D2.3 Water Potential

IB DP Biology Β· Continuity and Change β€” Cells

Kingdom
Cell Type
External Solution Concentration
mol L⁻¹ sucrose 0.28
0.000.501.001.50
Simulation Speed
1Γ— Lab shows realistic pace (minutes). Real osmosis is slow.
Membrane Permeability
Selectively permeable: only water crosses freely.
Legend
Water molecule (Hβ‚‚O)
Solute molecule
Selectively permeable membrane
Cell wall (plant only)
Solution Type
Isotonic
Cell State
Normal
Net Water Movement
β‡Œ No net movement
Hβ‚‚O Crossings per Second
β†’ Entering
12
← Leaving
12
Net: 0 β€” no net movement
At equilibrium, crossings continue in both directions. Net flux β‰ˆ 0; crossings never stop.
πŸ”¬ β†’ 🌿 Consequences at Each Level
πŸ”¬
Cell
β€”
🧬
Tissue
β€”
🌿
Organism
β€”
πŸ“‹ Prediction / Checkpoint
Explore the simulation. A question will appear.

Virtual Osmosis Practical

Potato cylinders (uniform mass ~2.0 g) placed in sucrose solutions for 30 minutes. One trial per concentration. Notice the flagged outlier β€” consider whether it should be excluded and why.

ψ Water Potential Calculator

Water potential (ψw) is the potential energy of water per unit volume (kPa), measured relative to pure water at atmospheric pressure and 20°C. Water moves from higher to lower ψw.

ψw  =  ψs  +  ψp
ψs β€” solute potential. Always ≀ 0. More dissolved solute = more negative. Fewer free water molecules per unit volume.
ψp β€” pressure potential. Positive in turgid cells (turgor). Zero in flaccid/plasmolysed. Negative in xylem (tension).
Cell A
ψsβˆ’800 kPa
ψp+400 kPa
ψw = βˆ’400 kPa
β†’
A β†’ B
Cell B / Solution
ψsβˆ’1500 kPa
ψp+0 kPa
ψw = βˆ’1500 kPa
Preset Scenarios
πŸ“‹ HL Questions

1. A cell has ψs = βˆ’900 kPa and ψp = +600 kPa. Calculate ψw. In which direction will water move if placed in pure water? Explain why.

2. A turgid cell stops gaining water despite being in a hypotonic solution. Use ψw = ψs + ψp to explain why net uptake ceases.

3. In xylem vessels ψp is negative (under tension). Using the equation, explain how this facilitates long-distance water transport.

4. Guard cells take up K⁺ ions when stomata open, making ψs more negative. Predict the effect on ψw and direction of water movement into guard cells.