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The contractile vacuole allows free-living protists to expel extra water. <b>The contractile vacuole allows free-living protists to expel extra water <br /></b>.
When moisture is plentiful, the guard cells swell with water, forcing the opening of the stoma open and allowing gas exchange to occur. When the plant loses too much water or water in the environment becomes less plentiful, the guard cells deflate, closing the stoma and preventing further water loss or gas exchange.