All design choices and example data
What should be varied?
- Vary temperature between otherwise comparable respirometer treatments. Temperature is the intended independent variable. This is the suitable option for the stated question.
- Use progressively narrower capillary tubing at higher temperatures. For a given gas-volume change, displacement depends on capillary cross-sectional area. Apparatus geometry must be comparable.
- Increase the mass of respiring material with each higher temperature. More material can take up more oxygen. Mass would be confounded with temperature.
Which processing method supports the stated temperature comparison?
- Multiply corrected displacement by capillary diameter to obtain oxygen volume. Volume is displacement multiplied by cross-sectional area. For circular tubing, calculate πr²; multiplying by diameter has the wrong dimensions.
- Use live displacement over five minutes without subtracting the matched blank. The blank helps account for displacement unrelated to respiration, including temperature or pressure effects.
- 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. This is the suitable option for the stated question.
Which control approach is useful?
- Match temperature and apparatus, but use a different mass of live material in each condition. Different masses provide different amounts of respiring material. Standardise mass for this comparison.
- Use equal live mass, comparable apparatus and matched temperature blanks. This supports comparison while accounting for non-respiratory displacement. This is the suitable option for the stated question.
- Use the blank measured at the lowest temperature to correct every temperature condition. Non-respiratory displacement may depend on temperature. Use the matched blank supplied for each condition.
Matched example
Live mass and apparatus geometry are comparable; matched blanks are supplied.
Calculation questions and worked method
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.
Live mass increases with temperature
The higher-temperature readings use more live material. Blank correction does not remove this live-mass confound.
Calculation questions and worked method
Calculate the blank-corrected mean displacement at 20 °C, in mm.
Mean = (4.4 + 4.5 + 4.6) ÷ 3 = 4.5. Corrected displacement = 4.5 − 0.5 = 4 mm.
Calculate the corrected displacement rate at 20 °C, in mm min⁻¹.
Corrected displacement rate = 4 ÷ 5 = 0.8 mm min⁻¹. Apparatus geometry would be needed for a volume rate.
Evaluation prompt
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.
Example explanation and self-check points
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.
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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.
