During transpiration the movement of potassium ions in and out of the guard cells causes the opening and closing of stomata. During day time when light falls on the leaves, the starch present in them absorbs the light. Due to this formation of malice acid takes place in the guard cells. The malice acid then dissociates to H+ ions and malate. The H+ ions move out of the guard cells and as a result K+ ions move inside the gaurd cells. Due to this the water potential in the guard cells decreases and water moves inside the guard cells causing them to swell up and become turgid which in turn causes the opening of stomata pores. At night K+ ions move out of them and water potential increases in the guard cells which causes water to move out of the cell and making them shrunk and closing the stomata pores.