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Resource Acquisition and Transport in Vascular Plants

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  • Figure 36.1 Plants or pebbles?
  • Figure 36.2 An overview of resource acquisition and transport in a vascular plant.
  • Figure 36.3 Emerging phyllotaxy of Norway spruce.
  • Figure 36.4 Leaf area index.
  • Figure 36.5 A mycorrhiza, a mutualistic association of fungus and roots.
  • Figure 36.6 Cell compartments and routes for short-distance transport.
  • Figure 36.7 Solute transport across plant cell plasma membranes.
  • Figure 36.7 Solute transport across plant cell plasma membranes.
  • Figure 36.7 Solute transport across plant cell plasma membranes.
  • Figure 36.7 Solute transport across plant cell plasma membranes.
  • Figure 36.7 Solute transport across plant cell plasma membranes.
  • Figure 36.8 Effects of solutes and pressure on water potential (  ) and water movement.
  • Figure 36.10 Transport of water and minerals from root hairs to the xylem.
  • Figure 36.11 Guttation.
  • Figure 36.12 Generation of transpirational pull.
  • Figure 36.13 Ascent of xylem sap.
  • Figure 36.13 Ascent of xylem sap.
  • Figure 36.13 Ascent of xylem sap.
  • Figure 36.13 Ascent of xylem sap.
  • Figure 36.14 An open stoma (left) and closed stoma (LMs).
  • Figure 36.15 Mechanisms of stomatal opening and closing.
  • Figure 36.15 Mechanisms of stomatal opening and closing.
  • Figure 36.15 Mechanisms of stomatal opening and closing.
  • Figure 36.16 Some xerophytic adaptations.
  • Figure 36.17 Loading of sucrose into phloem.
  • Figure 36.18 Bulk flow by positive pressure (pressure flow) in a sieve tube.
  • Figure 36.20 Virus particles moving cell to cell through a plasmodesma connecting turnip leaf cells (TEM).
  • Figure 36.UN01 In-text figure, p. 770
  • Figure 36.UN02 In-text figure, p. 770
  • Figure 36.UN03 Summary figure, Concept 36.1
  • Figure 36.UN05 Appendix A: answer to Test Your Understanding, question 10
  • 36 lecture presentation

    1. 1. Chapter 36Resource Acquisition andTransport in Vascular Plants
    2. 2. Overview: Underground Plants • Stone plants (Lithops) are adapted to life in the desert – Two succulent leaf tips are exposed above ground; the rest of the plant lives below ground
    3. 3. Figure 36.1
    4. 4. • The success of plants depends on their ability to gather and conserve resources from their environment• The transport of materials is central to the integrated functioning of the whole plant
    5. 5. Adaptations for acquiring resources werekey steps in the evolution of vascularplants • The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water • Early nonvascular land plants lived in shallow water and had aerial shoots • Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport
    6. 6. • The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis• Xylem transports water and minerals from roots to shoots• Phloem transports photosynthetic products from sources to sinks
    7. 7. Figure 36.2-3 CO2 O2 Light H2O Sugar O2 H2O and CO2 minerals
    8. 8. • Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss
    9. 9. Shoot Architecture and Light Capture • Stems serve as conduits for water and nutrients and as supporting structures for leaves • There is generally a positive correlation between water availability and leaf size
    10. 10. • Phyllotaxy, the arrangement of leaves on a stem, is specific to each species• Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral• The angle between leaves is 137.5° and likely minimizes shading of lower leaves
    11. 11. Figure 36.3 42 24 32 29 40 16 11 19 21 3 27 34 8 6 13 14 26 Shoot 1 5 apical 22 18 meristem 9 Buds 10 2 4 31 17 23 7 12 15 20 25 28 1 mm
    12. 12. • Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows• Self-pruning is the shedding of lower shaded leaves when they respire more than photosynthesize
    13. 13. Figure 36.4 Ground area covered by plant Plant A Plant B Leaf area = 40% Leaf area = 80% of ground area of ground area (leaf area index = 0.4) (leaf area index = 0.8)
    14. 14. • Leaf orientation affects light absorption• In low-light conditions, horizontal leaves capture more sunlight• In sunny conditions, vertical leaves are less damaged by sun and allow light to reach lower leaves
