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GCSE level biology exam revision notes on RESPIRATION

Respiration: Part 6. School/college experiments to investigate the factors controlling the rate of anaerobic respiration in yeast - apparatus, variables explained, experiment procedure and processing results

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Description and explanation of how to investigate anaerobic respiration in yeast

Key points and learning objectives for this page, after the main body of notes

Practise exam questions on experiments to investigate the anaerobic respiration rate of yeast

INDEX of biology notes on respiration

(6) Experiment to investigate the anaerobic respiration rate of yeast

Introduction

GCSE level biology experiment on how to investigate the rate of anaerobic respiration of yeast cells using a sugar molecule substrate (it is a fermentation type of reaction).

Descriptions of the apparatus with diagram and the experimental procedures to be followed e.g. to vary temperature or concentration.

The treatment of results e.g. data and graphs is described and dependent, independent and control variables fully explained.
 

Reminders about variables in quantitative investigations

The independent variable - the factor you choose to study

The dependent variable - the measurement you make from the experiments, which depend on everything else!

In all these experiments described it is the volume of carbon dioxide evolved and its rate of formation.

The controlled variables - everything you should keep constant for fair test control, this basically means you keep all these variables constant except for the chosen independent variable.

The variables for the experiment are

strain of yeast - there are different yeasts which do the same fermentation

mass of yeast - which should be well dispersed in the sugar solution

temperature - temperature control of the whole apparatus and reaction mixture

sugar - different sugars that can be fermented are available

concentration of enzyme - which should be well dispersed in the sugar solution

concentration of sugar - sugar readily dissolves in water, so dispersion is not a problem

volume of reactants - keep the same what ever the independent variable

apparatus perform experiment in the same way with the same apparatus

stirring - the rate of swirling the mixture round in the flask/boiling tube of the reactants in the thermostated water bath.

In the method description these variables will be assigned as independent, dependent or controlled.

 

Investigating the chemistry of the anaerobic respiration of yeast cells

You can also do the anaerobic respiration experiments with germinating seeds, but yeast produces more consistent results.

If you start with sucrose, the enzyme invertase hydrolyses sucrose and breaks it down into glucose and fructose.

sucrose  +  water  == enzyme invertase ==> glucose + fructose

C12H22O11(aq)  +  H2O(l)  ===> C6H12O6(aq)  +  C6H12O6(aq) 

The actual anaerobic fermentation reaction is facilitated by the enzyme zymase.

glucose/fructose (sugar) == enzyme zymase ==> ethanol + carbon dioxide

C6H12O6(aq) ===> 2C2H5OH(aq) + 2CO2(g) 

You can following the speed of the reaction by measuring the volume of carbon dioxide formed.

 

The apparatus, experimental procedure and analysis of results

apparatus for investigating rate of anaerobic respiration of yeast cells, college/school student lab experiment

Using a thermostated bath you can investigate the effect of temperature on the rate of fermentation.

You must keep the concentration of the sugar plus yeast mixture constant - fixed volumes of previously prepared stock solutions of the sugar or yeast suspension.

Keep to the same sugar and same yeast strain.

Check out the discussion on variables in the introduction.

You can start at 20oC and repeat the experiments several times for each temperature, and then raise the temperature by 5oC at a time to see the effect.

You can measure the rate of respiration in terms of the rate of evolution of gas e.g. cm3 CO2/min.

You must take great care in preparing the solutions and getting the experiment going.

You can place the stock solutions of yeast or sugar in the water bath to ensure they are all at the correct investigation temperature.

 

Other investigations of the anaerobic respiration of yeast cells

Using the above apparatus, or that described below (gas syringe) you can measure the rate of respiration under varying conditions, BUT make sure you only vary one factor, to measure its quantitative effect on the rate of respiration, everything else must be constant (see introduction on variables).

(i) with different substrate sugars of identical concentration, same enzyme concentration, same constant temperature, same apparatus etc.

(ii) for a fixed substrate sugar, the effect of changing its concentration at constant temperature and constant enzyme concentration, same apparatus etc.

(iii) for a specific yeast, the effect of changing its concentration at constant temperature and constant sugar concentration, same apparatus etc.

(iv) for different yeast strains, keeping everything constant including the yeast concentration, sugar concentration and temperature etc.

 

Note on test for carbon dioxide

If you bubble the gas from the reaction mixture through a limewater you get a white precipitate ('milkyness'), a positive test for carbon dioxide from the yeast anaerobic respiration..

You get exactly the same result if you blow some of your expelled breath through limewater - the same carbon dioxide from your aerobic respiration.

