Question

Are protozoans presenting contractile vacuoles (also called pulsatile vacuoles) more commonly found in fresh or in salt water?

Answer 1

Fresh water has a low concentration of solutes than salt water and fresh water tends to be less concentrated than the intracellular environment, causing cells to swell. Salt water, on the other hand, since it is very concentrated, tends to dehydrate the cell. So protozoans with contractile vacuoles more commonly found in fresh water

The vacuoles of protozoans are internal structures specialized in water storage that release water into cytoplasm when necessary. Therefore,vacuoles can dilute the cytoplasm to put it into osmotic equilibrium with the environment. As a result fresh water protozoa need vacuoles more, since their intracellular environment is hypertonic in relation to the exterior. Without the dilution mechanism provided by the vacuoles, fresh water protozoa would absorb too much water and would die.