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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>.
Amoeba proteus has one contractile vacuole which slowly fills with water from the cytoplasm (diastole), then, while fusing with the cell membrane, quickly contracts (systole), releasing water to the outside by exocytosis. This process regulates the amount of water present in the cytoplasm of the amoeba.