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a space or cavity in the protoplasm of a cell.
contractile vacuole a small fluid-filled cavity in the protoplasm of certain unicellular organisms. It gradually increases in size and then collapses; its function is thought to be respiratory and excretory.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
a cavity formed by the accumulation of fluid in the ectoplasm of a protozoan; after increasing for a time it empties itself externally by a sudden contraction; it functions as an osmoregulatory mechanism for water balance, especially in freshwater protozoans.
Farlex Partner Medical Dictionary © Farlex 2012
A membrane-bound organelle found in certain protists that pumps fluid in a cyclical manner from within the cell to the outside by alternately filling and then contracting to release its contents at various points on the surface of the cell. It functions in maintaining osmotic equilibrium.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
contractile vacuolea membrane-surrounded vacuole that periodically fills with liquid, expands and then actively voids the contents to the outside of the organism. Particularly found in protozoans and freshwater sponges, the contractile vacuole appears to have an osmoregulatory function involving active transport. The vacuoles are surrounded by MITOCHONDRIA to produce ATP which drives a sodium pump, enabling water to pass into the vacuole against the osmotic gradient. This theory is supported by the relative absence of such vacuoles in marine amoebae, whereas they are common in freshwater forms that take in water along an osmotive gradient because of the higher (less negative) osmotic pressure inside the cell compared with outside.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005