Maintenance of internal environment of the cell or body at constant called homeostasis.Homeostasis within cells- In protists (e.g. Amoeba), the salt and water concentration in the protoplasm remains constant. In fresh water forms water constantly enter by osmosis through the plasma membrane diluting cytosol. The water contractile vacuole. The contractile vacuole periodically contracts to remove excess water to outside. Thus the salt concentration in cytosol remains constant. This is a homeostatic mechanism in cells.