Which of the following means is used by fresh water organisms to prevent net gain of water or net loss of body salts?

Options

A. (a) Contractile vacuole
B. (b) Large volume of dilute urine
C. (c) Ionocytes
D. (d) All of these

Answer

To answer the question about how freshwater organisms prevent net gain of water or net loss of body salts, we can analyze the options provided:

1. **Understanding the Problem**: Freshwater organisms are constantly faced with the challenge of excess water entering their bodies due to osmosis, while also needing to retain essential salts. Therefore, they have developed various mechanisms to maintain osmotic balance.

2. **Option A: Contractile Vacuole**:
- Contractile vacuoles are organelles found in many freshwater organisms, such as sponges and hydras.
- They function by collecting excess water that enters the organism and expelling it out, thus preventing a net gain of water.
- This mechanism helps maintain the internal environment by regulating water levels.

3. **Option B: Large Volume of Dilute Urine**:
- Freshwater organisms tend to excrete large volumes of dilute urine.
- This process helps in eliminating excess water while retaining salts in the body.
- The production of dilute urine is a common adaptation in freshwater fish and amphibians to manage osmotic pressure.

4. **Option C: Ionocytes**:
- Ionocytes, also known as mitochondria-rich cells, are specialized cells found in the gills of freshwater fishes.
- They play a crucial role in actively transporting ions (salts) into the body, counteracting the loss of salts that occurs due to the surrounding dilute environment.
- This mechanism is essential for maintaining ionic balance.

5. **Conclusion**:
- All three options (contractile vacuoles, large volume of dilute urine, and ionocytes) are mechanisms used by freshwater organisms to prevent net gain of water and net loss of body salts.
- Therefore, the correct answer is **D: All of these**.