You cannot see respiration happening, so you measure something it changes — usually the oxygen being taken in. A respirometer does that by turning a tiny pressure drop into a drop of liquid you can watch move along a scale.
📚 What you need to know
Rate of respiration depends on how metabolically active the cells are, the organism’s size, and the supply of oxygen and substrate.
Temperature raises the rate up to the optimum of the enzymes involved, then the rate falls as they denature.
A respirometer measures the rate of oxygen consumption.
An alkali such as potassium hydroxide absorbs the carbon dioxide released, so the pressure drop is caused by oxygen uptake alone.
Lower pressure pulls the manometer fluid towards the organisms.
Volume of oxygen taken up = πr2h, where r is the radius of the capillary and h is the distance the fluid moved.
A control tube, a water bath and repeat readings are what make the results trustworthy.
What changes the rate
How active the cells are. Muscle tissue respires far faster than fat tissue because it needs more ATP.
Size of the organism. Small organisms have a large surface area to volume ratio, so they lose heat quickly and respire faster to replace it.
Oxygen supply. When oxygen is short, cells switch to anaerobic respiration and the ATP yield collapses.
Substrate supply. Less glucose available means a lower rate of respiration.
Temperature. The reactions are enzyme-controlled, so the rate rises to an optimum and then drops sharply as the enzymes denature.
pH. Carbon dioxide dissolving in tissue fluid lowers the pH, which can also start to denature the enzymes involved.
Every one of those bullets is really an enzyme point in disguise. Respiration is a pathway of enzyme-controlled steps, so anything that affects enzymes affects respiration.
The respirometer
The idea is simple. Seal some living organisms in a container with air. As they respire aerobically they take oxygen out of that air and put carbon dioxide back in. If you do nothing, the two roughly cancel out and the pressure barely changes.
So you add an alkali — potassium hydroxide or soda lime — which absorbs the carbon dioxide as fast as it is produced. Now gas is only ever being removed, the pressure inside falls, and the fluid in the capillary tube is pulled towards the organisms. How far it moves tells you how much oxygen was used.
The control tube is the part students forget. Without it, a change in room temperature or air pressure would look exactly like respiration.
What each part is for
Part
Job it does
Sealed tube
Stops air getting in or out, so any pressure change is caused by the organisms
Potassium hydroxide or soda lime
Absorbs carbon dioxide, so the pressure drop is due to oxygen uptake only
Gauze platform
Keeps the organisms away from the corrosive alkali
Capillary tube and scale
A narrow bore makes a small volume change show as a large, readable movement
Control tube with glass beads
Compensates for changes in room temperature or atmospheric pressure
Syringe and screw clip
Reset the fluid to the start position between trials
Thermostatic water bath
Holds temperature steady, since even small changes alter the air pressure
Use seeds, not animals. Germinating seeds respire fast and give excellent data, so there is no reason to use woodlice or maggots. Choosing the option that avoids using animals is a valid point in an ethics question.
🧩 Running a respirometer
Set both tubes up identically. Same alkali, same volume of air — the glass beads should match the volume of the seeds.
Put the whole apparatus in a water bath at the temperature you want, and leave it to equilibrate before taking any readings.
Open the screw clip to let the pressure settle, then close it to seal the system.
Note the starting position of the manometer fluid against the scale.
Time a fixed interval and record how far the fluid moved.
Reset with the syringe and repeat at least three times, then take a mean and discard any obvious anomaly.
Turning a distance into a volume
The manometer fluid sits inside a tube, so the volume of gas that disappeared is just the volume of the cylinder the fluid moved through.
Volume of oxygen taken up
volume = πr2h
A narrow tube is deliberate. Halving the radius quarters the area, so the same tiny volume of gas moves the fluid four times further and is much easier to read.
Two things trip people up in this calculation, every single time: the question gives the diameter and you need the radius, and the distance is in centimetres while the diameter is in millimetres. Sort both out before you touch the calculator.
Using technology instead
Oxygen sensors and carbon dioxide monitors measure gas concentration directly and in real time.
They avoid handling strong alkalis, which is safer.
Dataloggers record continuously over hours, so you get far more readings than you could take by hand.
Careful with wording: these measure respiration rate, which is not the same as breathing rate.
Worked examples
WORKED EXAMPLE
In a respirometer the fluid moved 24 mm in 12 minutes. The capillary tube has an internal diameter of 1.0 mm. Calculate the rate of oxygen uptake in mm3 min−1. Use π = 3.142.
Step 1: diameter to radiusr = 1.0 ÷ 2 = 0.5 mmStep 2: cross-sectional areaπr² = 3.142 × 0.5² = 0.7855 mm²Step 3: volume of oxygen used0.7855 × 24 = 18.85 mm³Step 4: divide by the time18.85 ÷ 12 = 1.571Rate = 1.57 mm³ min−¹square the radius, not the diameter — getting that wrong makes the answer four times too big
WORKED EXAMPLE
The seeds in the tube above had a total mass of 2.5 g. Calculate the rate of oxygen uptake per gram of seed, and explain why this figure is more useful for comparing samples.
Step 1: divide the rate by the mass1.571 ÷ 2.5 = 0.62840.63 mm³ min−¹ g−¹Step 2: say why it matters
A bigger sample of seeds would use more oxygen simply because there is more tissue.
dividing by mass removes that difference, so two samples can be compared fairly
WORKED EXAMPLE
Explain why a respirometer must contain an alkali, and why a second tube of glass beads is used. [4]
Step 1: what the organisms do to the air
They remove oxygen but release carbon dioxide, so the volume would barely change.
Step 2: what the alkali fixesalkali absorbs CO₂ → pressure drop is due to oxygen uptake aloneStep 3: what the beads are for
They give an identical tube with no respiration, matched for volume of air.
Step 4: why that helps
Room temperature or atmospheric pressure changes affect both tubes equally, so they cancel out.
The alkali isolates oxygen uptake; the control tube cancels out changes in the surroundingsfour marks means four separate points — two for each half of the question
💡 Exam tip
Halve the diameter first. Then square. Writing r = d ÷ 2 as your first line makes the error much less likely.
Convert everything to millimetres before you start, so the volume comes out in mm3.
If asked to improve reliability, the three safe answers are water bath, control tube and repeats with a mean.
Say the respirometer measures oxygen consumption, not “respiration” in general. Be specific about what is measured.
Rates per gram or per organism are easier to compare than raw rates — mention this if a question asks about fair comparison.
For a temperature question, use the enzyme reasoning: rate up to the optimum, then denaturation.
⚠ Common mix-up
Squaring the diameter instead of the radius. The most common calculation error on this whole topic.
Mixing centimetres and millimetres. 2.3 cm is 23 mm — convert before substituting.
Forgetting to divide by time. πr2h gives a volume; a rate needs the time as well.
Saying the alkali absorbs oxygen. It absorbs carbon dioxide.
Thinking the control tube measures respiration too. It contains no living material, on purpose.
Confusing respiration rate with breathing rate. One is chemistry inside cells; the other is air moving in and out.
Leaving out the water bath. Air expands when it warms, so an unheated respirometer drifts and the readings mean nothing.
That completes Cell Respiration. Up next: The Process of Photosynthesis — the reaction that puts the energy into glucose in the first place, and the one place carbon dioxide stops being waste.
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