Question

Water potential is equal to solute potential in the following

• A. A cell soaked in isotonic solution
• B. A cell soaked in hypertonic solution
• C. A cell soaked in hypotonic solution
• D. A cell with incipient plasmolysis

Hint:

In hypertonic solutions, cells lose water, pressure potential drops to zero, so water potential equals solute potential.

Answer 1

The Correct Option is B

Water potential (\( \Psi_w \)) of a system is the sum of solute potential (\( \Psi_s \)) and pressure potential (\( \Psi_p \)): \[ \Psi_w = \Psi_s + \Psi_p \] In a hypertonic solution, water moves out of the cell, leading to plasmolysis, and the pressure potential becomes zero or negligible (as the cell membrane pulls away from the cell wall).
At this point, the water potential of the cell is almost entirely determined by its solute potential, and hence: \[ \Psi_w \approx \Psi_s \] Thus, in a hypertonic solution where a cell is fully plasmolysed or approaching full plasmolysis, the pressure potential is negligible or zero, making water potential equal to solute potential.

Answer 2

Water potential is equal to solute potential in the following:

Water potential (Ψ) of a solution or cell is determined by two main components: solute potential (Ψs) and pressure potential (Ψp). The relationship is given by the formula:
Ψ = Ψs + Ψp

Solute potential (also called osmotic potential) is always negative or zero because solutes reduce the free energy of water.
Pressure potential (also called turgor pressure) can be positive, negative, or zero depending on the situation.

When a cell is soaked in a hypertonic solution (i.e., a solution with higher solute concentration than the cell sap), water moves out of the cell, causing the cell to lose turgor pressure. Eventually, the pressure potential inside the cell becomes zero because the cell membrane shrinks away from the cell wall (plasmolysis).

At this point:
- Pressure potential (Ψp) = 0
- Therefore, water potential (Ψ) = solute potential (Ψs)

This means the total water potential equals the solute potential only when the cell is in a hypertonic solution and has lost turgor pressure completely.

Correct Answer: A cell soaked in hypertonic solution