How this instrument works
Water potential, denoted with the Greek letter psi (Ψ) and measured in megapascals, is plant physiology's way of predicting which direction water will move into or out of a cell, tissue or soil compartment. Pure water at atmospheric pressure is defined as the zero baseline; any solution or cell with a lower (more negative) value than its surroundings will draw water in, and any compartment with a higher one will lose it — water always flows down that gradient, from high toward low.
Two components combine to produce that total in a typical living cell. Pressure potential, Ψp, also called turgor potential, comes from the physical push of the cell wall resisting the swollen protoplast inside it and is usually positive in a healthy, turgid cell, though it can turn negative under tension in xylem. Osmotic potential, Ψs, is always zero or negative, falling further below zero as a solution's dissolved solute concentration rises — pure water sits at zero on this same scale, and every dissolved solute pulls it lower.
A fourth component, matric potential (Ψm), captures the additional pull that soil particles or cell-wall surfaces exert on water through adhesion and capillary forces — OpenStax Biology lists it alongside pressure, solute and gravitational components as one of the full set that can contribute to the total. In living cells suspended in solution, this matric term is typically small enough to treat as negligible next to pressure and osmotic potential, which is why this calculator, like most introductory treatments of cell water relations, sums only those two terms.
- Enter the cell's pressure (turgor) potential into Pressure potential, Ψp (MPa) — positive for a turgid, swollen cell, and 0 for a flaccid cell with no wall pressure.
- Enter the cell's osmotic potential into Osmotic (solute) potential, Ψs (MPa, typically negative) — more negative for cell sap with a higher dissolved-solute concentration.
- Read Water potential, Ψ (MPa) beneath both fields; it updates instantly as either input changes.
- To predict flow between two adjacent cells, compute Ψ for each — water moves from the higher (less negative) value toward the lower (more negative) one.
Worked example — a turgid cell at Ψp = 0.5, Ψs = −0.8 MPa
A plant cell is turgid, with its cell wall pushing back against the swollen protoplast at a turgor pressure of 0.5 MPa, while its cell sap carries enough dissolved solute to register −0.8 MPa of osmotic pull. Enter 0.5 into Pressure potential, Ψp (MPa) and −0.8 into Osmotic (solute) potential, Ψs (MPa, typically negative); Water potential, Ψ (MPa) reads −0.3.
The result is negative even though the cell is under positive turgor pressure, because the pressure term only partly offsets the more negative osmotic term — 0.5 + (−0.8) = −0.3 MPa. That negative total means the cell would still draw in water from an adjacent compartment closer to zero, such as pure water at atmospheric pressure, until the two potentials equalized.
Questions
What does a negative water potential actually mean?
It means the cell or solution has a lower value than pure water at atmospheric pressure, which is defined as zero. Anything with a more negative water potential than its surroundings will pull water in, which is exactly why cell sap — rich in dissolved solutes and therefore strongly negative in its solute component — draws water in from the soil and xylem in the first place.
Why is pressure potential usually positive in a living cell but negative in xylem?
In a living, turgid cell, the rigid cell wall physically resists the swollen protoplast pushing outward, generating a positive pressure much like an inflated balloon's wall pushing back on the air inside. In xylem conduits, water is instead often under tension — being pulled upward by transpiration — which produces a negative pressure potential, the opposite mechanical situation from a turgid living cell.
What happened to matric potential in this calculation?
It's omitted as typically negligible. OpenStax Biology's treatment of this topic lists matric potential (Ψm) alongside pressure and solute terms as a fourth possible component, arising from water's adhesion to soil particles or cell-wall surfaces. In a living cell suspended in solution it's usually small enough to ignore next to pressure and osmotic potential, though it becomes significant in dry soil, which this calculator does not model.
How does water potential predict which way water moves?
Water always moves from a region of higher (less negative) water potential toward a region of lower (more negative) one, whether that's between two cells, a cell and its surrounding soil solution, or two points along a plant's xylem. Comparing the computed Ψ for two adjacent compartments directly tells you the direction water will flow between them, without needing to know anything else about either compartment.
Is water potential the same thing as osmotic pressure?
Related, but not identical. Osmotic pressure is conventionally expressed as a positive magnitude describing a solution's pulling strength, while osmotic (solute) potential, as used in this equation, is that same quantity expressed with a negative sign by convention — so a solution's osmotic potential is numerically the negative of its osmotic pressure, not a separate measurement.