D2.3.11 (HL)—Water potential in plant tissue
Water entering plant tissue increases pressure potential and dilutes solutes, while water loss lowers pressure and concentrates solutes in cells.
- Syllabus
- First assessment 2025
- Objective
- D2.3.11
- Level
- HL
Bathing Solutions Change Both Potentials in Plant Cells
HL onlyWhen plant tissue is bathed in hypotonic or hypertonic solution, water movement changes both solute potential and pressure potential until equilibrium or plasmolysis is reached.
| External solution | Initial water movement | Change inside the plant cell | Result |
|---|---|---|---|
| Hypotonic (higher ψw outside) | Water enters | Cell sap is diluted, so ψs becomes less negative; expanding contents raise positive ψp | Cell becomes turgid and rising ψp opposes further entry |
| Hypertonic (lower ψw outside) | Water leaves | Cell sap becomes more concentrated, so ψs becomes more negative; ψp falls toward zero | Cell becomes flaccid and may plasmolyse |
A cell initially at ψs = −600 kPa and ψp = +300 kPa has ψw = −300 kPa. In an external solution below −300 kPa, water leaves, turgor falls and the internal potentials change.
Do not describe only solute concentration: in a walled cell, increasing pressure potential can stop net entry even while the cell sap remains more concentrated than the outside solution.
HL Water Potential
HL onlyWater potential is potential energy of water per unit volume, measured in kPa; pure water at standard conditions has water potential of 0 kPa. Water moves from higher water potential to lower water potential; solutes lower water potential by restricting water molecule movement. Water potential equals solute potential plus pressure potential; solute potential is zero or negative and pressure potential is often positive in walled cells. Water entering plant cells increases pressure potential and dilutes solutes; water leaving plant cells lowers pressure potential and makes solute potential more negative.
Concept essentials
- Water entering plant tissue increases pressure potential.
- Water entry dilutes solutes, making solute potential less negative.
- Water loss lowers pressure potential.
- Water loss concentrates solutes, making solute potential more negative.