Potato Osmosis: When Length Says 'No Change' but Mass Says Otherwise
An HS first-year practical group is studying osmosis with potato cylinders cut to 4.0 cm and blotted dry. Four cylinders of equal initial mass (2.00 g each) are placed for 30 minutes in 100 mL of distilled water, 5% sucrose, 10% sucrose, and 1.0 M sucrose. After removal and gentle blotting, the masses are 2.18 g, 2.06 g, 1.96 g, and 1.62 g respectively. Lengths are measured with a ruler marked in millimetres; the 10% cylinder appears unchanged in length, so a student writes in the record book that 10% sucrose is isotonic to potato tissue. The teacher notes that the 10% cylinder lost mass but not measurable length and asks the group to re-evaluate. The group has no balance more precise than 0.01 g, and the room temperature is 28 °C. They must decide what conclusion is justified and how to improve the evidence.
1. Calculate the percentage mass change for each cylinder and arrange the four solutions in increasing order of water potential (most negative first).
Read the worked answer
Percentage change = (final mass − initial mass)/initial mass × 100.
- Distilled water: (2.18 − 2.00)/2.00 × 100 = +9.0%
- 5% sucrose: +3.0%
- 10% sucrose: −2.0%
- 1.0 M sucrose: (1.62 − 2.00)/2.00 × 100 = −19.0%
Water potential becomes more negative as solute concentration rises. Order from most negative to least negative: 1.0 M sucrose < 10% sucrose < 5% sucrose < distilled water. The 10% solution is therefore hypertonic, not isotonic: the tissue lost water to it. The sign of the mass change, not the length alone, shows the direction of net water movement.
2. Explain why the 10% cylinder can lose mass while showing no measurable change in length. What does this imply about using length alone to define isotonicity?
Read the worked answer
Potato tissue is not a simple cylinder of water. Cells have rigid walls and air spaces; water loss first reduces turgor pressure and cell volume, which can change diameter or thickness more than length. The ruler has 1 mm resolution, so a small length change may be below detection, while a 0.04 g mass loss on a 2.00 g cylinder is measurable. Also, the cylinder may have been slightly turgid initially, so shrinkage is elastic and anisotropic. Isotonicity means no net water movement at equilibrium, i.e. zero mass change, not zero change in one dimension. Length alone is therefore an insensitive and potentially misleading indicator; mass change or a plasmolyse score is better.
3. Suggest one experimental change that would make the isotonic point estimate more reliable, and state the expected observation at the true isotonic concentration.
Read the worked answer
Use a concentration series, e.g. 0.2, 0.3, 0.4 and 0.5 M sucrose, with three or four cylinders per concentration, all blotted identically and weighed on the same balance. Plot mean percentage mass change against sucrose concentration and find the concentration at which the line crosses zero. At the true isotonic concentration the mean mass change should be approximately 0%, with no net plasmolysis in a companion onion-epidermis check. Replicates reduce the effect of biological variation and blotting error, and a series brackets the isotonic point instead of guessing from one solution.