ABSTRACT
Protozoa, or protistans as a whole, are single-celled organisms that live in water or a moist environment. The percentage of solutes dissolved in the water varies from mere trace amounts in freshwater streams to increasing concentrations in sewage treatment plants, in brackish water, and in the ocean. Many wall-less species such as Paramecium rely on a contractile vacuole complex (CVC) to maintain their water balance both under normal environmental conditions as well as during dramatic hypoosmotic changes in their environment. Freshwater protozoa have a cytosolic osmolarity under normal growth conditions that ranges roughly between 50 and 110 mOsmol/L and is hyperosmotic to the environment. The contractile vacuole of the CVC appears in wall-less, single-celled organisms to be a single-membrane-lined compartment. Only in the peniculine ciliates, to which Paramecium belongs, are such elaborate CVCs found. Thus the complexity of the CVC seems to have reached its apex in Paramecium and its close relatives.
