By the end of this lesson
- Plan a valid investigation of anaerobic respiration in yeast.
- Collect and process data on carbon dioxide production.
- Evaluate the method, including the control experiment, control variables and limitations.
BIG QUESTION
How can you measure how fast yeast respires?
Carbon dioxide released can be counted as bubbles or collected and measured as a volume.
Variables
Choose one independent variableThe variable that is deliberately changed in an investigation.: temperature (for example 20, 30, 40, 50 °C water baths) or glucose concentration. The dependent variableThe variable that is measured in an investigation to see how it is affected by the independent variable. is the rate of carbon dioxide production.
Control the mass or volume of yeast suspension, the volume and concentration of glucose solution (if temperature is varied), the type of yeast, the acclimation time and the measuring time.
Method
Mix a fixed volume of yeast suspension with glucose solution in a boiling tube. Cover the surface with a thin layer of liquid paraffinA layer of oil poured over a liquid to stop oxygen entering, keeping the conditions anaerobic. (or oil) to keep oxygen out so respiration is anaerobic. Place the tube in a water bath at the chosen temperature and leave it for 5 minutes to equilibrate.
Connect a delivery tube and count bubbles released into water per minute, or collect gas in an inverted measuring cylinder or gas syringe over a fixed time. Repeat three times at each value.
Control experiment
A control experimentA matched set-up without the factor being tested, such as boiled yeast or no glucose, so any difference in results can be linked to that factor., such as a tube with boiled (killed) yeast or no glucose, shows that gas is produced by living yeast respiring sugar.
Using boiled glucose solution (cooled) removes dissolved oxygen before adding yeast.
Data handling and interpretation
Calculate the mean volume of gas per minute at each value, identify anomalies and plot rate against temperature or concentration.
Illustrative pattern: rate rises from 20 °C to about 35–40 °C then falls as yeast enzymes denature. Link the pattern to enzyme activity, not to “yeast liking warmth”.
Safety and limitations
Take care with hot water baths and glassware. Yeast is low hazard but wash hands and do not ingest solutions.
Limitations: bubbles vary in size so counting is less accurate than measuring volume; some CO₂ dissolves in the solution; temperature may drift; yeast suspension can settle unevenly.
Graph skills: SLAPUK and choosing a scalePlotting gas volume per minute? Check your graph with SLAPUK.
Measuring carbon dioxide release
Choose the most accurate method available.
| Method | Advantage | Limitation |
|---|---|---|
| Counting bubbles | Simple, cheap. | Bubble size varies; less accurate. |
| Gas syringe | Measures volume accurately. | Plunger may stick; more expensive. |
| Inverted measuring cylinder | Measures volume. | Some CO₂ dissolves in water. |
| Limewater or indicator colour change | Shows CO₂ presence. | Only qualitative or semi-quantitative. |
Language that earns precision
- rate of respiration
- The amount of carbon dioxide produced (or glucose used) per unit time.
- equilibrate
- Leave apparatus until it reaches the water-bath temperature.
- liquid paraffin
- An oily layer used to stop oxygen dissolving into a solution.
Common misconceptions
Claim: The paraffin layer feeds the yeast.
Correction: It excludes oxygen so respiration is anaerobic.
Claim: Counting bubbles measures volume accurately.
Correction: Bubble size varies; collecting gas gives a volume.
Claim: Yeast dies at 40 °C.
Correction: The rate falls above the optimum as enzymes denature; the exact temperature varies with yeast strain.
Match the answer to the command word
State independent, dependent and at least two controlled variables, how you exclude oxygen and how you measure CO₂.
Suggest using a gas syringe instead of counting bubbles, and repeats to identify anomalies.
Recall, apply, then justify
Describe how you could investigate the effect of temperature on the rate of anaerobic respiration in yeast.
Hint
Set-up, anaerobic conditions, temperatures, measurement, control experiment and control variables.
Suggested marking points
- Mix yeast suspension and glucose solution in a test tube.
- Cover with a layer of oil/paraffin to exclude oxygen.
- Place in water baths at a range of temperatures (and allow to equilibrate).
- Measure CO₂ released over a fixed time (gas syringe or count bubbles).
- Keep yeast mass/volume and glucose concentration constant and repeat.
Explain why the glucose solution is boiled and cooled before the yeast is added.
Hint
Dissolved gas; temperature.
Suggested marking points
- Boiling removes dissolved oxygen (so respiration is anaerobic).
- Cooling prevents the yeast being killed / enzymes denaturing.
Evaluate counting bubbles as a method for comparing the rate of yeast respiration at different glucose concentrations, and suggest improvements.
Planning hint
Accuracy, precision, CO₂ dissolving, repeats.
What a strong design includes
- Counting is quick and needs simple equipment.
- Bubbles vary in size, so equal counts may not mean equal volumes.
- Fast bubbling is hard to count accurately.
- Some CO₂ dissolves before forming bubbles.
- Use a gas syringe to measure volume instead.
- Repeat each concentration and calculate a mean.
- Allow the same equilibration time before measuring.