Question

Suggest the theories to explain the changes that occur in the osmotic pressure of the guard cells during the mechanism of opening and closing of stomata.

Answer 1

Osmotic pressure:

1. The osmotic pressure is the minimal pressure required to prevent the inward movement of a solution's pure solvent through a semipermeable barrier.

Stomata:

1. The small apertures or pores in the epidermis of leaves and young stems are known as stomata.
2. They help in gas exchange and transpiration (water loss through evaporation).

Guard cells:

1. The epidermis of leaves, stems and other organs contains specialized plant cells known as guard cells.
2. They are utilized to control gas exchange by enclosing the stomatal pore.

A theory that explains stomatal opening and closure:

1. Lioyd's theory explains the opening and closure of stomata on the basis of change in the osmotic pressure of the guard cell.
2. According to this concept, guard cells contain the enzyme phosphorylase, which converts soluble carbohydrates to insoluble starch (carbohydrate) at night.
3. As a result, the osmotic pressure of guard cells remains constant and the stomata remain closed.
4. During the day, however, this enzyme turns insoluble starch into soluble sugar, increasing the osmotic pressure of the guard cell and opening the stomata.