    15. 15. • Shoot height and branching pattern also affect light capture• There is a trade-off between growing tall and branching
    16. 16. Root Architecture and Acquisition ofWater and Minerals • Soil is a resource mined by the root system • Taproot systems anchor plants and are characteristic of gymnosperms and eudicots • Root growth can adjust to local conditions – For example, roots branch more in a pocket of high nitrate than low nitrate • Roots are less competitive with other roots from the same plant than with roots from different plants
    17. 17. • Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae• Mutualisms with fungi helped plants colonize land• Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphate
    18. 18. Figure 36.5 Roots Fungus
    19. 19. Different mechanisms transport substancesover short or long distances • There are two major pathways through plants – The apoplast – The symplast
    20. 20. The Apoplast and Symplast: TransportContinuums • The apoplast consists of everything external to the plasma membrane • It includes cell walls, extracellular spaces, and the interior of vessel elements and tracheids • The symplast consists of the cytosol of the living cells in a plant, as well as the plasmodesmata
    21. 21. • Three transport routes for water and solutes are – The apoplastic route, through cell walls and extracellular spaces – The symplastic route, through the cytosol – The transmembrane route, across cell walls
    22. 22. Figure 36.6 Cell wall Apoplastic route Cytosol Symplastic route Transmembrane route Key Plasmodesma Plasma membrane Apoplast Symplast
    23. 23. Short-Distance Transport of Solutes AcrossPlasma Membranes • Plasma membrane permeability controls short- distance movement of substances • Both active and passive transport occur in plants • In plants, membrane potential is established through pumping H+ by proton pumps • In animals, membrane potential is established through pumping Na+ by sodium-potassium pumps
    24. 24. Figure 36.7 (a) H+ and membrane potential CYTOPLASM EXTRACELLULAR FLUID − + − H+ Hydrogen ion ATP + − + H+ H+ H+ H+ H + − + H + Proton pump − H+ + − + + (b) H+ and cotransport of neutral S H+ − H solutes + H+ H+ − + H+ H+ H+ S S H+ H+ H+ S S S − + H+ − + Sucrose H+/sucrose cotransporter − + (neutral solute) − + (c) H+ and cotransport of ions H+ − H+ NO 3 − + − − + H+ NO 3 H + H+ H+ Nitrate H+ NO 3 − H+ − NO − NO3 − NO 3 − + 3 − + H+ H+NO3− H+ H + − + cotransporter − + (d) Ion channels K+ Potassium ion − + K+ K+ − + K+ K+ K+ K+ − + Ion channel − +
    25. 25. Figure 36.7a CYTOPLASM EXTRACELLULAR FLUID − + H+ Hydrogen ion ATP − + − + H+ H+ H+ H+ H+ − + H+ Proton pump − H+ + (a) H+ and membrane potential
    26. 26. • Plant cells use the energy of H+ gradients to cotransport other solutes by active transport
    27. 27. Figure 36.7b − + S H − + H+ + H+ H+ − + H+ H+ H+ S S H+ H+ H+ S S S − + H+ − + H+/sucrose Sucrose cotransporter − + (neutral solute) (b) H+ and cotransport of neutral solutes
    28. 28. Figure 36.7c − + H + − H+ NO 3 − + − − + H+ NO 3 H+ H+ H+ Nitrate H+ − H+ NO 3 − NO NO3 − NO 3 − + 3 − − + H+ H+NO3− H+ − + H+ cotransporter (c) H+ and cotransport of ions
    29. 29. • Plant cell membranes have ion channels that allow only certain ions to pass
    30. 30. Figure 36.7d − + K+ Potassium ion − + K + K+ − + K+ K+ K+ K+ − + Ion channel − + (d) Ion channels
    31. 31. Short-Distance Transport of Water AcrossPlasma Membranes • To survive, plants must balance water uptake and loss • Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure
    32. 32. • Water potential is a measurement that combines the effects of solute concentration and pressure• Water potential determines the direction of movement of water• Water flows from regions of higher water potential to regions of lower water potential• Potential refers to water’s capacity to perform work
    33. 33. • Consider a U-shaped tube where the two arms are separated by a membrane permeable only to water• Water moves in the direction from higher water potential to lower water potential