 

Processing the rate data:  The volume and rate of formation of the carbon dioxide gas.

apparatus for collecting & measuring the volume of carbon dioxide from the anaerobic respiration of yeast cells with sugar

You can use a gas syringe system to make more accurate experiments.

e.g. doing the experiments at constant room temperature, you can keep the yeast concentration constant and vary the concentration of the sugar OR you can vary the substrate sugar (but keeping the sugar concentration constant).

Graph A graph of carbon dioxide volume versus time for anaerobic respiration of yeast cells with sugar investigations VCO2 versus time

Graph A represents the results of two experiment runs to show the tangential method to estimate the initial rate of evolution of carbon dioxide, though in reality you need a set of at least four sets of results for e.g. different concentrations or temperature.

What I'm describing here applies to ANY of the experimental data.

 

Graph B graph of rate of anaerobic respiration of yeast cells with increase in temperature, speed of reaction versus temperature constant concentrations, Graph C optimum temperature for the rate of anaerobic respiration of yeast cells with a sugar substrate

Graph B would be a typical set of results for the rate of anaerobic respiration of yeast cells with a sugar substrate i.e. a general increase with increase in temperature, with everything else kept constant.

However, you results cab complicated due to denaturing of the protein enzymes in the yeast cells giving an optimum temperature graph such as Graph C.

For full discussion on this see GCSE level biology Enzymes 5. Effect of temperature - What is the optimum temperature for an enzyme catalysed reaction?

 

Graph D graph of rate of anaerobic respiration of yeast cells with increasing concentration of sugar or yeast, speed of reaction versus concentration at constant temperature

Graph D shows typical results when varying the concentration of the sugar solution or the concentration of the yeast cells.

There is often a linear relationship between the speed of a chemical reaction and the concentration of one of the reactants - as long as everything else in the investigation is kept constant.

 

Whichever experimental apparatus you use the graphs illustrate typical graphical results you might obtain based on a rate of evolution of carbon dioxide e.g. cm3 CO2/minute.

 

See also ENZYMES - structure, function, optimum conditions, investigation experiments 

See also Enzymes and Biotechnology (gcse chemistry notes)

Practice exam questions on experiments to investigate the anaerobic respiration rate of yeast

A set of 10 randomized multiple-choice questions focused on school/college experiments investigating the anaerobic respiration rate of yeast.

Jot down your responses and then check out the answers

These are tailored for WJEC, CCEA, AQA, Edexcel, OCR, and IGCSE GCSE Biology specifications.

Anaerobic Respiration in Yeast: GCSE/IGCSE Biology Questions

Each question includes:

  • Four answer options (A–D)
  • Correct answer with explanation
  • Distractor analysis
  • Exam tips and common misconceptions

Q1. What is the main gas produced during anaerobic respiration in yeast?

  1. Oxygen
  2. Carbon dioxide
  3. Nitrogen
  4. Hydrogen

Q2. Which of the following best indicates the rate of anaerobic respiration in yeast during an experiment?

  1. Temperature of the water bath
  2. Number of bubbles produced per minute
  3. Colour change of limewater
  4. Volume of ethanol produced

Q3. Why is glucose added to the yeast suspension in anaerobic respiration experiments?

  1. To increase the temperature
  2. To act as a pH buffer
  3. To provide a substrate for respiration
  4. To kill bacteria

Q4. Which condition must be ensured to make the yeast respire anaerobically in a school experiment?

  1. High temperature
  2. Excess oxygen
  3. Absence of oxygen
  4. Presence of light

Q5. What is the role of the water bath in yeast respiration experiments?

  1. To sterilize the yeast
  2. To maintain a constant temperature
  3. To provide oxygen
  4. To measure ethanol concentration

Q6. Which piece of equipment is most suitable for collecting and measuring CO2 gas from yeast respiration?

  1. Thermometer
  2. Gas syringe
  3. Bunsen burner
  4. pH probe

Q7. What is the chemical equation for anaerobic respiration in yeast?

  1. Glucose → Carbon dioxide + Water
  2. Glucose → Ethanol + Carbon dioxide
  3. Glucose → Lactic acid
  4. Glucose → Oxygen + Ethanol

Q8. What would happen if the temperature in the yeast experiment is raised above 50°C?

  1. Respiration rate increases indefinitely
  2. Yeast cells divide faster
  3. Enzymes denature and respiration slows
  4. More oxygen is produced

Q9. Why is a layer of oil sometimes added on top of the yeast-glucose solution?

  1. To prevent evaporation
  2. To kill bacteria
  3. To block oxygen and ensure anaerobic conditions
  4. To increase CO2 production

Q10. Which two variables should be kept constant when investigating the effect of temperature on yeast respiration rate?

