Question

The conversion of starch to organic acid is essential for stomatal
A) Closure
B) Growth
C) Initiation
D) Opening

Answer 1

Hint: The stomata are the pores present in the leaves through which the exchange of gases take place. The opening and closing of stomata are regulated by the guard cells. In the stomata, the starch is converted into malic acid due to which the guard cells become turgid.

Complete answer:
The guard cells regulate the opening and closing of the stomata. The stomata open when they are turgid that is they are charged with water. The stomatal movement is regulated by the interconversion of starch to organic acids. This theory is put forward by Llyod.

According to this theory, the enzyme phosphorylase is present in the guard cells in which the soluble sugars is converted into starch at night. This leads to an osmotic pressure decrease and the stomata are closed. In case when light is available, photosynthesis takes place and the carbohydrate is produced. In the presence of an enzyme, the conversion of starch to malic acid takes place, the cells get turgid and further opens.

In this process, the dissociation of the organic acid helps in the increase of the proton concentration. As the protons move out of the guard cells and the potassium ions move into the guard cells. This increases the water potential in the guard cells. This leads to the endosmosis of water in the guard cells making them turgid. The closing of the stomata takes place when the cell is flaccid.

Thus, the correct answer is D.

Note: Turgidity is the condition in which the cell is charged with water. It takes place due to endosmosis. This leads to the swelling of the cell. Flaccidity is the opposite of turgidity. It leads to shrinkage of the cell.