All design choices and example data
What should be varied?
- Vary enzyme concentration alongside pH to keep the product readings similar. Two factors would change together. Similar product readings would not isolate the effect of pH.
- Vary pH between reactions with comparable starting concentrations. pH is the independent variable in the stated question. This is the suitable option for the stated question.
- Use longer collection periods at the pH values with smaller product volumes. Unequal periods make product volume depend on both pH and collection time.
Which recording method supports the stated pH comparison?
- Record product volume collected over the same 45 s interval. A fixed interval permits calculation of comparable mean product-volume rates. This is the suitable option for the stated question.
- Record the time to collect 1 cm³ at acidic pH but 2 cm³ at alkaline pH, and compare the raw times. The endpoint volumes differ, so raw times cannot isolate the effect of pH. A common fixed-volume endpoint could support a valid alternative rate method.
- Record product volume only when each reaction has finished, regardless of how long it takes. Final yield is not the amount produced in the specified 45 s. Similar yields can conceal different rates.
Which control approach is useful?
- Keep substrate concentration and time fixed, but allow reaction temperature to vary with pH. Temperature can affect enzyme activity, so it must also be comparable in this pH investigation.
- Add more enzyme at the pH values giving the lowest product volumes. Compensating with enzyme amount introduces a second cause of changes in rate.
- Keep temperature, starting concentrations and collection time comparable. These factors can affect product collection independently of pH. This is the suitable option for the stated question.
Matched example
Starting concentrations and temperature are comparable across pH conditions.
Calculation questions and worked method
Calculate the mean product-volume rate at pH 6, in cm³ min⁻¹.
Mean = (5.8 + 6 + 6.2) ÷ 3 = 6. 45 s = 0.75 min. Mean product-volume rate = 6 ÷ 0.75 = 8 cm³ min⁻¹.
Calculate the ratio of mean product volume at pH 7 to that at pH 4.
Volume ratio = 7.5 ÷ 1.5 = 5.
Enzyme concentration also changes
Higher pH conditions use more enzyme. The alternative values illustrate how that extra change could distort the pattern; they are not a quantitative prediction of concentration effects.
Calculation questions and worked method
Calculate the mean product-volume rate at pH 6, in cm³ min⁻¹.
Mean = (7 + 7.2 + 7.4) ÷ 3 = 7.2. 45 s = 0.75 min. Mean product-volume rate = 7.2 ÷ 0.75 = 9.6 cm³ min⁻¹.
Calculate the ratio of mean product volume at pH 7 to that at pH 4.
Volume ratio = 10.5 ÷ 1.5 = 7.
Evaluation prompt
Compare the matched example with the example in which enzyme concentration also changes. Explain why the second design cannot isolate a pH effect, give an improvement and explain what three assay readings can and cannot establish.
Example explanation and self-check points
The alternative data give larger volumes at the higher pH conditions, but both pH and enzyme concentration vary. The pattern therefore cannot be attributed to pH alone. Keep starting enzyme and substrate concentrations, temperature and collection time comparable while varying pH. Separate repeated assay runs show some variation and allow a mean; three results alone do not establish statistical significance or a universal optimum.
- I identify enzyme concentration as a second changing factor and connect it to the volume pattern.
- I propose comparable starting concentrations, temperature and duration for a pH comparison.
- I distinguish repeated measurements and a mean from proof of significance or a universal optimum.
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Original fixed datasets and reasoning tasks. Full requirements: AQA practical assessment · OCR A specification. These activities do not replace laboratory work or assessment of practical competence.