    34. 34. Figure 36.8 Solutes have a negative Positive pressure has a Solutes and positive Negative pressure effect on ψ by binding positive effect on ψ by pressure have opposing (tension) has a negative water molecules. pushing water. effects on water effect on ψ by pulling movement. water. Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium H2O H2O H2O H2O Adding solutes to the Applying positive In this example, the effect Applying negative right arm makes ψ lower pressure to the right arm of adding solutes is pressure to the right arm there, resulting in net makes ψ higher there, offset by positive makes ψ lower there, movement of water to resulting in net movement pressure, resulting in no resulting in net movement the right arm: of water to the left arm: net movement of water: of water to the right arm: Positive Positive Negative pressure pressure pressure Pure water Solutes Solutes Membrane H2O H2O H2O H2O
    35. 35. Water Movement Across Plant CellMembranes • Water potential affects uptake and loss of water by plant cells • If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis • Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall
    36. 36. Turgorpressureandplasmolysis
    37. 37. • If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid• Turgor loss in plants causes wilting, which can be reversed when the plant is watered
    38. 38. Aquaporins: Facilitating Diffusion ofWater • Aquaporins are transport proteins in the cell membrane that allow the passage of water • These affect the rate of water movement across the membrane
    39. 39. Long-Distance Transport: The Role of BulkFlow • Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure • Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem • Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm
    40. 40. Transpiration drives the transport ofwater and minerals from roots to shootsvia the xylem • Plants can move a large volume of water from their roots to shoots
    41. 41. Absorption of Water and Minerals by RootCells • Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water • Root hairs account for much of the surface area of roots • After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals
    42. 42. • The concentration of essential minerals is greater in the roots than soil because of active transport
    43. 43. Transport of Water and Minerals into theXylem • The endodermis is the innermost layer of cells in the root cortex • It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue
    44. 44. • Water can cross the cortex via the symplast or apoplast• The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder• Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder
    45. 45. Figure 36.10 Casparian strip Pathway along Endodermal apoplast cell Pathway through symplast Plasma membrane Casparian strip Apoplastic route Vessels (xylem) Symplastic Root route hair Epidermis Endodermis Vascular cylinder (stele) Cortex
    46. 46. • The endodermis regulates and transports needed minerals from the soil into the xylem• Water and minerals move from the protoplasts of endodermal cells into their cell walls• Diffusion and active transport are involved in this movement from symplast to apoplast• Water and minerals now enter the tracheids and vessel elements
    47. 47. Bulk Flow Transport via the Xylem • Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow • The transport of xylem sap involves transpiration, the evaporation of water from a plant’s surface • Transpired water is replaced as water travels up from the roots • Is sap pushed up from the roots, or pulled up by the leaves?
    48. 48. Pushing Xylem Sap: Root Pressure • At night root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential • Water flows in from the root cortex, generating root pressure • Root pressure sometimes results in guttation, the exudation of water droplets on tips or edges of leaves
    49. 49. Figure 36.11
    50. 50. • Positive root pressure is relatively weak and is a minor mechanism of xylem bulk flow
    51. 51. Pulling Xylem Sap: The Cohesion-TensionHypothesis • According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots • Xylem sap is normally under negative pressure, or tension
    52. 52. Transpirational Pull• Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata• As water evaporates, the air-water interface retreats further into the mesophyll cell walls• The surface tension of water creates a negative pressure potential
    53. 53. • This negative pressure pulls water in the xylem into the leaf • The transpirational pull on xylem sap is transmitted from leaves to roots© 2011 Pearson Education, Inc.
