Transpiration

• Excessive water loss through transpiration is a problem for plants, and leaves will minimise this loss of water
• The stomata is essential to allow carbon dioxide to diffuse into the leaf for photosynthesis
• Transpiration stream is the movement of water from the roots to the leaves through the xylem
• Transpiration stream - Water forms a continuous stream in the xylem vessels; from the roots, up the stem and into the leaf
• Transpiration is the loss of water vapour from plants, through the stomata of the leaves
• Evaporation: Water evaporates from the cell walls of the spongy mesophyll cells. This produces a higher concentration of water vapour in the air spaces inside the leaf.
• Diffusion: Water vapour diffuses from a higher concentration in the air spaces within the leaf, to a lower water concentration in the air outside the leaf.
• Factors affecting the rate of transpiration:

  1. Light intensity
  2. Temperature
  3. Air movement
• Light intensity:

  • As light intensity increases, the rate of transpiration increases
  • During times of high light intensity the stomata are open to allow carbon dioxide into the plant for photosynthesis. This allows transpiration to take place. When it is dark the plant cant photosynthesise to the stomata close, preventing transpiration.
• Temperature:

  • As temperature increases, the rate of diffusion increases
  • As the temperature increases the water molecules have more kinetic energy. This increases both rate of diffusion and the rate of evaporation
• Air movement (wind):

  • As air movement increases, the rate of transpiration increases
  • When there's little air movement, as water vapour diffuses out of the stomata it remains close to the stomata's lower surface. This increases the water vapour concentration in the air outside the leaf, decreasing the rate of diffusion
  • When it's windy, as water vapour diffuses out of the stomata, air movement removes it from the stomata's lower surface. This keeps the concentration of water vapour outside the leaf low, increasing the rate of diffusion.
• Translocation: The process by which sugars produced during photosynthesis are transported around the plant.
• Humidity:

  • As air humidity decreases, the rate of transpiration increases
  • The lower the air humidity is, the bigger the difference is between the concentration of water vapour in the air spaces within the leaf and the concentration of water vapour in the air outside the leaf. The greater the concentration gradient is, the faster the rate of diffusion is
  • As air humidity increases, the difference in water vapour concentration decreases, so the rate of diffusion decreases.
• A potometer is used to investigate the rate of transpiration
• When setting up a potometer you must:

  1. Cut the shoot under water to prevent air entering the xylem
  2. Make sure leaves are dry
  3. Check apparatus is air tight
• A potometer actually measures the rate of water uptake, which we assume to equal the rate of transpiration

  This assumption is incorrect because:

  1. Some water is used in photosynthesis
  2. Some water is used to make cells turgid
• Turgid ( cells full of water to prevent bursting )
• Turgid ( cells full of water to prevent bursting )