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Describe the mechanism of opening and closing of the stomata using the photosynthetic theory

Opening

  • During the day in presence of light, chloroplasts in the cytoplasm of the guard cells photosynthesize and form glucose. The glucose formed in the cytoplasm of the guard cells is osmotically active and thus increases the osmotic pressure in the cytoplasm of the guard cells. The guard cells through osmosis then draw water from the adjacent epidermal cells and become turgid. The walls of the guard cells due to their differences in thickness stretch unevenly causing the stoma to open.

Closing

  • At night, there is no light and no photosynthesis takes place. The glucose in the cytoplasm of the guard cells is then converted to starch, which is osmotically inactive. This decreases the osmotic pressure in the cytoplasm of guard cells. The guard cells then lose water to the adjacent epidermal cells through osmosis and become flaccid. The inner and outer walls relax thus closing the stoma.
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