem, sucrose, other organiccompounds, and some mineral ions movethrough tubes formed by chains of cells, sieve-tube members.

–These are alive at functional maturity, although theylack the nucleus, ribosomes, and a distinct vacuole.

–The end walls, the sieve plates, have pores thatpresumably facilitate the flow of fluid between cells.

–A nonconducting nucleated companion cell,connected to the sieve-tube member, may assist thesieve-tube cell.


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• Primary growth produces the primary plantbody - the parts of the root and shoot systemsproduced by apical meristems.

Primary growth: Apicalmeristems extend shootsand roots by giving rise

to the primary plantbody

• An herbaceous plant and the youngest parts of awoody plant represent the primary plant body.

Fig. 35.14

Root Primary Growth


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• Most absorption of water and minerals in bothsystems occurs near the root tips, where vastnumbers of tiny root hairs increase the surfacearea enormously.

–Root hairs areextensionsof individualepidermalcells on the rootsurface.

•• Some plants have roots, adventitious roots,Some plants have roots, adventitious roots,arising aboveground from stems or evenarising aboveground from stems or evenfrom leaves.from leaves.

In some plants, suchIn some plants, suchas corn, adventitiousas corn, adventitiousroots function asroots function asprop rootsprop roots that help that helpsupport tall stems.support tall stems.

• The zone of cell division includes the apicalmeristem and its derivatives, primarymeristems.–The apical meristem produces the cells of the primarymeristems and also replaces cells of the root cap thatare sloughed off.

•Near the middle is the quiescent center, cells thatdivide more slowly than other meristematic cells.

–These cells are relatively resistant to damage fromradiation and toxic chemicals.–They may act as a reserve that can restore themeristem if it becomes damaged.


Fig. 35.14


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• Just above the apical meristem, the products ofits cell division form three concentric cylindersof cells that continue to divide for some time.

–These primary meristems: the protoderm,procambium, and ground meristem will produce thethree primary tissue systems of the root: dermal,vascular, and ground tissues.

• The zone of cell division blends into the zoneof elongation where cells elongate, sometimesto more than ten times their original length.

•In the zone of maturation, cells begin tospecialize in structure and function.

•The procambium gives rise to the stele,which in roots is a central cylinder ofvascular tissue where both xylem and phloemdevelop.


Fig. 35.14

X-Section


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Presence of pith• An established root may sprout lateral roots

from the outermost layer of stele, the pericycle.–Located just inside the endodermis, the pericycle is alayer of cells that may become meristematic and begindividing.

Endodermis

Pericycle

• The apical meristem of a shoot is a dome-shaped mass of dividing cells at the terminalbud.

–Leaves arise as leaf primordia on the flanks of theapical meristem.

–Axillary buds develop from islands ofmeristematic cells left by apical meristems at theleaf primordia base.

The Shoot


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Upper Epidermis

Palisade parenchyma

Spongy mesophyll

Lower Epidermis

Cuticle

Vein

• The epidermal barrier is interrupted only by thestomata, tiny pores flanked by specializedepidermal cells called guard cells.–Each stoma is a gap between a pair of guard cells.–The stomata allow gas exchange between thesurrounding air and the photosynthetic cells inside theleaf.–They are also the majoravenue of evaporativewater loss from theplant - a process calledtranspiration.

GuardCells

Stomates


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X-Section

•In most dicots, the vascularbundles are arranged in a ring,with pith on the inside andcortex outside the ring.

•In the stems of mostmonocots, the vascularbundles are scatteredthroughout the groundtissue.

Dicot

Monocot

Fiber cap

Primary Phloem

Vascular Cambium

Primary Xylem

Fibrous Sheath

Primary Phloem

Primary Xylem

(Note the absence of avascular cambium inmonocots.)

• The stems and roots, but not the leaves, of mostdicots increase in girth by secondary growth.

