When a fresh water protozoan is placed in marine water

Options

A. The contractile vacuole disappears
B. The contractile vacuole becomes bigger in the size
C. The number of contractile vacuoles increases
D. The contractile vacuoles remain unchanged.

Answer

**Step-by-Step Solution:**

1. **Understanding the Context:**
- A freshwater protozoan is an organism that lives in low salinity (freshwater) environments. It has adapted to maintain its internal water balance in such conditions.

2. **Function of the Contractile Vacuole:**
- The contractile vacuole is an organelle found in many freshwater protozoans. Its primary function is to expel excess water that enters the cell due to osmosis, helping to maintain osmotic balance.

3. **Effect of Placing in Marine Water:**
- Marine water has a higher concentration of solutes (salinity) compared to the internal environment of freshwater protozoans. When placed in marine water, the surrounding environment is hypertonic relative to the protozoan's internal environment.

4. **Osmoregulation Response:**
- In a hypertonic environment, water tends to move out of the cell to balance the solute concentration. To prevent dehydration, the protozoan must retain water instead of expelling it.

5. **Outcome for the Contractile Vacuole:**
- Since the primary role of the contractile vacuole is to expel excess water, in a marine environment, the protozoan will no longer need this function. Therefore, the contractile vacuole will disappear as it is no longer necessary for osmoregulation.

6. **Conclusion:**
- The correct answer to the question is that the contractile vacuole disappears when a freshwater protozoan is placed in marine water.

**Final Answer:**
- The contractile vacuole disappears.

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