Water will be absorbed by root hairs when

Options

A. the concentration of salts in soil is high
B. the concentration of solutes in the cell sap is high
C. the plant is rapidly transpring
D. they are separated from soil by a semipermeable membrane

Answer

To solve the question "Water will be absorbed by root hairs when?", we need to analyze the options provided and understand the principles of osmosis and water potential.

### Step-by-Step Solution:

1. **Understanding Water Absorption**:
Water absorption by root hairs occurs through a process called osmosis, which is the movement of water from an area of higher water potential to an area of lower water potential.

2. **Analyzing Option 1**:
- **Option 1** states that "the concentration of salts in soil is high".
- When the concentration of salts (solute) in the soil is high, the water potential of the soil becomes low.
- As a result, root hairs will lose water to the soil rather than absorb it.
- **Conclusion**: This option is incorrect.

3. **Analyzing Option 2**:
- **Option 2** states that "the concentration of solute in the cell sap is high".
- If the concentration of solutes in the root hair cells is high, the water potential inside the root hairs will be lower than that of the surrounding soil (assuming the soil has a lower solute concentration).
- Therefore, water will move into the root hairs from the soil.
- **Conclusion**: This option is correct.

4. **Analyzing Option 3**:
- **Option 3** states that "the plant is rapidly transpiring".
- While transpiration creates a negative pressure that can help pull water up from the roots, it is not a direct cause of water absorption by root hairs.
- This condition can facilitate water absorption, but it is not a guaranteed or necessary condition.
- **Conclusion**: This option is less definitive.

5. **Analyzing Option 4**:
- **Option 4** states that "they are separated from soil by a semi-permeable membrane".
- Root hairs are surrounded by a semi-permeable membrane, but this does not directly influence the absorption of water.
- Water absorption depends on the water potential gradient, not merely the presence of a semi-permeable membrane.
- **Conclusion**: This option is incorrect.

6. **Final Conclusion**:
Based on the analysis, the most accurate answer to the question is **Option 2**: "the concentration of solute in the cell sap is high".