Chapter 10


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  • The hyphae of mycorrhizal fungi extend into soil, where their large surface area and efficient absorption enable them to obtain mineral nutrients, even if these are in short supply or are relatively immobile. Mycorrhizal fungi seem to be particularly important for absorption of phosphorus, a poorly mobile element, and a proportion of the phosphate that they absorb has been shown to be passed to the plant.
  • Functions of the Symplast and Apoplast in Transport How do water and solutes move from one location to another within plant tissues and organs? For example, what mechanisms transport water and minerals from the root hairs to the vascular cylinder of the root? Such short–distance transport is sometimes called lateral transport because its usual direction is along the radial axis of plant organs, rather than up and down along the length of the plant. Three routes are available for this transport. By the first route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell in the pathway by the same mechanism. This transmembrane route requires repeated crossings of plasma membranes as the solutes exit one cell and enter the next. The second route, via the symplast, the continuum of cytosol within a plant tissue, requires only one crossing of a plasma membrane. After entering one cell, solutes and water can then move from cell to cell via plasmodesmata. The third route for short–distance transport within a plant tissue or organ is along the apoplast, the pathway consisting of cell walls and extracellular spaces. Without entering a protoplast, water and solutes can move from one location to another within a root or other organ along the byways provided by the continuum of cell walls.
  • The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem.
  • The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem.
  • The transpiration–cohesion–tension mechanism that transports xylem sap against gravity is an excellent example of how physical principles apply to biological processes. In the long–distance transport of water from roots to leaves by bulk flow, the movement of fluid is driven by a water potential difference at opposite ends of a conduit. In a plant, the conduits are vessels or chains of tracheids. The water potential difference is generated at the leaf end by transpirational pull, which lowers the water potential (increases tension) at the “upstream” end of the xylem. On a smaller scale, water potential gradients drive the osmotic movement of water from cell to cell within root and leaf tissue. Differences in both solute concentration and turgor pressure contribute to this short–distance transport. In contrast, bulk flow depends only on pressure. Another contrast with osmosis, which moves only water, is that bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water. The plant expends no energy to lift xylem sap by bulk flow. Instead, the absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by lowering the water potential in the air spaces within a leaf. Thus, the ascent of xylem sap is ultimately solar powered.
  • Leaves generally have broad surface areas and high surface area–to–volume ratios. The broad surface area is a morphological adaptation that enhances the absorption of light needed to drive photosynthesis. The high surface area–to–volume ratio aids in the uptake of carbon dioxide during photosynthesis as well as in the release of oxygen produced as a by–product of photosynthesis. Upon diffusing through the stomata, CO2 enters a honeycomb of air spaces formed by the spongy parenchyma cells. Because of the irregular shape of these cells, the internal surface area of the leaf may be 10 to 30 times greater than the external surface area we see when we look at the leaf. Although broad surface areas and high surface area–to–volume ratios increase photosynthesis, they also have the serious drawback of increasing water loss by way of the stomata. Thus, a plant’s tremendous requirement for water is part of the cost of making food by photosynthesis. By opening and closing the stomata, guard cells help balance the plant’s requirement to conserve water with its requirement for photosynthesis
  • Guard cells arbitrate the photosynthesis–transpiration compromise on a moment–to–moment basis by integrating a variety of internal and external stimuli. Even the passage of a cloud or a transient shaft of sunlight through a forest canopy can affect the rate of transpiration.
  • Chapter 10

    1. 1. Chapter 10Transport in multicellular plants
    2. 2. Why do multicellular plantsneed a transport system?  like animals – need O2 and nutrients  differ from animals – nature of the nutrients needed, gases required, and RATE
    3. 3. Particular requirements of plant cells CO2 - during daylight- photosynthesis 6 CO2 + 6 H2O → C6H12O6 + 6 O2
    4. 4. Particular requirements of plant cells Oxygen (O2) – Photosynthesizing cells make enough O2 for their own needs, but at a slower rate than animals
    5. 5. Particular requirements of plant cells Organic nutrients (containing carbon) – some cells make their own but other parts must be supplied
    6. 6. Particular requirements of plant cells Inorganic ions & H2O → NH4+, NO3-, H20, get from roots
    7. 7. Transport of H20 H20 is transported in xylem tissue
    8. 8. Transport of H20, the Process H20 uptake near root tips (root hairs) H20 enters xylem H20 moves up xylem H20 from xylem to leaf cells evaporation of H20 into leaf air spaces transpiration of H20 vapor through open stoma into the atmosphere
    9. 9. Overall H20 moves from high to low water potential (ψ) from soil to root from root to leaf
    10. 10. From Soil to Root Hair remember Chapter 4, page 61?
