12.1.7—Respirometer investigations

A respirometer links a measurable gas-volume change to respiration by comparing an organism-containing tube with a suitable control. Oxygen uptake can be estimated over a defined time; with carbon dioxide absorbed, the remaining gas change is attributed to oxygen consumption.

1. Prepare and check the apparatus: Set up the experimental and control tubes, use an equal volume of inert material in the control where appropriate, check that the system is sealed, and bring the manometer or gas-volume indicator to a known starting position. Keep the same reading side and scale convention throughout.
2. Standardise the conditions: Use a measured mass and comparable biological material. Keep temperature, apparatus volume, measurement interval and other relevant conditions constant; a controlled water bath can reduce temperature variation.
3. Measure oxygen uptake: Run the matched tubes with a suitable carbon-dioxide absorbent present. Record the gas-volume change or manometer movement over the chosen time and convert it using the calibrated geometry of the apparatus. Call this oxygen-consumption estimate x when the setup supports that interpretation.
4. Measure the second gas condition: Reset the apparatus safely, remove the absorbent from both matched tubes, and repeat for the same time interval. Call the corresponding change y. The difference between the two readings estimates the carbon dioxide produced, so CO₂ output = x − y under these assumptions.
5. Calculate or compare: Use the matched oxygen and carbon-dioxide estimates to calculate RQ = CO₂ produced / O₂ consumed, or compare rates between substrates or conditions. Repeat independent runs and calculate a mean; interpret differences only when the controls and time basis are comparable.
6. Check the conclusion: Inspect for leaks, drift, inconsistent starting positions, unequal biological mass or temperature changes. A respirometer reading is an indirect estimate and does not by itself prove a substrate or mechanism; unusual results require checking the control, calibration and biological state.

Data cue: with the same time and scale basis, x = O₂ consumed when carbon dioxide is absorbed, and y = O₂ consumed − CO₂ produced without the absorbent. Therefore CO₂ produced = x − y, followed by RQ = (x − y) / x. Use this relationship only when the control and apparatus assumptions are satisfied.

The investigation teaches how a respirometer produces comparable gas data; it does not require invented apparatus dimensions, readings or dangerous procedures. Keep organism mass, temperature, time, sealing, absorbent/control treatment and repeats matched before attributing a change to respiration.