Transpiration and Translocation

• Xylem


  Vessels that carry water and mineral ions from the roots to the stem and leaves
• Phloem


  Vessels that transport food substances (mainly dissolved sugars) made in the leaves to the rest of the plant for immediate use or storage
• Xylem and phloem vessels go to every part of the plant, but they are totally separate
• Translocation


  The transport of food substances (mainly dissolved sugars) in the phloem
• Phloem tubes


  • Made of columns of elongated living cells with small pores in the end walls to allow cell sap to flow through
  • They transport food substances (mainly dissolved sugars) made in the leaves to the rest of the plant for immediate use or storage
  • The transport goes in both directions
• Xylem tubes


  • Made of dead cells joined end to end with no end walls between them and a hole down the middle
  • Strengthened with a material called lignin
  • They carry water and mineral ions from the roots to the stem and leaves
• Transpiration stream


  The movement of water from the roots, through the xylem and out of the leaves
• Transpiration is the loss of water from the plant
• Transpiration


  1. Caused by the evaporation and diffusion of water from a plant's surface, mostly the leaves

  2. Creates a slight shortage of water in the leaf, drawing more water up from the rest of the plant through the xylem vessels to replace it

  3. Leads to a constant transpiration stream of water through the plant
• Transpiration is a side-effect of the way leaves are adapted for photosynthesis, as they need stomata to allow gas exchange
• A big tree can lose about a thousand litres of water from its leaves every single day
• Factors affecting transpiration rate

  • Light intensity
  • Temperature
  • Air flow
  • Humidity
• Using a potometer to estimate transpiration rate

  1. Measure the uptake of water by the plant, as this is directly related to water loss by the leaves (transpiration)

  2. Record the distance moved by an air bubble in the apparatus over a set time period

  3. Keep conditions constant throughout the experiment
• Guard cells


  • Have a kidney shape which opens and closes the stomata
  • When the plant has lots of water, the guard cells fill with it and go plump and turgid, opening the stomata
  • When the plant is short of water, the guard cells lose water and become flaccid, closing the stomata
  • Thin outer walls and thickened inner walls make the opening and closing work
  • Sensitive to light and close at night to save water
  • More stomata on the undersides of leaves than the top
• The opening and closing of stomata allows plants to survive