The respirometer and RQ values
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of respiration from GCSE and A-level. You can test your knowledge on these below.
What are the products of aerobic respiration?
Carbon dioxide, water, and ATP.
What is the role of oxygen in aerobic respiration?
It acts as the final electron acceptor in the electron transport chain.
What is the general equation for aerobic respiration?
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the respirometer and RQ values or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Measuring Respiration with a Respirometer
1. Using a Respirometer to Measure Respiration
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A respirometer measures the rate of oxygen uptake by an organism.
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A typical setup includes:
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A sealed chamber containing the organism (e.g., germinating seeds or woodlice).
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A syringe or fluid-filled manometer connected to a capillary tube.
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A chemical such as soda lime or potassium hydroxide to absorb CO₂. If carbon dioxide was not absorbed the net volume of gas produced or consumed would be 0.
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As oxygen is used in aerobic respiration, the pressure inside the chamber decreases, causing the manometer fluid to move towards the chamber with the respiring organism.
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A control chamber with glass beads the same mass as the organisms is also needed. This demonstrates that movement of the fluid is due to the respiring organisms
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The syringe is used to recalibrate the manometer, so repeated measurements can be taken, by adding more air into the chamber.
2. How the Manometer Works
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A manometer is used to detect changes in gas volume or pressure.
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The movement of the liquid in the manometer towards the respirometer chamber with the respiring organism indicates oxygen uptake.
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The volume of oxygen used is calculated by measuring the distance moved by the fluid and using the diameter of the capillary tube. Volume = π x r² x distance
3. Control Variables in the Experiment
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Temperature must be kept constant as it affects enzyme activity.
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The same species and developmental stage of the organism should be used.
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Volume and concentration of CO₂ absorber, e.g. potassium hydroxide, must be consistent.
4. Calculating the Rate of Respiration
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Rate = Volume of oxygen consumed / Time
- Units are typically cm³ min⁻¹ or mm³ min⁻¹.
Respiratory Quotient (RQ)
What is RQ?
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RQ = volume CO₂ produced ÷ volume O₂ consumed
How is it calculated?
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Measure both the volume of carbon dioxide produced and oxygen consumed (in separate experiments or using a dual-chambered respirometer).
RQ Values for Different Substrates
|
Substrate |
RQ Value |
Explanation |
|
Carbohydrates |
1.0 |
Equal volumes of CO₂ and O₂ are exchanged. |
|
Lipids |
~0.7 |
More oxygen is needed per CO₂ molecule produced due to longer hydrocarbon chains that need oxidising. |
|
Proteins |
~0.9 |
Intermediate between carbohydrates and lipids. |
Interpreting RQ > 1
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Indicates anaerobic respiration is occurring.
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More CO₂ is being produced than O₂ consumed, as anaerobic pathways (e.g., fermentation) do not require oxygen.
Key Terms
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Respirometer: Apparatus used to measure the rate of respiration.
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Manometer: A U-shaped tube filled with liquid, used to measure gas pressure or volume change.
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Respiratory Quotient (RQ): A ratio indicating the substrate used in respiration.
No answer provided.
Exam Tips
Always state that CO₂ is absorbed by soda lime / potassium hydroxide when using a respirometer to ensure pressure changes are only due to oxygen uptake.
The control tube is an example of a negative control and shows that any movement of the fluid in the manometer is due to the respiring organisms.
No answer provided.
A student used a respirometer to measure the rate of respiration in germinating seeds. They found that 1.2 cm³ of oxygen was consumed over 10 minutes. Calculate the rate of oxygen consumption and explain how the use of soda lime ensures accurate results. (3 marks)
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Rate = 1.2 cm³ / 10 min = 0.12 cm³ min⁻¹
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Soda lime absorbs carbon dioxide produced during respiration.
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This ensures any change in gas volume is due to oxygen uptake only.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!