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: