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Vacuoles can be seen by light microscopy in the cytoplasm of many protozoa. Some of these vacuoles periodically fill with a fluid substance that is then expelled. Evidence is strong that these contractile vacuoles function principally in osmoregulation. They are more prevalent and fill and empty more frequently in freshwater protozoa than in marine and endosymbiotic species, where their surrounding medium would be more nearly isosmotic (having the same osmotic pressure) to their cytoplasm. Smaller species, which have a greater surface-to-volume ratio, generally have more rapid filling and expulsion rates in their contractile vacuoles. Excretion of metabolic wastes, on the other hand, is almost entirely by diffusion. The main end product of nitrogen metabolism is ammonia, which readily diffuses from the small bodies of protozoa.
Although it seems clear that contractile vacuoles function to remove excess water that has entered cytoplasm by osmosis, a reasonable mechanism for such removal has been elusive. A recent hypothesis suggests that proton pumps on the vacuolar surface and on tubules radiating from it actively transport H and cotransport bicarbonate (HCO3), which are osmotically active particles. As these particles accumulate within a vacuole, water would be drawn into the vacuole. Fluid within the vacuole would remain isosmotic to the cytoplasm.
Then as the vacuole finally joins its membrane to the surface membrane and empties its contents to the outside, it would expel water, H, and HCO3. These ions can be replaced readily by action of carbonic anhydrase on CO2 and H2O. Carbonic anhydrase is present in the cytoplasm of amebas.
Some ciliates, such as Blepharisma, have contractile vacuoles with structure and filling mechanisms apparently similar to those described for amebas. Others, such as Paramecium, have more complex contractile vacuoles. Such vacuoles are located in a specific position beneath the cell membrane, with an “excretory” pore leading to the outside,and surrounded by ampullae of about six feeder canals. Feeder canals, in turn, are surrounded by fine tubules about 20 nm in diameter, which connect with the canals during filling of ampullae and at their lower ends connect with the tubular system of endoplasmic reticulum. Ampullae and contractile vacuoles are surrounded by bundles of fibrils, which may function in contraction of these structures. Contraction of ampullae fills the vacuole. When the vacuole contracts to discharge its contents to the outside, the ampullae become disconnected from the vacuole, so that backflow is prevented. Tubules, ampullae, or vacuoles may be supplied with proton pumps to draw water into their lumens by the mechanism already described.
Useful External Links
- Osmoregulation and contractile vacuoles of protozoa by NIH
- Osmoregulation and contractile vacuoles of protozoa by ReasearchGate
- OSMOREGULATION OR HOMEOSTASIS IN PROTOZOA by BIOZOOM
