Stomata

• Guard cells have a kidney shape which opens and closes the stomata in a leaf
• When the plant has lots of water the guard cells fill with it and go plump and turgid, which makes the stomata open so gases can be exchanged for photosynthesis
• When the plant is short of water the guard cells lose water and become flaccid, making the stomata close to help stop too much water vapour escaping
• Characteristics of guard cells:

  • thin outer walls and thickened inner walls make the opening and closing work
  • sensitive to light and close at night to save water without losing out on photosynthesis
• There are usually more stomata on the undersides of leaves than on top because the lower surface is shaded and cooler, so less water is lost through the stomata than if they were on the upper surface
• Guard cells are adapted for gas exchange and controlling water loss within a leaf