Respiratory quotient and respiration practicals — Edexcel International A Level Biology
The respiratory quotient, and the two core practicals on respiration in yeast and with a simple respirometer.
Revision notes
3 short notes, in the order of the specification. Each one in short:
The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in a given time. Its value shows which substrate is being respired: about 1.0 for carbohydrate, about 0.7 for lipid and about 0.9 for protein. A value above 1.0 suggests anaerobic respiration.
In this core practical, a redox indicator such as methylene blue or DCPIP acts as an artificial hydrogen carrier in a yeast suspension. As the yeast respires, dehydrogenase enzymes pass hydrogen to the indicator, which loses its blue colour. The time taken to decolourise measures the rate of respiration.
A simple respirometer measures the rate of respiration of germinating seeds or small invertebrates from the movement of fluid along a capillary tube. With carbon dioxide absorbed by potassium hydroxide, the movement shows oxygen consumed. Without the absorber, the net change allows carbon dioxide produced to be found, and so the RQ.
Exam questions
7 exam-style questions (21 marks), each with its mark scheme.
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8 cards: flip them, mark what you knew, and practise the rest.
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The whole of respiration, muscles and the internal environment on one page, so you can see where this subtopic fits.
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Quick check questions
What is the formula for the respiratory quotient?
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RQ = volume of CO₂ produced ÷ volume of O₂ consumed.
What is the typical RQ when carbohydrate is respired?
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About 1.0.
In Core practical 15, why does the redox indicator change colour?
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It accepts hydrogen from dehydrogenase enzymes, so it is reduced and loses its blue colour.
In a respirometer, what is the purpose of potassium hydroxide?
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It absorbs carbon dioxide, so that the movement of fluid shows the volume of oxygen consumed.
Frequently asked questions
What is the respiratory quotient and what does it tell you?
The respiratory quotient is the volume of carbon dioxide produced divided by the volume of oxygen consumed. It shows which substrate is being respired. Carbohydrate gives about 1.0, protein about 0.9 and lipid about 0.7, because lipids contain less oxygen, so more oxygen is used per carbon dioxide made.
How do you calculate the RQ using a respirometer?
Measure the oxygen consumed with potassium hydroxide present, which absorbs carbon dioxide. Then repeat without it to find the net volume change, which is oxygen used minus carbon dioxide produced. Subtract to find the carbon dioxide produced, then divide this by the oxygen consumed.
Why does methylene blue lose its colour when yeast respires?
Methylene blue acts as an artificial hydrogen carrier. Dehydrogenase enzymes in respiring yeast remove hydrogen from the substrate, and the indicator accepts it in place of NAD. Once reduced, it turns from blue to colourless, so the faster it decolourises, the faster the rate of respiration.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026