Lesson focus: A controlled osmosis investigation uses change in mass or length to estimate tissue water balance while recognising measurement and biological variation.
Design, analyse and evaluate an osmosis investigation using plant tissue and percentage changeHow much something has changed compared with what it started as, written as a percentage..
Build on earlier learning
Osmosis and active transportThe previous lesson distinguished osmosis from diffusion and active transport. This lesson turns the model into practical evidence.
Planning an investigation?
Design an investigation with CORMMSSA step-by-step Logic Lab guide to planning a fair test, with a worked example and practice.
Practical
Osmosis in potato
- Cut equal potato cylinders and weigh each.
- Place them in sugar solutions of different concentrations for 30 minutes.
- Blot dry, reweigh and calculate the percentage change in mass.
Record: A graph of concentration against percentage change in mass.
Safety: Cut away from your hand on a tile.
You will be successful when you can…
- Identify the independent, dependent and control variables.
- Calculate percentage mass change.
- Estimate an isotonic concentration.
- Evaluate uncertainty and biological variation.
Words that carry the science
- independent variable
- The factor deliberately changed in an investigation.
- dependent variable
- The response that is measured.
- control variable
- A factor kept sufficiently constant for a fair comparison.
- percentage change
- Change divided by initial value, multiplied by 100%.
- isotonic
- Describes solutions giving no net water movement across the membrane under the stated conditions.
- replicate
- An independent repeat used to estimate variation and improve reliability.
A fair method isolates concentration
01A fair method isolates concentration
Cut plant-tissue cylinders to equal dimensions, record initial mass, place them in equal volumes of different sucrose concentrations for the same time and temperature, then blot them consistently before recording final mass. Concentration is the independent variableThe one thing you choose to change in an investigation, such as the strength of the sugar solution.; percentage mass change is the dependent variableThe thing you measure to see if it changes, such as how much mass a potato piece gains or loses..
Use several cylinders at each concentration from comparable tissue. Random allocation reduces bias from natural variation within the potato or plant organ.
02Percentage change allows fair comparison
Calculate percentage change = (final mass − initial mass) ÷ initial mass × 100. Positive values indicate net water gain; negative values indicate net water loss.
percentage changeHow much something has changed compared with what it started as, written as a percentage. is more comparable than raw change when starting masses differ. Keep signs: a negative percentage contains directional information.
03The zero crossing is an estimate, not proof
Plot mean percentage mass change against solution concentration. The concentration where a best-fit trend crosses zero estimates an isotonicDescribes two solutions with the same solute concentration, so there is no net movement of water between them by osmosis. condition for the tissue under those conditions.
Variation, limited concentration intervals, tissue damage, timing, temperature and inconsistent blotting create uncertainty. Zero mass change does not mean water molecules stopped moving; movement may be equal in both directions.
Osmosis in potato cylinders
Potato cylinders are left in sucrose solutions of different concentrations for 30 minutes, then blotted and weighed. The concentration where the mass does not change matches the potato cells.
Results
| Sucrose / mol/dm³ | Start / g | Now / g | Change / % |
|---|---|---|---|
| 0.0 | 1.90 | 1.90 | 0.0 |
| 0.2 | 2.04 | 2.04 | 0.0 |
| 0.4 | 1.94 | 1.94 | 0.0 |
| 0.6 | 2.06 | 2.06 | 0.0 |
| 0.8 | 2.03 | 2.03 | 0.0 |
| 1.0 | 1.98 | 1.98 | 0.0 |
Masses are illustrative. Percentage change lets you compare cylinders that started at different masses.
Calculate percentage mass change
A potato cylinder changes from 3.20 g to 2.88 g.
- Calculate change: 2.88 − 3.20 = −0.32 g.
- Divide by initial mass: −0.32 ÷ 3.20 = −0.10.
- Multiply by 100: −0.10 × 100 = −10%.
- Interpret the sign: the tissue lost water overall.
How to make the reasoning visible: The percentage mass change is −10%, consistent with the external solution having lower water concentration than the tissue.
Osmosis investigation
Analyse mean results from equal potato cylinders left for 45 minutes at constant temperature.
| Sucrose / mol dm⁻³ | Initial mass / g | Final mass / g | Percentage change |
|---|---|---|---|
| 0.00 | 3.00 | 3.36 | +12.0% |
| 0.20 | 3.10 | 3.29 | +6.1% |
| 0.40 | 3.05 | 3.06 | +0.3% |
| 0.60 | 3.00 | 2.85 | −5.0% |
| 0.80 | 3.20 | 2.88 | −10.0% |
Graph skills: SLAPUK and choosing a scaleOsmosis: plotting gains and losses (negative values).
Estimate the isotonic concentration.
Why calculate a mean from replicates?
Check your understanding
Answer from memory, then check against the suggested answer.
What is the percentage-change equation?
Why blot tissue before weighing?
What does zero mean percentage change suggest?
One cylinder in 0.40 mol dm⁻³ gains 8% while four others change by less than 1%. What should the student do?
Why might two potato varieties have different zero-crossing concentrations?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student investigates osmosis using one potato cylinder at each sugar concentration. The cylinders have different starting lengths, and the final mass is measured without blotting.
Evaluate the method. Identify three weaknesses and how to fix them.
(3)Describe how to process the results to find the concentration with no change in mass.
(3)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., identify each limitation, state its likely effect and give a matched improvement.