3 Figure 36.2 An overview of transport in a vascular plant (layer 1) MineralsH2O
4 Figure 36.2 An overview of transport in a vascular plant (layer 2) MineralsH2OCO2O2
5 Figure 36.2 An overview of transport in a vascular plant (layer 3) CO2O2LightH2OSugarH2OMinerals
6 Figure 36.2 An overview of transport in a vascular plant (layer 4) MineralsH2OCO2O2SugarLight
7 Figure 36.3 Proton pumps provide energy for solute transport CYTOPLASMEXTRACELLULAR FLUIDATPH+Proton pump generatesmembrane potentialand H+ gradient.–+
8 Figure 36.4 Solute transport in plant cells +CYTOPLASMEXTRACELLULAR FLUIDCations ( forexample) are driven into the cell by the membrane potential.Transport proteinK+–(a) Membrane potential and cation uptakeH+NO3–NO3 –(b) Cotransport of anionsPlant cells canalso accumulate aneutral solute,such as sucrose( ), bycotransportingdown thesteep protongradient.S(c) Cotransport of a neutral soluteCell accumulatesanions (NO3 –, forexample) bycoupling their transport to the inward diffusion of H+ through acotransporter.
9 Figure 36.5 Water potential and water movement: an artificial model Y = –0.23 MPa(a)0.1 Msolution(d)(c)(b)YP = 0H2OYS = –0.23Y= –0.23 MPaYS = –0.23Y = 0 MPaYP =Y = –0.07 MPaYP =YS = 0Y = –0.30 MPaYP = –0.30YP = 0Y = 0 MPaPure water
10 Figure 36.6 Water relations in plant cells 0.4 M sucrose solution: = 0s = –0.9 = –0.9 MPa = 0s = –0.7 = –0.7 MPaInitial flaccid cell: = 0s = 0 = 0 MPaDistilled water:Plasmolyzedcell at osmoticequilibriumwith itssurroundings = 0 = –0.9 MPa = 0.7s = –0.7 = 0 MPaTurgid cellat osmoticInitial conditions: cellular > environmental . The cellloses water and plasmolyzes. After plasmolysis is complete,the water potentials of the cell and its surroundings are thesame.Initial conditions: cellular < environmental . Thereis a net uptake of water by osmosis, causing the cell tobecome turgid. When this tendency for water to enter isoffset by the back pressure of the elastic wall, waterpotentials are equal for the cell and its surroundings.(The volume change of the cell is exaggerated in thisdiagram.)(b)
11 Figure 36.7 A watered Impatiens plant regains its turgor
12 Figure 36.8 Cell compartments and routes for short-distance transport Transport proteins inthe plasma membraneregulate traffic ofmolecules betweenthe cytosol and thecell wall.the vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.PlasmodesmaVacuolar membrane(tonoplast)Plasma membraneCell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.KeySymplastApoplastThe symplast is thecontinuum ofcytosol connectedby plasmodesmata.The apoplast isthe continuumof cell walls andextracellularspaces.Transmembrane routeSymplastic routeApoplastic routeTransport routes between cells. At the tissue level, there are three passages:the transmembrane, symplastic, and apoplastic routes. Substances may transferfrom one route to another.Cell wallCytosolVacuole(a)(b)
13 Figure 36.9 Lateral transport of minerals and water in roots Casparian stripEndodermisPathway alongapoplastPathwaythroughsymplast1Uptake of soil solution by thehydrophilic walls of root hairsprovides access to the apoplast.Water and minerals can thensoak into the cortex alongthis matrix of walls.Casparian stripPlasmamembrane2Minerals and water that crossthe plasma membranes of roothairs enter the symplast.Apoplasticroute13Vessels(xylem)2453As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove inward via the symplast.RoothairSymplasticrouteEpidermisCortexEndodermisVascular cylinder4Within the transverse and radial walls of each endodermal cell is theCasparian strip, a belt of waxy material (purple band) that blocks thepassage of water and dissolved minerals. Only minerals already inthe symplast or entering that pathway by crossing the plasmamembrane of an endodermal cell can detour around the Casparianstrip and pass into the vascular cylinder.5Endodermal cells and also parenchyma cells within thevascular cylinder discharge water and minerals into theirwalls (apoplast). The xylem vessels transport the waterand minerals upward into the shoot system.
