Contractile Vacuoles of Amoeba
Microscopic observations revealed that in protozoans contractile vacuole exhibits a cyclic functioning. It gradually gets filled with fluid and increases in volume (diastole) till it reaches a critical size. Then it moves towards the cell boundary, suddenly expels its contents to the outside and decreases in size (systole). The mechanisms of filling and discharge of the vacuole are not yet clearly understood. Electron microscopic studies show that the contractile vacuole contains a single membrane enclosing the lumen.
Figure: Contractile Vacuoles of Amoeba
This membrane is surrounded by a layer of densely packed vesicles around which there is a layer of mitochondria which presumably supply energy for the osmotic work done by the vacuole. The vesicles empty their contents into the contractile vacuole by fusion of their membranes. According to current theories, the small vesicles initially contain a fluid isosmotic to the cytoplasm. The fluid later becomes hypoosmotic due to the removal of salts, particularly potassium, by active transport. The hypoosmotic vesicles now fuse and open into the contractile vacuole leading to diastole. It is also believed that contractian of myofibrils present in the vacuolar membrane is probably responsible for the expulsion of the fluid during systole.