Please describe the closing mechanism of stomata?

Answer 1

A basic answer is that the guard cells have chloroplasts and can carry on photosynthesis while the surrounding epidermal cells have no chloroplasts.

In light:
-- Guard cells photosynthesize and make glucose.
-- More glucose in guard cells causes more water to diffuse into the cells than out of the cells.
-- Guard cells swell up as water pressure builds inside them.
-- Guard cells have a thicker cell wall on the inner side (next to the stoma) and a thinner cell wall on the outer side (away from the stoma), so a guard cell swells unevenly and becomes kind of curved or bent. The pair of guard cells bend in such a way that it leaves a gap between them which is the stoma.

In dark:
-- No photosynthesis. Guard cells use their glucose during cellular respiration.
-- More water diffuses out of the guard cells than into them.
-- The guard cells relax into a rather straight shape, so the gap between them (the stoma) closes.