Question

Explain the mechanism of opening and closing of stomata in leaves.

Answer 1

Opening and closing of stomata depend upon the activity of guard cells. guard cells are crescent like or kidney shaped. Their concave wall is thicker than convex which face each other. The opening and closing of stomata depend upon the turgidity of the stomata when guard cells become turgid stomata open while guard cells become flaccid stomata closed. Guard cells contain chloroplast hence they synthesize food by photosynthesis, using $C{O}_2$ during day time. Thus, due to lack of $C{O}_2$ the pH value of the cell increases (7.5 pH). At this pH starch produced during photosynthesis reacts with inorganic phosphate in presence of phosphorylase enzyme to form glucose-I-phosphate, which is not soluble in water thus concentration of the cell sap of the guard cell increases. This lead to absorption of water by the guard cells from neighboring cells by osmosis, thus increasing turgor pressure due to which the outer thinner wall of the guard cells stretches out and stomata open for diffusion of water vapor.
Starch + Phosphate $\frac{\underset{LightpH7.5}{Phosphorylase}}{DarkpH5}$ Glucose- I-phosphate
In dark, the reverse reaction occurs in the guard calls. In dark, photosynthesis does not occur and respiration release $C{O}_2$ thus pH value of the guard cell decreases (pH 5). At this $P^H$ glucose-1- phosphate converts into insoluble starch. This causes rapid fall of the osmotic concentration of cell sap of guard cell due to which water from guard cell moves towards the neighbouring cells by osmosis process due to which guard cells become flaccid, thus stomatal opening closes.