Water potential equals solute potential plus pressure potential, combining negative solute effects and positive pressure effects in plant cells and tissues.
Coverage 2025–2025 · Updated 16 Jul 2026
In cells with walls, total water potential is the sum of solute potential and pressure potential.
\psi_w=\psi_s+\psi_p
| Component | Sign and meaning |
|---|---|
| Solute potential, ψs | 0 for pure water and increasingly negative as dissolved solute lowers water's potential |
| Pressure potential, ψp | Usually positive inside turgid walled cells; can be negative in xylem sap under tension |
If ψs = −600 kPa and ψp = +100 kPa, then ψw = −600 + 100 = −500 kPa. Water tends to enter from a neighbouring region at −400 kPa.
Keep signs and units through the calculation. Pressure potential is not always zero or positive: tension in functioning xylem gives a negative value.
This objective is assessed through multiple choice, commonly using Explain.
Explain
Build the answer around this relationship: Water potential equals solute potential plus pressure potential.
Representative question
The water potential of a plant cell is -0.24 kPa . If the pressure potential of the cell is 0.46 kPa , what is the solute potential of that cell?
0.22 kPa
-0.22 kPa
0.70 kPa
-0.70 kPa
Water 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.
