B6A5PlantTransportS10 - Exchange & Transport in...

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Unformatted text preview: Exchange & Transport in Plants Exchange & Transport in Vascular Plants Plant Exchange & Transport I. Water and Solute Uptake by Cells II. Local Transport III. Long Distance Transport IV. Gas Exchange Water and Solutes - Uptake by Cells Passive Transport (Diffusion) • Net movement of molecules from a region of high concentration to a region of low concentration Caused by random (Brownian) movements of molecules (Increase entropy) Each type of molecule follows its own concentration gradient At equilibrium, movement is equal in both directions Osmosis: simple diffusion of the solvent (water) Osmotic pressure • Water diffuses according to its concentration gradient Water moves across lipid bilayer and through aquaporins (membrane gate proteins) Move the water by moving the solutes! Hypertonic solution: higher concentration of solutes • ↑Osm → ↓ [water] ↓ Osm →↑[water] Hypotonic solution: lower concentration of solutes • Osmosis can generate force (osmotic pressure) Semipermeable membrane Heyer Water and Solute Uptake by Cells Isotonic solution: equal solute concentrations 1 Exchange & Transport in Plants Water and Solute Uptake by Cells Hypotonic solution Isotonic solution Water and Solute Uptake by Cells Hypertonic solution Plant cells flaccid Animal cell Plant cell Plant cells turgid Turgor p ressure: pressure exerted on wall of turgid cell = 75–100psi Water Potential (Ψ) • Osmotic pressure pulls water to the right. • Osmotic potential (=solute potential [Ψ S ]): solution on left has potential energy to push water to the right • ↓Osm→↑ ΨS • Physical pressure (= pressure potential [ΨP]): solution on right has potential energy to push water to the left • If ΨS & Ψ P are equal but opposite→ no net flow ⇒ Ψ = ΨS + ΨP = 0 Water and Solute Uptake by Cells Water Potential (Ψ) • Pure water, Ψ S = 0 M Pa. 1 M Pa = 10 atm = ~160 p si • pressure potential [ΨP] could be negative • solutes lower ΨS 0.1 Osm = – 0.23 MPa • Water moves from high Ψ to low Ψ. Selective permeability Water moves across lipid bilayer and through aquaporins • Except for water and small n onpolar solutes, permeability of cell membranes is selective and regulated. • Permeability determined by transporter proteins. Ions and small molecules use protein channels and pumps – Channels and carriers are solute specific – If no transporter, than that solute cannot cross membrane • (Artificial membranes are only semipermeable —i.e., only discriminate based upon molecular size.) Heyer 2 Exchange & Transport in Plants Types of cellular transport • Passive transport: driven by Brownian motion Water and Solute Uptake by Cells – Simple diffusion & osmosis – Facilitated diffusion (carrier mediated passive transport) • Active transport: requires chemical energy (ATP) – Carrier mediated – Can transport a gainst concentration gradient Water and Solute Uptake by Cells Water and Solute Uptake by Cells Water and Solute Uptake by Cells Water and Solutes — Local Transport Tissue compartments & membranes Heyer 3 Exchange & Transport in Plants Water and Solutes — Local Transport Transmembrane T ransport: f rom cell-to-cell across plasma membranes [SLOW!] Water and Solutes — Local Transport Roots S ymplastic Transport: from cell-to-cell through plasmadesmata A poplastic T ransport: around cells through porous cell walls Water and Solutes — Long Distance Transport Water and Solutes — Xylem Transport Apoplastic movement of xylem sap—pushing Via xylem and phloem Bulk Flow: movement of fluids through vessels Must generate big pressure differences Where’s the pump? Root Pressure: active transport in roots → minerals accumulate in xylem → water follows → ↑ pressure Limited to 1–2 m, if at all Guttation: root pressure pushes water out leaves Water and Solutes — Xylem Transport Water — a polar molecule δ– Apoplastic movement of xylem sap —pulling TranspirationCohesion-Tension Mechanism Heyer δ+ • Polar: one end slightly positive (δ +), the other end slightly negative (δ– ) • Cohesion: – water molecules stick to each other 4 Exchange & Transport in Plants Water — a polar molecule • Adhesion (wetting): Water and Solutes — Xylem Transport Pop quiz! – water molecules are