All subjects · Biology · WBI15 (Unit 5)

Core practical 16: respirometers and RQ

Core practical 16 uses a simple respirometer to measure oxygen uptake by small organisms and to calculate their respiratory quotient.

How the respirometer works

Small organisms (such as germinating seeds or woodlice) are placed on gauze in a sealed tube above potassium hydroxide solution or soda lime, which absorbs CO2.

As the organisms take in O2, and the CO2 they release is absorbed, the volume of gas in the tube falls. Coloured fluid in an attached capillary tube (manometer) moves towards the organisms.

Volume of O2 used = πr²l (r = radius of the capillary bore, l = distance moved)
Rate = volume / time, often divided by mass for comparison.

A control tube with glass beads of the same volume compensates for changes in temperature and pressure. Keep the apparatus in a water bath at constant temperature and allow it to equilibrate before sealing.

Measuring RQ

1. Run the respirometer with CO2 absorber: the fluid moves a distance x, proportional to O2 used.

2. Run it again without absorber (water instead): the fluid moves y, proportional to O2 used minus CO2 produced.

3. CO2 produced = x - y

RQ = (x - y) / x

If the fluid does not move without absorber (y = 0), CO2 produced equals O2 used and RQ = 1.

Safety and ethics

Potassium hydroxide and soda lime are corrosive: wear eye protection and keep them away from the organisms with the gauze. Handle invertebrates gently, keep the time in the apparatus short and return them to their habitat.

Worked example

With KOH the fluid moves 25 mm in 10 minutes; without KOH it moves 5 mm in 10 minutes. Calculate the RQ.

O2 used is proportional to 25.
CO2 produced is proportional to 25 - 5 = 20.
RQ = 20 / 25 = 0.8

Answer: RQ = 0.8

Key idea: KOH or soda lime absorbs CO2, so fluid movement shows O2 uptake: V = πr²l. Without absorber, movement = O2 used - CO2 produced. RQ = (x - y) / x. Use a control tube and constant temperature.

Check yourself

Fluid in a capillary of radius 0.5 mm moves 30 mm. Calculate the volume of oxygen used in mm³ to 3 significant figures.
Show answer
23.6 mm³. π x 0.25 x 30 = 23.6 mm³.
Germinating seeds of mass 2.0 g use 36 mm³ of oxygen in 6 minutes. Calculate the rate of oxygen uptake in mm³ g-1 min-1.
Show answer
3 mm³ g-1 min-1. 36 / 6 = 6 mm³ per min; 6 / 2.0 = 3.
With soda lime the fluid moves 40 mm; without it the fluid moves 12 mm in the same time and direction. Calculate the RQ.
Show answer
0.7. CO2 = 40 - 12 = 28; RQ = 28 / 40 = 0.7.
What is the role of potassium hydroxide in the respirometer?
- To absorb carbon dioxide
- To supply oxygen
- To kill microorganisms
- To act as a respiratory substrate
Show answer
To absorb carbon dioxide. Removing CO2 means movement shows only O2 uptake.
With CO2 absorber present, why does the fluid move towards the organisms?
- Carbon dioxide is produced
- The temperature rises
- Oxygen is used and the CO2 released is absorbed, so gas volume falls
- Water is produced
Show answer
Oxygen is used and the CO2 released is absorbed, so gas volume falls. Lower gas volume and pressure pull the fluid in.
Why is a control tube with glass beads used?
- To compensate for changes in temperature and atmospheric pressure
- To provide extra oxygen
- To absorb more CO2
- To measure the mass of the organisms
Show answer
To compensate for changes in temperature and atmospheric pressure. Any movement in the control is due to physical changes, not respiration.
Without CO2 absorber the fluid does not move at all. What is the RQ?
- 0
- 0.7
- 1.0
- Greater than 2
Show answer
1.0. CO2 produced equals O2 used.

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Last updated 7 November 2026. Spotted a mistake? Tell us.