What should be varied?
- Vary external sucrose concentration between comparable tissue treatments.
- Use progressively thinner tissue pieces as sucrose concentration increases.
- Use tissue from a different plant source for each sucrose concentration.
AQA RP3 context · practical reasoning
Osmosis: compare tissue and solution
Investigate the effect of external sucrose concentration on the mass change of comparable plant-tissue pieces.
Each piece starts at 2.00 g. All pieces spend the same stated interval in solution. The example uses separate pieces from comparable tissue; it does not represent independent plants. Final masses are measured with a consistent blotting method.
Questions and answers · Matched example · 3 readings per condition.
Plan the comparison
Which calculation gives percentage mass change?
- (Final mass − initial mass) ÷ final mass × 100.
- Final mass ÷ initial mass × 100.
- (Final mass − initial mass) ÷ initial mass × 100.
Which control approach is useful?
- Weigh each piece straight from solution without removing surface liquid.
- Use comparable tissue dimensions, exposure time and consistent blotting.
- Leave the larger pieces immersed for longer to compensate for their size.
Calculate from evidence
Pieces are measured after consistent blotting.
| Sucrose concentration / mol dm⁻³ | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 0 | 2.23 | 2.24 | 2.25 |
| 0.2 | 2.13 | 2.14 | 2.15 |
| 0.4 | 2.03 | 2.04 | 2.05 |
| 0.6 | 1.93 | 1.94 | 1.95 |
| 0.8 | 1.83 | 1.84 | 1.85 |
Percentage change = (mean final mass − initial mass) ÷ initial mass × 100. Estimate the zero crossing by local linear interpolation between the adjacent opposite signs. Give numerical answers to about three significant figures.
Calculate percentage mass change at 0.40 mol dm⁻³, using the mean final mass.
Estimate the zero-mass-change concentration by local linear interpolation, in mol dm⁻³.
Evaluate the comparison
Explain how the different blotting assumptions change the apparent zero-mass-change estimate. Suggest an improvement and explain why even a well-measured zero crossing does not establish identical solute composition inside and outside the tissue.
Use the alternate example below alongside the data above.
| Sucrose concentration / mol dm⁻³ | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 0 | 2.23 | 2.24 | 2.25 |
| 0.2 | 2.16 | 2.17 | 2.18 |
| 0.4 | 2.09 | 2.1 | 2.11 |
| 0.6 | 2.02 | 2.03 | 2.04 |
| 0.8 | 1.95 | 1.96 | 1.97 |
Answers and reasoning
What should be varied?
Vary external sucrose concentration between comparable tissue treatments. The intended comparison changes external solution concentration.
Which calculation gives percentage mass change?
(Final mass − initial mass) ÷ initial mass × 100. Subtract initial mass to find the change, then use initial mass as the reference. A loss gives a negative result.
Which control approach is useful?
Use comparable tissue dimensions, exposure time and consistent blotting. These reduce differences unrelated to external concentration.
Calculate percentage mass change at 0.40 mol dm⁻³, using the mean final mass.
Mean = (2.03 + 2.04 + 2.05) ÷ 3 = 2.04. Percentage change = (2.04 − 2) ÷ 2 × 100 = 2%.
Estimate the zero-mass-change concentration by local linear interpolation, in mol dm⁻³.
The adjacent bracket is 0.4 mol dm⁻³ (2%) to 0.6 mol dm⁻³ (-3%). Local interpolation = 0.4 − (2 × 0.2 ÷ -5) = 0.48 mol dm⁻³. This estimate does not establish identical solute composition.
Evaluation example
Surface fluid raises the measured final mass in the biased example, moving its apparent zero crossing towards a higher external concentration. Consistent blotting, tissue dimensions and exposure time make the comparison more defensible. A local zero crossing estimates a condition with no net mass change under the method used; it does not show that the tissue and solution contain identical solutes. Separate pieces can reveal variation, but pieces from one tissue source do not represent every plant.
- I link retained surface fluid to a shifted apparent zero crossing.
- I propose consistent blotting with comparable tissue and exposure conditions.
- I limit the inference from zero mass change and from pieces sharing one tissue source.
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.