Stomata

• Control of stomatal opening:

  Opening

  • the chloroplasts in the guard cells photosynthesise producing ATP
  • this ATP provides the energy for active transport of K+ into the guard cells from the surrounding epidermal cells
  • stored starch is converted to malate
  • the K+ and malate are soluble and therefore lower the water potential in the guard cells
  • water enters via osmosis
  • the guard cells expand and the thinner cell walls on the outside stretch, and the thicker cell wall on the inside stretches less as it is inelastic
  • the pair of cells curve away from each other creating a pore (stomata)
• Control of Stomatal opening:

  Closure

  • the K+ in the guard cell diffuse out passively into epidermal cells down the concentration gradient
  • water moves out of the guard cell by osmosis
  • the guard cells are now plasmolysed
  • the stoma closes
• Stomata role - regulate water loss and allow gas exchange