1. Gas Exchange in Plants Structure and function of stomata and guard cells

2. Structure and function of stomata  Generally stomata open during day and close during the night. The guard cells on each side of the stoma pore control the size of the poor by changing their shape.

3. Structure and function of stomata  Make your own clear and labelled diagram of the stomata and guard cells from p28 of Systems & Their Maintenance  The process of drawing these structures will help you learn them!

4. Structure and function of stomata  If water enters the guard cells, they swell up and the stoma pore opens.  If water leaves the guard cells, they become flaccid and the stoma pore closes.

5. Structure and function of stomata  These changes are thought to be due to a reversible uptake and loss of potassium ions by the guard cells.

6. Structure and function of stomata  During the day the chloroplasts inside the guard cells photosynthesise.  As a result they produce ATP, which fuels an active transport mechanism that takes up potassium ions from the surrounding epidermal cells.

7. Structure and function of stomata  This lowers the water potential in the guard cells and water enters by osmosis, the guard cells become turgid and the stoma pore opens.

8. Structure and function of stomata  At night the chloroplasts in the guard cells do not photosynthesise.  Less ATP is available for the active uptake of potassium ions, which then leave the guard cells by diffusion.

9. Structure and function of stomata  The loss of the potassium ions raises the water potential of the guard cell, water passes out into the epidermal cells, which have a more negative water potential.  The guard cells become flaccid and the stoma pore closes.