The Guard cells control the stomatal openings in the epidermis of the leaf.

Three environmental factors regulate these cells.  These factors are light, CO₂ concentration and water availability.

When the guard cells are activated, K⁺ pumps actively transport K⁺ into the guard cells, resulting in a high [K⁺] in the cells.

As a result, water enters the cells by osmosis.  This causes the guard cells to swell.  The one side of the guard cells is thicker than the other and does not stretch.  As the guard cells swell up they bend.

When the stoma is open CO₂ can diffuse into the leaf and enter the Calvin Cycle.  The oxygen produced in photolysis, diffuses out ot the open stoma.

Water vapour also escapes from the stoma by the process of  Transpiration.  As water transpires, other water molecules are pulled up through the plant behind it.