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Chapter 36

Chapter 36. Transport in Vascular Plants 維管植物的運輸. Overview: Pathways for Survival of vascular plant ( 維管植物存活途徑 ) For vascular plants, the evolutionary journey ( 演化之旅 ) onto land involved the differentiation of the plant body into roots and shoots ( 植物體分化為根與枝條 )

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Chapter 36

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  1. Chapter 36 Transport in Vascular Plants 維管植物的運輸

  2. Overview: Pathways for Survival of vascular plant (維管植物存活途徑) • For vascular plants, the evolutionary journey (演化之旅) onto land involved the differentiation of the plant body into roots and shoots(植物體分化為根與枝條) • Vascular tissue transports nutrients throughout a plant; such transport may occur over long distances (長途運輸) Figure 36.1

  3. Key Concepts • Concept 36.1:Physical forces drive the transport of materials in plants over a range of distances • Concept 36.2:Roots absorb water and minerals from the soil • Concept 36.3: Water and minerals ascend from roots to shoots through xylem (木質部) • Concept 36.4:Stomata help regulate the rate of transpiration (蒸散作用) • Concept 36.5: Organic nutrients are translocated through the phloem (韌皮部)

  4. Concept 36.1: Physical forces drive the transport of materials in plants over a range of distances • Transport in vascular plants occurs on three scales (三種尺度/三種層次) • Transport of water and solutes by individual cells, such as root hairs (個別細胞水與溶質的運輸) • Short-distance transport of substances from cell to cell at the levels of tissues and organs (組織與器官內細胞間物質的短距離運輸) • Long-distance transport within xylem and phloem at the level of the whole plant (整株植物木質部與韌皮部的長距離運輸)

  5. Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for photosynthesis. Some O2produced by photosynthesis is used in cellular respiration. 1 2 3 4 CO2 O2 Sugars are produced by photosynthesis in the leaves. Light H2O Sugar Transpiration, the loss of water from leaves (mostly through stomata), creates a force within leaves that pulls xylem sap upward. 6 Sugars are transported as phloem sap to roots and other parts of the plant. 5 7 Water and minerals are transported upward from roots to shoots as xylem sap. Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars. Roots absorb water and dissolved minerals from the soil. O2 H2O CO2 Minerals • A variety of physical processes (物理過程) are involved in the different types of transport (眾 多物理過程介入或參與不同型式的運輸) (蒸散作用) (韌皮液) (木質液) (木質液) Figure 36.2

  6. Selective Permeability of Membranes: A Review膜的選擇通透性----各種膜系統 • The selective permeability(選擇通透性) of a plant cell’s plasma membrane (細胞膜) • Controls the movement of solutes into and out of the cell • Specific transport proteins/transportor(專一性輸送蛋白) • Enable plant cells to maintain an internal environment different from their surroundings • Solutes in cell: cation (陽離子), anion (陰離子), neutral solute (中性溶質)

  7. EXTRACELLULAR FLUID CYTOPLASM – + H+ + – ATP H+ – + H+ Proton pump generates membrane potential and H+gradient. Proton pump H+ H+ H+ – H+ + H+ – + The Central Role of Proton Pumps (質子唧筒) • Proton pumps (質子唧筒) in plant cells • Create a hydrogen ion gradient (氫離子梯度) that is a form of potential energy(潛能) that can be harnessed (=used) to do work (作功) • Contribute to a voltage known as a membrane potential (膜電位), also a kind of potential energy 細胞外液 細胞質 Figure 36.3

  8. 細胞質 + 細胞外液 CYTOPLASM EXTRACELLULAR FLUID – + – Cations ( , for example) are driven into the cell by themembrane potential. K+ K+ + – K+ K+ K+ transporter K+ K+ – + K+ – + Transport protein (transportor) (a) Membrane potential and cation uptake (陽離子的吸收) • Plant cells use energy stored in the proton gradient(氫離子梯度) and membrane potential (膜電位), both of which are potential energy • To drive the transport of many different solutes Figure 36.4a

  9. NO3– • In the mechanism called cotransport (共同運輸) • A transport protein(cotransporter) couples the passage of one solute to the passage of another + 細胞外液 – 細胞質 H+ H+ NO3 – + – NO3– + 高濃度H+ 低濃度NO3- – H+ H+ 低濃度H+ 高濃度NO3- H+ Cell accumulates anions (, for example) by coupling their transport to theinward diffusion H+ H+ cotransporter H+ NO3– – NO3 – + NO3 – H+ of through a cotransporter. – + H+ NO3– – H+ + H+ 共同運輸蛋白 H+ (b) Cotransport of anions (陰離子的共同運輸) Figure 36.4b

  10. 細胞外液 細胞質 + – H+ H+ H+ S + – Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. H+ – + H+ H+ S cotransporter H+ – S H+ H+ H+ S S S – H+ + – + H+ S H+ + – (c) Cotransport of a neutral solute (中性溶質的共同運輸) • The “coattail” effect of cotransport (共同運輸) • Is also responsible for the uptake of the sugar sucrose (neutral solute) by plant cells 低濃度H+ 高濃度sugar 高濃度H+ 低濃度sugar Figure 36.4c

