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Transcript
Transport in Vascular Plants Chapter 36 p. 738-755 Proton Pumps in Plant Cells Move H+ ions OUT of cell (inside cell more negative) “Membrane Potential” Creates potential energy that can be used to do work: 1) Helps roots take up K+ ions into cell 2) Cotransport of ions (NO3-) 3) Cotransport of neutral substances (sucrose) Transport of H2O & Solutes H2O will move from area of ↓ [solutes] to area of ↑ [solutes] Plant cells have cell walls that create physical pressure within a cell Water Potential (Ψ): created by [solutes] and physical pressure Determines direction of H2O flow (in or out of cell) H2O moves from ↑ Ψ to ↓ Ψ Measured in Megapascals (MPa) – pure H2O is 0.0MPa How Solutes & Pressure Affect Ψ Ψ = Ψs + Ψp Ψs = Solute/Osmotic Potential = amount of dissolved solutes By adding solutes, you ↓ ability of H2O to do work, therefore Ψs is ALWAYS negative Ψp = Pressure Potential = physical pressure on a solution Can be + or – compared to atmos. pressure Water Uptake & Loss in Plants If cell (Ψ=-0.7MPa) is placed pure H2O, water moves INTO cell If cell (Ψ=-0.7MPa) is placed in sucrose solution, water moves OUT of cell Cell becomes turgid Cell plasmolyzes & plant will wilt Aquaporins: proteins through which H2O travels across membranes Control rate of H2O flow Plant Cell Compartmentalization Plant cells have 3 compartments, separated by membranes: 1) Cell Wall 2) Cell Membrane 3) Vacuole Surrounded by tonoplast membrane Cell Sap: contents of vacuole Proton pumps move H+ ions INTO vacuole, creating gradient Short-Distance Transport Symplast: continuum of cytosol btwn cells connected by plasmodesmata Apoplast: continuum of cell walls & extracellular space btwn cells Move cell to cell Move location Transmembrane Route: mvmnt of substances across cell walls into neighboring cells Long Distance Transport Bulk Flow: mvmnt of fluids driven by pressure in xylem & phloem 1) in Xylem: transpiration creates (–) pressure in leaves, pulling xylem sap upwards 2) in Phloem: loading sugar into sieve tubes creates (+) pressure at leaf end Forces sap into rest of plant body Functions of Transport in Plants Plants must use intracellular, short distance, and long distance transport to perform: 1) 2) 3) 4) H2O & mineral absorption in roots Mvmnt of xylem sap upwards Control of transpiration Mvmnt of organic nutrients w/in phloem H2O & Mineral Absorption in Plants 1) H2O & minerals move through root epidermis & cortex via apoplastic or symplastic pathways Some H2O & minerals move from apoplastic to symplastic as transported inward Mycorrhizae: symbiotic structure of roots & fungi that help ↑ H2O & mineral absorption from soil H2O & Mineral Absorption in Plants (con’t) 2) At endodermis, Caspian Strip creates waxy barrier that blocks transport of materials outside symplastic pthwy H2O & minerals not already in symplastic pthwy can not cross into vascular cylinder 3) H2O & minerals move into xylem vessels & upward throughout plant Movement of Xylem Sap Upwards Xylem sap is moved in 2 ways: 1) As H2O & minerals enter xylem vessels in roots, creates (+) root pressure, pushing sap upwards Minor effect 2) As H2O transpires from leaves, creates (-) pressure, pulling sap upwards Major effect Uses Transpiration-Cohesion-Tension Mechanism Transpiration-Cohesion-Tension 1) Transpirational Pull: as H2O diffuses into air pockets of mesophyll, it evaporates & is lost thru stomata Creates –Ψ in leaves, which keeps pulling H2O in direction out stoma 2) Cohesion-Adhesion: cohesion of H2O molecules to each other & adhesion to cell walls pulls xylem sap ↑ Only works if H2O chain is unbroken If air bubble forms, vessel is useless & H2O must be redirected ...only youngest, outermost secondary xylem actively transports H 2O Control of Transpiration # stomata & size of pores regulate H2O loss in plants Guard cells change shape when take in K+ Affected by both genetics & evolution (i.e.: CO2 levels in atmosphere) K+ in: guard cells bend, stomata open K+ out: guard cells straighten, stomata close Stomata close at night, open during day because: 1) light stimulates accumulation of K+ in guard cells 2) ↓ CO2 in leaf air spaces 3) “Internal Clock” of guard cells Stomata may close during day if under environmental stress (i.e.: H2O deficiency) Xerophyte Transpiraiton Xerophyte: plant adapted to dry, arid climates small, thick leaves thick cuticle reflective or hairy leaves that trap H2O CAM Plants: Mesophyll incorporates CO2 into organic cmpds at night (stomata open) During day, org. cmpds release CO2 into plant for photosynthesis (stomata close) Translocation of Organic Nutrients in Phloem Phloem sap ALWAYS flows from sugar source → sugar sink Sugar Sink: plant organ that consumes sugars (i.e.; root, stem, fruit, etc.) Driven by (+) pressure created at source end Sugars (sucrose) move from mesophyll → sieve tube cells via symplastic & apoplastic pthwys Sucrose cotransports with H+ ions into sieve tube & companion cells