attracted to and stick to other polar molecules – Like the cellulose of xylem walls What are the big pipes called? What are the smaller pipes called? • Capillary action – “lead” water molecules attracted to “dry” cellulose for adhesion – Pull rest of water along by cohesion Regulating Transpiration Water and Solutes — Xylem Transport Guard cells turgid: stoma open Transpiration: the loss of water vapor K+ pumped into central vacuole; water follows Guard cells flaccid: stoma closed Gates allow K+ diffusion out of cell; water follows from the stomata of leaves Regulating Transpiration Negative ψp → negative ψ in leaves Transpiration pull moves xylem sap upward Guard cells turgid: stoma open Heyer Guard cells flaccid: stoma closed 5 Exchange & Transport in Plants Water and Solutes — Phloem Transport Water and Solutes — Phloem Transport Symplastic Flow Solute [osmotic] potential (Ψ s ) creates pressure gradient Movement of phloem sap — pushing only Source tissue •Photosynthesis or starch breakdown → ↑sucrose [solute] in phloem sap solution → ↓ Ψs → Absorb water from xylem → ↑ ΨP in phloem Translocation: moving sugar from sources to sinks Sink organ •Starch synthesis → ↓sucrose [solute] in phloem sap solution → ↑ Ψs →phloem loses water to xylem → ↓ ΨP in phloem Fig. 36.19 Leaf local transport: sucrose loading into phloem Bulk flow from ↑ΨP to ↓ΨP in phloem, from source to sink Secondary effect: With leaf-to-root translocation, xylem flow is increased without transpiration Gas Exchange Gas Exchange Why must plants do gas exchange? Photosynthesis (chlorenchyma): CO2 + H20 + energy CH2O + O2 Respiration (all tissues): CH2O + O2 CO2 + H20 + energy Photosynthesis (chlorenchyma): CO2 + H2 0 + energy CH2 O + O2 Respiration (all tissues): CH2 O + O2 CO2 + H2 0 + energy Photosynthetic mesophyll (chlorenchyma): cells are inside the leaf epidermis Photosynthetic mesophyll (chlorenchyma): cells are inside the leaf • • Need to deliver adequate CO2 from air to chlorenchyma But exposure to dry air causes water loss epidermis mesophyll mesophyll epidermis epidermis Heyer 6 Exchange & Transport in Plants Air: composition & Gas Exchange & Water Loss partial pressures N2: 78%; PN = 0.78 atm High demand for CO2 in leaves in daylight; but water loss is a big problem. Cuticle limits water loss through epidermis. O2: 21%; PO = 0.21 atm (↑ H 2O/↓ CO2): S tomata open to let air circulate. (↓ H 2O/↑ CO2): S tomata close to limit water loss. 2 2 CO2: 0.03%; PCO = 0.0003 atm 2 Other gases bring total up to 1 atmosphere. Gas Exchange & Water Loss Gas Exchange & Water Loss Plants have tricks to balance gas exchange & water loss. Xerophytes: plants adapted for low-moisture habitats Desert, windy, seashore • Shoot epidermis of the epiphytic cactus R hipsalis – Surface view: The crater-shaped depressions with a guard cell each at their base can be recognized. – Cross-section: The guard cells are deeply countersunk, the cuticle is extremely thick. Oleander: stratified epidermis & stomata in hairy pits. Surface view Gas Exchange & Water Loss Layer of dead, air-filled cells in epidermis Air-pockets are silvery and insulating → keep leaves cool Living tissues displaced from surface → r educe moisture loss Cross-section Gas Exchange & Water Loss No leaves! Trichomes make hairy surface Dense hairs trap humid air Old man cactus Maui silversword Heyer Cactus “leaf” primordia → spines Photosynthetic stem Ocotillo Leafless most of year Small, short-lived leaves in rainy season 7 Exchange & Transport in Plants Gas Exchange & Water Loss What about Oxygen? Some plants store CO2 at night so they can keep stomata closed all day. • Lenticels: elongated parenchyma creating air gaps in the peridermal c ork – permit gas-exchange between the atmosphere and the metabolically active cells below the bark – Often develop under site of stoma in primary epidermis This is covered in 6B! What about Oxygen? What about Oxygen? Special issues for submerged plants Mangroves: Pneumatophores c arry O2 to roots in mud 5°C 35°C % O 2 in air 21% 21% % O 2 in water 0.9%* 0.5%* O2 in water/air * 1/ 25 x 1/ 40 x A t equillibrium w ith air. Stagnation may decrease to 0 . What about Oxygen? Some aquatic plants need special tricks for oxygen. Heyer 8 ...
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