CIE A Level Biology (9700) 2019-2021

Revision Notes

14.2.3 Abscisic Acid & Stomatal Closure

Abscisic Acid & Stomatal Closure

  • During times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata
    • Certain environmental conditions can cause water stress, such as very high temperatures or reduced water supplies
  • Guard cells have ABA receptors on their cell surface membranes
  • ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells
  • ABA also causes calcium (Ca2+) ions to move into the cytoplasm of the guard cells through the cell surface membranes
  • The calcium ions act as second messengers:
    • They cause channel proteins to open that allow negatively charged ions to leave the guard cells
    • This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells
    • The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells
  • This loss of ions increases the water potential of the guard cells
  • Water leaves the guard cells by osmosis
  • The guard cells become flaccid, causing the stomata to close


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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