The mechanism of opening and closing

• what happens to the stomata during the day?

  • if water enters the guard cells, they become turgid and swell, and the pore opens
  • if water leaves the guard cells, they become flaccid and the pore closes
• why do the changes to the stomata during the day happen?

  • in light, chloroplasts in guard cells photosynthesise, producing ATP
  • this ATP provides energy for active transport of K$^+$into guard cells from surrounding epidermal cells
  • stored starch is converted to malate ions
  • cell walls of guard cells are thinner in some places than others-guard cells expand as they absorb water but less in the areas where cell wall is thick-these areas are opposite each other on the 2 guard cells and, as guard cell stretches, a pore appears between these areas with less stretching-this is the stoma
• what happens to the stomata at night?

  the reverse process of what happens in the day occurs and the pore closes
• what does K$^+$mean?


  potassium ion
• what do malate and potassium ions do?

  they lower the water potential of guard cells
• how do plants lose water?

  by evaporation through their stomata in a process called transpiration
• what happens if a plant loses too much water?

  it wilts
• what happens to leaves held horizontally in terms of transpiration?

  on leaves held horizontally, sunlight on the upper surface would increase evaporation, so confining stomata to the lower surface minimises the water loss
• what does the waxy cuticle do?

  it reduces water loss
• what is transpiration?

  the evaporation of water from the leaves (mainly through the stomata) to the atmosphere
• what is the transpiration stream?

  the upward movement of water through the plant
• what is this an image of?

  an open stoma

  
• what is this a diagram of?

  a closed stoma

  
• what is this a diagram of?

  an open stoma

  
• where does gas exchange and water loss happen in a plant?

  it both happens through the stomata
• when do stomata close?

  • at night - to prevent water loss when theres insufficient light for photosynthesis
  • in very bright light, as this generally is accompanied by intense heat, which could increase evaporation
  • if theres excessive water loss
• why do stomata need to close?

  to control water loss