Preparation: Use a cork borer to cut uniform cylinders of potato tissue, ensuring they have the same diameter. Trim them to an identical length using a scalpel and ruler to standardize surface area.
Initial Measurement: Blot each cylinder dry with a paper towel to remove surface moisture, then record the initial mass using a high-precision balance (e.g., to g).
Immersion: Place the cylinders into separate boiling tubes containing a range of sucrose concentrations (e.g., to mol/dm³). Ensure the volume of solution is sufficient to fully submerge the tissue.
Incubation: Leave the samples for a set period (at least 30 minutes) in a temperature-controlled environment to allow osmosis to reach a measurable state.
Final Measurement: Remove the cylinders, blot them dry again to ensure only internal water changes are measured, and record the final mass.
Percentage Change Calculation: Calculate the percentage change in mass for each sample using the formula: \\text{% Change} = \\frac{\\text{Final Mass} - \\text{Initial Mass}}{\\text{Initial Mass}} \\times 100.
Graphing: Plot the percentage change in mass (y-axis) against the concentration of the solute (x-axis). Draw a line of best fit through the data points.
Finding the Intercept: Identify the x-intercept where the line crosses the axis (0% change). This concentration is equivalent to the water potential of the potato cells.
Conversion: Use a standard reference table to convert the molar concentration found at the x-intercept into the corresponding water potential in kPa.
| Feature | Hypotonic Environment | Hypertonic Environment |
|---|---|---|
| Water Movement | Net movement into the cell | Net movement out of the cell |
| Cell State | Turgid (Firm) | Plasmolysed (Flaccid/Shrivelled) |
| Mass Change | Positive % Change | Negative % Change |
| Appearance | Swollen, rigid tissue | Soft, floppy tissue |
Blotting Importance: Always emphasize the need to blot cylinders dry before weighing. Failure to do so adds the mass of surface liquid to the results, creating a significant systematic error.
Percentage vs. Absolute: Explain why percentage change is used instead of absolute mass change. It allows for a fair comparison between samples that may have had slightly different starting masses.
Control Variables: Identify key controls such as temperature (using a water bath), volume of solution, and the source of the plant tissue (to ensure consistent age and type).
Anomalous Results: If a data point falls far from the line of best fit, it should be identified as an anomaly and excluded from the mean calculation or line of best fit construction.
