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Vascular tissue Syllabus Theme A Plant Structure and Function Transports nutrients throughout a plant; such transport may occur over long distances A3: Transport in Vascular Plants Campbell & Reece Chap. 36 pp. 738-755 TRANSPORT CO2 O2 Sugars are produced by photosynthesis in the leaves. STOMATA Light H2O Transport of water and solutes by individual Sugar cells, such as root hairs Short-distance transport of substances from cell to cell at the levels of tissues and organs Long-distance transport within xylem and phloem at the level of the whole plant Transpiration, creates a force within leaves that pulls xylem sap upward. Sugars are transported as phloem sap to roots and other parts of the plant. Water and minerals are transported upward from roots to shoots as xylem sap. Roots absorb water and dissolved minerals from the soil. O2 H2O Minerals Selective Permeability of Membranes: A Review The selective permeability of a plant cell’s plasma membrane Controls the movement of solutes into and out of the cell Specific transport proteins Enable plant cells to maintain an internal environment different from their surroundings CO2 Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars. Effects of Differences in Water Potential To survive Plants must balance water uptake and loss Osmosis Determines the net uptake or water loss by a cell Is affected by solute concentration and pressure 1 Effects of Differences in Water Potential Water potential Is a measurement that combines the effects of solute concentration and pressure Determines the direction of movement of water Water How Solutes and Pressure Affect Water Potential Both pressure and solute concentration Affect water potential The solute potential of a solution Is proportional to the number of dissolved molecules from regions of high water potential to regions of low water potential Pressure potential Is the physical pressure on a solution Quantitative Analysis of Water Potential Application of physical pressure Flows Reduces water potential (c) (a) The addition of solutes Increases (b) 0.1 M solution water potential ψ = ψS + ψp Pure water H2O H2O H2O ψ = 0 MPa Negative pressure Decreases water potential (d) ψP = 0 ψS = −0.23 ψ = −0.23 MPa ψ = 0 MPa ψP = 0.23 ψS = −0.23 ψ = 0 MPa ψ = 0 MPa ψP = 0.30 ψS = −0.23 ψ = 0.07 MPa Water potential Affects uptake and loss of water by plant cells If a flaccid cell is placed in an environment with a higher solute concentration The cell will lose water and become plasmolyzed Initial flaccid cell: ψP = 0 ψS = −0.7 0.4 M sucrose solution: ψP = 0 ψS = −0.9 H2O ψP = −0.30 ψS = 0 ψ = −0.30 MPa ψP = 0 ψS = −0.23 ψ = −0.23 MPa Plasmolyzed cell at osmotic equilibrium with its surroundings ψ = − 0.7 MPa ψ = −0.9 MPa ψP = 0 ψS = −0.9 ψ = −0.9 MPa 2 If the same flaccid cell is placed in a solution with a lower solute concentration The Turgor loss in plants causes wilting Which can be reversed when the plant is watered cell will gain water and become turgid Initial flaccid cell: ψP = 0 ψS = −0.7 ψ = − 0.7 MPa Distilled water: ψP = 0 ψS = 0 ψ = 0 MPa Turgid cell at osmotic equilibrium with its surroundings ψP = 0.7 ψS = −0.7 ψ = − 0 MPa Pathway of water and minerals Key Symplast In most plant tissues The cell walls and cytosol are continuous from cell to cell The cytoplasmic continuum Is called the symplast The apoplast Is the continuum of cell walls plus extracellular spaces Apoplast Transmembrane route Apoplast The symplast is the continuum of cytosol connected by plasmodesmata. The apoplast is the continuum of cell walls and extracellular spaces. Symplast Symplastic route Apoplastic route Casparian strip Functions of the Symplast and Apoplast in Transport Endodermal cell Pathway along apoplast Pathway through symplast Water and minerals can travel through a plant by one of three routes of one cell, across a cell wall, and into another cell Via the symplast Along the apoplast Casparian strip Out 1 Plasma membrane Apoplastic route Vessels (xylem) 2 Symplastic route Root hair Epidermis Cortex Endodermis Vascular cylinder Roots absorb water and minerals from the soil 3 The Roles of Root Hairs and Cortical Cells The Endodermis: A Selective Sentry The endodermis Root hairs? Innermost Mycorrhizae - mutual beneficial relationship with fungi, which facilitate the absorption of water