Stomata & Guard Cells
Biology: Cambridge International A Level, topic 14.5.1
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.
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.
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.
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.
Test yourself on Stomata & Guard Cells
An increased concentration of which ion causes guard cells to close?
An increased concentration of which ion causes guard cells to become turgid?