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Water Transport in Vascular Plants

Water Transport in Vascular Plants. Key Concepts. Concept.1: Physical forces drive the transport of materials in plants over a range of distances Concept.2: Roots absorb water and minerals from the soil Concept.3: Water and minerals ascend from roots to shoots through xylem ( 木質部 )

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Water Transport in Vascular Plants

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  1. Water Transport in Vascular Plants

  2. Key Concepts Concept.1:Physical forces drive the transport of materials in plants over a range of distances Concept.2:Roots absorb water and minerals from the soil Concept.3: Water and minerals ascend from roots to shoots through xylem (木質部) Concept.4:Stomata help regulate the rate of transpiration (蒸散作用)

  3. Non-vascular plants Vs Vascular plants • The evolutionary journey onto land involved the differentiation of the plant body into roots and shoots Moss are non-vascular plants

  4. Figure 36.1 • Vascular tissue • Transports nutrients throughout a plant; such transport may occur over long distances

  5. Anatomy of a woody stem

  6. Water-conducting cells of xylem

  7. Concept 1: Physical forces drive the transport of materials in plants over a range of distances • Transport in vascular plants occurs on two scales • 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

  8. 1 2 3 4 . 6 5 7 . • A variety of physical processes • are involved in the different types of transport CO2 O2 Light H2O Sugar O2 H2O CO2 Minerals Figure 36.2

  9. A variety of physical processes Are involved in the different types of transport 1 2 4 3 Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for photosynthesis. Some O2produced by photosynthesis is used in cellular respiration. Sugars are produced by photosynthesis in the leaves. Transpiration, the loss of water from leaves (mostly through stomata), creates a force within leaves that pulls xylem sap upward. 6 5 7 Water and minerals are transported upward from roots to shoots as xylem sap. Roots absorb water and dissolved minerals from the soil. Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars. CO2 O2 Light H2O Sugar Sugars are transported as phloem sap to roots and other parts of the plant. O2 H2O CO2 Minerals Figure 36.2

  10. Selective Permeability of Membranes: A Review • The selective permeability of a plant cell’s plasma membrane • Controls the movementofsolutes into and out of the cell • Specific transport proteins • Enable plant cells to maintain an internal environmentdifferent from their surroundings

  11. Bulk Flow in Long-Distance Transport In bulk / mass flow (巨流) Movement of fluid (sap) in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and phloem sieve tubes

  12. Absorption of water and minerals from the soil • Concept 2: Roots absorb water and minerals from the soil • Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable (without cuticle) to water and where root hairs are located • Root hairs account for much of the surface area of roots

  13. 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

  14. Water potential Is a measurement that combines the effects of solute concentration and pressure Determines the directionofmovement of water Water Flows from regions of high water potential to regions of low water potential

  15. 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 • Pressure potential • Is the physical pressure on a solution

  16. Quantitative Analysis of Water Potential The addition of solutes Reduces water potential (a) 0.1 M solution Pure water H2O P= 0 S= 0.23 = 0.23 MPa = 0 MPa Figure 36.5a

  17. Application of Positive physical pressure Increases water potential (b) (c) H2O H2O P= 0.23 S= 0.23 = 0 MPa P= 0.30 S= 0.23 = 0.07 MPa = 0 MPa = 0 MPa Figure 36.5b, c

  18. Negative pressure Decreases water potential (d) H2O P= 0.30 S= 0 = 0.30 MPa P= 0 S= 0.23 = 0.23 MPa Figure 36.5d

  19. water vapour water A plant loses water in the form ofwater vapour from the surface of the plant into the atmosphere This process is called transpiration

  20. Ascend from roots to shoots through the xylem • Concept 3: Water and mineralsascend from roots to shoots through the xylem • Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant • The transpired water must be replaced by water transported up from the roots

  21. The Ascent of Xylem Sap • Xylem sap • Rises to heights of more than 100 m in the tallest plants Water-conducting cells

  22. Pushing Xylem Sap: Root Pressure • At night, when transpiration is very low • Root cells continue pumping mineral ions into the xylem of the vascular cylinder, loweringthe water potential • Waterflows in to the xylem from the root cortex • Generating root pressure

  23. Figure 36.11 Guttation- a demonstration of root pressure • Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous plants

  24. Root pressure • In most plants, root pressure is notthe major mechanism driving the ascent of xylem sap. • At most, root pressure can force water upward only a few meters, and many plants generate no root pressure at all. • For the most part, xylem sap is not pushed from below by root pressure butpulledupward by the leaves themselves.

  25. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism • Water is pulled upward by negativepressure in the xylem Transpirational Pull starts with water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata

  26. Water Movement from the Leaf to the Atmosphere Transpiration = the evaporation of water from leaf surfaces

  27. Transpiration produces negative pressure (tension) in the leaf • Which exerts a pulling force on water in the xylem, pulling water into the leaf

  28. Hydrogen Bond between water molecules Water is a polar molecule, In water, the negative regions on one molecule are attracted to the positive regions on another, and the molecules form hydrogen bonds.                    

  29. The Process of Transpiration

  30. Movement of Water Up Xylem Vessels When water enters the roots, hydrogen bonds link each water molecule to the next so the molecules of water are pulled up the thin xylem vessels like beads on a string. The water moves up the plant, enters the leaves, moves into air spaces in the leaf, and then evaporates (transpires) through the stomata (singular, stoma).

