difference between opening and closing stomatal …

Opening of stomata: Solutes from neighbouring epidermal and mesophyll cells enter the guard cells lowering its osmotic potential and water potential. This lowered water potential and osmotic potential will allow movement of water into guard cells from neighbouring cells. Guard cells become turgid due to water accumulation in them which results in the opening of the guard cells. 
Closing of stomata: As the somata open the solute concentration is reduced. This makes the water from the guard cells to move away into neighbouring cells. Now, guard cells becom flaccid with no water. They collapse against each other and result in the closing of stomata. 

Each stoma is surrounded by two guard cells. The guards are of kidney shaped and contain chloroplasts. There is a respiratory chamber present in each stoma. The mechanism of the closing and opening of the stomata depends upon the presence of sugar and starch present in the guard cells. In the presence of light, the guard cells of the stomata contain sugar which is synthesized by their chloroplasts. The sugar is soluble and increases the concentration of guard cells. Due to higher concentration of the cytoplasm of guard cells, the water enters into these cells from the neighbouring cells by osmosis. Hence the stomata remains open.

in the absence of light the sugar present in guard cells converts into the starch. The starch is insoluble, and this way the guard cells remains in lower concentration than the of neighbouring cells, and the neighbouring cells take out the water from the guard cells by osmosis making them flaccid and the stomata closed.