Opening and Closing of Stomata

According to the active $K^+$ transport theory (Levitt), in light $H^+$ ions are pumped out of the guard cells, and $K^+$ ions enter the guard cells from the surrounding subsidiary cells (influx of $K^+$).
$Cl^-$ ions and malate also accumulate, so the solute concentration of the guard cells rises and their water potential falls.
Water enters the guard cells by osmosis, they become turgid, and because of their thick inner walls and radially arranged microfibrils they bend apart, opening the stoma.
Efflux of $K^+$ causes closure (e.g. under the influence of ABA).
So stomata open due to influx of potassium ions.

Each stoma is surrounded by two guard cells.
When water enters the guard cells (after $K^+$ ions move in), they become turgid. Because their inner walls are thick and inelastic and the cellulose microfibrils are arranged radially, they bend outward and the stoma opens.
When the guard cells lose water and become flaccid, the stoma closes.
So the opening and closing of stomata is due to the change in turgor pressure of the guard cells.

According to Zelitch's glycolate theory, glycolic acid is formed in the guard cells during photosynthesis when the $CO_2$ concentration is low (in light).
Glycolate is oxidised, producing ATP, which is used to pump $K^+$ ions into the guard cells. Water then enters, the guard cells become turgid, and the stoma opens.
So glycolate induces opening of stomata at low $CO_2$ concentration.