Contractile vacuole

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Figure 1: A paramecium. The contractile vacuole can be seen as the multiple 'armed' structure on the right-hand side of the ciliate.

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

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

Basically, the contractile vacuole stores extra water. If the cell has a need for water, the contractile vacuole can release more water into the cell. But if water is in excess, the contractile vacuole will remove it.

Contractile vacuoles are often used as a cell's method of propulsion.