Chapter 11: Transport in Vascular Plant

• Physical forces


  • Drive the transport of materials in plants over a range of distances
• Scales of transport in vascular plants

  • Transport of water and solutes by individual cells, such as root hairs
  • 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

  
• Three levels of transport across plants


  • Uptake of water and solutes by individual cells
  • Short-distance transport of solutes and water from cell to cell at the level of tissues and organs
  • Long-distance transport of sap by xylem and phloem at whole plant level (bulk flow)
• Passive transport


  Molecules tend to move down their concentration gradient<|>Transport proteins embedded in the membrane can speed movement across the membrane
• Active transport


  Solutes are pumped across membranes against their electrochemical gradients<|>The cell must expend metabolic energy, usually in the form of ATP, to transport solutes "uphill" - counter to the direction in which the solute diffuses<|>Active transporters are a special class of membrane proteins, each responsible for pumping specific solutes

  
• ATP-dependent proton pumps


  Used to pump H+ out of the cell<|>Contribute to membrane potential and establishment of pH gradient across membrane

  
• Cotransport


  A transport protein couples the passage of one solute to the passage of another
• Roots


  • Absorb water and minerals from the soil
  • Enter the plant through the epidermis of roots and ultimately flow to the shoot system
• Short-distance transport routes


  • Across plasma membrane through cell walls
  • Apoplastic route → move along the cell walls and extracellular spaces (not entering the cell)
  • Symplastic route → move along cytosol and from cell to cell via plasmodesmata

  
• Apoplast


  Everything external to the plasma membranes of living cells (include cell walls, extracellular spaces, interior of dead cells such as vessel elements and tracheids)
• Symplast


  Consist of entire mass of cytosol of all living cells in a plant (including plasmodesmata)
• Endodermis


  • Is the innermost layer of cells in the root cortex
  • Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue
• Casparian strip


  • Forces water & minerals that are passively moving through the apoplast to cross the plasma membrane of the endodermal cell and enter the stele via symplast
• Lateral transport routes in roots

  • Apoplastic route
  • Symplastic route
  • Transmembrane route
• Transpiration drives the transport of water and minerals from roots to shoots via the xylem
• Xylem sap


  Dilute solution of water & dissolved mineral
• Transpiration-cohesion-tension mechanism


  1. Water is pulled upward by negative pressure in the xylem

  2. Transpiration provides the pull and that the cohesion of water due to hydrogen bonding transmits the pull along the entire length of the xylem from shoots to the roots
• Air outside the leaf is drier

  Water moves from areas of higher (more positive) potential to lower (more negative)
• Transpiration produces negative pressure (tension) in the leaf


  Exerts a pulling force on water in the xylem, pulling water into the leaf
• Cohesion and adhesion


  • Facilitate the transpirational pull on xylem sap
  • Cohesion of water due to hydrogen bonding makes it possible to pull a column of xylem sap from above
  • Strong adhesion of water molecules (hydrogen bonds) to hydrophilic walls of xylem cells helps offset the downward force of gravity

  
• The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism
• Transpiration


  1. Water vapour diffuses from the moist air spaces of the leaf to the drier air outside via stomata

  2. Water from the xylem is pulled into the surrounding cells and air spaces to replace the water that was lost
• Cohesion and adhesion in the ascent of xylem sap

  The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution<|>Is facilitated by cohesion and adhesion<|>The cohesion of water due to hydrogen bonding makes it possible to pull a column of xylem sap from above<|>The strong adhesion of water molecules (hydrogen bonds) to hydrophilic walls of xylem cells helps offset the downward force of gravity

  
• Ascent of xylem sap


  The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism
• Pushing xylem sap: root pressure

  1. At night, when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential

  2. Water flows in from the root cortex, generating root pressure, a push of xylem sap that sometimes causes more water to enter the leaves than is transpired, resulting in guttation

  
• Guttation


  The exudation of water droplets on tips of grass blades or the leaf margins
• Stomata


  • Leaves generally have broad surface areas and high surface-to-volume ratios, which increase photosynthesis and water loss through stomata
  • About 90% of the water a plant loses escapes through stomata
  • Each stoma is flanked by guard cells which control the diameter of the stoma by changing shape

  
• Changes in turgor pressure that open and close stomata

  Result primarily from the reversible uptake and loss of potassium ions by the guard cells
• Xerophyte adaptations that reduce transpiration

  • Xerophytes are plants adapted to arid climates and have various leaf modifications that reduce the rate of transpiration
  • The stomata of xerophytes are concentrated on the lower leaf surface and are often located in depressions that shelter the pores from the dry wind
• Translocation


  1. Is the transport of organic nutrients in the plant

  2. Phloem sap is an aqueous solution that is mostly sucrose and travels from a sugar source to a sugar sink

  
• Sugar source


  Is a plant organ that is a net producer of sugar, such as mature leaves
• Sugar sink


  Is an organ that is a net consumer or storer of sugar, such as a tuber or bulb
• Movement of sugar from sources to sinks

  1. Sugar must be loaded into sieve-tube members before being exposed to sinks

  2. In many plant species, sugar moves by symplastic and apoplastic pathways

  3. Phloem loading requires active transport, with proton pumping and co-transport of sucrose and H+ enabling the cells to accumulate sucrose

  
• Researchers have concluded that in angiosperms, sap moves through a sieve tube by bulk flow driven by positive pressure
• The pressure flow model explains why phloem sap always flows from source to sink