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Transport in Vascular Plants
Chapter 36 Transport in Vascular Plants
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A. Physical Forces CO2 O2 light H2O sugar O2 H2O CO2 minerals
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A. Physical Forces major substances transported are: H2O and minerals transport in xylem moves water because of transpiration evaporation, cohesion and adhesion sugars transport in phloem bulk flow gas exchange
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short-distance transport
transport occurs on three scales cellular from environment into plant cells transport of into root hairs H2O and solutes short-distance transport from cell to cell loading of from photosynthetic leaves into phloem sieve tubes sugar long-distance transport transport in throughout whole plant xylem and phloem
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membranes selective permeability diffusion, passive transport, active transport phospholipid bilayer, protein channels
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Cellular Transport solutes are moved into plant cells by active transport proton pumps active transport protein in cell membrane mechanism that uses the energy stored in a concentration gradient to drive cellular work chemiosmosis – use to pump against the concentration gradient the cell ATP H+ (hydrogen) ions out of sets up a separation of across a membrane membrane potential – opposite charge
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The Proton Pump
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both the proton pump and membrane potential have which is used to drive the transport of many different solutes stored energy
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Water Potential water uptake and loss must be balanced water moves by osmosis add which affects osmosis cell walls physical pressure water potential, , takes both and into account solute (dissolved substances) concentration Ψ physical pressure measured in megapascals, MPa (or bars)
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where: Ψ = water potential ΨS = solute potential (osmotic potential)
Ψ = ΨS + ΨP where: Ψ = water potential ΨS = solute potential (osmotic potential) ΨP = pressure potential the ΨS of pure water is zero Pure water = 0 MPa
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Addition of solutes adding solute the water potential (because there is less free water molecules less capacity to do work) and ΨS is lowers 0.1 M solution negative Pure water H2O P = 0 S = –0.23 = 0 MPa = –0.23 MPa
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ΨP can be relative to atmospheric pressure positive or negative
Applying physical pressure Applying physical pressure Pure water Pure water H2O H2O P = P = S = –0.23 S = –0.23 = 0 MPa = 0 MPa = 0 MPa = MPa
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water under (pulling) gives pressure eg) water in xylem tension
negative Negative pressure Pure water H2O P = –0.30 P = 0 S = 0 S = –0.23 = –0.30 MPa = –0.23 MPa
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water gives pressure eg) turgor pressure pushing out positive
water always moves from areas of to areas of high Ψ low Ψ water moves through the phospholipids bilayer and through transport proteins called aquaporins cells will be or depending on the environment plasmolyzed turgid plasmolyzed turgid
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loss of turgor causes wilting
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Short-Distance Transport plant cells are compartmentalized cell wall
cell membrane – cytosol vacuole Cell wall Cytosol Vacuole Vacuolar membrane (tonoplast) Plasmodesma Plasma membrane
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transport routes for water and solutes transmembrane route
repeated of plasma membrane crossing Transmembrane route
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symplast route movement within cytosol
plasmodesmata junctions connect cytosol of neighboring cells Key Symplast Transmembrane route Symplast Symplastic route
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apoplast route movement through the continuum of from cell to cell
cell walls no cell membranes are crossed Key Symplast Apoplast Transmembrane route Apoplast Symplast Symplastic route Apoplastic route
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Long-Distance Transport which is the movement of fluid driven by
bulk flow pressure flow in xylem tracheids and vessels creates which xylem sap upwards from roots negative pressure transpiration pulls loading of sugar from photosynthetic leaf cells generates high positive pressure which pushes phloem sap through sieve tubes lack of some organelles in phloem cells and the complete lack of cytoplasm in xylem cells makes them very efficient tubes for transport
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B. Roots much of the absorption of takes place at the root tips water and minerals root hairs extensions of epidermal cells walls are hydrophilic huge amount of surface area
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soil solution moves into
apoplast flows through walls into cortex solution moves into of root cells symplast water moves from Ψ in soil to Ψ in root high low active transport concentrates certain molecules in the root cells eg) K+ ions