    11. 11. Root hairs extensions of epidermis H20 moves into root hair with the water potential (ψ) gradient Soil Root cytoplasm Low solute High solute (proteins, ions, sugars) High ψ Low ψ H20
    12. 12. Root hairs large # of root hairs – large surface area - increase rate of osmosis - root hairs constantly being replaced
    13. 13. Mycorrhizas fungi which function like root hairs, form a mutualistic symbiotic relationship with the root - useful in poor soil that can’t grow root hairs - absorb nutrients, especially phosphate
    14. 14. Mycorrhizae
    15. 15. Root Hair to Xylem
    16. 16. H2O movement - H2O moves into the root hair through epidermal cells - H2O must move across cortical cells to the xylem tissue in the center of the root (Ψ gradient is responsible) H2O movement can take two pathways - which pathway depends on the type of plant and condition
    17. 17. Routes from cell to cell Moving water & solutes between cells  transmembrane route  repeated crossing of plasma membranes  slowest route but offers more control  symplast route  move from cell to cell within cytosol  apoplast route  move through connected cell wall without crossing cell membrane  fastest route but never enter cell
    18. 18. Apoplast H2O moves through the cortex by traveling along criss-cross fibers of cellulose fibers located in cortical cell walls - never enters the cells
    19. 19. Apoplast
    20. 20. Symplast H2O enters the cortical cells cytoplasm and vacuole - moves to the next cell by interconnecting plasmodesmata channels - enters the cells
    21. 21. Symplast
    22. 22. Water & mineral uptake by roots Mineral uptake by root hairs  dilute solution in soil  active transport pumps  this concentrates solutes (~100x) in root cells Water uptake by root hairs  flow from high H2O potential to low H2O potential  creates root pressure
    23. 23. Route water takes through root Water uptake by root hairs  a lot of flow can be through cell wall route  apoplasty
    24. 24. H2O reaches the stele Once the H2O reaches the stele (center), apoplast pathway stops - endodermal cells have a thick waterproof waxy band of suberin in the cell walls – Casparian strip - can only pass into these cells by symplast or cytoplasm movment
    25. 25. Controlling the route of water in root Endodermis  cell layer surrounding vascular cylinder of root  lined with impervious Casparian strip  forces fluid through selective cell membrane & into symplast  filtered & forced into xylem vessels Aaaaah… Structure-Function yet again!
    26. 26. H2O reaches the stele in a young root the suberin deposits form bands in the cell walls called Caspian strips The symplast path remains open
    27. 27. H2O reaches the stele in older root, entire cells become suberised, closing the symplast path too Only passage cells are then permeable to water
    28. 28. Suberin deposits - as cell ages, some cells become completely blocked and can only use passage cells - allows the plant to control incoming nutrients - builds root pressure - H2O crosses the pericycle into the xylem
    29. 29. H2O reaches the stele
    30. 30. Xylem dual function (support & transport) Made up of four different types of cells  parenchyma cells – standard plant cells, no chloroplast  fibers – elongated cells with dead lignifies walls  vessel elements  tracheids
    31. 31. Xylem
    32. 32. Plant tissues Dermal  “skin” of plant  single layer of tightly packed cells that covers & protects plant Vascular  transport materials between roots & shoots  xylem & phloem Ground  everything else: storage, photosynthetic  bulk of plant tissue
    33. 33. Plant cell types in tissues
    34. 34. Plant cell types in Those would’ve tissues been great names for my kids! Parenchyma  “typical” plant cells = least specialized  photosynthetic cells, storage cells  tissue of leaves, stem, fruit, storage roots Collenchyma  unevenly thickened primary walls = support Sclerenchyma  very thick, “woody” secondary walls = support  rigid cells that can’t elongate  dead at functional maturity
    35. 35. Plant cell types in tissues
    36. 36. ParenchymaParenchyma cells are relatively unspecialized, thin,flexible & carry out many metabolic functions all types of cells develop from parenchyma
    37. 37. Xylem - vessel elements make up the xylem vessels began as normal cell, but lignin in cells walls impermeable to water - as it builds up, cells die, empty lumen left - no lignin at plasmodesmata – kept open = pits
    38. 38. Xylem - tracheids - non living lignified walls - not open ended, not tubes - H2O moves through pits - primitive plants, ferns & conifers
    39. 39. Xylemdead cells →water-conductingcells of xylem
    40. 40. Xylem
    41. 41. Xylem
    42. 42. H2O movement in the xylem H2O crossed cortex, epidermis, pericycle into xylem vessels - moves up vessels to the leaves - in root - xylem in center - in stem – near the outer region
    43. 43. How does H20 get from the xylem to the leaf? - H20 evaporates from cell walls of mesophyll - H20 replaced from xylem
    44. 44. Ascent of xylem “sap”Transpiration pull generated by leaf
    45. 45. Rise of water in a tree by bulk flow Transpiration pull  adhesion & cohesion  H bonding  brings water & minerals to shoot Water potential  high in soil → low in leaves Root pressure push  due to flow of H2O from soil to root cells  upward push of xylem sap
    46. 46. From leaf to atmosphere – transpiration - inside leaf mesophyll – air spaces that are saturated with H20 vapor - H20 evaporates fro mesophyll cell walls - internal leafs spaces are in direct contact with the atmosphere through the stomata - diffuse down the gradient to the atmosphere
    47. 47. transpiration loss of H20 - low humidity – transpiration HIGH, large gradient - high humidity – transpiration LOW - high temperature/wind – transpiration rate HIGH
    48. 48. Control of transpiration Stomata function  always a compromise between photosynthesis & transpiration  leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis  a corn plant transpires 125 L of water in a growing season
    49. 49. Regulation of stomata Microfibril mechanism  guard cells attached at tips  microfibrils in cell walls  elongate causing cells to arch open = open stomata  shorten = close when water is lost Ion mechanism  uptake of K+ ions by guard cells  proton pumps  water enters by osmosis  guard cells become turgid  loss of K+ ions by guard cells  water leaves by osmosis  guard cells become flaccid
    50. 50.  Other cues  light trigger  blue-light receptor in plasma membrane of guard cells triggers ATP-powered proton pumps causing K+ uptake  stomates open  depletion of CO2  CO2 is depleted during photosynthesis (Calvin cycle)  circadian rhythm = internal “clock”  automatic 24-hour cycle
    51. 51. stomata stomata open/close – control rate of transpiration - CO2 must come in, so H20 will leaves (HIGH transpiration) - bright day, CO2 more needed for photosynthesis - but then more water loss through transpiration - plant will have to compromise
    52. 52. Hydrostatic pressure pressure exerted by liquid - H20 removed from the xylem reduces pressure - creates a high to low gradient - (e.g. suck on a straw, reduces pressure at the top, liquid rises) - H20 flows up the xylem
    53. 53. Hydrostatic pressure - H20 moves by mass flow (moves as one body of liquid) -H20 molecules attracted to each other (H bonds) – cohesion -H20 molecules attracted to lignin (H bonds) – adhesion - air bubbles stop movement (break water column) but pits allow H20 to move around
    54. 54. Root pressure transpiration reduces hydrostatic pressure at top of xylem - plants also increases root pressure to increase pressure differences - active secretion of solutes, mineral ions, into the water of the xylem vessels in the root
    55. 55. Root pressure cells surrounding the xylem use active transport to pump solutes across their membranes and into the xylem - the presence of solutes lowers the water potential drawing water from the surrounding root cells - increases pressure at the base of the xylem vessels - root pressure is not essential
    56. 56. Comparing rates of transpiration - hard to measure H2O vapor leaving the plant - since most H2O lost by a plant was taken in by the roots - a measurement of transpiration could be approximated by measuring the amount of water uptake
    57. 57. potometer apparatus used to measure uptake - must be airtight - plant stem in water with rest of plant above the airtight seal - cut the stem with a slanting cut - as water evaporates it is drawn into the xylem from the capillary tubing and can be measured - an experiment can expose the plant to varying conditions to compare the rate of transpiration
    58. 58. Xerohytes plants that live in places where water is scarce - adaptations include -dune grass - eaves can roll up exposing a tough waterproof cuticle to the outside air while the stomata remain open in the enclosed middle of the roll
    59. 59. Xerohytes hairs help trap a layer of moist air close to the leaf surface - cactus – flattened photosynthetic stem, stores water, leaves are reduced to spines which reduces surface area for transpiration - waxy coating cuts down on water loss
    60. 60. All Cacti are xerophytes
    61. 61. Transverse Section Through Leaf of Xerophytic Plant
    62. 62. Left and right Epidermis of the cactus Rhipsalisdissimilis.Left: View of the epidermis surface. The crater-shapeddepressions with a guard cell each at their base can beseen.Right: X-section through the epidermis & underlyingtissues. The guard cells are countersunk, the cuticle isthickened. These are classic xerophyte adaptations.
    63. 63. Marram grass possesses:rolled leaves, leaf hairs andsunken stomata. Theseadaptations make it resistantto dry conditions and ofcourse sand-dunes whichdrain very quickly retain verylittle water.
    64. 64. BYB3 June 2001 Question 8 part c
    65. 65. BYB3 June 2001 Question 8 part c ANSWERS
    66. 66. Translocation - term used to describe the transport of soluble organic substances (assimilates) – such as sugars  - assimilates are transported in sieve tube elements in the phloem tissue
    67. 67. Phloem tissue types of cells  sieve (tube) elements  companion cells  parenchyma  fibers
    68. 68. Sieve elements sieve tube – made up of many sieve elements joined end to end vertically to form a continuous column a living cell with a cellulose cell wall, a plasma membrane, cytoplasm containing ER and mitochondria
    69. 69. Sieve elements amount of cytoplasm is very small, only providing a thin layer inside the cell wall, no nucleus or ribosomes sieve plate – end walls made of perforated sieve plate pores allow free movement of liquids
    70. 70. Companion cells each sieve elements has at least on companion cell lying close beside it same organelles of other plant cells, including small vacuole and nucleus
    71. 71. Companion cells contain more mitochondria and ribosomes, are metabolically very active numerous plasmodesmata pass through their cell walls making direct contact between the cytoplasm of the companion cell and sieve element
    72. 72. The contents of phloem sieve tubes liquid called phloem sap or sap not easy to collect phloem sap when phloem tissue is cut, the sieve elements respond by rapidly blocking the sieve pores in a process called clotting
    73. 73. phloem sieve tubes pores become blocked first by plugs of phloem protein strands and then the carbohydrate callose callose – molecular structure similar to cellulose, long chains of glucose units linked by β 1-3 glycosidic bonds
    74. 74. Phloem: food-conducting cells sieve tube elements & companion cells
    75. 75. Aaaaah… Phloem Structure-Function again! Living cells at functional maturity  lack nucleus, ribosomes & vacuole  more room: specialized for liquid food (sucrose) transport Cells  sieve tubes  end walls, sieve plates, have pores to facilitate flow of fluid between cells  companion cells  nucleated cells connected to the sieve- tube  help sieve tubes
    76. 76. sieve plate Phloemsievetubes
    77. 77. Phloem: food-conducting cells sieve tube elements & companion cells
    78. 78. Vascular tissue in herbaceous dicot stems monocottrees & shrubs grasses & lilies
    79. 79. Root structure: monocot
