What should be varied?
- Select a different colorimeter wavelength for each treatment temperature.
- Increase the mass of tissue as treatment temperature increases.
- Vary treatment temperature between otherwise comparable tissue samples.
AQA RP4 context · practical reasoning
Membranes: evaluate a pigment-leakage comparison
Investigate how treatment temperature relates to pigment released from comparable coloured plant tissue over a fixed interval.
Absorbance is measured after 10 min with the same instrument settings and suitable blanking. Readings are dimensionless. Separate tissue pieces are used; the colour reading is a leakage indicator, not a directly measured membrane permeability coefficient.
Questions and answers · Matched example · 3 readings per condition.
Plan the comparison
Which recording method supports the stated temperature comparison?
- Record how many tissue pieces show a visible colour change.
- Record solution absorbance after the same 10 min treatment.
- Record solution colour by eye under different lighting at each temperature.
Which control approach is useful?
- Standardise tissue amount, cut surface, preparation and treatment duration.
- End each treatment when the solution reaches the same visible colour.
- Rinse the pieces for cooler treatments, but leave pieces for hotter treatments unrinsed.
Calculate from evidence
Comparable tissue amount and preparation are used.
| Temperature / °C | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 20 | 0.05 | 0.06 | 0.07 |
| 30 | 0.09 | 0.1 | 0.11 |
| 40 | 0.17 | 0.18 | 0.19 |
| 50 | 0.39 | 0.4 | 0.41 |
| 60 | 0.69 | 0.7 | 0.71 |
Mean absorbance = sum of readings ÷ number of readings. A difference subtracts the lower-temperature mean from the higher-temperature mean. Give numerical answers to about three significant figures.
Calculate the mean absorbance at 40 °C.
Calculate mean absorbance at 50 °C minus mean absorbance at 20 °C.
Evaluate the comparison
Explain why a larger absorbance at a higher temperature does not by itself identify a particular molecular membrane change. Evaluate the tissue-amount confound and propose a stronger comparison.
Use the alternate example below alongside the data above.
| Temperature / °C | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 20 | 0.05 | 0.06 | 0.07 |
| 30 | 0.14 | 0.15 | 0.16 |
| 40 | 0.29 | 0.3 | 0.31 |
| 50 | 0.69 | 0.7 | 0.71 |
| 60 | 0.99 | 1 | 1.01 |
Answers and reasoning
What should be varied?
Vary treatment temperature between otherwise comparable tissue samples. Temperature is the intended independent variable.
Which recording method supports the stated temperature comparison?
Record solution absorbance after the same 10 min treatment. Comparable instrument conditions and duration support a pigment comparison.
Which control approach is useful?
Standardise tissue amount, cut surface, preparation and treatment duration. These factors affect leakage independently of the intended temperature comparison.
Calculate the mean absorbance at 40 °C.
Mean = (0.17 + 0.18 + 0.19) ÷ 3 = 0.18. Mean absorbance = 0.18; absorbance is dimensionless.
Calculate mean absorbance at 50 °C minus mean absorbance at 20 °C.
Absorbance difference = 0.4 − 0.06 = 0.34. This is not a permeability coefficient.
Evaluation example
A larger absorbance is consistent with more pigment in the sampled liquid under suitable measurement conditions. It is not a direct permeability coefficient and does not distinguish lipid changes from particular protein changes. Increasing tissue amount with temperature provides another reason for higher readings. Standardise tissue amount, cut surface, preparation, duration and instrument conditions; use separate pieces to assess variation. More repeats do not remove a systematic tissue-amount confound.
- I interpret absorbance as a pigment signal and limit the molecular claim.
- I explain how unequal tissue amount confounds the temperature comparison.
- I propose standardisation and distinguish repeat variation from a systematic confound.
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.