Sample Preparation: Plant tissue (often potato) is cut into uniform cylinders using a cork borer to ensure a consistent surface area to volume ratio, which standardizes the rate of diffusion across all samples.
Serial Dilution: A range of sucrose or salt concentrations (e.g., to ) is prepared to create a calibration curve. This allows for the precise identification of the point where mass change is zero.
Standardization of Procedure: Samples must be left for a sufficient, identical duration (e.g., 30–60 minutes) to allow osmosis to reach a measurable state. Temperature should be controlled as it affects the kinetic energy of water molecules.
Drying Technique: Before weighing, samples must be gently 'dabbed' with a paper towel. This removes excess surface liquid that would otherwise add inaccurate mass to the final measurement without being part of the internal cellular change.
| Feature | Hypotonic Solution | Hypertonic Solution |
|---|---|---|
| Water Potential | Higher than the cell | Lower than the cell |
| Net Water Movement | Into the cell | Out of the cell |
| Plant Cell State | Turgid (Firm) | Flaccid/Plasmolysed (Floppy) |
| Mass Change | Positive (%) | Negative (%) |
Mass vs. Length: While both can be measured, mass is generally more accurate as it accounts for 3D changes in volume, whereas length only measures one dimension and can be affected by the sample bending.
Animal vs. Plant Cells: Plant cells have a rigid cell wall that prevents bursting (lysis) in hypotonic solutions by exerting pressure potential. Animal cells lack this and will burst if too much water enters.
Why use Percentage Change?: Students must explain that initial masses of tissue samples are never identical. Using percentage change allows for a fair comparison between samples regardless of their starting size.
Finding the Isotonic Point: On a graph of percentage change (y-axis) vs. concentration (x-axis), the isotonic point is the x-intercept. This is where the net movement of water is zero.
Verification of Results: If the line of best fit does not cross the x-axis, the range of concentrations used was insufficient. A wider range of concentrations should be tested to find the intercept.
Precision: Always ensure the balance used is accurate to at least two decimal places () to detect small osmotic changes.
Surface Moisture: Failing to dry the potato cylinders properly is the most common source of error. Liquid on the surface adds 'fake' mass, leading to an overestimation of water gain or an underestimation of water loss.
Evaporation: Leaving the beakers uncovered during incubation can lead to water evaporation, which increases the solute concentration and lowers the water potential of the solution, skewing the results.
Tissue Consistency: Using different parts of a plant (e.g., the skin vs. the center of a potato) can introduce variables because different tissues may have different starting water potentials.