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  12. Effects of Differences in Water Potential水勢差的效應 • To survive plants must balance water uptake and loss • Differences in water potential drive water transport in plant cells (水勢差驅動植物細胞中的水份運輸) • Osmosis (滲透作用) • Determines the net uptake or water loss by a cell • Is affected by solute concentration and pressure

  13. water solute water • Osmotic pressure (滲透壓) in animal cells low osmotic pressure=low [solute]=high [water] high osmotic pressure=high [solute]=low [water] • Direction of moving water low osmoticpressure=low [solute]=high [water] high osmotic pressure=high [solute]=low [water] • Isotonic solution(等張溶液)、 hypertonic solution(高張溶液)、 hypotonic solution(低張溶液)

  14. Water potential (水勢) • Is a measurement that combines the effects of solute concentration and pressure • Determines the direction of movement of water • Water • Flows from regions of high water potential to regions of low water potential • Solute (溶質) In Membrane Water Out Solute

  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. (a) 0.1 M solution Pure water H2O P= 0 S= 0.23 = 0.23 MPa = 0 MPa Quantitative Analysis of Water Potential (水勢) • The addition of solutes reduces water potential High [solute] =Low water potential Low [solute] =High water potential Figure 36.5a

  17. (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 • Application of physical pressure(物理性壓力) • Increases water potential(水勢) Figure 36.5b, c

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

  19. Initial flaccid cell: P= 0 S= 0.7 0.4 M sucrose solution: = 0.7 MPa P= 0 S= 0.9 Plasmolyzed cell at osmotic equilibrium with its surroundings = 0.9 MPa P= 0 S= 0.9 = 0.9 MPa Figure 36.6a • Water potential(水勢) • Affects uptake and loss of water by plant cells • If a flaccid cell(鬆弛細胞) is placed in an environment with a higher solute concentration • The cell will lose water and become plasmolyzed(膜壁分離)

  20. If the same flaccid cell is placed in a solution with a lower solute concentration • The cell will gain water and become turgid (膨脹) Initial flaccid cell: P= 0 S= 0.7 Distilled water: = 0.7 MPa P= 0 S= 0 Turgid cell at osmotic equilibrium with its surroundings = 0 MPa P= 0.7 S= 0.7 = 0 MPa Figure 36.6b

  21. Turgor (膨壓) loss in plants causes wilting (萎凋) • Which can be reversed when the plant is watered (澆水) Figure 36.7

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  23. Aquaporin Proteins and Water Transport • Aquaporins (水孔蛋白) • Are transport proteins(運轉蛋白) in the cell membrane that allow the passage of water • Do not affect water potential

  24. Three Major Compartments (區間) of Vacuolated Plant Cells • Transport is also regulated • By the compartmental structure of plant cells • Compartmentation(區間化/區隔化/間隔化) • Cell wall (細胞壁) • Cytosol (細胞質液) • Vacuole (液胞)

  25. The plasma membrane(細胞膜) • Directly controls the traffic of molecules into and out of the protoplast • Is a barrier between two major compartments (區間), the cell wall and the cytosol

  26. Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. (a) Cell compartments (細胞間隔) ：同一細胞內 • The third major compartment in most mature plant cells is the vacuole, a large organelle that can occupy as much as 90% of more of the protoplast’s volume • The vacuolar membrane(液胞膜) regulates transport between the cytosol and the vacuole Three compartments Cell wall Transport proteins in the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. Cytosol Vacuole 1. Vacuolar membrane (tonoplast) (液胞膜) 2. Plasma membrane 3. Plasmodesma (細胞質連絡絲) Figure 36.8a

  27. In most plant tissues • The cell walls and cytosol are continuous from cell to cell • The cytoplasmic continuum • Is called the symplast (共質體/合胞體) • The apoplast (離質體/非原質體/質外體) • Is the continuum of cell walls plus extracellular spaces

  28. Key Symplast (穿越細胞膜途徑) Apoplast 1. Transmembrane route (質外體) Apoplast The symplast is the continuum of cytosol connected by plasmodesmata. The apoplast is the continuum of cell walls and extracellular spaces. (共質體) Symplast 2. Symplastic route 3. Apoplastic route (共質體途徑) (質外體途徑) Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another. (b) Tissue compartments (組織間隔) ：不同細胞間 Figure 36.8b

  29. Functions of the Symplast and Apoplast in Transport • Water and minerals can travel through a plant by one of three routes • Out of one cell, across a cell wall, and into another cell • Via the symplast (共質體/合胞體) • Along the apoplast (離質體/非原質體/質外體)

  30. Bulk Flow in Long-Distance Transport長距離運輸的巨流 • In bulk flow(巨流) • Movement of fluid (sap) in the xylem and phloem is driven by pressure differences(壓力差) at opposite ends of the xylem vessels (木質部導管) and phloem sieve tubes (韌皮部篩管)

