8.3 transpiration

• transpiration is evaporation of water molecules from the surface of mesophyll cells to air spaces (1)
• transpiration is diffusion of water vapour out of stomata - when there is a concentration gradient (2)
• water vapour loss is related to:

  • large internal surface area provided by air spaces between mesophyll cells
  • size & number of stomata
• cohesion - forces of attraction between water molecules that hold them together
• transpiration pull draws up a column of water molecules, up the xylem vessels
• as water evaporates at leaf & diffuses out of stomata, more water is drawn up the plant from roots
• factors affecting transpiration rate:

  temperature, wind speed, humidity
• temperature increase = transpiration rate increase


  due to kinetic energy of water molecules increase, so they evaporate & diffuse faster from the mesophyll cells
• wind speed increase = transpiration rate increase


  because the wind removes the water vapour surrounding the leaf faster
• humidity increase = transpiration rate decrease


  if the air surrounding the leaf has more water vapour, there will be a weak concentration gradient for diffusion
• if the amount of water that a plant loses is greater than the amount of water that is available in the soil (to travel into the plant through its roots), then wilting will occur
• wilting - plant will become soft and droop, because the cell walls become flaccid and is unable to support the plant
• plant wilted because more water has evaporated from the leaves, and there's not enough water in the soil