Lesson 5 of 5 / Respiration and ATP production
Measure respiration fairly
What does a moving respirometer liquid actually measure?
I can investigate temperature, oxygen and substrate effects on respiration.
I can investigate substrate, oxygen and temperature effects on respiration.Syllabus
Syllabus 9477, Core 3(k). Investigate substrate, oxygen and temperature effects on respiration.
About 7 min
Make a guess. You are not marked.
In a respirometer with germinating seeds, the liquid moves towards the seeds. Does that prove the seeds used oxygen?
Show the answer
Not alone; temperature and pressure need a control.
A matched tube of non-respiring material shows any non-biological movement to subtract.
The key idea
A valid respiration measurement separates biological gas exchange from temperature, pressure and sample-size effects.
Explore the idea
Separate oxygen uptake from physical gas changes
The living sample shows 1.00 cm3 apparent gas uptake. The marker alone cannot distinguish respiration from physical gas changes. Inspect the matched non-respiring control before calculating the biological rate.
These are calculation scenarios, not a model predicting how a real sample changes. Capillary volume change = displacement x cross-sectional area; correction must preserve movement direction. Use comparable displaced volume in the control, allow temperature equilibration, check seals and repeat measurements. The simplified apparatus omits connections used to equalise pressure before readings. Practise laboratory setup and reagent safety with supervision.
Explanation
A simple respirometer can measure oxygen uptake by respiring material. If carbon dioxide is absorbed by a suitable reagent, a fall in gas volume or pressure mainly reflects oxygen removal. A marker in a capillary then moves, allowing volume change to be estimated from distance and cross-sectional area.
Temperature and atmospheric pressure can also move gas or liquid. Use a matched control with non-respiring material of comparable displaced volume and allow equilibration in a water bath. Keep the apparatus sealed, compare equal living mass or normalise rate to mass, and repeat independent measurements.
Substrate supply and oxygen availability can limit respiration, while temperature changes enzyme activity and can eventually damage cells. Changing one factor while controlling others is essential. In intact organisms, uptake and transport of substrate can also matter, so a response is not automatically the direct effect on one enzyme.
For yeast under defined oxygen-limited conditions, CO2 or ethanol production can indicate fermentation rate. It is not directly comparable to oxygen uptake without specifying the pathway. Use suitable supervised laboratory materials and risk controls for caustic CO2 absorbents, hot water and pressure; a lesson model is not a substitute for laboratory training.
Step by step
- 1
Define the biological signal
Oxygen uptake or fermentation product formation?
- 2
Remove physical confounds
Control temperature, pressure and leaks.
- 3
Calculate a comparable rate
Use volume per time per mass when appropriate.
Worked example
Work through the evidence
After correction with a control, a 2 g seed sample takes up 0.8 cm3 O2 in 4 minutes. Calculate rate per gram.
One way to explain it
0.8/(4 x 2) = 0.10 cm3 O2 min-1 g-1.
Why this answer works
- Divide by time and living mass.
- Use the corrected gas change, not an uncorrected marker displacement.
Watch out for this
A student says: "Every gas-volume fall proves oxygen consumption by living cells." What is wrong with this?
Show the answer
Cooling, pressure changes and leaks can also affect gas volume; controls are needed.
Check your understanding
Why include a matched non-respiring control?
Show the answer
To estimate non-biological effects on gas movement
This helps correct for temperature or pressure changes.
Quick recall
What does a moving respirometer liquid actually measure?