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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. 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. CVs should not be confused with vacuoles which store food or water. A CV is found predominantly in protists and in unicellular algae. In freshwater environments, the concentration of solutes inside the cell is higher than outside the cell. Under these conditions, water flows from the environment into the cell by osmosis.
Thus, the CV acts as a protective mechanism against cellular expansion and possibly explosion from too much water; it expels excess water from the cell by contracting. However, not all species that possess a CV are freshwater organisms; some marine and soil microorganisms also have a CV.
The CV is predominant in species that do not have a cell wall, but there are exceptions. Through the process of evolution, the CV was mostly eliminated in multicellular organisms; however it still exists in the unicellular stage of several multicellular fungi and in several types of cells in sponges, including amoebocytes, pinacocytes, and choanocytes.
The stage in which water flows into the CV is called diastole.
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