Contractile vacuole

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A contractile vacuole is a type of vacuole involved in osmoregulation. It pumps excess water out of a cell and is found prominently in freshwater protists.

In a Paramecium, a common freshwater protist, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm which can be opened and closed. This pore has, in some forms of paramecium, disappeared entirely when not in use, leading to the idea of a "second cell" in paramecium.

Other protists, such as Amoeba, have contractile vacuoles that move to the surface of the cell when full and undergo exocytosis.