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 only · Matched example · 3 readings per condition.

Return to the workbench

Plan the comparison

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.

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.

Absorbance (dimensionless) after 10 min · 3 readings per condition · Matched example
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.

Absorbance (dimensionless) after 10 min · 3 readings per condition · Tissue amount increases with temperature
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

Original authored examples, not real experimental predictions, official marks or a laboratory protocol.