Question

Which process is responsible for controlling the movement of guard cells?
A. Transpiration
B. Photosynthesis
C. Osmosis
D. Diffusion

Answer 1

Hint: Specialized plant cells are guard cells that are used for gas exchange regulation in the epidermis of leaves, stems and other organs. They are formed in pairs that form a stomatal pore, with a gap between them. When water is readily available, the guard cells become turgid, and the stomatal pores open. The guard cells become flaccid when water availability is less.

Complete answer: Osmosis is the movement of a liquid towards a greater concentration of solvent over a semi-permeable membrane (lower concentration of solvent). The solvent is usually water in biological systems, but osmosis can occur in other liquids, supercritical liquids and even gases. Osmosis is the mechanism by which the opening and closing of stomata by guard cells are controlled. The water potential decreases as potassium ions accumulate in the cells, and endosmosis occurs, leaving the cell turgid. This allows the cells to take a form that will open up the stomatal pore. The cells become flaccid and the pores close as the water flows out of the cells. Guard cells are cells that each stoma is surrounded by opening and closing the stomata, they help control the rate of transpiration. The principal cause for the opening or closure is light. There is a relatively thick cuticle on the pore-side of each guard cell and a thin one opposite.
So, option C is the correct answer.

Note: Stomatal pore closure is triggered by ion release from guard cells: Other ion channels that mediate the release of ions from guard cells have been identified, resulting in osmotic water efflux from guard cells due to osmosis, shrinking of guard cells and closing of stomatal pores.