Xylem

• What is transpiration?

  The process by which plants release water vapor into the atmosphere through their leaves
• What does the xylem do?

  Transport water and nutrients in plants
• What is the role of the stomata?

  Guard cells open the stomata when they are turgid

  When the stomata are open there is a greater transpiration rate
• What is the structure of the xylem?

  • Xylem vessels
  • Lignin
• Movement of water across a leaf:

  • Mesophyll cells lose water vapour by evaporation
  • Water enters by osmosis
  • Loss of water by surrounding cells lowers their water potential
  • They take in water from surrounding cells
• Movement of water up the xylem:

  • Water evaporates from mesophyll cells leading to transpiration
  • Water has high cohesive forces (due to hydrogen bonds formed with other water molecules)
  • Water also has high adhesive forces, so the molecules stick to the mesophyll xyelm walls
  • So water forms a column up the xylem called the transpiration pull
• What is the difference between transpiration and the transpiration stream?

  Transpiration is the loss of water vapour from the stomata, the transpiration stream is the movement of water through the xylem and mesophyll cells
• Water evaporates from the mesophyll cells into air spaces in the leaf, then water vapour diffuses through the stomata
• What are the factors affecting transpiration rate?

  • Air movement (more transpiration)
  • Humidity (less transpiration)
  • Light intensity (more transpiration
  • Temperature (more transpiration)
• What do potometer experiments do?

  Measure transpiration rate
• Why do we cut the shoot underwater in a potometer experiment?

  To prevent air from entering the xylem
• Process of the potometer experiment:

  • Allow one air bubble to form and place the tube back in the water
  • Record the starting location of the air bubble
  • Record the end location of the air bubble after leaving it for a set period of time
  • Change the factor being investigated
  • Repeat
  • The faster the air bubble travels, the faster transpiration is occurring