14 Figure 36.10 Mycorrhizae, symbiotic associations of fungi and roots 2.5 mm
16 Figure 36.12 The generation of transpirational pull in a leaf Evaporation causes the air-water interface to retreat farther intothe cell wall and become more curved as the rate of transpirationincreases. As the interface becomes more curved, the water film’spressure becomes more negative. This negative pressure, or tension,pulls water from the xylem, where the pressure is greater.CuticleUpperepidermisMesophyllLowerWater vaporCO2O2XylemStomaEvaporationAt first, the water vapor lost bytranspiration is replaced byevaporation from the water filmthat coats mesophyll cells.In transpiration, water vapor (shown as blue dots)diffuses from the moist air spaces of the leaf to thedrier air outside via stomata.AirspaceCytoplasmCell wallVacuoleWater filmLow rate oftranspirationHigh rate ofAir-waterinterfaceY = –0.15 MPaY = –10.00 MPa312
17 Figure 36.13 Ascent of xylem sap Outside air Y= –100.0 MPaLeaf Y (air spaces)= –7.0MPaLeaf Y (cell walls)= –1.0 MPaTrunk xylem Y= – 0.8 MPaWater potential gradientRoot xylem Y= – 0.6 MPaSoil Y= – 0.3 MPaMesophyllcellsStomaWatermoleculeAtmosphereTranspirationAdhesionCellwallCohesion,byhydrogenbondingRoothairSoilparticleCohesionand adhesionin the xylemWater uptakefrom soil
18 Figure 36.14 Open stomata (left) and closed stomata (colorized SEM)
19 Figure 36.15 The mechanism of stomatal opening and closing Cells turgid/Stoma openH2ORadially orientedcellulose microfibrilsCellwallVacuoleGuard cellK+Changes in guard cell shape and stomatal openingand closing (surface view). Guard cells of a typicalangiosperm are illustrated in their turgid (stoma open)and flaccid (stoma closed) states. The pair of guardcells buckle outward when turgid. Cellulose microfibrilsin the walls resist stretching and compression in thedirection parallel to the microfibrils. Thus, the radialorientation of the microfibrils causes the cells to increasein length more than width when turgor increases.The two guard cells are attached at their tips, so theincrease in length causes buckling.(a)Role of potassium in stomatal opening and closing.The transport of K+ (potassium ions, symbolizedhere as red dots) across the plasma membrane andvacuolar membrane causes the turgor changes ofguard cells.(b)Cells flaccid/Stoma closed
20 Figure 36.16 Structural adaptations of a xerophyte leaf Lower epidermaltissueTrichomes(“hairs”)CuticleUpper epidermal tissueStomata100m
21 Figure 36.17 Loading of sucrose into phloem Sucrose manufactured in mesophyll cells cantravel via the symplast (blue arrows) tosieve-tube members. In some species, sucroseexits the symplast (red arrow) near sievetubes and is actively accumulated from theapoplast by sieve-tube members and theircompanion cells.(a)Mesophyll cellCell walls (apoplast)Plasma membranePlasmodesmataCompanion(transfer) cellSieve-tubememberPhloemparenchyma cellBundle-sheath cellHigh H+ concentrationCotransporterProtonpumpATPKeySucroseApoplastSymplastH+A chemiosmotic mechanism is responsible forthe active transport of sucrose into companion cellsand sieve-tube members. Proton pumps generatean H+ gradient, which drives sucrose accumulationwith the help of a cotransport protein that couplessucrose transport to the diffusion of H+ back into the cell.(b)Low H+ concentrationS
22 Figure 36.18 Pressure flow in a sieve tube Vessel(xylem)H2OSieve tube(phloem)Source cell(leaf)SucroseSink cell(storageRoot)1Loading of sugar (green dots)into the sieve tube at thesource reduces waterpotential inside thesieve-tube members.This causes the tubeto take up waterby osmosis.243This uptake ofwater generatesa positive pressurethat forces thesap to flow alongthe tube.The pressure isrelieved by theunloading of sugar and the consequentloss of water from the tubeat the sink.In the case ofleaf-to-roottranslocation,xylem recycleswater from sinkto source.Transpiration streamPressure flow
23 Figure 36.19 What causes phloem sap to flow from source to sink? Aphid feedingStylet in sieve-tubememberSevered styletexuding sapSieve-TubeTo test the pressure flow hypothesis, researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink.EXPERIMENTThe closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.RESULTSThe results of such experiments support the pressure flow hypothesis.CONCLUSIONSap dropletStyletSapdroplet25 mSieve- tube member
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