What should be varied?
- Vary enzyme concentration alongside pH to keep the product readings similar.
- Vary pH between reactions with comparable starting concentrations.
- Use longer collection periods at the pH values with smaller product volumes.
AQA RP1 context · practical reasoning
Enzymes: plan a pH comparison
Investigate how pH affects a supplied enzyme reaction, using the volume of product collected over 45 seconds.
An original assay example uses the same starting substrate and enzyme concentrations. Product volume is collected for 45 s in each separate reaction. Three readings represent separate assay runs, not a claim about three different organisms.
Questions only · Matched example · 3 readings per condition.
Plan the comparison
Which recording method supports the stated pH comparison?
- Record product volume collected over the same 45 s interval.
- Record the time to collect 1 cm³ at acidic pH but 2 cm³ at alkaline pH, and compare the raw times.
- Record product volume only when each reaction has finished, regardless of how long it takes.
Which control approach is useful?
- Keep substrate concentration and time fixed, but allow reaction temperature to vary with pH.
- Add more enzyme at the pH values giving the lowest product volumes.
- Keep temperature, starting concentrations and collection time comparable.
Calculate from evidence
Starting concentrations and temperature are comparable across pH conditions.
| pH | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 4 | 1.3 | 1.5 | 1.7 |
| 5 | 2.8 | 3 | 3.2 |
| 6 | 5.8 | 6 | 6.2 |
| 7 | 7.3 | 7.5 | 7.7 |
| 8 | 2.8 | 3 | 3.2 |
Mean volume = sum of readings ÷ number of readings. Rate = mean volume ÷ 0.75 min. Give numerical answers to about three significant figures.
Calculate the mean product-volume rate at pH 6, in cm³ min⁻¹.
Calculate the ratio of mean product volume at pH 7 to that at pH 4.
Evaluate the comparison
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.
Use the alternate example below alongside the data above.
| pH | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 4 | 1.3 | 1.5 | 1.7 |
| 5 | 3.4 | 3.6 | 3.8 |
| 6 | 7 | 7.2 | 7.4 |
| 7 | 10.3 | 10.5 | 10.7 |
| 8 | 8.8 | 9 | 9.2 |
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.