Edexcel A Level (IAL) Biology -7.7 Core Practical 15: Investigation of Respiration in Yeast- Study Notes- New Syllabus

CORE PRACTICAL 15

Investigating Respiration in Yeast Using a Redox Indicator

🌱 Introduction

This practical uses an artificial hydrogen carrier (a redox indicator such as methylene blue or TTC) to show when respiration is occurring in yeast.
Yeast release hydrogen during respiration, which reduces the indicator and causes a colour change.
This provides a simple visual way to measure respiratory activity.

Principle of the Experiment

In normal respiration, hydrogen released from glucose is accepted by NAD.
Here, an artificial hydrogen acceptor replaces NAD. When reduced, the indicator changes colour:

• Methylene blue: blue to colourless
• TTC (triphenyl tetrazolium chloride): colourless to red

Faster colour change means faster respiration.

Materials Used:

• Yeast suspension
• Glucose solution
• Redox indicator (methylene blue / TTC)
• Water bath (30–35°C)
• Test tubes / boiling tubes
• Timer
• Pipettes
• Cotton wool plug

Method:

• Add yeast suspension to a test tube.
• Add glucose solution so yeast has a substrate.
• Add a few drops of the redox indicator.
• Place the tube in a warm water bath (30–35°C).
• Record the time taken for the colour to change.
• Compare colour change speed under different conditions such as:
  • Temperature
  • Presence or absence of glucose
  • Different substrate concentrations

What You Are Actually Measuring

You measure the time taken for the indicator to be reduced.
Shorter time means faster respiration because hydrogen is released more quickly.

Variables

Independent variable

• Temperature
• Glucose concentration
• Type of sugar

Dependent variable

• Time taken for the indicator to change colour

Controlled variables

• Volume of yeast
• Volume of glucose
• Volume of indicator
• Temperature (if not under test)
• pH
• Oxygen availability

📌 Expected Results

• With glucose present, indicator changes colour more quickly.
• At optimum temperature, the colour change is fastest.
• Boiled yeast shows no colour change because it cannot respire.

📋 Summary Table