and minerals from the soil 2.5 mm Water can cross the cortex Via layer of cells in the root cortex the vascular cylinder Last checkpoint for the selective passage of minerals from the cortex into the vascular tissue Surrounds the symplast or apoplast Rise of water up the stem The waxy Casparian strip of the endodermal Plants lose an enormous amount wall Blocks apoplastic transfer of minerals from the cortex to the vascular cylinder Root Pressure of water through transpiration Define transpiration! The transpired water must be replaced by water transported up from the roots Guttation Root pressure At night, when transpiration is very low Root cells continue pumping mineral ions into the xylem of the vascular cylinder Lowers the water potential Water flows in from the root cortex Generating sometimes results in guttation Exudation of water droplets root pressure 4 The Transpiration-CohesionTension Mechanism Water is pulled upward by negative pressure in the xylem Water vapour in the airspaces of a leaf Diffuses down its water potential gradient and exits the leaf via stomata Transpiration produces negative pressure (tension) in the leaf Ψ = –0.15 MPa Ψ = –10.00 MPa Cell wall Air-water interface Airspace Low rate of transpiration Cuticle High rate of transpiration Upper epidermis Cytoplasm Evaporation Mesophyll Airspace Airspace Cell wall Lower epidermis Evaporation Vacuole Water film Which exerts a pulling force on water in the xylem, pulling water into the leaf Cuticle CO2 O2 Xylem O2 Stoma Water vapor Water vapor Cohesion and Adhesion CO2 Xylem sap Outside air Ψ = –100.0 MPa Mesophyll cells Stoma Leaf Ψ (air spaces) = –7.0 MPa The transpirational pull on xylem sap Transpiration Leaf Ψ (cell walls) = –1.0 MPa Atmosphere Xylem cells Is Trunk xylem Ψ = – 0.8 MPa Water potential gradient transmitted all the way from the leaves to the root tips and even into the soil solution Is facilitated by cohesion and adhesion Water molecule Adhesion Cohesion and adhesion in the xylem Cell wall Cohesion, by hydrogen bonding Water molecule Root xylem Ψ = – 0.6 MPa Root hair Soil Ψ = – 0.3 MPa Soil particle Water uptake from soil The Control of Transpiration Leaves generally have broad surface Effects of Transpiration on Wilting and Leaf Temperature Plants lose a areas High Water large amount of water by transpiration If the lost water is not replaced by absorption through the roots surface-to-volume ratios These characteristics Increase photosynthesis Increase water loss through stomata 20 µm The plant will lose water and wilt 5 Effects of Transpiration on Wilting and Leaf Temperature Transpiration also results in evaporative cooling Why Stomata: Major Pathways for Water Loss About 90% of the water a plant loses Escapes must the temperature be lowered? through stomata Each stoma is flanked by guard cells Which control the diameter of the stoma by changing shape Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in turgor pressure that open and close stomata from the reversible uptake and loss of K+ ions by the guard cells Sugar Translocation Result H2O H2O H2O H2O H2O K+ H2O Translocation Is the transport of organic nutrients in the plant In the phloem tissue Phloem sap Is an aqueous solution that is mostly sucrose Travels from a sugar source to a sugar sink H2O H2O H2O H2O 6 Sugar must be loaded into sieve-tube A sugar source Is a plant organ that is a net producer of sugar, such as mature leaves members before being exposed to sinks In many plant species, sugar moves by symplastic and apoplastic pathways A sugar sink Mesophyll cell Cell walls (apoplast) Is an organ that is a net consumer or storer of sugar, such as a tuber or bulb Plasmodesmata Mesophyll cell loading requires active transport Proton pumping and cotransport of sucrose and H+ Enable the cells to accumulate sucrose High H+ concentration Cotransporter H+ Proton pump S Phloem parenchyma cell Pressure Flow: The Mechanism of Translocation in Angiosperms Vessel (xylem) In angiosperms Sap moves through a sieve tube by bulk flow driven by positive pressure Sieve tube (phloem) H2 O Source cell (leaf) Sucros e 1 H2 O 2 Pressure flow Phloem Bundlesheath cell Transpiration stre am In many plants Sieve-tube member Companion (transfer) cell Plasma membrane Sink cell (storage root) Key ATP 4 3 Sucros e H+ Low H+ concentration H+ Sucrose S Apoplast H2 O Symplast 7