Core practical 15: redox indicator and yeast

Respiration, Muscles and the Internal Environment · Respiratory quotient and respiration practicals · note 2 of 3

Spec 7.7

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Core practical 15: redox indicator and yeastSpec 7.7

In short

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.

Core practical:

Core practical 15: use an artificial hydrogen carrier (redox indicator) to investigate respiration in yeast.

During respiration, dehydrogenase enzymes remove hydrogen from the substrate and normally pass it to NAD. A redox indicator (for example methylene blue or DCPIP) can accept this hydrogen instead, so it acts as an artificial hydrogen carrier. When it is reduced, it changes colour (blue to colourless). The faster the yeast respires, the faster the indicator is decolourised.

Make a suspension of yeast in a glucose solution of known concentration.
Place equal volumes of the suspension in boiling tubes in water baths at different temperatures (the independent variable), and let them reach the temperature.
Add the same volume of the redox indicator to each tube and mix. A layer of oil may be added to exclude oxygen, which would re-oxidise the indicator.
Start a stopwatch and record the time taken for the blue colour to disappear, or until it matches a colour standard.
Repeat at each temperature and calculate a mean. Rate of reaction is proportional to 1 ÷ time.

Independent variable: temperature (or glucose concentration, or yeast concentration).
Dependent variable: time for the indicator to lose its colour.
Control variables: volume and concentration of yeast, glucose and indicator; pH; the colour end-point used.
Control experiment: boiled and cooled yeast should not decolourise the indicator.

Risks: hot water can scald; use a thermometer and a water bath that is not boiling. Methylene blue stains skin and clothes; wear eye protection and gloves, and wipe up spills.
Limitation: judging the colour end-point is subjective, so use the same colour standard, or a colorimeter for the endpoint.

Exam tip:

If asked why the indicator changes colour, say that it accepts hydrogen released by the dehydrogenase enzymes in place of NAD, so a faster colour change means a faster rate of respiration.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

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.