    80. 80. Root structure: dicotphloem xylem
    81. 81. How are aphids used to sample sap? - aphids have a tubular mouthpart – stylets - stylet is inserted through the surface of the plant’s stem/leaf into the phloem - phoem sap flows through the stylet
    82. 82. Why doesn’t the stylet clot? - the diameter of the stylet is so small it does not allow the sap to flow out rapidly enough to switch on the plant’s phloem clotting mechanism stimulus-response
    83. 83. How translocation occurs - like water movement in the xylem, sap moves by mass flow - mass flow in xylem resulted from pressure difference created by the water potential gradient between the soil and air, this required no energy from the plant - to create the pressure differences needed for mass flow in the phloem, the plant has to use energy, an active process
    84. 84. How translocation occurs - active loading – sucrose is actively loaded into the sieve elements , usually where photosynthesis takes place - as sucrose is loaded, this decreases the water potential in the sap - through osmosis water enter the sap, moving down the water potential gradient - in the root, (or other area along the way), sucrose may be removed, again water follows by osmosis
    85. 85. How translocation occurs - this creates a pressure difference: hydrostatic pressure is high in the part of the sieve tube in the leaf and lower in the part of the root - water flows from high to low taking dissolved solutes with it source – area of the plant where sucrose is loaded sink - area of the plant where sucrose is taken out
    86. 86. Loading sucrose into phloem photosynthesis produces trioses which are converted into sucrose sucrose, in solution, moves from the mesophyll cells to the phloem - moves by the symplast pathway (cell-to-cell via the plamodesmata) or the apoplast pathway traveling along the cell walls
    87. 87. Loading sucrose into phloem - sucrose is loaded into companion cells by active transport - ATP is used to pump H+ ions outside the companion cell - H+ reenter the cell traveling down their concentration gradient traveling with sucrose (against its through carrier proteins (co-transporter proteins) in the plasma membrane
    88. 88. Unloading sucrose from phloem little is known about this process - believed sucrose moves from the phloem though diffusion down a concentration gradient - this gradient is maintained through cells converting sucrose to something else to maintain the gradient - invertase – enzyme that hydrolyzes sucrose to glucose and fructose
    89. 89. Evidence for the mechanism of phloem transport - the role of sieve pores and phloem proteins were topics of considerable arguments in the 70’s and 80’s - now known phloem proteins are not present in living active phloem tissue - evidence that sap moves by mass flow is considerable - rate of phloem transport is 10,000 X faster than diffusion - actual rates of transport measured match closely with those calculated from measured pressure differences at source and sink
    90. 90. Evidence for the mechanism of phloem transport - evidence of ‘active’ loading of sucrose into sieve elements in sources such as leaves - - phloem sap always has a relatively high pH about 8, expected if H+ ions are being transported out of the cells - - electrical potential of -150mV, consistent with an excess of H+ ions outside the cell compared to inside - - ATP is present in phloem sieve elements in large amounts, expected it is required for active transport
    91. 91. Comparison of sieve elements and xylem vessels  Similarities  - liquid moves in mass flow along a pressure gradient  - liquid moves in tubes formed from cells stacked end to end
    92. 92. Comparison of sieveelements and xylem vessels Differences - water transported through dead xylem vessels - translocation through live phloem sieve tubes requires active loading of sucrose at sources thus requiring living cells - xylem has lignified walls, phloem does not - lignified, dead xylem vessels are entirely empty providing a tube through which water can flow unimpeded
    93. 93. Comparison of sieve elements and xylem vessels  - lignified, dead xylem vessels have strong walls for plant support  - end walls of elements disappear where phloem sieve elements form sieve plates which prevent the phloem sieve elements from collapsing  - sieve plates allow the phloem to seal itself if damaged, i.e. part of a blade of grass is eaten by a herbivore
    94. 94. Comparison of sieveelements and xylem vessels - phloem sap has a high turgor pressure because of its high solute content, it would leak out quickly if the sieve elements did not seal - phloem sap contains valuable substances such as sucrose, which clotting protects from loss - clotting may also prevent the entry of microorganisms which might feed on the nutritious sap or cause disease