    54. 54. Figure 36.12 Cuticle Xylem Upper epidermis Microfibrils in cell wall of Mesophyll mesophyll cell Air space Lower epidermis Cuticle Stoma Microfibril Water Air-water (cross section) film interface
    55. 55. Figure 36.13 Xylem sap Outside air ψ Mesophyll cells = −100.0 MPa Stoma Leaf ψ (air spaces) Water molecule = −7.0 MPa Transpiration Atmosphere Leaf ψ (cell walls) Adhesion by = −1.0 MPa Xylem hydrogen bonding Water potential gradient cells Cell wall Trunk xylem ψ Cohesion by = −0.8 MPa Cohesion and hydrogen bonding adhesion in the xylem Water molecule Root hair Trunk xylem ψ = −0.6 MPa Soil particle Soil ψ Water uptake Water = −0.3 MPa from soil
    56. 56. Figure 36.13a Water molecule Root hair Soil particle Water Water uptake from soil
    57. 57. Figure 36.13b Adhesion by Xylem hydrogen bonding cells Cell wall Cohesion by Cohesion and hydrogen bonding adhesion in the xylem
    58. 58. Figure 36.13c Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Transpiration
    59. 59. Adhesion and Cohesion in the Ascent ofXylem Sap• Water molecules are attracted to cellulose in xylem cell walls through adhesion• Adhesion of water molecules to xylem cell walls helps offset the force of gravity
    60. 60. • Water molecules are attracted to each other through cohesion• Cohesion makes it possible to pull a column of xylem sap• Thick secondary walls prevent vessel elements and tracheids from collapsing under negative pressure• Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules
    61. 61. Xylem Sap Ascent by Bulk Flow: A Review • The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism • Bulk flow is driven by a water potential difference at opposite ends of xylem tissue • Bulk flow is driven by evaporation and does not require energy from the plant; like photosynthesis it is solar powered
    62. 62. • Bulk flow differs from diffusion – It is driven by differences in pressure potential, not solute potential – It occurs in hollow dead cells, not across the membranes of living cells – It moves the entire solution, not just water or solutes – It is much faster
    63. 63. The rate of transpiration is regulated bystomata • Leaves generally have broad surface areas and high surface-to-volume ratios • These characteristics increase photosynthesis and increase water loss through stomata • Guard cells help balance water conservation with gas exchange for photosynthesis
    64. 64. Figure 36.14
    65. 65. Stomata: Major Pathways for Water Loss • About 95% of the water a plant loses escapes through stomata • Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape • Stomatal density is under genetic and environmental control
    66. 66. Mechanisms of Stomatal Opening andClosing • Changes in turgor pressure open and close stomata – When turgid, guard cells bow outward and the pore between them opens – When flaccid, guard cells become less bowed and the pore closes
    67. 67. Figure 36.15 Guard cells turgid/ Guard cells flaccid/ Stoma open Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view) H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O (b) Role of potassium in stomatal opening and closing
    68. 68. Figure 36.15a Guard cells turgid/ Guard cells flaccid/ Stoma open Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view)
    69. 69. • This results primarily from the reversible uptake and loss of potassium ions (K+) by the guard cells
    70. 70. Figure 36.15b Guard cells turgid/ Guard cells flaccid/ Stoma open Stoma closed H2O H2O H2O H 2O K+ H2O H2O H2O H2O H2O H2O (b) Role of potassium in stomatal opening and closing
    71. 71. Stimuli for Stomatal Opening and Closing • Generally, stomata open during the day and close at night to minimize water loss • Stomatal opening at dawn is triggered by – Light – CO2 depletion – An internal “clock” in guard cells • All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles
