Question

When a fresh water protozoan possessing a contractile vacuole, is placed in a glass containing marine water, the vacuole will

• A. Disappear
• B. Increase in size
• C. Decrease in size
• D. Increase in number

Answer 1

Answer: (A)

Disappear

Answer 2

Freshwater protozoans thrive in hypotonic solutions, which means they live in environments where the external solution has a lower concentration of solutes than their internal protoplasm. To cope with the influx of excess water that enters their protoplasm through endosmosis, these protozoans develop contractile vacuoles. However, when these protozoans are introduced into marine water, which is hypertonic (meaning it has a higher solute concentration than their internal environment), they begin to lose water through the process of exosmosis. Consequently, as they lose water, the contractile vacuoles within the cells disappear.