Question

Why does the sugar present in guard cells turn into starch at night??????

Answer 1

The mechanism of the closing and opening of the stomata depends upon the presence of sugar and starch in the guard cells. During day time or in the presence of light, the guard cells of the stomata contain sugar synthesized by their chloroplasts.

The sugar is soluble and increases the concentration of the sap of guard cells. Due to higher concentration of the cytoplasm of guard cells, the water comes to them from the neighbouring cells by osmosis and they become turgid. With the result the stomata remain open.

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

The conversion of sugar into starch during night and vice-versa in day time depends upon the acidity (pH) and alkalinity of the cell sap of guard cells. During night there is no photosynthesis and the carbon dioxide accumulates in the guard cells, converting the cell sap into weak acidic starch.

During day time the carbon dioxide is used in the process of photosynthesis, the cell sap becomes alkaline and the starch converts into sugar.