Water Potential & Osmosis
Predict which way water moves and read the zero crossing in potato-core data.
What to do when the question looks unfamiliar
- Step 1Compute total water potential on each side.
- Step 2Move net water from higher Ψ to lower Ψ.
- Step 3For a mass-change graph, find where the y-value crosses zero.
Compare total Ψ, including pressure
Water potential is commonly modeled as Ψ = Ψs + Ψp. Solute potential is zero for pure water and negative in a solution; positive pressure can offset a negative solute potential.
A cell with Ψs = −0.8 MPa and Ψp = +0.5 MPa has Ψ = −0.3 MPa. In a solution at −0.4 MPa, net water leaves the cell. The more negative value is lower.
From a potato-core graph to a biological claim
Calculate percent change as (final mass − initial mass) ÷ initial mass × 100. Positive change indicates net water gain under the assay assumptions; negative change indicates loss.
The concentration at 0% mass change estimates the external solution where there was no net water movement under those conditions. Interpolate between nearby points and report an estimate, rather than pretending the graph is more precise than it is.
Use replicates and match core size, exposure time, and temperature. Blot consistently so droplets do not count as gained tissue mass.