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A cavity in the cytoplasm of many freshwater unicellular algae that enlarges as it fills with water and contracts as water is expelled from it. The external medium is hypotonic with respect to the algal cell contents. To help prevent osmotic lysis, energy from ATP is used to pump water into the contractile vacuoles, the pulsations being visible with a light microscope. Water is released through the cell membrane or, in the case of Euglena, into the gullet. Such vacuoles are particularly important in those algae that do not have a rigid cell wall to help limit water intake.
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