GCSE osmosis potato practical

Explain the method, calculate signed percentage changes and make sense of the zero-crossing graph.

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Explain what moves

Osmosis is the net movement of water through a partially permeable membrane from a more dilute solution towards a more concentrated solution. In this experiment, it is water moving by osmosis—not sugar moving into the potato. The cell membrane is the partially permeable barrier.

Mass gain

Net water movement into the cells increases the mass of the tissue.

Mass loss

Net water movement out of the cells decreases its mass.

No net change

Water can still move both ways. No net movement means the opposing movements balance; it does not mean every water molecule has stopped.

Identify the variables

Independent variable

The concentration of the sugar solution. Use clearly labelled concentrations with units.

Dependent variable

The percentage change in potato mass, calculated from the initial and final masses.

Control variables

Use comparable skin-free cylinder lengths and diameters, the same immersion duration and a consistent blotting method. Keep the temperature and tissue source as consistent as possible so they do not become alternative explanations for the change.

Follow the method—and know why

Use your teacher’s approved method and risk assessment. Cork borers and scalpels can cut: use prepared cylinders or the supervised cutting procedure your teacher provides. Keep liquid away from the electrical balance. This is a school practical, not an instruction to use cutting equipment unsupervised at home.

Prepare and label
Use a range of sugar concentrations, including water as the zero-concentration comparison. Label containers and samples so every final mass can be matched to its initial mass and solution.
Measure the starting mass
Use comparable skin-free potato cylinders. Gently blot surface liquid consistently, then weigh each cylinder and record its initial mass in grams.
Immerse for the same duration
Fully immerse the cylinders in their assigned solutions. Follow the stated time in your school method and keep it equal across concentrations. There is no single universal soaking time: published example methods differ.
Blot and reweigh
Remove each cylinder, gently blot off adhering solution in the same way, then record its final mass. Do not count drops of solution as potato tissue.
Repeat and summarise
Use separate cylinders as repeats at every concentration. Calculate each cylinder’s percentage change, then a mean for each concentration. Reweighing the same cylinder several times checks the reading but does not provide independent tissue repeats.

Original illustrative results

These constructed teaching values show one cylinder per concentration. They are not real school results or means of repeats; an investigation should include repeats.

Illustrative sugar-solution concentrations and potato masses
Concentration (mol dm⁻³)	Initial mass (g)	Final mass (g)	Mass change (%)
0.00	4.00	4.48	+12
0.20	4.00	4.16	+4
0.40	4.00	3.84	−4
0.60	4.00	3.68	−8

Read the graph and its zero crossing

Choose the axes
Plot sugar concentration in mol dm⁻³ on the x-axis and percentage mass change on the y-axis. Include negative values below zero. With repeats, plot the mean percentage changes and examine their spread.
Describe the pattern
In the illustrative data, increasing concentration changes the response from a gain to a loss. Draw an appropriate best-fit line or curve; do not force the whole relationship into a straight line.
Estimate the crossing
The example changes from +4% at 0.20 mol dm⁻³ to −4% at 0.40 mol dm⁻³. Approximating this local part as straight puts zero halfway between them: 0.20 + 0.5 × (0.40 − 0.20) = 0.30 mol dm⁻³.
Interpret cautiously
About 0.30 mol dm⁻³ is the estimated solution concentration giving no net mass change for this illustration. It is not a directly measured point, an exact concentration for every potato, or evidence that water stops moving. Real tissue varies, and the duration and measurement uncertainty limit the estimate.
Refine the estimate
Test closer concentrations around the observed sign change, with repeats. If all your points show gains or all show losses, the crossing is not bracketed: extend the tested range rather than presenting a distant extrapolation as a measurement.

Evaluate an error, not just “human error”

Surface solution left on the potato

Extra liquid makes the final mass too high. A gain looks larger or a loss looks smaller. Use consistent gentle blotting; merely adding more repeats will not remove a bias repeated every time.

Different cylinder dimensions or soaking times

These weaken the comparison because concentration is no longer the only relevant difference. Standardise dimensions and duration rather than simply saying “be more careful”.

Variation between tissue samples

Separate repeats reveal scatter. Compare results before calculating means and investigate an unexpected value instead of deleting it just because it spoils the pattern.

Widely spaced concentrations

A broad gap between the last positive and first negative result makes the crossing uncertain. Add concentrations within that interval; extra decimal places on the graph do not create extra evidence.

Frequently asked questions

Straight answers to the questions people ask most often.

Is this the same practical in AQA Biology and Combined Science?

The investigation concerns solution concentration and plant-tissue mass in both courses, but its number differs: Biology 8461 practical 3 and Combined Science: Trilogy 8464 practical 2. Use the wording and method for your own course.

Does repeating the experiment fix poor blotting?

Not if every sample is weighed with surface solution still attached. Repeats help assess random variation; consistent blotting addresses the extra mass caused by adhering liquid.

Official guidance

Other sources

1.
Pearson Edexcel GCSE Biology Foundation revision guide sample: Osmosis in potatoes