Guard cells (stoma): Osmosis

Credit: McGraw-Hill Companies, Inc Osmosis in guard cells: Potassium ions are being actively pumped from the surrounding epidermal cells to the guard cells. Water moves into the guard cells by osmosis. This increases the turgor pressure in the guard cells, causing them to become turgid. The inner cell walls (nearer to the stoma) of the guard cells are thicker. The outer cell walls (further away) of the guard cells are thinner. The thinner outer walls stretch more than the inner walls, causing the stoma to open. When potassium ions diffuse out of the guard cells, water moves out of the guard cells by osmosis. The guard cells become flaccid. The stoma closes. Stomatal closure is a natural response to darkness or drought as a means of conserving water.