Water Potential & Osmosis Activity

Goal: Calculate solute potential with Ψs = −iCRT and water potential with Ψ = Ψs + Ψp, and predict which way water moves by osmosis.

Open the tool at thebiologylab.org/tools/water-potential/

What to do

1. On the Single solution tab, press “0.1 M sucrose at 25 °C”, then “0.3 M NaCl (i = 2) at 25 °C” and “Turgid plant cell (0.3 M, Ψp = +0.5 MPa)”. Record Ψs, Ψp and Ψ for each.
2. Switch to Compare two cells and press “Compare: plant cell vs 0.5 M NaCl”. Note Ψ for both cells and the net water movement.
3. Press “Compare: red blood cell vs pure water” and note the same readouts.

Record your results

Questions

1. 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).

2. Why is Ψs for 0.3 M NaCl six times more negative than for 0.1 M sucrose?

3. The turgid plant cell has Ψp = +0.5 MPa. What is its water potential, and what does the positive pressure potential represent?

4. 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?

5. 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.

6. Challenge: what concentration of sucrose at 25 °C has the same water potential as the turgid plant cell (−0.243 MPa)?

Free from Biology Lab · thebiologylab.org