Question

Active ${{\text{K}}^ + }{\text{ - }}{{\text{H}}^ + }$ explain theories of
A. Ascent of sap
B. Phloem conduction
C. Ion absorption
D. Stomata movement

Answer 1

The epidermis of leaves consist of pores that help in the exchange of gases with the external environment. They are known as stomata and they are present on the epidermis of all the plant shoot systems including parts of flowers.

Complete answer:
Guard cells are present on the border of stomatal opening, these guard cells are surrounded by subsidiary cells. These subsidiary cells act as a reservoir for water and ions that pass into and out of the guard cells as the stomata open and close. The guard cells regulate the stomatal aperture thereby controlling the uptake of ${\text{C}}{{\text{O}}_{\text{2}}}$and water loss by indirectly controlling the process of photosynthesis.
Several theories explain the process of opening and closing of stomata including the active ${{\text{K}}^{\text{ + }}}{\text{ - }}{{\text{H}}^{\text{ + }}}_{^{^{}}}$ theory which is also known as the potassium pump theory. So the opening and closing of stomata is based on the turgor pressure change in the guard cells. According to this theory ${{\text{K}}^{\text{ + }}}$ ions are transferred into the guard cells in the presence of light and then the following steps take place –
-Protons are formed by the dissociation of malic acid in the presence of light and they move from the cytoplasm of the guard cells to the chloroplast.
-In response to the exit of protons ${{\text{K}}^{\text{ + }}}$ ions move into the guard cells from the surrounding mesophyll cells.
-When ${{\text{K}}^{\text{ + }}}$ reach the guard cells they react with malate ions present in the guard cells and form potassium malate.
-Formation of guard cells leads to the increase in the osmotic potential of the guard cells because of which water enters into it and makes it turgid leading to opening of the stomata.
-During night time potassium malate dissociates and ${{\text{K}}^{\text{ + }}}$ exit out of the guard cell due to which the water is lost from the guard cell leading to the closing of stomata.

Hence, the correct answer is option (D).

Note: Stomata are present in abundance in the lower side of the leaf and their number varies from species to species. Some desert plants keep their stomata closed during day time to prevent the loss of water and open in the night.