Guard cells open and close stoma in different conditions. When light intensity is high, potassium ions move into guard cells. Describe how this movement of potassium ions causes the stoma to open.

When the potassium ions move into the guard cells they increase the concentration. This causes water to move into the guard cells from more dilute areas by osmosis. The additional water leads to the guard cells swelling unevenly because the thicker, inner walls are less flexible than the thinner, outer walls. The stoma opens due to this uneven swelling.