What should be varied?
- Vary temperature between otherwise comparable respirometer treatments.
- Use progressively narrower capillary tubing at higher temperatures.
- Increase the mass of respiring material with each higher temperature.
Respiration reasoning; AQA RP9 connection, not its identical protocol · practical reasoning
Respiration: correct a displacement measurement
Investigate temperature and gas-displacement readings for comparable respiring material over five minutes.
The model apparatus has carbon dioxide absorbed, so a corrected displacement can indicate oxygen uptake under its stated conditions. A matched blank is supplied at each temperature. Displacement in mm is not oxygen volume without apparatus geometry, and is not ATP production. This is a data exercise, not a laboratory protocol.
Questions and answers · Matched example · 3 readings per condition.
Plan the comparison
Which processing method supports the stated temperature comparison?
- Multiply corrected displacement by capillary diameter to obtain oxygen volume.
- Use live displacement over five minutes without subtracting the matched blank.
- Subtract matched-blank displacement from live displacement over the same five minutes.
Which control approach is useful?
- Match temperature and apparatus, but use a different mass of live material in each condition.
- Use equal live mass, comparable apparatus and matched temperature blanks.
- Use the blank measured at the lowest temperature to correct every temperature condition.
Calculate from evidence
Live mass and apparatus geometry are comparable; matched blanks are supplied.
| Temperature / °C | Reading 1 | Reading 2 | Reading 3 | Matched blank / mm |
|---|---|---|---|---|
| 10 | 1.1 | 1.2 | 1.3 | 0.2 |
| 15 | 1.9 | 2 | 2.1 | 0.4 |
| 20 | 2.9 | 3 | 3.1 | 0.5 |
| 25 | 3.9 | 4 | 4.1 | 0.6 |
| 30 | 4.9 | 5 | 5.1 | 0.8 |
Corrected displacement = mean live displacement − matched blank. Corrected rate = corrected displacement ÷ 5 min. Give numerical answers to about three significant figures.
Calculate the blank-corrected mean displacement at 20 °C, in mm.
Calculate the corrected displacement rate at 20 °C, in mm min⁻¹.
Evaluate the comparison
Explain what the matched blank corrects and why it cannot remove the unequal-live-mass confound. State an improvement and the limit of reporting displacement as a measure of respiration.
Use the alternate example below alongside the data above.
| Temperature / °C | Reading 1 | Reading 2 | Reading 3 | Matched blank / mm |
|---|---|---|---|---|
| 10 | 1.1 | 1.2 | 1.3 | 0.2 |
| 15 | 2.4 | 2.5 | 2.6 | 0.4 |
| 20 | 4.4 | 4.5 | 4.6 | 0.5 |
| 25 | 6.9 | 7 | 7.1 | 0.6 |
| 30 | 9.9 | 10 | 10.1 | 0.8 |
Answers and reasoning
What should be varied?
Vary temperature between otherwise comparable respirometer treatments. Temperature is the intended independent variable.
Which processing method supports the stated temperature comparison?
Subtract matched-blank displacement from live displacement over the same five minutes. Subtracting the matched blank helps account for non-respiratory displacement. The corrected displacement can then be converted to gas volume using capillary cross-sectional area.
Which control approach is useful?
Use equal live mass, comparable apparatus and matched temperature blanks. This supports comparison while accounting for non-respiratory displacement.
Calculate the blank-corrected mean displacement at 20 °C, in mm.
Mean = (2.9 + 3 + 3.1) ÷ 3 = 3. Corrected displacement = 3 − 0.5 = 2.5 mm.
Calculate the corrected displacement rate at 20 °C, in mm min⁻¹.
Corrected displacement rate = 2.5 ÷ 5 = 0.5 mm min⁻¹. Apparatus geometry would be needed for a volume rate.
Evaluation example
The matched blank helps account for movement caused by the apparatus and environmental conditions rather than uptake by the live sample. Subtracting it does not standardise live mass: more respiring material can produce a larger total uptake. Use comparable live mass and apparatus at each temperature with matched blanks. Corrected mm per minute is a displacement rate; converting it to oxygen volume needs apparatus geometry, and it is not an ATP yield.
- I distinguish non-respiratory blank movement from live-sample uptake.
- I explain why unequal live mass remains a confound after blank correction.
- I propose comparable live mass and limit mm-rate, oxygen-volume and ATP claims.
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.