5 original questions · Application and evidence

Respiration: apparatus, controls and interpretation

Use the context and data to draft a response, then compare your reasoning with an explanatory example.

Context and data

An illustrative respirometer comparison uses equal masses of germinating seeds at two temperatures. Carbon dioxide is absorbed in the apparatus. Matched control apparatus contains inert material of comparable displaced volume. Each displacement is measured over six minutes, with positive displacement defined as movement consistent with gas uptake. Use test minus control for the stated correction.

Six-minute displacements in matched apparatus
Temperature / °C	Seed apparatus displacement / mm	Control displacement / mm
15	2.4	0.3
25	4.8	0.6

Original illustrative data. Use the stated assumptions and units; these are not examination-board questions.

This worksheet includes questions, data, explanatory responses and self-check points.

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Five questions

1. Calculate the control-corrected displacement rate at each temperature.

2. Compare the corrected rates and explain why the correction matters to interpreting the apparatus readings.

3. Explain how oxygen consumption can produce the measured displacement when carbon dioxide is absorbed.

4. A student claims that the table proves ATP production exactly doubled at 25 °C. Evaluate this conclusion.

5. Suggest two changes that would strengthen the comparison and explain their purposes.

Explanatory responses and self-checks

These are original revision check points, not official examination marks.

1. Calculate

At 15 °C the corrected displacement is 2.1 mm, giving 2.1/6 = 0.35 mm min⁻¹. At 25 °C it is 4.2 mm, giving 0.70 mm min⁻¹. These are displacement rates; a calibrated capillary area or equivalent information would be needed to express gas volume per minute.

Subtract control: 2.4 − 0.3 = 2.1 mm and 4.8 − 0.6 = 4.2 mm.
Divide each corrected displacement by six minutes.
Give 0.35 and 0.70 mm min⁻¹, without relabelling them as gas volumes.

Common confusion: A length reading alone does not supply an oxygen volume.

2. Compare

The corrected rate at 25 °C is twice that at 15 °C: 0.70/0.35 = 2.0. Control displacement indicates that some movement occurs without respiring seeds, for example through physical changes in gas or apparatus conditions. The correction seeks to separate that contribution in the stated design.

Calculate 0.70/0.35 = 2.0.
State that the higher-temperature corrected displacement rate is twice the lower.
Explain that control movement can reflect non-biological apparatus or environmental changes.

Common confusion: A control correction depends on comparability; it does not guarantee that every non-biological effect is identical.

3. Explain

Seeds consume oxygen during aerobic respiration. The produced carbon dioxide is absorbed, so it does not replace the consumed gas in the measured gas phase. The net change can generate a pressure or volume difference that moves the indicator fluid, subject to the apparatus design and controls.

Relate aerobic respiration to removal of oxygen from the apparatus gas.
Explain that absorbed carbon dioxide does not fully replace the consumed gas.
Connect the net gas change with a pressure or volume difference moving the fluid.

Common confusion: The indicator movement is not caused by ATP travelling along the capillary.

4. Evaluate

The corrected displacement rate doubles, which can support increased oxygen uptake under comparable apparatus conditions. ATP was not measured. Substrate use, coupling and the relationship between displacement and gas uptake also matter, so the evidence does not establish that ATP production exactly doubled.

Distinguish corrected apparatus displacement from direct ATP measurement.
Recognise the connection with oxygen uptake under the method’s assumptions.
Explain that ATP yield, substrate use and apparatus limitations prevent an exact ATP conclusion.

Common confusion: A proxy measurement must not be presented as a direct measurement of every linked metabolic process.

5. Suggest

Use independent replicates at both temperatures to assess spread rather than relying on one comparison. Keep seed mass and physiological state comparable and allow apparatus to equilibrate under the supervised method. Calibration and leak checks help establish whether displacement reflects comparable gas changes.

Use independent repeats at each temperature and summarise their variation.
Maintain matched biomass, physiological state and temperature equilibration.
Calibrate or check apparatus comparability and leaks to support the displacement interpretation.

Common confusion: Repeats improve evidence about variability; they do not automatically eliminate a systematic apparatus error.