Stomata (Cambridge International A-Level Biology)

Spec point — 14.2.1 · explain that stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration

Stomata

• Plants carry out homeostasis

  • Just like animals, they need to maintain a constant internal environment

• 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

Exam Hint 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

Opening & Closing of Stomata

Spec point — 14.2.2 · explain that stomata have daily rhythms of opening and closing

• 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