When a fresh-water protozoan possessing a contractile vacuole, is placed in a glass containing marine water, the vacuole will
- increase in number
- disappear
- increase in size
- decrease in size
Answer
(4) decrease in size
Solution
In fresh water (hypotonic medium) water keeps entering the protozoan by osmosis, and the contractile vacuole works actively to pump out the excess water.
⇒ Marine water is hypertonic, so little or no water enters the cell.
∴ The contractile vacuole has less water to remove; it fills slowly and decreases in size (its rate of pulsation falls).
Answer: (4)