Bài giảng Biology - Chapter 36: Transport in Vascular Plants

Chapter 36Transport in Vascular PlantsOverview: Pathways for SurvivalFor vascular plantsThe evolutionary journey onto land involved the differentiation of the plant body into roots and shootsVascular tissueTransports nutrients throughout a plant; such transport may occur over long distancesFigure 36.1Concept 36.1: Physical forces drive the transport of materials in plants over a range of distancesTransport in vascular plants occurs on three scalesTransport of water and solutes by individual cells, such as root hairsShort-distance transport of substances from cell to cell at the levels of tissues and organsLong-distance transport within xylem and phloem at the level of the whole plantMineralsH2OCO2O2CO2O2H2OSugarLightA variety of physical processesAre involved in the different types of transport Sugars are produced byphotosynthesis in the leaves.5 Sugars are transported asphloem sap to roots and otherparts of the plant.6 Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.4Transpiration, the loss of waterfrom leaves (mostly throughstomata), creates a force withinleaves that pulls xylem sap upward.3 Water and minerals aretransported upward fromroots to shoots as xylem sap.2Roots absorb waterand dissolved mineralsfrom the soil.1Figure 36.2 Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.7Selective Permeability of Membranes: A ReviewThe selective permeability of a plant cell’s plasma membraneControls the movement of solutes into and out of the cellSpecific transport proteinsEnable plant cells to maintain an internal environment different from their surroundingsThe Central Role of Proton PumpsProton pumps in plant cellsCreate a hydrogen ion gradient that is a form of potential energy that can be harnessed to do workContribute to a voltage known as a membrane potentialFigure 36.3CYTOPLASMEXTRACELLULAR FLUIDATPH+H+H+H+H+H+H+H+Proton pump generates membrane potentialand H+ gradient.–––––+++++Plant cells use energy stored in the proton gradient and membrane potentialTo drive the transport of many different solutes+CYTOPLASMEXTRACELLULAR FLUIDCations ( , for example) are driven into the cell by themembrane potential.Transport proteinK+K+K+K+K+K+K+K+–––++(a) Membrane potential and cation uptake––++Figure 36.4aIn the mechanism called cotransportA transport protein couples the passage of one solute to the passage of anotherFigure 36.4bH+H+H+H+H+H+H+H+H+H+H+H+NO3– NO3 – NO3– NO3– NO3 – NO3 – –––+++–––+++NO3–(b) Cotransport of anions H+of through acotransporter.Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion H+H+H+H+H+H+H+H+H+H+SSSSSPlant cells canalso accumulate a neutral solute,such as sucrose( ), bycotransporting down thesteep protongradient.SH+–––+++––++–Figure 36.4cH+H+S+–(c) Contransport of a neutral solute The “coattail” effect of cotransportIs also responsible for the uptake of the sugar sucrose by plant cellsEffects of Differences in Water PotentialTo survivePlants must balance water uptake and lossOsmosisDetermines the net uptake or water loss by a cellIs affected by solute concentration and pressureWater potentialIs a measurement that combines the effects of solute concentration and pressureDetermines the direction of movement of waterWaterFlows from regions of high water potential to regions of low water potentialHow Solutes and Pressure Affect Water PotentialBoth pressure and solute concentrationAffect water potentialThe solute potential of a solutionIs proportional to the number of dissolved moleculesPressure potentialIs the physical pressure on a solutionQuantitative Analysis of Water PotentialThe addition of solutesReduces water potentialFigure 36.5a0.1 MsolutionH2OPurewaterP = 0S = 0.23 = 0.23 MPa = 0 MPa(a)Application of physical pressureIncreases water potentialH2OP = 0.23S = 0.23 = 0 MPa = 0 MPa(b)H2OP = 0.30S = 0.23 = 0.07 MPa = 0 MPa(c)Figure 36.5b, cNegative pressureDecreases water potentialH2OP = 0S = 0.23 = 0.23 MPa(d)P = 0.30S = 0 = 0.30 MPaFigure 36.5dWater potentialAffects uptake and loss of water by plant cellsIf a flaccid cell is placed in an environment with a higher solute concentrationThe cell will lose water and become plasmolyzedFigure 36.6a0.4 M sucrose solution:Initial flaccid cell:Plasmolyzed cellat osmotic equilibriumwith its surroundingsP = 0S = 0.7P = 0S = 0.9P = 0S = 0.9 = 0.9 MPa = 0.7 MPa = 0.9 MPaIf the same flaccid cell is placed in a solution with a lower solute concentrationThe cell will gain water and become turgidDistilled water:Initial flaccid cell:Turgid cellat osmotic equilibriumwith its surroundingsP = 0S = 0.7P = 0S = 0P = 0.7S = 0.7Figure 36.6b = 0.7 MPa = 0 MPa = 0 MPaTurgor loss in plants causes wiltingWhich can be reversed when the plant is wateredFigure 36.7Aquaporin Proteins and Water TransportAquaporinsAre transport proteins in the cell membrane that allow the passage of waterDo not affect water potentialThree Major Compartments of Vacuolated Plant CellsTransport is also regulatedBy the compartmental structure of plant cellsThe plasma membrane Directly controls the traffic of molecules into and out of the protoplastIs a barrier between two major compartments, the cell wall and the cytosolThe third major compartment in most mature plant cellsIs the vacuole, a large organelle that can occupy as much as 90% of more of the protoplast’s volume The vacuolar membraneRegulates transport between the cytosol and the vacuoleTransport proteins inthe plasma membraneregulate traffic ofmolecules betweenthe cytosol and thecell wall.Transport proteins inthe vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.PlasmodesmaVacuolar membrane(tonoplast)Plasma membraneCell wallCytosolVacuoleCell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.