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TL;DR Cut uniform potato cylinders with a cork borer, weigh them, immerse each in a different sucrose concentration for 30 minutes, then re-weigh and calculate percentage change in mass. Plot percentage change (y-axis) against concentration (x-axis) and read off the isotonic point where the curve crosses the x-axis. Always use percentage change instead of absolute change so that differences in starting mass do not skew your comparison.
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Three terms describe the relationship between two solutions separated by such a membrane:
Hypotonic solution has a higher water potential than the cell sap. Water moves into the cell by osmosis, and the cell gains mass. In a plant cell, the cell wall prevents bursting and the cell becomes turgid.
Hypertonic solution has a lower water potential than the cell sap. Water moves out of the cell by osmosis, and the cell loses mass. In a plant cell, the cell membrane pulls away from the cell wall (plasmolysis).
Isotonic solution has the same water potential as the cell sap. There is no net movement of water, so the cell mass stays the same.
The potato osmosis experiment exploits these three situations. Each sucrose concentration creates a different water-potential gradient across the potato cell membranes. By measuring mass change at each concentration, you can identify the isotonic point and confirm that osmosis follows the direction predicted by water potential theory.
2 | Apparatus list
Item
Purpose
Large potato
Provides uniform plant tissue with living cells and partially permeable membranes.
Provides the range of water potentials needed to observe net water gain and net water loss.
Paper towels
Used to blot surface moisture from each cylinder before weighing.
Stopwatch
Ensures every cylinder is immersed for the same duration (30 minutes).
Labels or marker pen
Identifies each beaker or tube with its concentration.
3 | Step-by-step method
Prepare the solutions. Label six beakers or boiling tubes with the concentrations 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol/dm3. Pour enough sucrose solution into each to fully submerge a potato cylinder (approximately 20--30 cm3).
Cut the potato cylinders. Push the cork borer through the potato to produce six cylinders of the same diameter. Trim each cylinder to exactly the same length (for example 4.0 cm) using the knife and white tile. Wipe any starch residue off each cylinder.
Record the initial measurements. Blot each cylinder gently with a paper towel to remove surface moisture. Measure and record the initial mass (to 0.01 g) and initial length (to the nearest mm) of each cylinder.
Immerse the cylinders. Place one cylinder into each labelled beaker or tube. Start the stopwatch as soon as the last cylinder is submerged.
Wait 30 minutes. Leave the setup undisturbed at room temperature. Do not move or shake the beakers.
Remove and re-measure. After 30 minutes, remove each cylinder in the same order it was placed in. Blot the surface gently with a paper towel using the same pressure each time. Record the final mass and final length of each cylinder.
Calculate percentage change. For each cylinder, use the formula:
Repeat. If time allows, repeat the experiment with fresh cylinders and fresh solutions to obtain a second set of results. Average the percentage change values for each concentration.
4 | Raw data table template
Copy this layout into your practical report. Column headings must include the quantity name and the unit separated by a forward slash.
Concentration / mol dm−3
Initial mass / g
Final mass / g
Change in mass / g
% change in mass
0.0
0.2
0.4
0.6
0.8
1.0
If you also measure length, add columns for initial length, final length, change in length, and percentage change in length. The same percentage-change formula applies.
5 | Graph plotting
Axes
x-axis: Concentration of sucrose solution / mol dm−3.
y-axis: Percentage change in mass / %.
What you should see
At low concentrations (0.0 and 0.2 mol/dm3), the solution is hypotonic relative to the potato cell sap. Water enters the cells by osmosis, so percentage change is positive. At high concentrations (0.8 and 1.0 mol/dm3), the solution is hypertonic. Water leaves the cells, so percentage change is negative. Between these extremes, the curve crosses the x-axis. This crossing point is the isotonic point -- the concentration at which the sucrose solution has the same water potential as the potato cell sap.
Drawing the line
Draw a smooth best-fit curve (not dot-to-dot) through the plotted points. Do not force the line through any single point. If one point sits far from the trend, circle it and note it as an anomaly.
Reading off the isotonic point
Draw a horizontal dashed line from the y-axis at 0% across to the curve. Drop a vertical dashed line from the intersection down to the x-axis. The concentration you read off is the estimated solute concentration of the potato cell sap.
From the table above, the percentage change crosses zero between 0.2 and 0.6 mol/dm3. On a plotted graph, the smooth curve passes through 0% at approximately 0.4 mol/dm3. This means the water potential of the potato cell sap is equivalent to a 0.4 mol/dm3 sucrose solution.
In practice, the crossing point rarely falls exactly on a measured concentration. You would read the value from the graph to one decimal place (for example 0.38 mol/dm3) and state it as your best estimate.
