Transport In Plants

Stomatal movement depends on changes in guard cell turgor, a mechanism explained in NCERT Class 11, Chapter 11 (Transport in Plants). When stomata open, potassium ions are actively transported into the guard cells, lowering their solute potential and hence their water potential. As a result, water flows into the guard cells by osmosis, raising their turgor. Because the inner wall of a guard cell facing the pore is thicker and less elastic than the outer wall, the swelling guard cells bow outward, and the stomatal pore opens. The option stating water leaves and cells shrink yet the stoma opens is self-contradictory, since loss of turgor closes stomata. Solute potential does not rise when ions accumulate; adding solutes makes it more negative, so the claim of immediate closure is wrong. Uniform wall thickening is also incorrect because the differential thickness of guard cell walls is precisely what enables the pore to open. A biological consistency check confirms the logic: turgid, water-filled guard cells with unequal wall thickness must curve apart, widening the pore for gas exchange during photosynthesis, whereas the loss of these ions and water at night reverses the bowing and closes the stoma to conserve water.