Stomata (Cambridge (CIE) A Level Biology): Revision Note

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

• Carbon dioxide enters leaves and oxygen diffuses out of leaves during gas exchange for photosynthesis

Regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration, e.g.:

• When stomata are open, leaves:

  • gain carbon dioxide for photosynthesis, but

  • lose large amounts of water by transpiration

• When stomata are closed, leaves:

  • retain water, but

  • have a reduced supply of carbon dioxide so the rate of photosynthesis decreases

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