How stomata open and close
Stomata are the tiny pores in the epidermis of a leaf through which gases diffuse into or out of the plant. Each stomata is flanked by two “guard cells” which control the opening and closing of the stomata. During the day, light reacts with the guard cells (somehow), causing potassium ions (K+) to flow into the cells. Water then moves by osmosis into the guard cells because the water potential in these cells has been lowered by the increase of K+ ions.
Brief little digression on osmosis…
Osmosis is the movement or diffusion of water molecules from an area of higher concentration of water (i.e. outside the guard cells, I’m guessing in the cell wall) to an area of lower concentration (i.e. inside the guard cells) in an attempt to reach equilibrium (i.e. the same arrangement of molecules outside the cell as inside). The principle behind osmosis, is water potential and I’ll defer to wikipedia for now until I get a chance to get my head around it…
And now, back to stomatal opening and closing…
The influx of water makes the guard cells ‘turgid’, changing their shape such that the pore between them is exposed. Late in the day or at night, the process reverses. K+ ions flow out of the guard cells, increasing the water potential so that water moves out of these cells again, and they lose their turgidity, closing the stoma.
Turgidity also causes other types of movement in plants.