3. Secondary growth: Lateral meristems addgirth by producing secondary vascular tissue and

periderm

–The secondary plant body consists of the tissuesproduced during this secondary growth in diameter.

–The vascular cambium acts as a meristem for theproduction of secondary xylem and secondary phloem.

–The cork cambium acts as a meristem for a tough thickcovering for stems and roots that replaces the epidermis.


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• Vascular cambium is a cylinder of meristematiccells that forms secondary vascular tissue.–The accumulation of this tissue over the yearsaccounts for most of the increase in diameter of awoody plant.–Secondary xylem forms to the interior and secondaryphloem to the exterior of the vascular cambium.

D = Derivative

• The meristematic bands make a continuouscylinder of dividing cells surrounding theprimary xylem and pith of the stem.

• As secondary growth continues over the years,layer upon layer of secondary xylem accumulates,producing the tissue we call wood.

–Wood consists mainly of tracheids, vessel elements(in angiosperms), and fibers.

–These cells, dead at functional maturity, have thick,lignified walls that give wood its hardness and strength.


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• Bark refers to all tissues external to thevascular cambium, including secondary phloem,cork cambium, and cork.

•While cork initially develops from specializationof cells from the cortex, this supply is eventuallyexhausted and new cork cambium then developsfrom parenchyma cells in the secondary phloem.

•Only the youngest secondary phloem, internal tothe cork cambium, functions in sugar transport.

–Older secondary phloem dies and helps protect thestem until it is sloughed off as part of the bark duringlater seasons of secondary growth.

Older at Base

Youngertoward apex

AnnualGrowth Rings

•The cork plus thecork cambium formsthe periderm, aprotective layer thatreplaces theepidermis.

The cork cambiumreforms inward eachyear, cutting offmore secondaryphloem.

• In temperate regions, secondary growth inperennial plants ceases during the winter.–The first tracheids and vessels cells formed in thespring (early wood) have larger diameters and thinnerwalls than cells produced later in the summer (latewood).–The structure of the early wood maximizes deliveryof water to new, expanding leaves.–The thick-walled cells of later wood provide morephysical support.

•This pattern of growth: cambium dormancy, earlywood production, and late wood production, eachyear produces annual growth rings.


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• After several years of secondary growth,several zones are visible in a stem.

––These include twoThese include twozones of zones of secondarysecondaryxylemxylem ( (heartwoodheartwoodandand sapwoodsapwood), the), thevascular cambiumvascular cambium,,living secondaryliving secondaryphloemphloem, , corkcorkcambiumcambium, and, andcorkcork..

• The heartwood no longer conducts water but itslignified walls of its dead cells form a centralcolumn that supports the tree.–These cells are clogged with resins and othercompounds that help protect the core from fungi andwood-boring insects.

•The sapwood functions in the upward transportof water and minerals, called the xylem sap.

–Because each new layer of secondary xylem has alarger circumference, secondary growth enables thexylem to transport more sap each year, providingwater and minerals to an increasing number ofleaves.


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PLANT WATERPLANT WATERRELATIONSRELATIONS

WATER CONTENT OF PLANT TISSUESWATER CONTENT OF PLANT TISSUES

1. Growing tissues = 80-95%1. Growing tissues = 80-95%

2. Sapwood = 35%2. Sapwood = 35%

3. Heartwood = < 30%3. Heartwood = < 30%

4. Seeds = 5-15%4. Seeds = 5-15%


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HYDROGEN BONDING BETWEENWATER MOLECULES

Polarity and Hydrogen Bonds Affect thePolarity and Hydrogen Bonds Affect theProperties of WaterProperties of Water

2.2.Water has a high specific heat.Water has a high specific heat.

3.3.Water has a high latent heat of vaporization.Water has a high latent heat of vaporization.

4.4.Water has strong adhesive and cohesiveWater has strong adhesive and cohesiveproperties. (Causes capillarity)properties. (Causes capillarity)

5.5.Water has high tensile strength.Water has high tensile strength.