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mycorrhizae symbiotic structures plant roots with fungus greatly increases surface area for water and mineral absorption greatly increases volume of soil reached by plant
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endodermis layer surrounding vascular cylinder of root lined with impervious Casparian strip forces solution through selective cell membrane and into symplast also prevents leakage of xylem sap back into soil solution in endodermis and parenchyma cells is discharged into cell walls (apoplast) by active and passive transport this allows the solution to then move to the xylem cells
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Casparian strip Pathway along apoplast Endodermal cell Pathway through symplast Casparian strip Plasma membrane Apoplastic route Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex
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C. Ascent of Xylem Sap root pressure in xylem of roots the Ψ mineral ions lowers water flows causing in root pressure pressure positive of xylem sap upward push accounts for of ascent of sap very small part
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water vapour leaves the leaf through the stomata (transpiration)
transpiration pull generated by leaf powered solar Ψ in leaf is than Ψ in higher atmosphere water vapour leaves the leaf through the stomata (transpiration) water pulled up Ψ is in roots and in leaves, moves water plant high low up adhesion, cohesion, hydrogen bonding
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Water potential gradient
Xylem sap Outside air Ψ = –100.0 MPa Mesophyll cells Stoma Leaf Ψ(air spaces) = –7.0 MPa Water molecule Transpiration Atmosphere Leaf Ψ(cell walls) = –1.0 MPa Xylem cells Adhesion Water potential gradient Cell wall Trunk xylem Ψ = –0.8 Mpa Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root xylem Ψ = –0.6 MPa Root hair Soil particle Soil Ψ = –0.3 MPa Water Water uptake from soil
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D. Stomata photosynthesis and transpiration compromise in and out but also out CO2 O2 H2O leaf transpires more than its weight in a day xylem sap can flow at 75 cm/min O2, H2O CO2
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H2O evaporation takes place even with
closed stomata drought will cause wilting transpiration causes of the leaves evaporative cooling
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microfibril mechanism
regulation of stomata microfibril mechanism guard cells attached at tips contain microfibrils in cell walls guard cells elongate and bow out when turgid guard cells shorten and become less bowed when flaccid Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell
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ion mechanism proton pumps are used to move into guard cells (stored in vacuoles) K+ ions Ψ in cells than surrounding cells H2O moves lower in guard cells become and turgid open of K+ ions causes H2O to move of guard cells loss out become and flaccid close Cells turgid/Stoma open Cells flaccid/Stoma closed H2O K+
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other cues light depletion of CO2
blue-light receptors in plasma membrane triggers ATP-powered proton pumps causing K+ uptake stomata open depletion of CO2 CO2 in air spaces in mesophyll is used for photosynthesis depletion causes stomata to open
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circadian rhythm automatic 24-hour cycle
stomata open in day, close at night
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xerophytes plants adapted for arid regions adapted to water loss reduce small, thick leaves reflective leaves hairy leaves stomata in pores on underside of leaves alternative photosynthetic pathway (CAM)
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E. Organic Nutrients is the transport of organic nutrients translocation phloem contains: sap water sugar (sucrose) (30% by weight) minerals amino acids hormones
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sieve tubes carry sap from to
sugar source (leaves) sugar sink (growing roots, buds, stems and fruit) variable direction of flow sap flow rate can be as high as 1 m/hr sugars are loaded into the phloem flow through via symplast plasmodesmata active of sucrose into phloem cells with H+ ions in proton pump cotransport
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Key Apoplast Symplast Cotransporter High H+ concentration
Mesophyll cell Companion (transfer) cell Sieve-tube member Proton pump Cell walls (apoplast) Plasma membrane Plasmodesmata Sucrose Bundle- sheath cell Phloem parenchyma cell Low H+ concentration Mesophyll cell
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pressure flow Ψ in is than in the xylem at because of the that takes place phloem lower sugar source sugar loading H2O diffuses from xylem into phloem is generated which causes the through phloem sieve tubes positive pressure sap to move Ψ in is than in the xylem at because of the from the phloem phloem higher sugar sinks sugar being removed H2O diffuses from phloem back into xylem
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low Ψ high Ψ low Ψ high Ψ Vessel (xylem) Sieve tube (phloem) H2O
Sucrose Source cell (leaf) H2O stream flow Pressure Transpiration Sink cell (storage root) Sucrose low Ψ H2O high Ψ
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