AQA GCSE Biology · Cell Biology
Osmosis required practical
Investigate the effect of different sugar or salt solutions on the mass of plant tissue.
Potato cylinders gain mass in dilute solutions and lose it in concentrated ones. Where the line crosses zero is the concentration that matches the inside of the cells.
Equipment
- Potato, cork borer and ruler
- Scalpel and white tile
- Sucrose solutions: 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³
- Boiling tubes and a test tube rack
- Top-pan balance (0.01 g)
- Paper towels and a timer
Variables
- Independent you change
- Concentration of the sucrose solution
- Dependent you measure
- Percentage change in mass of the potato cylinder
- Control you keep the same
- Same potato and cylinder size
- Same volume of solution
- Same time in the solution
- Same temperature
Method
- 1
Use a cork borer to cut potato cylinders, then trim them all to 3 cm with a scalpel.
- 2
Blot each cylinder dry and measure its mass on the balance. Record it.
- 3
Add 10 cm³ of each sucrose solution to its own labelled boiling tube.
- 4
Put one cylinder in each tube and leave them all for the same time (e.g. 24 hours).
- 5
Take the cylinders out, blot them gently dry and measure their mass again.
- 6
Work out the percentage change in mass for each, then repeat and calculate a mean.
Worked example
One cylinder per concentration, left for 24 hours.
| Concentration (mol/dm³) | Start mass (g) | End mass (g) | Change (%) |
|---|---|---|---|
| 0.0 | 2.50 | 2.80 | +12.0 |
| 0.2 | 2.46 | 2.61 | +6.1 |
| 0.4 | 2.52 | 2.50 | −0.8 |
| 0.6 | 2.48 | 2.29 | −7.7 |
| 0.8 | 2.51 | 2.18 | −13.1 |
| 1.0 | 2.47 | 2.07 | −16.2 |
Working
% change = (end mass − start mass) ÷ start mass × 100
0.0 mol/dm³: (2.80 − 2.50) ÷ 2.50 × 100 = +12.0%
0.8 mol/dm³: (2.18 − 2.51) ÷ 2.51 × 100 = −13.1%
Conclusion
Below about 0.38 mol/dm³ the solution is more dilute than the cell sap, so water moves into the potato by osmosis and its mass goes up. Above it, water moves out. The concentration inside the potato cells is about 0.38 mol/dm³.
Risks
- Scalpel and cork borer can cutCut downwards onto a white tile, away from your fingers.
- Spilt solutions make the floor slipperyWipe up spills straight away.
Common mistakes
- Using change in mass (g) instead of percentage change. The cylinders don’t start at exactly the same mass, so only % change is a fair comparison.
- Forgetting to blot the cylinders before weighing. Solution left on the outside adds mass that didn’t go into the cells.
- Saying “sugar moves into the potato”. It’s water that moves, by osmosis, across the partially permeable membranes.
Better results
- Do three cylinders at each concentration and work out a mean, ignoring anomalies.
- Add more concentrations close to where the line crosses zero, to pin the cell sap concentration down more precisely.
- Blot every cylinder the same way (e.g. roll once on a paper towel) so the drying is consistent.
Launching November 2026
7 exam questions on this practical.
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