Measuring respiration rate

The big idea: Cell respiration releases energy from glucose. In aerobic respiration it uses up oxygen and releases carbon dioxide.

The respiration rate is simply how fast this happens — for example how much oxygen is used (or carbon dioxide is made) per minute.

Because we can't see respiration directly, we measure it indirectly by tracking one of the things that changes: the gases, the temperature, or the mass of the organism.

Respiration rate

How quickly an organism respires — usually measured as the volume of oxygen used, or carbon dioxide produced, in a given time (e.g. per minute).

Respirometer

A piece of apparatus that measures respiration rate by detecting the change in gas volume (usually the oxygen used up) as organisms respire.

Indicator (variable)

A measurable quantity that changes as respiration happens — oxygen used, carbon dioxide produced, temperature rise or mass lost — that lets us estimate the rate.

Control (dead organisms)

An identical set-up using non-respiring (dead or boiled) organisms, used to show that any change measured is caused by living respiration and not by something else.

Why we measure indirectly: You can't watch a cell respire. But every time it respires it takes in oxygen, gives out carbon dioxide, warms up slightly and loses a little mass (as carbon leaves in CO₂).

Each of these is something we can measure — so measuring any one of them tells us the respiration rate.

The classic way to measure respiration rate is a respirometer. A common school version uses germinating seeds (or small invertebrates) sealed in a tube, connected to a thin tube containing a drop of coloured liquid.

As the seeds respire, they use up oxygen from the sealed air. This lowers the gas volume, so the coloured liquid moves towards the seeds. How far it moves in a set time measures the oxygen used — the respiration rate.

The role of potassium hydroxide (KOH): There's a problem: respiration uses oxygen but also releases carbon dioxide. If both gases changed, the volume might barely move and you couldn't tell what was happening.

So a small amount of potassium hydroxide (KOH) is added. KOH absorbs all the carbon dioxide the seeds release.

Now the only gas change left is the oxygen being used up — so the coloured liquid moves in cleanly, and its movement measures exactly that.

Why you need a dead-seed control: The coloured liquid could also move if the temperature or air pressure in the room changed — nothing to do with respiration.

To rule this out, an identical tube is set up with dead (boiled) seeds, which do not respire.

If the liquid in the control tube stays still, you know any movement in the live tube is caused by respiration, not by the surroundings. This makes the experiment a fair test.

A memory hook: KOH = 'Kills Off the H' — think of it as removing the unwanted gas (CO₂) so the only thing the respirometer 'sees' is oxygen disappearing.

Liquid moves towards the seeds because oxygen is being used up (volume shrinks).

How this is tested: Respirometers are a Paper 1B / Paper 2 data favourite. A 1-mark question often shows a respirometer with germinating versus dead seeds and KOH and asks you to interpret what the moving coloured liquid demonstrates — that living seeds use up oxygen as they respire.

A Suggest question may ask how a graph shows an inhibitor (such as cyanide) slowing respiration — the liquid moves less, so less oxygen is used.

Another Suggest format asks you to propose another quantity that could measure respiration rate — carbon dioxide produced, temperature rise or mass lost are all valid.

✓ Why this scores full marks: It links each observation to a reason: oxygen used → liquid moves; KOH → only oxygen is measured; dead control → movement must be respiration.

A common slip is to say the liquid moves 'because of carbon dioxide' — but KOH removes the CO₂, so it is the oxygen being used that moves the liquid.