Single-celled culture comparisons
If temperature or substrate is varied, use comparable amounts of active culture and consistent timing. Carbon dioxide measurements can reflect different metabolic conditions; oxygen availability and substrate use matter when interpreting the result.
Respirometer reasoning
In a typical oxygen-consumption respirometer, carbon dioxide is absorbed so it does not replace the oxygen removed. A change in gas volume can then move fluid in a capillary. Temperature and pressure changes can also cause movement, so use an appropriate matched control and allow equilibration.
Convert movement to volume
For a cylindrical capillary, volume change = πr² × distance. Use the internal radius and consistent units. A length per minute is not a gas-volume rate until you have accounted for the cross-sectional area. Apply any stated control correction before attributing the movement to the organisms.
Compare like with like
Divide volume change by time. To compare samples of different mass, also divide by mass, while recognising that tissue type and physiological state may differ. Keeping masses equal alone does not guarantee otherwise identical samples.
Use RQ under its assumptions
RQ is CO₂ produced divided by O₂ consumed over a comparable interval. Different fuel mixtures and metabolic conditions can affect it. A gas-volume ratio is not sufficient by itself to prove that one named substrate was the only respiratory fuel.
Worked data example
A corrected measurement shows that 4 g of organisms consumes 2.4 cm³ of oxygen in 3 min. Find the oxygen-consumption rate per gram.
Whole-sample rate = 2.4/3 = 0.8 cm³ min⁻¹. Per-gram rate = 0.8/4 = 0.20 cm³ g⁻¹ min⁻¹.
Why use a matched apparatus control?
It helps identify or correct movement caused by non-biological factors such as temperature or pressure changes rather than respiration.