vacuole

vacuole - in biology, a space within a cell that is empty of cytoplasm, lined with a membrane, and filled with fluid. Especially in protozoa, vacuoles are cytoplasmic organs (organelles), performing functions such as storage, ingestion, digestion, excretion, and expulsion of excess water. The large central vacuoles often found in plant cells enable them to attain a large size without accumulating the bulk ...

contractile vacuole - regulatory organelle, usually spherical, found in freshwater protozoa and lower metazoans, such as sponges and hydras, that collects excess fluid from the protoplasm and periodically empties it into the surrounding medium. It may also excrete nitrogenous wastes. In amoebas it changes position with the animal's movement; in most ciliates it follows a definite path through the cell; in the Euglena ...

cell - Most plant cells contain one or more membrane-bound vesicles called vacuoles. Within the vacuole is the cell sap, a water solution of salts and sugars kept at high concentration by the active transport of ions through permeases in the vacuole membrane. This high concentration causes the entry, via osmosis, of water into the vacuole, which in turn expands the vacuole and generates a hydrostatic ...

excretion - Some protozoan animals possess an organelle having the form of an internal sac, or vacuole, which enlarges by the accumulation of a clear fluid and then discharges its contents to the exterior. The cycle of filling and emptying may be repeated as frequently as every half minute. The chief role of the contractile vacuole appears to be in osmotic regulation, not in nitrogen excretion.

digestive system, invertebrate - Unicellular organisms that ingest food particles via vacuoles rely on intracellular digestion to prepare the nutrients for use. The enzymes that catalyze this digestion, being very potent chemicals capable of breaking down the cell substance itself, are held until needed in special packets, or vesicles, called lysosomes; the membrane of a lysosome is both impermeable to the enzymes and capable of ...