1.1.Water is an excellent solvent.Water is an excellent solvent.

USING A SYRINGE TO CREATEPOSITIVE AND NEGATIVE PRESSURES CAVITATIONCAVITATION

Sudden explosive expansion of a gasSudden explosive expansion of a gasbubble in liquid when the liquid isbubble in liquid when the liquid isplaced under tension (negativeplaced under tension (negativepressure).pressure).


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THE UNIT OF PRESSUREUSED TO DESCRIBE THESTATE OF WATER IN PLANTCELLS IS THE MEGAPASCAL(MPa)

WHAT DRIVES THEWHAT DRIVES THEMOVEMENT OF WATER?MOVEMENT OF WATER?

Driving Forces for DifferentDriving Forces for DifferentTypes of Water MovementTypes of Water Movement

1.1.∆∆P P ⇒⇒ Bulk Flow (Mass Flow)Bulk Flow (Mass Flow)

2. 2. ∆∆C C ⇒⇒ Diffusion Diffusion

3. 3. ∆∆ΨΨww ⇒⇒ Osmosis (movement across Osmosis (movement acrosssemipermeable semipermeable membranes)membranes)


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Main DrivingMain DrivingForces for WaterForces for WaterFlow from Soil toFlow from Soil toPlant toPlant toAtmosphere.Atmosphere.

1.

2.

3.

4.Transpiration

THERMAL MOTION LEADS TODIFFUSION AND MIXING

DiffusionDiffusion

DIFFUSION IS RAPID OVERDIFFUSION IS RAPID OVERSHORT DISTANCES BUTSHORT DISTANCES BUT

EXTREMELY SLOW OVEREXTREMELY SLOW OVERLONG DISTANCES.LONG DISTANCES.

Pressure-Driven Bulk FlowPressure-Driven Bulk FlowDrives Long-Distance WaterDrives Long-Distance Water

TransportTransport

1) In the soil.In the soil.

2)2) In the xylem.In the xylem.

3)3) In the phloem.In the phloem.


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““OSMOSISOSMOSIS”” - WATER MOVEMENT - WATER MOVEMENTACROSS SEMIPERMEABLEACROSS SEMIPERMEABLE

MEMBRANESMEMBRANES

1.1.DIFFUSION DIRECTLY ACROSS THEDIFFUSION DIRECTLY ACROSS THELIPID BILAYER.LIPID BILAYER.

2. DIFFUSION AND BULK FLOW2. DIFFUSION AND BULK FLOWTHROUGH THROUGH AQUAPORINAQUAPORIN CHANNELS CHANNELS

Two Mechanisms:Two Mechanisms:

(Aquaporin)

OSMOSIS is driven by the Water PotentialOSMOSIS is driven by the Water PotentialGradient.Gradient.

THE WATER POTENTIAL (THE WATER POTENTIAL (ΨΨww) IS THE) IS THECHEMICAL POTENTIAL OR FREE ENERGYCHEMICAL POTENTIAL OR FREE ENERGYOF WATER EXPRESSED IN PRESSUREOF WATER EXPRESSED IN PRESSUREUNITS (MEGAPASCALS).UNITS (MEGAPASCALS).

WATER POTENTIAL EQUATION

Ψw = Ψs + Ψp

SOLUTES PRESSURE


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Solute Potential or Osmotic PotentialSolute Potential or Osmotic Potential((ΨΨss))

The effect that solutes have onThe effect that solutes have onlowering the water potential.lowering the water potential.

Thus, the solute potential isThus, the solute potential isusually negative.usually negative.

ΨΨpp = Hydrostatic Pressure or = Hydrostatic Pressure orPressure Potential (referred toPressure Potential (referred toas as ““turgor turgor pressurepressure””))

ΨΨpp = 0 MPa for water at = 0 MPa for water atatmospheric pressure.atmospheric pressure.


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U-TUBEEXPERIMENTS

TURGID CELL TURGID CELL FLACCID CELL (PLASMOLYZED) FLACCID CELL (PLASMOLYZED)


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WILTING (Reversible) Permanent WiltingPermanent Wilting

•Plasmodesmata connect the cytosolic compartments ofneighboring cells.•This cytoplasmic continuum, the symplast, forms acontinuous pathwayfor transport.

•The walls ofadjacent plant cellsare also in contact,forming a secondcontinuouscompartment, theapoplast.

CAPILLARITYCAPILLARITY

Movement of water through narrowMovement of water through narrowtubes.tubes.

Driven by adhesion, cohesion, andDriven by adhesion, cohesion, andsurface tension.surface tension.


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CAPILLARITY - in tubes and soil

(SURFACE TENSION)

GRAVITY

Meniscus

Tube Soil

(Suberin)Radial Path of Waterand Ions into Root

SOLUTE ACCUMULATION IN THE XYLEM AT NIGHTSOLUTE ACCUMULATION IN THE XYLEM AT NIGHTCAN GENERATE CAN GENERATE ROOT PRESSUREROOT PRESSURE

SOLUTESSOLUTESIN XYLEMIN XYLEMH20

H20

H20

H20

HIGH Soil HIGH Soil ΨΨww

LowLowXylem Xylem ΨΨwwΨΨpp

H20

ROOT PRESSURE CANROOT PRESSURE CANRESULT IN RESULT IN GUTTATION.GUTTATION.

Guttation Guttation is the exudation of xylem fluidis the exudation of xylem fluid((““guttation guttation dropletsdroplets””) through specialized) through specializedpores calledpores called hydathodes hydathodes at the margins ofat the margins ofleaves .leaves .


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GUTTATIONDROPLETS

XYLEMXYLEMTRANSLOCATIONTRANSLOCATION

XYLEMCONDUCTINGCELLS

•• The high tensile strength of water The high tensile strength of waterallows high allows high tensionstensions to build up in the to build up in thexylem.xylem.

•• Cavitation Cavitation (breakage of the water chain)(breakage of the water chain)occurs at very high tensions.occurs at very high tensions.

• Because of strong cohesion of waterBecause of strong cohesion of watermolecules due to hydrogen bonding,molecules due to hydrogen bonding,water can be pulled up the xylem like awater can be pulled up the xylem like achain.chain.

•• A water vapor embolism (gas bubble) A water vapor embolism (gas bubble)forms in the xylem, blocking flow.forms in the xylem, blocking flow.


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WATER RELATIONSOF THE LEAF

Water moves intothe leaf via thevascular system(veins)

The Midribbranches intoprogressivelysmaller veins.

TensionsTensions(negative(negativepressures)pressures)originate inoriginate inthe cell wallsthe cell wallsof theof themesophyllmesophyllcellscells


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As water evaporates from the wall, tiny menisci form inAs water evaporates from the wall, tiny menisci form inthe interstices of the cellulose the interstices of the cellulose microfibrilsmicrofibrils: these: thesegenerate a surface tensiongenerate a surface tension


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Fig. 36.13a


Fig. 36.11

Transpiration:solar-poweredbulk flow

SUGAR TRANSLOCATIONSUGAR TRANSLOCATIONIN THE PHLOEMIN THE PHLOEM


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1.1. THE EVOLUTION OF AERIAL SHOOTS ANDTHE EVOLUTION OF AERIAL SHOOTS ANDSUBTERRANEAN ROOTS NECESSITATED ASUBTERRANEAN ROOTS NECESSITATED AMECHANISM FOR LONG-DISTANCE TRANSPORT OFMECHANISM FOR LONG-DISTANCE TRANSPORT OFSUGARS.SUGARS.

