14.5.1

Stomata & Guard Cells

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Stomata & Guard Cells

Plants regulate gaseous exchange through their stomata. Stomata are apertures found predominantly on the underside of leaves. Each stomatal aperture is formed by two guard cells.

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Daily rhythm

  • Stomata regulate gaseous exchange and water loss through transpiration.
  • Stomata tend to be open during the day to allow gaseous exchange for photosynthesis. Plants need CO2 for photosynthesis.
  • But, having stomata open means water is lost through transpiration.
  • So, stomata tend to close at night to prevent excessive water loss.
    • There is also no sunlight at night, so the plant has no requirement for carbon dioxide for respiration.
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Guard cells

  • Two guard cells form a stomatal aperture.
  • Guard cells have a thick cell wall in their middle portion. So, when guard cells are full of water (turgid) they bend away from the thick middle portion.
    • Each turgid guard cell forms a "C" shape, so the stomatal aperture is open.
  • Following water loss, guard cells become flaccid. Flaccid guard cells are linear and lie side-by-side.
  • This closes the guard cell, preventing further water loss.
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Regulating water

  • Potassium (K+) ions are used to regulate guard cell opening.
  • Turgidity is caused by an increase in K+ ion concentration within guard cells.
  • A higher K+ ion concentration lowers the water potential of the guard cell. Water then follows by osmosis.
  • This makes guard cells turgid and opens the stomatal aperture.
  • Stomata close following excessive water loss. This is usually due to a lower rate of photosynthesis following loss of sunlight.
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Water stress

  • Water stress is communicated in plants by the hormone abscisic acid (ABA).
  • The roots of plants produce ABA in response to stress or low water potentials.
  • ABA travels in water to guard cells. Once in guard cells, ABA triggers the release of Ca2+ ions.
  • Ca2+ ions act as a second messenger. It binds to and activates proteins.
    • Ca2+ ions initiate a signalling cascade that results in the stomatal aperture closing.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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