  1. Type of sugar
  2. Volume of yeast suspension
  3. Number of bubbles
  4. Temperature

Jot down your responses and then check out the answers

Key points - Experiment to measure the rate of anaerobic respiration in yeast

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Investigating Anaerobic Respiration in Yeast

This experiment demonstrates that yeast respires anaerobically, producing carbon dioxide and ethanol, and releases energy.

Word Equation:

Glucose → Ethanol + Carbon dioxide + Energy (as 'energy releasing ATP molecules)

Apparatus for investigating the anaerobic respiration of yeast cells.

  • Yeast suspension (yeast + glucose solution)

  • Boiling tube or conical flask

  • Limewater or hydrogen carbonate indicator (to test CO2)

  • Delivery tube

  • Water bath

  • Bung

  • Stopwatch

  • Measuring cylinder

  • Graduated syringe or gas syringe (optional for CO2 collection)

  • Oil layer (optional, to block oxygen)

Method (Basic Setup) for investigating the anaerobic respiration of yeast cells.

  1. Mix yeast and glucose solution in a boiling tube or conical flask.

  2. Seal with a bung fitted with a delivery tube leading to:

    • A test tube of limewater (turns cloudy if CO2 is released), or

    • A gas syringe to measure the volume of gas produced.

  3. Optionally, add a layer of oil over the yeast mixture to prevent oxygen entering.

  4. Place the setup in a warm water bath (e.g. 30–40°C) to maintain optimal temperature.

  5. Start the stopwatch and begin recording:

    • Time intervals

    • Volume of gas collected (if using a gas syringe)

    • Or time taken for limewater to turn cloudy

  6. Compare results under different conditions (e.g. temperature, sugar concentration).

Variables when investigating the anaerobic respiration of yeast cells.

Variable Type

Example

Independent

Temperature, glucose concentration

Dependent

Volume of CO2 produced, time to change indicator

Control

Volume of yeast solution, time, apparatus used

Recording and Calculating Rate

If using a gas syringe:

Rate of respiration} = Volume of CO2 collected (cm³) / Time (minutes)

Record results in a table and plot a graph of CO2 volume versus time for analysis.

Interpreting results of investigating the anaerobic respiration of yeast cells.

  • More CO2 means higher anaerobic respiration rate.

  • Optimal temperature (around 37°C) gives fastest rate—beyond this, enzymes may denature.

  • A low sugar concentration may limit respiration rate.

Conclusion

This experiment shows that:

  • Yeast respires anaerobically when oxygen is absent.

  • The products are carbon dioxide and ethanol.

  • Conditions like temperature and sugar availability affect the rate of anaerobic respiration.

Keywords, phrases and learning objectives the anaerobic respiration of yeast cells.

Be able to describe an experimental method to investigate the rate of anaerobic respiration of yeast in the fermentation of sugar.

Fully appreciate the difference and necessity of independent, dependent and controlled variables for investigation the anaerobic respiration of yeast cells.

Know the apparatus required from a diagram and describe an outline of the procedure.

Know how to record the results and from the data graphs and compute the rate of reaction.

Hence to show the variation of the rate of aerobic respiration with temperature or varying the concentration of the sugar (or yeast).

ANSWERS to the practice exam questions on experiments to investigate the anaerobic respiration rate of yeast

Q1. What is the main gas produced during anaerobic respiration in yeast?

  1. Oxygen
  2. Carbon dioxide
  3. Nitrogen
  4. Hydrogen

Correct Answer: B. Carbon dioxide
Yeast respires anaerobically to produce ethanol and carbon dioxide.

Distractor Analysis:

    1. Oxygen – Yeast does not produce oxygen; it’s used in aerobic respiration.
    1. Nitrogen – Not involved in respiration.
    1. Hydrogen – Not a product of yeast respiration.

Exam Tip:
Don’t confuse aerobic and anaerobic respiration. Anaerobic in yeast = ethanol + CO2.

Q2. Which of the following best indicates the rate of anaerobic respiration in yeast during an experiment?

  1. Temperature of the water bath
  2. Number of bubbles produced per minute
  3. Colour change of limewater
  4. Volume of ethanol produced

Correct Answer: B. Number of bubbles produced per minute (accurate volume preferentially!)
CO2 bubbles are a direct measure of respiration rate.

Distractor Analysis:

    1. Temperature affects rate but doesn’t measure it.
    1. Limewater is used to test for CO2 but not quantify rate.
    1. Ethanol is hard to measure in school labs.

