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Osmosis in Potato Cylinders

Middle schoolBiologyIdealized potato tissue with conserved solute, 80% initial water and elastic wall pressure
Net water entry0.0 min
02.5123.4244.2365.0485.9606.8Sample mass (g)Time (min)

About this experiment

Place an idealized potato cylinder in a large sugar-solution bath and follow its mass change. Water responds to the difference between outside and inside water potential. The model preserves internal solute and dry matter, while cell-wall pressure resists swelling. It demonstrates osmosis trends rather than predicting measurements for a particular potato variety.

Learning goals

  • Predict water entry or loss from water-potential differences
  • Relate bath concentration to the direction and percentage of mass change
  • Distinguish effects on the rate of change from effects on equilibrium mass
  • Plan the classic potato cylinder experiment and find the isotonic point from a graph of percentage mass change

How to use it

  1. 1Start with a 0.10 mol/L bath and play the first several minutes. Compare inside and outside water potential.
  2. 2Choose the isotonic preset at 0.30 mol/L. Explain why the mass curve is flat.
  3. 3Try a 0.60 mol/L bath and compare the direction of water movement and mass change.
  4. 4At one bath concentration, change hydraulic conductance and compare the time to approach the same predicted equilibrium mass.
  5. 5Record the mass change after 60 minutes for baths of 0, 0.1, 0.2, 0.3, 0.4 and 0.6 mol/L. Plot percentage change against concentration and find where the line crosses zero.

The model

Bath water potential in MPa for concentration in mol/L, at 298 K
Inside potential with conserved solute and a 2 MPa elastic wall coefficient
Relative water-volume change driven by the water-potential difference
Dry matter stays fixed and initial water contributes 80% of mass

Assumptions

  • The bath is large enough to keep a constant concentration. Solute does not cross the membrane; only water changes the sample mass.
  • Initial cell-sap concentration is 0.30 mol/L, temperature is 298 K, and the initial water-volume ratio r is 1.
  • Wall pressure is linear above the initial water volume and zero below it. Tissue damage, metabolism, detailed cell geometry and active transport are omitted.
  • The model uses RK4 with a 0.05 minute integration step. Hydraulic conductance is relative, and sample-size surface-area effects are not included.
  • Parameters are illustrative. Real isotonic concentration and mass-change curves must be measured for the actual tissue.

Try this in class

  1. Q1Why does water enter a sample even when the surrounding solution contains sugar?
  2. Q2Why does changing conductance affect the approach rate but not the equilibrium condition?
  3. Q3Why can elastic wall pressure stop swelling before the inside and outside solute concentrations become equal?
  4. Q4Why do school experiments record percentage change in mass rather than the change in grams?
  5. Q5Why does a lettuce leaf go limp in salty dressing but crisp in cold water?

Frequently asked questions

Is 0.30 mol/L the isotonic concentration of every potato?

No. It is the chosen starting concentration of this idealized model. Real potatoes vary with variety, growth conditions and storage, so an actual experiment is needed.

Why does doubling the initial mass leave percentage change unchanged?

This model uses the same relative hydraulic conductance and water fraction for every sample. It intentionally omits how real surface area and cylinder dimensions affect water transport.

Why do the potato cylinders need to be blotted dry before weighing?

Water clinging to the surface would add mass that never entered the tissue. Blotting each sample the same way makes the measured change reflect water that actually moved by osmosis.