1
• Plasmodesmata
• Channelsconnec+ngneighboringcells
• Cellmembraneandcytosolarecon+nuousfromcelltocell
• Symplast
• Cytoplasmiccon+nuum
• Throughplasmodesmata
• Apoplast
• con+nuumofcellwallsandextracellularspacesincludinginteriorofdeadcells
CompartmentsandTransport
• Waterandmineralscantravelthroughaplantbythreeroutes:
1. Transmembraneroute
• outofonecell,acrossacellwall/membrane
• intoanothercell
2. Symplas7croute
• viathecon+nuumofcytosol
3. Apoplas7croute
• insidecellwallsandextracellularspaces
ThreeMajorPathwaysofTransport
Cell wall Cytosol
Vacuole
Plasmodesma Vacuolar membrane Plasma membrane
(a) Cell compartments Key
Transmembrane route Apoplast
Symplast Apoplast
Symplast
Apoplast
Symplastic route
Apoplastic route
(b) Transport routes between cells
Absorp+onofWaterandMineralsbyRootCells• Bulkflow
• Mostwaterandmineralabsorp+on
• Occursnearroot+ps
• Epidermisispermeableto
water
• Loca+onofroothairs
• Addsurfacearea
• Soilsolu+onenterstheroots
• Viasymplas+corapoplas+c
route
2
• Endodermis
• Innermostlayerofcellsintherootcortex
• surroundsthevascularcylinder
• lastcheckpointforselec+vepassageofminerals
• fromthecortexintothevascular+ssue
TransportofWaterandMineralsintotheXylem
• Watercancrossthecortexviathesymplastorapoplast
• Casparianstrip
• Suberinlayerintheendodermalwall
• Blocksapoplas+ctransferofminerals
• fromthecortextothevascularcylinder
• Forcesthesymplas+croute
• Allowsfilteringofsoilsolu+onbyselec+vepermeability
TransportofWaterandMineralsintotheXylem
Fig.36-12
Pathway along apoplast
Casparian strip
Endodermal cell
Pathway through symplast
Casparian strip
Plasma membrane
Apoplastic route
Symplastic route
Root hair
Epidermis
Cortex
Endodermis
Vessels (xylem)
Stele (vascular cylinder)
3
BulkFlowDrivenbyNega+vePressureintheXylem• Transpira7on
• evapora+onofwaterfromaplant’ssurface
• Plantslosealargevolumeofwaterinthisway
• Waterreplacedbythebulkflowofwaterandminerals
• calledxylemsap
• fromthestelesofrootstothestemsandleaves
• Issapmainlypushedupfromtheroots,orpulledupbytheleaves?
PushingXylemSap:RootPressure
• Atnight
• transpira+onisverylow
• rootcellscon+nuepumpingmineral
ions
• intothexylemofthevascular
cylinder
• loweringthewater
poten+al
• Waterflowsinfromtherootcortex
• genera+ngposi+verootpressure
• Gu?a7on
• Rootpressure
• some+mesresultsintheexuda+onofwaterdropletson+psoredgesofleaves
PushingXylemSap:RootPressure
4
• Posi+verootpressure
• isrela+velyweak
• minormechanismofxylembulkflow
• Nega+vepressure
• Waterispulledupwardinthexylem
• Transpira+onalpull
• producesnega+vepressure(tension)intheleaf
• Exertsapullingforceonwaterinthexylem,pullingwaterintotheleaf
PushingXylemSap:RootPressure
Cuticle Xylem
Upper epidermis
Mesophyll
Lower epidermis
Cuticle
Air space
Microfibrils in cell wall of
mesophyll cell
Stoma
Microfibril (cross section)
Water film
Air-water interface
• Cavita+on
• Forma+onofawatervaporpocketinxylem
• Byabreakinthechainofwatermolecules
• Causedbydroughtstressorfreezing
CohesionandAdhesionintheAscentofXylemSap
Stomataregulatetherateoftranspira+on
• Leavesgenerallyhavebroadsurfaceareas
• andhighsurface-to-volumera+os
• increasesphotosynthesis
• Butalsoincreaseswaterlossthroughstomata
• ~95%ofplantwaterloss
• Eachstoma
• Flankedbyapairofguardcells
• Controlopeningofstomabychangingshape
5
MechanismsofStomatalOpeningandClosing
• Changesinturgorpressureinguardcells
• openandclosestomata
• primarilyfrom
reversibleuptake/
lossofpotassium
ions
• K+co-transportedbyprotonpumps
Radially oriented cellulose microfibrils
Cell wall
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Vacuole Guard cell
(a) Changes in guard cell shape and stomatal opening and closing (surface view)