2. THE PRIMARY FUNCTION OF THE PHLOEM IS TO2. THE PRIMARY FUNCTION OF THE PHLOEM IS TOCARRY OUT THE LONG DISTANCE TRANSLOCATIONCARRY OUT THE LONG DISTANCE TRANSLOCATIONOF SUGARS AND OTHER PHOTOSYNTHETICOF SUGARS AND OTHER PHOTOSYNTHETICPRODUCTS.PRODUCTS.

3. PHLOEM TRANSLOCATION OCCURS IN SIEVE TUBE3. PHLOEM TRANSLOCATION OCCURS IN SIEVE TUBEELEMENTS STACKED INTO SIEVE TUBES.ELEMENTS STACKED INTO SIEVE TUBES.

11oo

XYLEMXYLEM

11oo

PHLOEMPHLOEM

VASCULARBUNDLE

Girdling a stem (removing the bark andsecondary phloem) kills the tree because the

roots starve for sugars.

Aftergirdling,which diesfirst, theshoot orthe root?

2o

PHLOEM

2o XYLEM

VASCULARCAMBIUM

Cork

Corkcambium

1° XYLEM

PITH


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SLIME PLUGSLIME PLUG

DAMAGEDDAMAGEDSEIVESEIVETUBETUBEELEMENTSELEMENTS WHAT IS

THEFUNCTIONOF SLIMEPLUGS?

Companioncell

Sieve tubeelements

Parenchyma cell

Unobstructed sieveplate pores

Sieve element

Companioncell

Sieveplate

Parenchyma cell


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PATTERNS OF PHLOEMPATTERNS OF PHLOEMTRANSLOCATION: SOURCE TO SINKTRANSLOCATION: SOURCE TO SINK

1. Phloem sap is not 1. Phloem sap is not translocated translocated exclusivelyexclusivelyin either an upward or downward directionin either an upward or downward directionand is not influenced by gravity.and is not influenced by gravity.

2.2. Phloem translocation occurs from sourcePhloem translocation occurs from sourceto sink.to sink.

3. Sources include any exporting organ,3. Sources include any exporting organ,typically mature leaves exportingtypically mature leaves exportingphotosynthate photosynthate (sugar).(sugar).

3. Storage organs (roots, tubers, seeds, etc.)3. Storage organs (roots, tubers, seeds, etc.)can also serve as sources.can also serve as sources.

4. 4. SinksSinks include any include any nonphotosyntheticnonphotosyntheticorgan or tissue that does not produceorgan or tissue that does not producesufficient sufficient photosynthate photosynthate to support itsto support itsown metabolic needs: roots, undergroundown metabolic needs: roots, undergroundstems, buds, immature leaves, flowers,stems, buds, immature leaves, flowers,fruits, etc.fruits, etc.

SOURCE TO SINK PATHWAYS FOLLOW ANATOMICALSOURCE TO SINK PATHWAYS FOLLOW ANATOMICALAND DEVELOPMENTAL PATTERNSAND DEVELOPMENTAL PATTERNS

1.1. Proximity is important: upper mature leavesProximity is important: upper mature leavessupply supply photosynthate photosynthate to growing shoot tip; lowerto growing shoot tip; lowerleaves supply the root; middle leaves supply both.leaves supply the root; middle leaves supply both.

2. Development influences transport:2. Development influences transport:

a. Young leaves begin as sinks, gradually becomea. Young leaves begin as sinks, gradually becomesources.sources.

b. Reproductive structures become dominantb. Reproductive structures become dominantsinks during flowering.sinks during flowering.


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CHANGES IN SOURCE-SINK RELATIONS DURING LEAFCHANGES IN SOURCE-SINK RELATIONS DURING LEAFDEVELOPMENTDEVELOPMENT

SINK

SOURCE

Young leaf Mature LeafIntermediate3. Vascular connections important; sourceleaves preferentially supply sinks to which theyhave vascular connections; typically, sourcesleaves supply sinks along the same side of thestem.

