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

1. increase in number
2. disappear
3. increase in size
4. decrease in size

Correct answer: disappear

Solution

A contractile vacuole helps freshwater protozoa expel excess water that constantly enters by osmosis. In marine water, the outside is hypertonic, so water does not rush into the cell; the vacuole becomes unnecessary and disappears.

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