Amoebae in Fresh Water

Question

Amoebae which live in fresh water take in water and store excess in a contractile vacuole, which occasionally expels the excess water. Why does water continually enter fresh water Amoebae? Marine Amoebae which inhabit the ocean do not have contractile vacuoles. Why is this so? (osmosis question)

Answer 1

Freshwater amoebae experience a net diffusion of water into the organism.  This is caused by a higher solute concentration within the amoeba relative to the external environment, resulting in osmosis across the membrane in an effort to reach osmotic equilibrium.  Marine amoebae do not have contractile vacuoles because the solute concentration in seawater is higher outside of the membrane, so there is a net diffusion of water out of the organism.