1414COCO22 -Labeling Experiment -Labeling Experiment

(Label mainly on same side.)(Label mainly on same side.)

(sucrose)


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• The pressure flow model explains why phloem sapalways flows from sugar source to sugar sink,regardless of their locations in the plant.

• Researchers have devised several experiments totest this model, including an innovative experimentthat exploits natural phloem probes::

••aphids that feed on phloem sapaphids that feed on phloem sap!!

“Honey dew”

“stylet”

APHIDS

Phloem sieve tube

“stylet”

Phloem sieve tube

Phloem Sieve Tube

“stylet”

SEVERED APHID STYLET


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•The closer the aphid’s stylet is to a sugar source, thefaster the sap will flow out and the greater its sugarconcentration.

THE PRESSURE-FLOW MODEL OFTHE PRESSURE-FLOW MODEL OFPHLOEM TRANSLOCATIONPHLOEM TRANSLOCATION

1.1. ACCORDING TO THE ACCORDING TO THE MMÜÜNCH PRESSURE FLOWNCH PRESSURE FLOW

MODELMODEL, SUGARS MOVE BY BULK FLOW IN SIEVE, SUGARS MOVE BY BULK FLOW IN SIEVE

TUBES IN RESPONSE TO AN OSMOTICALLYTUBES IN RESPONSE TO AN OSMOTICALLYGENERATED PRESSURE GRADIENT (GENERATED PRESSURE GRADIENT (ΔΨΔΨpp) BETWEEN) BETWEEN

THE SOURCE AND THE SINK.THE SOURCE AND THE SINK.

2. 2. TRANSLOCATION IS THUS TRANSLOCATION IS THUS PASSIVEPASSIVE..

MMüünch Pressure-Flow Modelnch Pressure-Flow Model


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THE PRESSURE-FLOW MODEL OFTHE PRESSURE-FLOW MODEL OFPHLOEM TRANSLOCATION REQUIRESPHLOEM TRANSLOCATION REQUIRESTWO ACTIVE STEPS:TWO ACTIVE STEPS:

1.1. ATP-DEPENDENT ATP-DEPENDENT PHLOEM LOADINGPHLOEM LOADING OF SUGARS OF SUGARS

OCCURS AT THE SOURCE;OCCURS AT THE SOURCE;

2. ATP-DEPENDENT 2. ATP-DEPENDENT PHLOEM UNLOADINGPHLOEM UNLOADING OCCURS AT OCCURS AT

THE SINK.THE SINK.

Sieve element

Companioncell

PREDICTIONS OF THE PRESSURE-FLOWMODEL HAVE BEEN CONFIRMED

1. Sieve plate pores must be unobstructedfor pressure-flow to occur between sievetube members.

CONFIRMED BY ELECTRONMICROSCOPY

PREDICTIONS (CONT.)

2. True bidirectional transport in a singlesieve tube cannot take place.

CONFIRMED BY APHID STUDIESUSING RADIOACTIVE TRACERSAND DYES.


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PREDICTIONS (CONT.)

3. PHLOEM TRANSLOCATION SHOULD BE APASSIVE PROCESS, NOT DIRECTLYDEPENDENT ON ATP.

CONFIRMED BY PHYSIOLOGICALSTUDIES USING COLDTREATMENT, ANOXIA, ANDINHIBITORS.

PREDICTIONS (CONT.)

4. THERE MUST BE PRESSURE GRADIENTSBETWEEN SOURCE AND SINK SUFFICIENT TODRIVE BULK FLOW.

CONFIRMED BY STUDIES USINGAPHID STYLETS ASMICROMANOMETERS.

• Overview: Transport in plants occurs on threelevels:(1) the uptake and loss of water and solutes by individual

cells

(2) short-distance transportof substances from cell tocell at the level of tissuesor organs

(3) long-distance transportof sap within xylem andphloem at the level ofthe whole plant.


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photosynthesis: the light...

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