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
(b) Role of potassium in stomatal opening and closing
H2O H2O
H2O
H2O H2O
H2O H2O
H2O
H2O
H2O
K+
S+muliforStomatalOpeningandClosing• Generally
• stomataopenduringtheday
• closeatnight
• tominimizewaterloss
• Stomatalopeningatdawnistriggeredby:
• Light
• Bluelightreceptorss+mulateK+uptake
• CO2deple+on
• Internal“clock”inguardcells
• Func+onseveninthedark
• circadianrhythm
• 24-hourcyclesofbehavior
Adapta+onsThatReduceEvapora+veWaterLoss• Xerophytes
• plantsadaptedto
aridclimates
• leafmodifica+ons
• reducetherate
oftranspira+on
• Crassulaceanacid
metabolism(CAM)
• Specializedformof
photosynthesis
• wherestomatal
gasexchange
occursatnight
Ocotillo (leafless)
Oleander leaf cross section and flowers
Cuticle
Upper epidermal tissue
Ocotillo leaves
Trichomes (“hairs”)
Crypt Stomata Lower epidermal tissue
100 µm
Ocotillo after heavy rain Old man cactus
6
SugarTransport
• Transloca7on
• Processoftranspor+ngtheproductsofphotosynthesisthroughphloem
MovementfromSugarSourcestoSugarSinks• Phloemsap
• Aqueoussolu+onthatishighinsucrose
• Travelsfromasugarsourcetoasugarsink
• Sugarsource
• Anorganthatisanetproducerofsugar
• matureleaves
• Sugarsink
• Anorganthatisanetconsumerorstorageunitofsugar
• tuberorbulb
• Astorageorgancanbebothasugarsinkinsummerandsugarsourceinwinter
• Sugar
• Mustbeloadedintosieve-tubeelementsbeforebeingtravelingtosinks
• Dependingonthespecies
• Maymovebysymplas+corbothsymplas+candapoplas+cpathways
• Transfercells
• Modifiedcompanioncells
• enhancesolutemovementbetweentheapoplastandsymplast
• Viaprotonpumpco-transport
MovementfromSugarSourcestoSugarSinks
Mesophyll cell Cell walls (apoplast)
Plasma membrane Plasmodesmata
Companion (transfer) cell
Sieve-tube element
High H+ concentration Cotransporter Proton pump
Low H+ concentration
Key
Apoplast
Symplast Mesophyll cell Bundle- sheath cell
Phloem parenchyma cell
Sucrose ATP
H+
H+ H+
S
S
7
• Inmanyplants
• Phloemloadingrequiresac+vetransport
• ProtonpumpingandcotransportofsucroseandH+
• Enablethecellstoaccumulatesucrose
• Atthesink
• Sugarmoleculesdiffuse(facilitated)fromthephloemtosink+ssues
• followedbywater
MovementfromSugarSourcestoSugarSinks
Mesophyll cell Cell walls (apoplast)
Plasma membrane Plasmodesmata
Companion (transfer) cell
Sieve-tube element
High H+ concentration Cotransporter Proton pump
Low H+ concentration
Key
Apoplast
Symplast Mesophyll cell Bundle- sheath cell
Phloem parenchyma cell
Sucrose ATP
H+
H+ H+
S
S
BulkFlowbyPosi+vePressure
• Sapmovesthrougha
sievetubebybulkflow
drivenbyposi+ve
pressure
• Calledpressureflow
• Increasing
pressureat
sourceend
• Reduced
pressureatsink
end
4
3
2
1
1
2
3 4
Vessel (xylem)
Sieve tube (phloem)
Source cell (leaf) Loading of sugar
Uptake of water
Unloading of sugar
Water recycled
Sink cell (storage root)
Sucrose
H2O
H2O
Bul
k flo
w b
y ne
gativ
e pr
essu
re
H2O
Sucrose
Bul
k flo
w b
y po
sitiv
e pr
essu
re
Youshouldnowbeableto:
1. Describehowprotonpumpsfunc+onintransportofmaterialsacrossmembranes
2. Definethefollowingterms:osmosis,waterpoten+al,flaccid,turgorpressure,turgid
3. Explainhowaquaporinsaffecttherateofwatertransportacrossmembranes
4. Describethreeroutesavailableforshort-distancetransportinplants
8
5. Relatestructuretofunc+oninsieve-tubecells,vesselcells,andtracheidcells
6. Explainhowtheendodermisfunc+onsasaselec+vebarrierbetweentherootcortexandvascularcylinder
7. Defineandexplainguia+on
8. Explainthisstatement:“Theascentofxylemsapisul+matelysolarpowered”
9. Describetheroleofstomataanddiscussfactorsthatmightaffecttheirdensityandbehavior
10. Tracethepathofphloemsapfromsugarsourcetosugarsink;describesugarloadingandunloading
Youshouldnowbeableto:
