Osmosis equilibrium in isotonic solution

Steps:

• Observe cell appearance in Q: unchanged, indicating no net water gain or loss.
• Recall osmosis: water moves across membrane from high to low water potential until equilibrium.
• Compare to P and R: P shows turgid cells (hypotonic, water enters); R shows plasmolysis (hypertonic, water exits).
• Conclude Q is isotonic, where water potentials balance, preventing change.

Why D is correct:

• Water potential equality (ψ_cell = ψ_solution) means no net osmosis, as water moves only down gradients (ψ = ψ_s + ψ_p).

Why the others are wrong:

• A: Solute concentration equality doesn't account for pressure potential in water potential.
• B: Cytoplasm concentration ignores vacuole, the main solute compartment in plant cells.
• C: Cytoplasm water potential differs from whole cell's, which includes vacuole and wall pressure.

Final answer: D