  31. Xylem Sap Ascent by Bulk Flow: A Review • The mechanism of transpiration depends on the generation of negative pressure (tension) in the leaf due to unique physical properties of water. • As water transpires from the leaf, water coating the mesophyll cells replaces water lost from the air spaces.. • Adhesion to the wall and surface tension causes the surface of the water film to form a meniscus, “pulling on” the water by adhesive and cohesive forces.

  32. adhesion and surface tension lowers the water potential • The tension generated by adhesion and surface tension lowers the water potential, drawing water from where its potential is higher to where it is lower. • Mesophyll cells will lose water to the surface film lining the air spaces, which in turn loses water by transpiration. • The water lost via the stomata is replaced by water pulled out of the leaf xylem.

  33. Transpiration pull on xylem sap is transmitted all the way…. • The transpirational pull on xylemsap is transmitted all the way from the leaves to the root tips and even into the soil solution. • Cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water separating. • Helping to fight gravity is the strong adhesion of water molecules to the hydrophilicwalls of the xylem cells. • The very small diameter of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls.

  34. tension within the xylem…. • The upward pull on the cohesive sap creates tension within the xylem • This tension can actually cause a decrease in the diameter of a tree on a warm day. • Transpiration puts the xylem under tension all the way down to the root tips, lowering the water potential in the root xylem and pulling water from the soil.

  35. Xylem sap Outside air Y = –100.0 MPa Mesophyll cells Stoma Water molecule Leaf Y (air spaces) = –7.0 MPa Transpiration Atmosphere Leaf Y (cell walls) = –1.0 MPa Xylem cells Adhesion Cell wall Water potential gradient Trunk xylem Y = – 0.8 MPa Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root xylem Y = – 0.6 MPa Root hair Soil Y = – 0.3 MPa Soil particle Water Water uptake from soil Figure 36.13 Cohesion and Adhesion in the Ascent of Xylem Sap • The transpiration pull on xylem sap • Is transmittedall the way from the leaves to the root tips and even into the soil solution • Is facilitated by cohesion and adhesion

  36. The Plant – Soil – Atmosphere Continuum Movement of water from soil through plant to atmosphere involves different mechanisms of transport: In the vapor phase, water moves by diffusion until it reaches outside air (and convection, a form of bulk flow, becomes dominant) In xylem, water moves bybulk flow in response to a pressure gradient (ΔΨp) For water transport across membranes, water potential difference across membrane is driving force (osmosis, e.g. when cells absorb water and roots transport water from soil to xylem) In all cases: water moves toward regions of low water potential (or free energy)

  37. Maple treeTranspiration: 200 liters/day 75 cm/min

  38. Stomata: Major Pathways for Water Loss About 90% of the water a plant loses Escapes through stomata

  39. Stomata Stomata from sedge (Carex) Cytosol and vacuole Pore Heavily thickened guard cell wall Guard cells Subsidiary cells Stoma from a grass Epidermal cell

  40. Stomata Stomata from onion epidermis Outside surface of the leaf with stomatal pore inserted into the cuticle Guard cells facing the stomatal cavity, toward the inside of the leaf Stomatal pore Guard cell

  41. The cell walls of guard cells have specialized features Two main types of guard cells - kidney-shaped stomata: for dicots, monocots, mosses, ferns, gymnosperms - grass-like stomata: grasses and a few other monocots (e.g. palms) Grass-like stomata Kidney-shaped stomata

  42. An increase in guard cell turgor pressure opens the stomata - guard cells function as multisensory hydraulic valves - guard cells sense changes in the environment: light intensity, light quality, temperature, leaf water status, intracellular CO2 - this process requires ion uptake and other metabolic changes in the guard cells (discussed later) - due to this ion uptake→ Ψs decreases → Ψw decreases → water moves into guard cells → Ψp (turgor pressure) increases → cell volume increases → opening of stomata (due to differential thickening of guard cell walls)

  43. The cell walls of guard cells have specialized features Atmosphere Portions of the guard cell wall are substantially thickened (up to 5 um across) Pore Nucleus Plastid Vacuole Substomatal cavity Inner cell wall

  44. Changes in turgor pressure that open and close stomata Result primarily from the reversible uptake and loss of potassium ions by the guard cells Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells. H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O

  45. Effects of Transpiration on Wilting and Leaf Temperature Plants lose a large amount of water by transpiration If the lost water is not replaced by absorption through the roots The plant will lose water and wilt

  46. Turgor and wilting • Turgor loss in plants causes wilting • Which can be reversed when the plant is watered Figure 36.7

  47. Importance of transpiration • absorption of water • transport of water and minerals

  48. Evaporative cooling? Transpiration also results in evaporative cooling Which can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processes But recent findings cast doubt on the actual significance of the cooling effects

  49. rate of transpiration relative humidity (%) Environmental factors affecting the rate of transpiration 1. Relative humidity Humidity • concentration gradient of water vapour between the inside of a leaf and the atmosphere  more water vapour diffuses out

  50. rate of transpiration temperature (oC) Environmental factors affecting the rate of transpiration 2. Temperature Temperature  rate of evaporation of water

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