D2.3 Water Potential | IB DP Biology

Cell Type

External Solution Concentration

mol L⁻¹ 0.30

0.000.751.50

Solution Type

Isotonic

Cell State

Normal

Net Water Movement

⇌ No net movement

Legend

Water molecule (H₂O)

Solute molecule

Selectively permeable membrane

Cell wall (plant only)

Cell Data

Internal Conc.

0.30 mol L⁻¹

External Conc.

0.30 mol L⁻¹

Membrane

Selectively permeable

H₂O crosses freely. Solutes cannot cross.

Cell Effect

Animal Cell

No net change in volume

📋 CHECKPOINT

Adjust the slider to explore. A question will appear.

Virtual Osmosis Practical

Potato cylinders are placed in sucrose solutions of increasing concentration. After 30 minutes, record the percentage change in mass to determine the isotonic concentration of potato cell sap.

ψ Water Potential Calculator

Water potential (ψw) is the potential energy of water per unit volume (kPa). Water always moves from higher to lower water potential — from less negative to more negative.

ψ_w = ψ_s + ψ_p

ψs — solute potential. Always ≤ 0. More dissolved solute = more negative. Represents reduced free water due to solute–water interactions.

ψp — pressure potential. Usually positive (turgor). Cell wall pushes back on expanding protoplast, increasing ψw. Negative in xylem under tension.

Cell A

ψs −800 kPa

ψp +400 kPa

ψw = −400 kPa

→

Water: A → B

Cell B / Solution

ψs −1500 kPa

ψp +0 kPa

ψw = −1500 kPa

Preset Scenarios

📋 HL Checkpoint Questions

1. A plant cell has ψs = −900 kPa and ψp = +600 kPa. Calculate ψw. If placed in pure water (ψw = 0), in which direction does water move? Explain.

2. A fully turgid cell stops gaining water even though it is in a hypotonic solution. Using ψw = ψs + ψp, explain why net water movement ceases.

3. In xylem, ψp is negative (tension generated by transpiration). Explain how this contributes to the ascent of water from roots to leaves.