All stoma are capable of opening and closing in response to the internal pressure of 2 sausage-shaped guard cells that surround it. These movements are affected by the light intensity and the rate of evaporation of water.

In the day, photosynthesis in plants occurs, sugars are formed. This lowers the water potential in the guard cells by osmosis as they swell and became turgid.

The inner wall of the guard cell is thicker than the outer wall. The swollen guard cells become more curved and pull the stoma open.

At night, the sugar is being used up and water leaves the guard cells, so they became flaccid and stomata pore closes. They reduce the amount of water vapour escaping from the leaf.

Guard cells work to control excessive water loss, closing on hot day, or windy days and opening when the conditions are more favorable for the gas exchange...