What is the role of stomata in photosynthesis and how do they open and close?

Stomata are tiny pores, mostly on the lower surface of leaves, surrounded by two bean-shaped guard cells. Their main role is gas exchange: they let carbon dioxide enter the leaf for photosynthesis and allow oxygen and water vapour to leave. The opening and closing is controlled by the guard cells. When water flows into the guard cells they become turgid and swell. Because their inner wall (facing the pore) is thicker and less elastic than the outer wall, they bend outward, opening the pore. When water leaves, the guard cells become flaccid and the pore closes. This usually happens at night to save water. So stomata are essential because without them carbon dioxide could not reach the photosynthesising cells inside the leaf.