7 | Why percentage change, not absolute change?
Different potato cylinders will never have exactly the same starting mass, even if you cut them to the same length and diameter. A cylinder that starts at 3.50 g and gains 0.28 g has undergone a different proportional change than a cylinder that starts at 2.80 g and gains the same 0.28 g.
3.500.28×100=8.0
Using percentage change normalises the data so that every cylinder is compared on the same scale. This makes the comparison fair and allows you to combine or compare results from different groups.
Examiners specifically penalise candidates who plot absolute change instead of percentage change, because absolute values do not account for the different starting conditions.
8 | Sources of error and improvements
Source of error
Effect on results
Improvement
Not blotting each cylinder consistently
Some cylinders retain more surface moisture than others, giving an artificially high final mass.
Use the same number of gentle blots with a clean paper towel for every cylinder.
Potato cylinders with different surface areas
Cylinders of unequal diameter or rough-cut ends have different rates of osmosis.
Use a single cork borer for uniform diameter, and trim both ends flat with a knife.
Temperature variation during the experiment
Higher temperature increases the rate of osmosis, so cylinders in warmer parts of the room exchange more water.
Conduct the experiment in a thermostatically controlled water bath, or place all beakers together in the same location.
Time not controlled precisely
Removing cylinders at different times means some are immersed longer than others.
Use a stopwatch and remove all cylinders at exactly 30 minutes, in the same order they were placed in.
Using potatoes from different batches
Different potato varieties or storage conditions lead to different starting water potentials.
Use cylinders cut from the same single potato.
Only one replicate per concentration
A single result could be anomalous and gives no indication of reliability.
Run at least three replicates per concentration and calculate a mean percentage change.
9 | Common exam mistakes
Mistake 1 -- Confusing osmosis with diffusion
Osmosis specifically involves water molecules moving through a partially permeable membrane. Diffusion is the net movement of any molecules (or ions) from a region of higher concentration to a region of lower concentration. In a free-response answer, always state "partially permeable membrane" when discussing osmosis.
The definition of osmosis must include this term. Answers that say "water moves from dilute to concentrated" without mentioning the membrane will not earn full marks.
Mistake 3 -- Plotting absolute change instead of percentage change
As explained in Section 7, absolute mass change does not account for differences in starting mass. Always calculate and plot percentage change.
Mistake 4 -- Not identifying the isotonic point
Many candidates plot a correct graph but fail to mark the isotonic point. Draw dashed construction lines to show where the curve crosses the x-axis and state the concentration value explicitly.
Mistake 5 -- Wrong axis labels
The x-axis should show concentration (with the unit mol dm−3), and the y-axis should show percentage change in mass (with the unit %). Missing units or swapped axes lose marks.
Mistake 6 -- Writing "semi-permeable" instead of "partially permeable"
The Cambridge/SEAB syllabus uses "partially permeable membrane." While "semi-permeable" is scientifically acceptable, using the syllabus wording avoids any risk of lost marks.
10 | Frequently asked questions
Why do we use a cork borer and not a knife?
A cork borer produces cylinders with a uniform and repeatable cross-sectional area. Cutting by hand with a knife produces irregular shapes, so the surface area exposed to the solution varies between cylinders. Controlling surface area is essential for a fair test.
How long should the cylinders stay in the solution?
Thirty minutes is the standard duration used in most O-Level practical sessions. This is long enough for a measurable change in mass without reaching full equilibrium. If you are designing a planning question, you could justify a longer time (for example 60 minutes) to allow the system to approach equilibrium more closely.
Can I use length change instead of mass change?
Yes. Length change also reflects the gain or loss of water in the potato cells. However, mass change is generally preferred because an electronic balance (to 0.01 g) gives more precise readings than a ruler (to 1 mm). If you measure both, you can present either or both in your report.
What if my graph does not cross the x-axis?
If all your percentage changes are positive, your highest concentration may still be hypotonic relative to the potato cell sap. You would need to extend the range with higher concentrations (for example 1.2 or 1.5 mol/dm3). If all values are negative, try including lower concentrations.
Why does the percentage change decrease at higher concentrations instead of staying constant?
As concentration increases, the water potential of the solution falls further below that of the cell sap. This steeper gradient drives more water out of the cells, so the mass loss continues to increase. The trend is not linear because at very high concentrations, most of the available water has already left the cells, so the rate of additional loss slows.
What is the expected isotonic point for a potato?
For a typical potato at room temperature, the isotonic point is usually between 0.3 and 0.5 mol/dm3 sucrose. The exact value depends on the potato variety, how long it has been stored, and the ambient conditions. There is no single correct answer -- examiners assess whether you can read the value accurately from your graph.
