A plant's guard cells regulate the opening and closing of the epidermal stomata by expanding or contracting in response to environmental signals. When a pair of guard cells surrounding a stoma receives the signal that the stomatal pore needs to open, the guard cell pair fill with water, changing the cell's shape and opening the pore. An inverse process occurs when the guard cells receive a signal to close the stoma, initiating a loss of water and causing them to shrink and close the pore.
The change in turgor, or hydrostatic pressure, within a guard cell pair is the result of the osmotic water flow across the cell walls. The water potential inside the cell pair changes as a result of the related movements of ions and sugar solutes, and when that potential decreases, it lets the cells absorb water, expand and open the stoma.
Although sugar solutes within the guard cells play a role in the expansion and contraction processes, the primary mediators are chlorine and potassium ions. The accumulation of potassium ions within a guard cell, triggered by an environmental signal such as sunlight, causes the osmotic pressure to decrease and attracts water into the cell. The triggered increase of chlorine ions and an additional anion called malate within the cell contribute to the opposite effect, causing water to exit and the guard cell pair to contract and close the stomatal pore.