For a solution / a cell

Options

A. `Psi_w` is negative but `Psi_s` is positive
B. `Psi_w` is positive but `Psi_s` is negative
C. Both `Psi_w` and `Psi_s` are negative
D. Both are positive

Answer

To solve the question regarding the relationship between water potential and solute potential in a solution or a cell, we can break it down step by step:

### Step-by-Step Solution:

1. **Understanding Water Potential**:
- Water potential (ψw) is a measure of the potential energy of water in a system and is influenced by two main components: solute potential (ψs) and pressure potential (ψp).
- The equation for water potential is given by:
\[ ψw = ψs + ψp \]

2. **Components of Water Potential**:
- **Solute Potential (ψs)**: This is the potential of water to move due to the presence of solutes. It is always negative or zero because the addition of solutes lowers the water potential.
- **Pressure Potential (ψp)**: This is the physical pressure exerted on the water in a system. It can be positive (when pressure is applied) or zero (when the system is at atmospheric pressure).

3. **Analyzing the Options**:
- **Option 1**: Water potential is negative but solute potential is positive. (Incorrect)
- **Option 2**: Water potential is positive but solute potential is negative. (Incorrect)
- **Option 3**: Both water potential and solute potential are negative. (Correct)
- **Option 4**: Both are positive. (Incorrect)

4. **Conclusion**:
- When considering a solution at atmospheric pressure (where ψp = 0), the water potential (ψw) becomes equal to the solute potential (ψs), which is negative. Therefore, both ψw and ψs are negative in this scenario.
- Hence, the correct answer is **Option 3**: Both water potential and solute potential are negative.