Mass Transport in Plants

• Xylem:

  • long cells/tubes with no end walls
  • continuous water columns
  • no cytoplasm/no organelles to obstruct water flow
  • lignin to withstand tension
• The main force that pulls water up the stem of a plant is the evaporation of water from leaves - a process called transpiration
• Cohesion - Tension Theory:

  • water evaporates/transpires from leaves
  • reduces water potential in cells
  • water potential gradient across cells
  • water is drawn out of the xylem due to the negative pressure i.e. there is tension
  • many hydrogen bonds mean that the water is cohesive
  • it is drawn up the xylem in a continuous column
• Evidence for C-T Theory:

  Tree trunks change diameter according to the rate of transpiration. During the day when transpiration is at its greatest, there is more tension and adhesion ( more negative pressure) in the xylem and the trunk decreases in diameter.
• Temperature - as temperature increases so does the rate of transpiration. This is because the water molecules have more kinetic energy and so evaporate out of the leaf at a faster rate.
• Wind (air movement) - an increases in wind will increase the rate of transpiration as the moist air surrounding the leaves - and reducing the water potential gradient - will be removed. The steeper gradient will therefore increase the transpiration rate
• Humidity - an increase in humidity increases the rate of transpiration as the water potential gradient is less steep so water will diffuse less quickly.
• Light Intensity - as this increases, the rate of transpiration increases. This occurs because of the increase in photosynthesis. There is more gas exchange so more stomata are open. There is more surface area for transpiration to occur.
• Potometer Precautions:

  • seal joints so that air bubble doesn't escape
  • cut shoot under water so no air bubbles formed
  • cut shoot at a slant increases area
  • dry off leaves to increase transpiration rate
  • insert apparatus underwater to prevent air entering
  • ensure no air bubbles are present only need one to follow easily
  • shut tap to allow air bubble to move to start
  • note where the air bubble starts for an accurate measurement
  • restart bubble at start for an accurate measurement
• Phloem:

  • living cells run close to xylem vessels
  • sieve pores, for ease of flow
  • companion cells have many mitochondria
  • sieve element has few organelles for ease of flow.
• Translocation Theory:

  1. Sucrose is pumped out of the source, by the companion cells, and into the sieve tube elements
  2. This lowers the water potential in the st element
  3. Water moves from the xylem into the phloem by osmosis, down its osmotic gradient causing a high hydrostatic pressure in phloem
  4. Water moves down its hydrostatic pressure gradient, taking the sucrose with it from the source area to the sink area.
  5. Sucrose is actively transported out of the phloem into the sink by AT. water follows by osmosis.