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 and answers · 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 |
Answers and reasoning
What should be varied?
Vary pH between reactions with comparable starting concentrations. pH is the independent variable in the stated question.
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.
Which control approach is useful?
Keep temperature, starting concentrations and collection time comparable. These factors can affect product collection independently of pH.
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.
Evaluation example
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.
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.