    72. 72. • Drought, high temperature, and wind can cause stomata to close during the daytime• The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata
    73. 73. Effects of Transpiration on Wilting andLeaf Temperature • Plants lose a large amount of water by transpiration • If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt • Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes
    74. 74. Adaptations That Reduce EvaporativeWater Loss • Xerophytes are plants adapted to arid climates
    75. 75. Figure 36.16 Ocotillo (leafless) Oleander leaf cross section Cuticle Upper epidermal tissue Ocotillo after heavy rain Oleander 100 µm flowers Trichomes Crypt Stoma Lower epidermal (“hairs”) tissue Ocotillo leaves Old man cactus
    76. 76. • Some desert plants complete their life cycle during the rainy season• Others have leaf modifications that reduce the rate of transpiration• Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night
    77. 77. Sugars are transported from sources to sinksvia the phloem • The products of photosynthesis are transported through phloem by the process of translocation
    78. 78. Movement from Sugar Sources to SugarSinks • In angiosperms, sieve-tube elements are the conduits for translocation • Phloem sap is an aqueous solution that is high in sucrose • It travels from a sugar source to a sugar sink • A sugar source is an organ that is a net producer of sugar, such as mature leaves • A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb
    79. 79. • A storage organ can be both a sugar sink in summer and sugar source in winter• Sugar must be loaded into sieve-tube elements before being exposed to sinks• Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways• Companion cells enhance solute movement between the apoplast and symplast
    80. 80. Figure 36.17 Key Apoplast Symplast Companion High H+ concentration Mesophyll cell Cotransporter (transfer) cell Proton H+ Cell walls (apoplast) Sieve-tube pump element S Plasma membrane Plasmodesmata ATP Sucrose H+ H+ Bundle- Phloem S Mesophyll cell sheath cell parenchyma cell Low H+ concentration (a) (b)
    81. 81. • In many plants, phloem loading requires active transport• Proton pumping and cotransport of sucrose and H + enable the cells to accumulate sucrose• At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water
    82. 82. Bulk Flow by Positive Pressure: TheMechanism of Translocation in Angiosperms • Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow
    83. 83. Figure 36.18 Sieve Source cell Vessel tube (leaf) (xylem) (phloem) 1 Loading of sugar H2O 1 Sucrose H2O 2 Bulk flow by positive pressure Bulk flow by negative pressure 2 Uptake of water 3 Unloading of sugar Sink cell (storage root) 4 Water recycled 4 3 Sucrose H2O
    84. 84. • The pressure flow hypothesis explains why phloem sap always flows from source to sink• Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms• Self-thinning is the dropping of sugar sinks such as flowers, seeds, or fruits
    85. 85. The symplast is highly dynamic • The symplast is a living tissue and is responsible for dynamic changes in plant transport processes
    86. 86. Changes in Plasmodesmata • Plasmodesmata can change in permeability in response to turgor pressure, cytoplasmic calcium levels, or cytoplasmic pH • Plant viruses can cause plasmodesmata to dilate so viral RNA can pass between cells
    87. 87. Figure 36.20 Plasmodesma Virus particles Cell wall 100 nm
    88. 88. Phloem: An Information Superhighway • Phloem is a “superhighway” for systemic transport of macromolecules and viruses • Systemic communication helps integrate functions of the whole plant
    89. 89. Electrical Signaling in the Phloem • The phloem allows for rapid electrical communication between widely separated organs – For example, rapid leaf movements in the sensitive plant (Mimosa pudica)
    90. 90. Figure 36.UN01 Turgid
    91. 91. Figure 36.UN02 Wilted
    92. 92. Figure 36.UN03 CO2 H2O O2 O2 CO2 Minerals H2O
    93. 93. Figure 36.UN05