transpiration and translocation

• Xylem:

  • Dead cells so no absorption of water

  • No end walls so continuous tube

  • Strengthened by lignin
• Phloem:

  • Living cells with companion cells to provide energy

  • Sieve plates forming tubes so sugars and amino acids flow easily
• Transpiration — loss of water by evaporation
• Transpiration:

  1. Water enters roots by osmosis.
  2. Water drawn up the stem through xylem and roots.
  3. Water drawn out of leaf cells and xylem.
  4. Water vapour evaporates through stomata.
• Stomata:

  • Guard cells take in water — swell and stoma open

  • Guard cells lose water — flaccid and stoma closes
• Translocation — transport of sucrose around plant.
• Translocation:

  1. Sucrose is produced in leaves from glucose that was formed during photosynthesis.
  2. Dissolved sucrose is carried around in the phloem.
  3. Dissolved sugars are converted to starch and are stored in storage organs.
• Factors affecting transpiration:

  • Temperature — increases rate of diffusion of water molecules evaporating from mesophyll because it increases

  • Light intensity — the stomata opens wider for increased gas exchange

  • Air movement — removes water vapour around leaves

  • Humidity — high concentration of water molecules so the diffuse slows down because the concentration gradient is lower
• Temperature:

  • Increases molecular movement

  • Increases rate of diffusion of water molecules evaporating from mesophyll
• Light intensity:

  • The stomata opens wider to increase diffusion of water & gas exchange
• Air movement:

  • Removes water vapour from leaf surfaces

  • More water diffuses from the leaves
• Humidity:

  • Decrease

  • Reduces the concentration of water molecules outside the leaf

  • Diffusion of water from the leaf increases
• Potometer:

  1. Cut a shoot underwater to prevent air entering xylem and dry the leaves.
  2. Place it in a tube with a rubber bung.
  3. Record the starting location of the air bubble.
  4. Record the end location after 30 minutes.
  5. Reset bubble by opening tap below reservoir and repeat.
  6. Divide distance the bubble travelled by time taken.
• Potometer variables:

  • Independent — time

  • Dependant — distance bubble moved along capillary tube

  • Control — temp, air flow, supply of water