We use cookies to give you the best experience possible. By continuing to use the site, you indicate that you accept cookies. See our cookies policy here.
Better Equipped Practical Teaching Guides
A complete GCSE Biology required practical guide for investigating osmosis using equal-sized potato cylinders in sucrose solutions of different concentrations, with apparatus, method, percentage-change calculations, graph skills, troubleshooting, risk assessment, technician notes and exam support.
Teacher note: This resource supports practical teaching and equipment planning. Teachers should adapt the procedure and risk assessment to match their exam board specification, local laboratory rules and departmental policies.
In this investigation, students place equal-sized potato cylinders in sucrose solutions of different concentrations and measure the percentage change in mass. The results show the net movement of water into or out of potato tissue by osmosis and can be used to estimate the isotonic concentration.
Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher water potential to a region of lower water potential.
The surrounding solution has a higher water potential than the potato cells. Water enters the cells, the tissue becomes more turgid and the cylinder gains mass.
Water moves in both directions at equal rates. There is no net movement of water and little or no overall change in mass.
The surrounding solution has a lower water potential than the potato cells. Water leaves the cells and the cylinder loses mass.
Key idea: at the isotonic point, the sucrose solution and potato tissue have approximately equal water potentials.
Safety goggles and access to hand washing facilities.
Independent variable: concentration of sucrose solution (% w/v).
Dependent variable: percentage change in mass of each potato cylinder after the fixed immersion time.
Control variables: potato source, cylinder diameter and length, removal of skin, solution volume, immersion time, temperature, blotting method, balance used and time between blotting and weighing.
Reliability: use at least three independently treated cylinders at each concentration. Calculate the percentage change for each cylinder first, then calculate the mean percentage change.
| Sucrose concentration (% w/v) | Repeat | Initial mass (g) | Final mass (g) | Change in mass (g) | Percentage change (%) |
|---|---|---|---|---|---|
| 0 | 1 | ||||
| 0 | 2 | ||||
| 0 | 3 | ||||
| 5, 10, 15 and 20 | Repeat rows as required |
Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100
Initial mass = 2.00 g
Final mass = 2.40 g
Change = +0.40 g
Percentage change = (+0.40 ÷ 2.00) × 100 = +20%
Calculate percentage change separately for every cylinder. Identify any anomalous result, then calculate the mean percentage change for each concentration. Do not average initial and final masses before calculating percentage change.
| Solution concentration | Expected outcome |
|---|---|
| Very dilute | Potato gains mass |
| Moderately dilute | Small mass gain |
| Isotonic concentration | Approximately no mass change |
| Concentrated | Mass decreases |
| Very concentrated | Large mass decrease |
X-axis: concentration of sucrose solution (% w/v)
Y-axis: mean percentage change in mass (%)
Plot the points as a scatter graph. Do not simply join each point to the next. Draw a best-fit line or smooth curve, depending on the pattern of the data.
Finding the isotonic concentration: read the concentration where the best-fit line or smooth curve crosses 0% change in mass. This is an estimate of the concentration at which the solution and potato tissue have approximately equal water potentials.
| Problem | Possible cause | Solution |
|---|---|---|
| Unexpected mass changes | Surface solution not removed consistently | Use a standardised blotting method before every weighing. |
| Little change observed | Concentrations too similar or immersion time too short | Check solution preparation, timing and measurements; repeat if possible. |
| Large variation between repeats | Cylinders not identical | Use one cork borer and cut every cylinder to the same measured length. |
| Inconsistent results | Different immersion times | Control when each cylinder is added and removed. |
| Anomalous result | Preparation or measurement error | Check the method and repeat that concentration. |
“Osmosis is movement of sugar.”
Osmosis describes the net movement of water molecules.
“Water moves from concentrated to dilute solutions.”
Water moves from higher water potential to lower water potential.
“Osmosis happens without a membrane.”
A partially permeable membrane is essential.
“No mass change means water has stopped moving.”
Water still moves both ways, but at equal rates.
Grade 8–9 improvement: use the preliminary graph to locate the approximate isotonic region, then repeat the investigation with smaller concentration intervals, such as 1%, around that crossing point.
Introduction and theory: 10 mins | Cylinder preparation: 15 mins | Initial measurements: 10 mins | Immersion: 40 mins | Final measurements: 10 mins | Calculations: 10 mins | Graphing and analysis: 15 mins | Evaluation: 10 mins
Overall risk: low. Standard laboratory procedures should be followed. Schools must complete their own risk assessment according to local policies.
| Hazard | Risk | Control measure |
|---|---|---|
| Scalpel or knife | Cuts during cylinder preparation | Use a cutting tile, keep the supporting hand away from the direction of force and cut under supervision. |
| Cork borer | Injury from pushing forcefully | Use a controlled twisting action and remove the core with a suitable pusher, not fingers. |
| Glassware | Cuts from breakage | Handle carefully and report breakages immediately. |
| Spillages | Slips or sticky work surfaces | Clean spillages promptly and dry the surface. |
| Biological material | Minor contamination risk | Do not eat practical material and wash hands after the activity. |
| Skill assessed | Typical evidence |
|---|---|
| Calculating percentage change | Show substitution, working, sign and % unit. |
| Graph interpretation | Describe the trend and estimate the isotonic concentration. |
| Variables | State the independent, dependent and control variables. |
| Evaluation | Link an error to its consequence and a justified improvement. |
Percentage change accounts for small differences in starting mass and makes the cylinders easier to compare fairly.
Solution remaining on the surface would artificially increase the recorded final mass.
Different potatoes, varieties and individual tissues can contain different concentrations of dissolved solutes and therefore have different initial water potentials.
No. Water molecules continue to move in both directions, but the rates are equal, so there is no net movement.
Explore more practical guides, equipment and laboratory planning resources from Better Equipped.
This guide was written and reviewed by Better Equipped's technical team, drawing on experience supplying practical science equipment to schools, colleges and laboratories throughout the UK. If you have feedback on this guide, please contact us.
Last reviewed and updated: July 2026
Browse balances, test tubes, measuring cylinders, timers, rulers, forceps and general GCSE Biology practical equipment.
Call us: 
Email us: 
