noun, plural: contractile vacuoles
(cell biology) A specialized vacuole of eukaryote cells, especially protozoa, involved in osmoregulation, i.e. it allows the flow of water from the cytoplasm and then discharges this externally by the opening of a permanent narrow neck
A contractile vacuole is a specialized type of vacuole in eukaryotic cells, particularly protozoa and certain unicellular algae. It is involved in osmoregulation. Osmoregulation is the process of regulating water potential in order to keep fluid and electrolyte balance within a cell or organism relative to the surrounding. It is essential because it maintains a constant optimal osmotic pressure in the cell. Through it, the cell is able to keep the right amount of solute concentration and water.
In aquatic unicellular organisms such as protozoa and algae, the cell is hypertonic relative to its surrounding, i.e. the inside of the cell has higher concentration of solutes than the environment. As a result, the water tends to flow into the cell through osmosis. The contractile vacuole helps prevent excessive water influx that could harm and cause rupture (lysis) to the cell. The contractile vacuole contracts to expel water out of the cell (thus, the name). The period of water expulsion of contractile vacuole is referred to as systole whereas the period at which water flows into it is called diastole.
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