CIE A Level Biology (9700) 2019-2021

Revision Notes

14.2.1 Stomata


  • Plants carry out homeostasis – just like animals they need to maintain a constant internal environment
    • For example, mesophyll cells in leaves require a constant supply of carbon dioxide for photosynthesis
  • Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves
    • This means stomata control the entry of carbon dioxide into leaves

Response of guard cells & stomata table

Stomata table 1, downloadable AS & A Level Biology revision notes

  • Regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration

Advantages & disadvantages of stomatal opening & closure table

Stomata table 2, downloadable AS & A Level Biology revision notes

  • Stomata open and close in a daily rhythm
    • Even when the plant is kept in constant light or constant darkness, the daily rhythm of opening and closing of the stomata continues
  • Opening of stomata during the day:
    • maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen
    • allows the outward diffusion of water vapour in transpiration
  • Closing of stomata at night when photosynthesis cannot occur:
    • reduces the rate of transpiration
    • conserves water

Exam Tip

A stoma is actually the aperture (hole) between two guard cells, but the term is often used to refer to the whole unit (the two guard cells and the hole between them).

Don’t forget – stoma (singular) refers to one of these units, whereas stomata (plural) refers to many!


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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