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  32. Concept 36.2: Roots absorb water and minerals from the soil • Water and mineral salts (礦物鹽) from the soil • Enter the plant through the epidermis of roots and ultimately flow to the shoot system

  33. Casparian strip (1) Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. Endodermal cell Pathway along apoplast (質外體途徑) (2) Minerals and water that cross the plasma membranes of root hairs enter the symplast. Pathway through symplast 5 4 (共質體途徑) 卡氏帶 (3) As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast. Plasma membrane Casparian strip Apoplastic route 1 (質外體途徑) 2 Vessels (xylem) 3 5 4 (4) Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder. Root hair Symplastic route (共質體 途徑) Vascular cylinder Epidermis Cortex 內皮 表皮 皮層 Endodermis (5)Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system. 卡氏帶 Lateral transport (側向運輸) of minerals and water in roots 導管 維管束 Figure 36.9

  34. The Roles of Root Hairs, Mycorrhizae, and Cortical Cells • Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located • Root hairs account for much of the surface area of roots

  35. 2.5 mm • Most plants form mutually beneficial relationships(互利共生) with fungi, which facilitate (促進) the absorption of water and minerals from the soil • Roots and fungi form mycorrhizae (菌根), symbiotic structures(共生結構) consisting of plant roots united with fungal hyphae (菌絲) 菌根 是真菌與根的共生結合 Figure 36.10

  36. Once soil solution enters the roots • The extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

  37. The Endodermis: A Selective Sentry (選擇性進入) • The endodermis (內皮層) • Is the innermost layer (最內層) of cells in the root cortex • Surrounds the vascular cylinder and functions as the last checkpoint(最後關卡) for the selective passage of minerals from the cortex (皮質) into the vascular tissue (維管束組織)

  38. Water can cross the cortex(皮層) • Via the symplastic (共質體的) or apoplastic route (質外體的路徑) • The waxy Casparian strip (卡氏帶) of the endodermal wall (內皮細胞壁) • Blocks apoplastic transfer of minerals from the cortex to the vascular cylinder

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  40. CO2 O2 Light H2O Sugar O2 H2O CO2 Minerals • Concept 36.3: Water and minerals ascend (升高) 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

  41. Factors Affecting the Ascent of Xylem Sap • Xylem sap (木質部汁液) • Rises to heights of more than 100 m in the tallest plants

  42. Pushing Xylem Sap: Root Pressure (根壓) • At night, when transpiration is very low • Root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential • Water flows in from the root cortex (根的皮層) • Generating root pressure(根壓) in the xylem

  43. Figure 36.11 • Root pressure (根壓) sometimes results in guttation (點泌作用), the exudation (滲出作用) of water droplets (小水滴) on tips of grass blades or the leaf margins (葉緣) of some small, herbaceous eudicots (草本的真雙子葉植物)

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  45. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism (拉升木質液：蒸散作用內聚力-附著力-張力的作用機制) • Water is pulled upward by negative pressure (負壓) in the xylem Transpirational Pull (蒸散作用的拉力) • Water vapor in the airspaces of a leaf • Diffuses down its water potential gradient and exits the leaf via stomata

  46. Transpiration produces negative pressure (tension) in the leaf which exerts a pulling force(拉力) on water in the xylem, pulling water into the leaf Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film’s pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater. 3 Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water interface Xylem Airspace Cuticle High rate of transpiration Low rate of transpiration Upper epidermis Cytoplasm Evaporation Mesophyll Airspace Air-space Cell wall Evaporation Lower epidermis Vacuole Water film CO2 O2 CO2 O2 Stoma Cuticle Water vapor Water vapor In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata. At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells. 1 2 Figure 36.12

  47. Cohesion and Adhesion in the Ascent of Xylem Sap (木質液上升時的凝聚力與附著力) • The transpirational pull (蒸散作用拉力) on xylem sap • Is transmitted (傳導/傳遞) all the way from the leaves to the root tips and even into the soil solution • Is facilitated by cohesion and adhesion(凝聚力與附著力)

  48. 水份在樹木中的上升作用 • Ascent of xylem sap (木質液的上升) Xylem sap Outside air Y = –100.0 MPa Mesophyll cells 水勢梯度 Stoma Leaf Y (air spaces)= –7.0 MPa Water molecule Transpiration Atmosphere Leaf Y (cell walls)= –1.0 MPa Xylem cells Cell wall Adhesion Water potential gradient Cohesion, by hydrogen bonding Trunk xylem Y= – 0.8 MPa Cohesion and adhesion in the xylem Water molecule Root hair Root xylem Y= – 0.6 MPa Soil particle Soil Y= – 0.3 MPa Water uptake from soil Water Figure 36.13

  49. Xylem Sap Ascent by Bulk Flow: A Review • The movement of xylem sap (木質液) against gravity (重力/萬有引力) • Is maintained by the transpiration-cohesion-tension mechanism

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