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Describe the mechanism of opening and closing of the stomata using the starch-sugar interconversion theory


  • During the day, in presence of light photosynthesis occursand uses up the carbon (IV) oxide in the cytoplasm thus raising the pH (raises alkalinity). Alkalinity activates enzymes that catalyze the hydrolysis of starch, which is osmotically inactive, to glucose, which is osmotically active. The guard cells then draw water from the adjacent epidermal cells through osmosis and become turgid. The thicker inner walls of the guard cells are pulled outwards and the stoma opens.



  • At night, photosynthesis does not take place. Respiration occurs producing carbon (IV) oxide that lowers the pH (increases acidity). Increased acidity inactivates the enzymes that catalyse the conversion of starch to glucose. The osmotic pressure of the guard cells reduces compared to that of the adjacent epidermal cells. Guard cells then lose water through osmosis to the adjacent epidermal cells and become flaccid. The inner thicker walls relax, straighten up and the stoma closes.
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