Pearson International A Level Biology Core Practical 16: Determine respiration rate and respiratory quotient with a respirometer

This guide practises interpreting matched respirometer measurements. It is not a complete validated gas-exchange protocol or a claim to reproduce Pearson's prescribed procedure. Use current course documents and an approved apparatus and welfare assessment; state the calibration, sign convention and assumptions behind each inferred gas quantity.

Aim and scientific principle

Use matched respirometers to estimate oxygen uptake, carbon-dioxide production and respiratory quotient while controlling temperature and organism quantity.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use suitable respiring material, matched respirometers, capillary tubes or manometers, an alkali carbon-dioxide absorbent isolated from the organisms, a thermostatic bath, timer and calibration scale. Consider measuring range, resolution, uncertainty, calibration and suitability for the sample, not the instrument name alone. Draw a labelled setup that shows equal chamber volumes, the absorbent condition and exactly where capillary movement is read. Include an inert-volume control for non-biological pressure drift.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Check the apparatus for leaks, equilibrate temperature and establish a stable baseline.
2. Start the run consistently and record position, volume or trace measurements at fixed intervals.
3. Use a control without respiring material or with inactive material to identify non-biological drift.
4. Repeat, calculate a rate from an appropriate gradient and apply any calibration factor.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls