Water Potential Calculator
Understand cellular water balance. Explore osmosis visually.
Cell & Environment Inputs
Water Balance Summary
Ψw = Ψs + Ψp
(Ψs)
(Ψp)
(Ψexternal)
Net Water Movement
Into the cellExternal 0.00 → Cell −0.50 MPa
Potential Difference
+0.50 MPaΔΨ = Ψexternal − Ψcell. Sign gives driving tendency, not a flow rate.
Water Potential Scale (MPa)
Cellular Osmosis Visualizer
Turgid example · illustrative configurationIllustrative cell shape · values alone do not determine cell volume or confirm plasmolysis.
Calculation & Worked Example
Frequently Asked Questions
What is water potential?
Water potential describes the potential energy of water per unit volume relative to a reference such as pure water. It is expressed here in pressure units. This simplified cellular model adds solute and pressure potential only: Ψw = Ψs + Ψp.
Which way does water move?
Across a water-permeable membrane, the net thermodynamic tendency is from higher water potential to lower water potential. The tool compares external and cell totals. A positive external-minus-cell difference indicates an inward tendency; a negative difference indicates an outward tendency. This does not determine a flow rate.
Can zero pressure prove plasmolysis?
No. Zero pressure alone cannot distinguish a flaccid cell from a plasmolyzed one. Plasmolysis involves retraction of the protoplast and separation of the plasma membrane from the cell wall; confirming it requires structural evidence and context. The preset drawings are examples, not diagnoses.
Why can pressure potential be negative?
Pressure potential can represent tension, as in water transport through xylem. Positive pressure in a living cell is associated with turgor. A negative input is allowed mathematically, with a note that this generic cell drawing is not a literal model of a cell under tension.
More questions (8)
Which values and units should I enter?
Enter compatible measurements of solute and pressure potential, and optionally external total water potential. Choose MPa, kPa or bar. One MPa equals 1000 kPa or 10 bar. Switching units converts all entered potentials without changing their physical meaning.
Why must solute potential be zero or negative?
Relative to pure water at the same reference conditions, dissolved solutes lower water potential. Pure water has zero solute potential; adding solutes makes it more negative. The tool rejects positive solute-potential values under this convention.
What do the example presets change?
They load illustrative values and cell configurations. Turgid: −0.8 MPa solute, +0.3 MPa pressure, 0 MPa outside. Flaccid: −0.8, 0, −0.8 MPa. Plasmolysis example: −0.8, 0, −1.2 MPa. These are teaching examples, not universal state thresholds or final equilibrium predictions.
Does the visualizer predict cell size or water volume?
No. Presets illustrate different protoplast configurations. Custom values select an illustrative turgid configuration for positive pressure and a flaccid configuration for zero or negative pressure; they do not infer cell state from two inputs. The rigid wall is retained. Absolute volume, quantitative deformation and time-dependent osmosis would require geometry, solute content, membrane properties and wall mechanics.
What if I do not know external water potential?
Leave the external field blank. Cell water potential is still calculated. The direction, external-to-cell difference and flow arrows are withheld because two compartments are needed for the comparison. An invalid nonblank external value must be corrected.
Does the result update automatically?
Yes. Valid input changes update the results, scale and schematic. Calculate refreshes the same model. Reset restores the initial MPa example. Copy exports the current valid summary; a manual-copy field appears if the clipboard is unavailable.
Why might another calculation give a different result?
Check units, reference conditions and which components are included. The more general total can also include gravitational and matric potential. Those components may matter in tall plants, soil, dry tissues or other systems and are intentionally omitted here.
Does equal potential mean water molecules stop moving?
No. Approximately equal total potentials mean no net driving tendency is resolved by this simplified comparison, not that individual water molecules stop exchanging. Very small numerical differences are compared within a stated computational tolerance; this is not a biological threshold.
Sources & References
OpenStax Biology 2eWater and solute transport · Section 30.5
PubMed Central · Plant Cell ResearchLang et al. (2014) · Plasmolysis: Loss of Turgor and Beyond
Educational two-component model · Original illustrative diagrams · No institutional endorsement
Explore cellular water potential and compare water-driving tendencies with a live plant-cell schematic.