How Stomata Open & Close

Question 1

How do Stomata Open?

Answer 1

Stomata Opening

1. A trigger such as light will open the stomata
2. Potassium (K+) is actively pumped (active transport) from the neighbouring subsidiary cells into the guard cell vacuoles
3. As there is then more K+ (solute) in the guard cells, water will move from the subsidiary cells by osmosis [from slightly negative water potential to more negative water potential into the guard cells
4. Guard cells become turgid and because of their wall thickenings and shape – the stoma opens!

Question 2

How does Stomata Close?

Answer 2

Stomata Closing

1. A trigger such as ABA (Abscisic acid) from the roots indicating plant is water stressed may close stomata.
2. Potassium (K+) is actively pumped (active transport) from the guard cells into the subsidiary cell vacuoles.
3. As there is now more K+ (solute) in the subsidiary cells water will move from the guard cells by osmosis [from slightly negative water potential to more negative water potential] into the subsidiary cells.
4. Guard cells lose water and become flaccid and because of their wall thickenings and shape – the stoma closes!