Plan a respirometer investigation of RQ
Describe how a simple respirometer can be used to estimate RQ in germinating seeds or small invertebrates.
Section 12.1 · Energy
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What gas does an alkali absorb in a respirometer?
What you will learn
- Describe respirometer measurements for oxygen uptake and carbon dioxide release
- Plan fair, safe measurements for estimating RQ
Build the explanation
A simple respirometer measures gas-volume or pressure change around a respiring organism while temperature is controlled. In one setup, carbon dioxide is absorbed by an alkali kept physically separate from the organisms, so the remaining volume change estimates oxygen uptake. A paired setup without carbon dioxide absorption can help estimate total gas change and hence carbon dioxide output; RQ can then be calculated as CO₂ produced ÷ O₂ consumed. A suitable control, equal organism mass, fixed time and repeated trials improve the comparison.
Apply it carefully
Respirometer readings are affected by temperature, pressure, leaks and organism activity. Allow apparatus and organisms to equilibrate, keep the absorbing chemical from contacting living material, and use an appropriate control such as inert beads of similar volume. A simple setup estimates gas exchange under its conditions; it does not directly reveal every metabolic pathway or guarantee a pure substrate.
Pause and explain
Why should temperature be kept constant during an RQ investigation?
Worked example
Why is carbon dioxide absorbed in one respirometer chamber when measuring oxygen uptake?
Show the worked solution
- Consider what would happen to total gas volume if both gases remained.
- Removing CO₂ means the remaining gas change reflects O₂ use more directly.
Answer Absorbing CO₂ prevents its production from masking oxygen uptake in the measured volume change.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline how to use respirometer measurements to estimate oxygen uptake and carbon dioxide output for RQ.
Give me a hint
Explain the role of a carbon-dioxide-absorbing chamber and a comparison chamber.
Compare my reasoning
- Absorb CO₂ in one chamber so volume decrease indicates O₂ uptake.
- Compare with a suitable chamber that retains CO₂ to estimate CO₂ output, then calculate CO₂/O₂.
Measure gas-volume changes under controlled temperature. With CO₂ absorbed, the decrease estimates oxygen uptake. A paired measurement in which CO₂ is not absorbed can estimate its production from the difference; then calculate RQ = CO₂ produced ÷ O₂ consumed. The exact setup and corrections must be specified for the apparatus used.
Look for these in your work
- CO₂ absorption role stated
- O₂ uptake identified
- Comparison used for CO₂
- RQ formula applied
A trial uses 4 cm³ O₂ and estimates 2 cm³ CO₂ produced. Calculate RQ and name one control.
Give me a hint
Divide CO₂ by O₂ and control a variable that changes gas readings or metabolism.
Compare my reasoning
- RQ = 2 ÷ 4.
- RQ = 0.50; keep temperature constant and check that the apparatus is airtight.
RQ = 2/4 = 0.50. Temperature should be kept constant; the apparatus should also be checked for leaks and allowed to equilibrate before readings are taken.
Look for these in your work
- Correct order of ratio
- Correct value
- Valid control
A student puts an alkali pellet in the same open tube as germinating seeds. Explain the risk and improve the plan.
Give me a hint
Protect the organism and separate gas absorption from contact.
Compare my reasoning
- The alkali can damage living material.
- Keep the CO₂ absorbent physically separate behind a barrier and use supervised, appropriate apparatus.
The alkali may harm the seeds, so it should be held in a separate part of the apparatus where it absorbs gas without contacting them. Use a sealed, leak-checked setup, suitable control and teacher-supervised handling.
Look for these in your work
- Biological hazard recognised
- Physical separation proposed
- Leak/control supervision considered
Common mistakes
- Reversing the RQ ratio
- Letting alkali contact organisms
- Ignoring gas leaks or temperature changes
What can you explain now?
What gas does an alkali absorb in a respirometer?
Compare with the explanation
Carbon dioxide
The change in remaining gas volume can then indicate oxygen uptake.
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Make it stick
How can paired respirometer measurements distinguish oxygen uptake from carbon dioxide release?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.