When water is present, the membrane potential created by the proton pump allow K⁺ (potassium ions) to enter guard cells.

Water follows the ions in, turgor increases and causes the cells to lengthen and bend, opening the stoma for gas exchange.

When water leaves the cells, they shorten and become flaccid, closing the stoma.

Stomata can be seen on the underside of some conifer leaves such as spruce.