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Marine amoeba are isotonic to water and therefore have no contractile vacuole (because the solute concentration in seawater is higher outside of the membrane, so there is a net diffusion of water out of the organism.) and to regulate osmotic pressure, freshwater amoeba have a contractile vacuole that expels excess water from the cell. This organelle is necessary because the surrounding water is hypotonic with respect to the contents of the cell.
Without a contractile vacuole, the cell would fill with excess water and eventually burst.
eddynightsy answered the question on April 15, 2018 at 11:43
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