Contractile Vacuoles in Amoeba

 The Amoeba lives in freshwater which is hypotonic to the contents of its cell. Water constantly enters the amoebic cell through its selectively permeable membrane by osmosis. This water would dilute the cell contents unless the Amoeba excretes it. The excess water collects in a contractile vacuole which is a small sac, lined with a membrane.

    The contractile vacuole expands as its contents to the exterior. Thus, the vacuole acts as a kind of water pump for getting rid of excess water. This water has almost no salt in it. For the contractile vacuole to work, energy is needed. Numerous mitochondrial lie close to the vacuole. If an Amoeba is treated with a metabolic poison, the contractile vacuole will have no energy to work. As a result the animal will take in so much water that it will swell and eventual burst.

    Thus, we see that getting rid of excess water is the main problem in freshwater organisms. The Amoeba overcomes this by

 • having a cell membrane with a low permeability to water , and

 • a specialized mechanism (contractile vacuole) just for the removal of excess water. 

     The frequency of the filling and discharging processes of the contractile vacuoles decreases as the salinity of the aquatic medium increases as an Osmoregulatory as it regulates the water content of the cell.

      Excretory products , like ammonia and carbon dioxide, leave the Amoeba by diffusion across the selectively permeable cell membrane to the exterior. this is an adequate mechanism for excretion of metabolic wastes since the surface area to volume ratio of the Amoeba is large.