State the main function of the following:
Guard cells
Topic: Guard cells
Answer
They REGULATE THE OPENING AND CLOSING OF THE STOMATA in the leaf, controlling transpiration and the exchange of gases.
HOW THEY WORK: when the guard cells take in water they become TURGID. Because their INNER wall (facing the pore) is THICKER and less elastic than the outer wall, they bow OUTWARDS as they swell and the pore OPENS. When they lose water they become flaccid, straighten, and the pore CLOSES.
That UNEQUAL THICKENING is the whole mechanism, and it is what examiners want explained — not simply that the cells swell.
Guard cells are the ONLY epidermal cells containing CHLOROPLASTS, which is what distinguishes them from the surrounding cells.
Note that the stoma is the only plant pore that can be REGULATED; lenticels and hydathodes are permanently open.