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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>.
Increase in permeability of the cells of the collecting tubule to water is due to an increase in the production of antidiuretic hormone (ADH).   Aquaporins are constant com â¦ ponents of the tubule cells membranes. Unless ADH is present, aquaporins are virtually absent in the luminal membranes of the collecting duct.