Exam Tip:
Use bubble count or gas syringe volume to measure respiration rate.

Q3. Why is glucose added to the yeast suspension in anaerobic respiration experiments?

  1. To increase the temperature
  2. To act as a pH buffer
  3. To provide a substrate for respiration
  4. To kill bacteria

Correct Answer: C. To provide a substrate for respiration
Glucose is the sugar yeast ferments anaerobically.

Distractor Analysis:

    1. Temperature is controlled separately.
    1. Glucose doesn’t buffer pH.
    1. Glucose doesn’t kill bacteria.

Exam Tip:
Always link glucose to energy production via respiration.

Q4. Which condition must be ensured to make the yeast respire anaerobically in a school experiment?

  1. High temperature
  2. Excess oxygen
  3. Absence of oxygen
  4. Presence of light

Correct Answer: C. Absence of oxygen
Anaerobic respiration occurs without oxygen.

Distractor Analysis:

    1. Temperature affects rate but doesn’t determine aerobic/anaerobic.
    1. Oxygen presence = aerobic respiration.
    1. Light is irrelevant to yeast respiration.

 Exam Tip:
Seal the flask or use a layer of oil to exclude oxygen.

Q5. What is the role of the water bath in yeast respiration experiments?

  1. To sterilize the yeast
  2. To maintain a constant temperature
  3. To provide oxygen
  4. To measure ethanol concentration

 Correct Answer: B. To maintain a constant temperature
Temperature affects enzyme activity and respiration rate.

 Distractor Analysis:

    1. Sterilization isn’t the goal.
    1. Oxygen is excluded in anaerobic setups.
    1. Ethanol isn’t measured via water bath.

 Exam Tip:
Keep temperature between 30–40°C for optimal yeast activity.

Q6. Which piece of equipment is most suitable for collecting and measuring CO2 gas from yeast respiration?

  1. Thermometer
  2. Gas syringe
  3. Bunsen burner
  4. pH probe

 Correct Answer: B. Gas syringe
Gas syringes accurately measure gas volume.

 Distractor Analysis:

    1. Measures temperature, not gas.
    1. Not used in anaerobic setups.
    1. Measures acidity, not gas.

 Exam Tip:
Gas syringe or inverted measuring cylinder are standard methods.

Q7. What is the chemical equation for anaerobic respiration in yeast?

  1. Glucose → Carbon dioxide + Water
  2. Glucose → Ethanol + Carbon dioxide
  3. Glucose → Lactic acid
  4. Glucose → Oxygen + Ethanol

 Correct Answer: B. Glucose → Ethanol + Carbon dioxide
This is fermentation, a type of anaerobic respiration in yeast.

 Distractor Analysis:

    1. Water is a product of aerobic respiration.
    1. Lactic acid is produced in animals, not yeast.
    1. Oxygen is not produced.

 Exam Tip:
Yeast = ethanol + CO2; humans = lactic acid.

Q8. What would happen if the temperature in the yeast experiment is raised above 50°C?

  1. Respiration rate increases indefinitely
  2. Yeast cells divide faster
  3. Enzymes denature and respiration slows
  4. More oxygen is produced

 Correct Answer: C. Enzymes denature and respiration slows
High temperatures damage enzymes, halting respiration.

 Distractor Analysis:

    1. Respiration rate peaks then drops.
    1. Yeast division is not the focus.
    1. Oxygen is not produced anaerobically.

 Exam Tip:
Enzyme activity peaks around 37°C; above 50°C = denaturation.

Q9. Why is a layer of oil sometimes added on top of the yeast-glucose solution?

  1. To prevent evaporation
  2. To kill bacteria
  3. To block oxygen and ensure anaerobic conditions
  4. To increase CO2 production

 Correct Answer: C. To block oxygen and ensure anaerobic conditions
Oil prevents oxygen from entering the solution.

 Distractor Analysis:

    1. Evaporation isn’t the main concern.
    1. Oil doesn’t sterilize.
    1. CO2 production depends on respiration, not oil.

 Exam Tip:
Oil layer = simple way to exclude oxygen in school labs.

Q10. Which two variables should be kept constant when investigating the effect of temperature on yeast respiration rate?

  1. Type of sugar
  2. Volume of yeast suspension
  3. Number of bubbles
  4. Temperature

 Correct Answer: A and B. Type of sugar and volume of yeast suspension
A Sugar type affects rate, so must be fixed.

B To ensure a fair test, keep yeast volume constant.

 Distractor Analysis:

    1. Bubbles are the dependent variable.
    1. Temperature is the independent variable.

 Exam Tip:
Control all variables except the one you’re testing for.

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