Answer key: Water Potential & Osmosis Activity
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Goal: Calculate solute potential with Ψs = −iCRT and water potential with Ψ = Ψs + Ψp, and predict which way water moves by osmosis.
Completed results table
| Solution | Solute potential Ψs | Pressure potential Ψp | Water potential Ψ |
|---|---|---|---|
| 0.1 M sucrose at 25 °C | −0.248 MPa | 0 MPa | −0.248 MPa |
| 0.3 M NaCl (i = 2) at 25 °C | −1.487 MPa | 0 MPa | −1.487 MPa |
| Turgid plant cell (0.3 M, Ψp = +0.5 MPa) | −0.743 MPa | +0.5 MPa | −0.243 MPa |
Answers
Show how the calculator finds Ψs for 0.1 M sucrose at 25 °C, using Ψs = −iCRT with R = 0.00831 L·MPa/(mol·K).
i = 1 (sucrose does not ionize), C = 0.1 mol/L and T = 25 + 273.15 = 298.15 K, so Ψs = −1 × 0.1 × 0.00831 × 298.15 = −0.248 MPa.
Why is Ψs for 0.3 M NaCl six times more negative than for 0.1 M sucrose?
The concentration is 3 times higher, and NaCl splits into Na⁺ and Cl⁻ ions (i = 2), which doubles the number of solute particles again: 3 × 2 = 6. Ψs = −1.487 MPa compared with −0.248 MPa.
The turgid plant cell has Ψp = +0.5 MPa. What is its water potential, and what does the positive pressure potential represent?
Ψ = Ψs + Ψp = −0.743 + 0.5 = −0.243 MPa. The positive Ψp is turgor pressure: the cell wall pushing back on the swollen cell, which raises its water potential.
The plant cell (Ψ = −0.243 MPa) is placed in 0.5 M NaCl. Which way does water move, and what happens to the cell?
The NaCl solution has Ψ = −2.478 MPa, lower than the cell’s −0.243 MPa, so water leaves the cell (water moves from higher to lower water potential). The cell loses turgor and can plasmolyze, its membrane pulling away from the wall.
A red blood cell (Ψ = −0.773 MPa at 37 °C) is placed in pure water (Ψ = 0). Predict what happens, and explain why a plant cell in pure water does not do the same.
Water moves into the red blood cell, from 0 MPa toward −0.773 MPa. With no cell wall, pressure cannot build up to stop it, so the cell swells and can burst (hemolysis). A plant cell’s wall lets Ψp rise until the water potentials inside and outside are equal, so the cell becomes turgid instead of bursting.
Challenge: what concentration of sucrose at 25 °C has the same water potential as the turgid plant cell (−0.243 MPa)?
Set −iCRT = −0.243 with i = 1: C = 0.243 ÷ (0.00831 × 298.15) = 0.098 mol/L, about 0.1 M.