Closing and opening of the stomata depends on the strength of the alert cells.
Each guard cell has a thick wall on one side facing the stoma and a small wall on the other side. The guard cells contain chloroplasts.
As a result of the fusion of glucose during photosynthesis and other chemical changes, the osmotic pressure of the cellular content increases and absorbs more water from neighboring cells, thus becoming turgid.
As a result of turgor, the guarded cells are directed more outward and the open space between them opens up, thus opening the stoma.
At night, or when there is a shortage of water in the leaf, the alert cells turn flaccid and their strong inner walls are straight, thus closing the abdominal cavity.