Investigating Gas Exchange

During aerobic respiration, organisms consume oxygen ($O_2$) and release an approximately equal volume of carbon dioxide ($CO_2$).

To measure $O_2$ consumption alone, a chemical absorbent such as potassium hydroxide (KOH) or soda lime must be placed in the experimental chamber to remove the $CO_2$ as it is produced.

Without an absorbent, the volume of $CO_2$ released would replace the volume of $O_2$ consumed, resulting in no net change in gas pressure and making measurement impossible.

The volume of oxygen consumed is calculated by measuring the distance the liquid drop moves within the capillary tube over a specific time interval.

The formula for the volume of a cylinder is applied: $V = \pi r^2 d$ where $r$ is the internal radius of the capillary tube and $d$ is the distance moved by the liquid.

To determine the rate of respiration, the calculated volume is divided by the time elapsed and the mass of the organism, typically expressed in units such as $cm^3 \, g^{-1} \, min^{-1}$.

Temperature control is critical because gas volume is highly sensitive to temperature changes (Charles's Law); experiments are often conducted in a thermostatically controlled water bath.

A control tube containing glass beads or boiled (dead) seeds of the same volume as the living specimen is used to account for fluctuations in atmospheric pressure or temperature during the experiment.

Three-way taps are integrated into the system to allow the internal pressure to equalize with the atmosphere before starting a trial and to reset the liquid indicator to its starting position.

Check the Units: Always ensure that the radius of the capillary tube and the distance moved are in the same units (e.g., mm) before calculating volume, and convert to $cm^3$ if required.

Identify the Absorbent: In exam questions, look for the presence of KOH or soda lime; if it is missing, the experiment is likely measuring the net change in gas volume, not just oxygen consumption.

Sanity Check: The liquid in a respirometer should always move towards the respiring organism because the consumption of gas reduces internal pressure.

Variable Isolation: When asked how to improve the experiment, suggest using a water bath for temperature stability or a syringe for precise resetting of the manometer fluid.