(a)Figure 36.8aIn most plant tissuesThe cell walls and cytosol are continuous from cell to cellThe cytoplasmic continuumIs called the symplastThe apoplastIs the continuum of cell walls plus extracellular spacesKeySymplastApoplastThe symplast is thecontinuum ofcytosol connectedby plasmodesmata.The apoplast isthe continuumof cell walls andextracellularspaces.ApoplastTransmembrane routeSymplastic routeApoplastic routeSymplastTransport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another.(b)Figure 36.8bFunctions of the Symplast and Apoplast in TransportWater and minerals can travel through a plant by one of three routesOut of one cell, across a cell wall, and into another cellVia the symplastAlong the apoplastBulk Flow in Long-Distance TransportIn bulk flowMovement of fluid in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubesConcept 36.2: Roots absorb water and minerals from the soilWater and mineral salts from the soilEnter the plant through the epidermis of roots and ultimately flow to the shoot systemLateral transport of minerals and water in rootsFigure 36.9123Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls.Minerals and water that crossthe plasma membranes of roothairs enter the symplast.As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove inward via the symplast.Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks thepassage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder.Endodermal 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.Casparian stripPathway alongapoplast PathwaythroughsymplastPlasmamembraneApoplasticrouteSymplasticrouteRoot hairEpidermisCortexEndodermisVascular cylinderVessels(xylem)Casparian stripEndodermal cell4521The Roles of Root Hairs, Mycorrhizae, and Cortical CellsMuch of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are locatedRoot hairs account for much of the surface area of rootsMost plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soilRoots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphaeFigure 36.102.5 mmOnce soil solution enters the rootsThe extensive surface area of cortical cell membranes enhances uptake of water and selected mineralsThe Endodermis: A Selective SentryThe endodermisIs the innermost layer of cells in the root cortexSurrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissueWater can cross the cortexVia the symplast or apoplastThe waxy Casparian strip of the endodermal wallBlocks apoplastic transfer of minerals from the cortex to the vascular cylinderConcept 36.3: Water and minerals ascend from roots to shoots through the xylemPlants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plantThe transpired water must be replaced by water transported up from the rootsFactors Affecting the Ascent of Xylem SapXylem sapRises to heights of more than 100 m in the tallest plantsPushing Xylem Sap: Root PressureAt night, when transpiration is very lowRoot cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potentialWater flows in from the root cortexGenerating root pressureRoot pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicotsFigure 36.11Pulling Xylem Sap: The Transpiration-Cohesion-Tension MechanismWater is pulled upward by negative pressure in the xylemTranspirational PullWater vapor in the airspaces of a leafDiffuses down its water potential gradient and exits the leaf via stomataTranspiration produces negative pressure (tension) in the leafWhich exerts a pulling force on water in the xylem, pulling water into the leaf Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film’s pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater.CuticleUpperepidermisMesophyllLowerepidermisCuticleWater vaporCO2O2XylemCO2O2Water vaporStomaEvaporation At first, the water vapor lost bytranspiration is replaced by evaporation from the water film that coats mesophyll cells. In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of theleaf to the drier air outside via stomata. AirspaceCytoplasmCell wallVacuoleEvaporationWater filmLow rate oftranspirationHigh rate oftranspirationAir-waterinterfaceCell wallAirspaceY = –0.15 MPaY = –10.00 MPa312Figure 36.12Air-spaceCohesion and Adhesion in the Ascent of Xylem SapThe transpirational pull on xylem sapIs transmitted all the way from the leaves to the root tips and even into the soil solutionIs facilitated by cohesion and adhesionAscent of xylem sapXylemsapOutside air Y = –100.0 MPaLeaf Y (air spaces) = –7.0 MPaLeaf Y (cell walls) = –1.0 MPaTrunk xylem Y = – 0.8 MPaWater potential gradientRoot xylem Y = – 0.6 MPaSoil Y = – 0.3 MPaMesophyllcellsStomaWatermoleculeAtmosphereTranspirationXylemcellsAdhesionCellwallCohesion,byhydrogenbondingWatermoleculeRoothairSoilparticleWaterCohesion and adhesionin the xylemWater uptakefrom soil Figure 36.13Xylem Sap Ascent by Bulk Flow: A ReviewThe movement of xylem sap against gravityIs maintained by the transpiration-cohesion-tension mechanismConcept 36.4: Stomata help regulate the rate of transpirationLeaves generally have broad surface areasAnd high surface-to-volume ratiosBoth of these characteristicsIncrease photosynthesisIncrease water loss through stomata20 µmFigure 36.14Effects of Transpiration on Wilting and Leaf TemperaturePlants lose a large amount of water by transpirationIf the lost water is not replaced by absorption through the rootsThe plant will lose water and wiltTranspiration also results in evaporative coolingWhich can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processesStomata: Major Pathways for Water LossAbout 90% of the water a plant losesEscapes through stomataEach stoma is flanked by guard cellsWhich control the diameter of the stoma by changing shapeCells flaccid/Stoma closedCells turgid/Stoma openRadially oriented cellulose microfibrilsCellwallVacuoleGuard cellChanges in guard cell shape and stomatal opening and closing (surface view). Guard cells of a typical angiosperm are illustrated in their turgid (stoma open)and flaccid (stoma closed) states. The pair of guard cells buckle outward when turgid. Cellulose microfibrils in the walls resist stretching and compression in the direction parallel to the microfibrils. Thus, the radial orientation 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 the increase in length causes buckling.(a)Figure 36.15aChanges in turgor pressure that open and close stomataResult primarily from the reversible uptake and loss of potassium ions by the guard cellsH2OH2OH2OH2OH2OK+Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane andvacuolar membrane causes the turgor changes of guard cells.(b)H2OH2OH2OH2OH2OFigure 36.15bXerophyte Adaptations That Reduce TranspirationXerophytesAre plants adapted to arid climatesHave various leaf modifications that reduce the rate of transpirationThe stomata of xerophytesAre concentrated on the lower leaf surfaceAre often located in depressions that shelter the pores from the dry windLower epidermaltissueTrichomes(“hairs”)CuticleUpper epidermal tissueStomata100 mFigure 36.16Concept 36.5: Organic nutrients are translocated through the phloemTranslocationIs the transport of organic nutrients in the plantMovement from Sugar Sources to Sugar Sinks Phloem sapIs an aqueous solution that is mostly sucroseTravels from a sugar source to a sugar sinkA sugar sourceIs a plant organ that is a net producer of sugar, such as mature leavesA sugar sinkIs an organ that is a net consumer or storer of sugar, such as a tuber or bulbFigure 36.17aMesophyll cellCell walls (apoplast)Plasma membranePlasmodesmataCompanion(transfer) cellSieve-tubememberMesophyll cellPhloem parenchyma cellBundle-sheath cellSucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells.(a)Sugar must be loaded into sieve-tube members before being exposed to sinksIn many plant species, sugar moves by symplastic and apoplastic pathwaysA chemiosmotic mechanism is responsible forthe active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell.(b)High H+ concentrationCotransporterProtonpumpATPKeySucroseApoplastSymplastH+H+Low H+ concentrationH+SSFigure 36.17bIn many plantsPhloem loading requires active transportProton pumping and cotransport of sucrose and H+Enable the cells to accumulate sucroseVessel(xylem)H2OH2OSieve tube(phloem)Source cell(leaf)SucroseH2OSink cell(storageroot)1SucroseLoading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis. 24312This uptake of water generates a positive pressure that forces the sap to flow along the tube.The pressure is relieved by the unloading of sugar and the consequent loss of water from the tubeat the sink.34In the case of leaf-to-roottranslocation, xylem recycles water from sinkto source.Transpiration streamPressure flowFigure 36.18Pressure Flow: The Mechanism of Translocation in AngiospermsIn studying angiospermsResearchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressureThe pressure flow hypothesis explains why phloem sap always flows from source to sinkExperiments have built a strong case for pressure flow as the mechanism of translocation in angiospermsAphid feedingStylet in sieve-tubememberSevered styletexuding sapSieve-TubememberEXPERIMENTRESULTSCONCLUSIONSap dropletStyletSapdroplet25 mSieve-tubemember To 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. The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.The results of such experiments support the pressure flow hypothesis.Figure 36.19