Biology 11–12 · Years 11–12

Yeast fermentation of glucose: carbon dioxide production and budding cells

Module 1: Cells as the Basis of Life (Cell Function); also Module 5 (Reproduction)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Yeast releases carbon dioxide as it respires glucose without oxygen, and the same culture shows cells reproducing by budding.

What you need

Glucose 5 g; fast-acting dried yeast 1 g; warm water 50 mL at 30 to 40 degrees Celsius (the source quantities)
Conical flask 100 mL; cotton wool plug; delivery tube and bung; boiling tube with 5 mL limewater
Optional for quantitative work: gas syringe 100 mL, or a balloon over the flask and a tape measure; balance
Thermometer; water baths at 20, 30 and 40 degrees Celsius; microscope, slide, coverslip, methylene blue 0.1 percent

How to do it

Dissolve 5 g glucose in 50 mL warm water in the flask, add 1 g yeast, plug loosely with cotton wool and note the time.
When bubbling is steady, lead the gas through the delivery tube into limewater and record the change.
For a rate measurement, attach the gas syringe and record the volume every 2 minutes for 20 minutes, or count bubbles per minute; repeat at three temperatures.
Weigh the flask at the start and after 24 hours to record the mass of carbon dioxide lost.
Place a drop of the culture on a slide with methylene blue, add a coverslip and view at 400x: count cells with buds among 100 cells.
Compare the measured gas volume with the stoichiometric maximum from 5 g glucose.

What you should see

The limewater turns milky within seconds of the gas reaching it. Gas production is fastest at 30 to 40 degrees Celsius and slowest at 20. Complete fermentation of 5.00 g glucose (C6H12O6 to 2 C2H5OH + 2 CO2) releases 0.0555 mol CO2, which is 1.36 L at 25 degrees Celsius or 1.40 L at 35 degrees Celsius at 101.3 kPa (computed), so over 24 hours the flask loses up to about 2.4 g; a 20 minute run captures a small fraction of this. Under the microscope some cells carry a smaller attached bud. The learner knows it worked when limewater clouds, the mass falls, and buds are visible.

What changes

What you change: temperature (degrees Celsius)
What you measure: volume of carbon dioxide collected in 20 minutes
What you keep the same: mass of glucose and yeast
water volume
flask
start time after mixing

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Yeast needs air to make carbon dioxide; fermentation occurs without oxygen.
The bubbles are air the yeast breathed in; they are carbon dioxide made from glucose carbon.
Budding is the same as binary fission; the bud is smaller than the parent and separates later.

Safety card

Low riskLearners carry it out

Hazards

Limewater is an irritant
Foam may overflow a fast fermentation
Hot water baths

Controls

Wear eye protection
Plug loosely, never seal the flask
Use a plastic tray under the flask
Yeast is a food-grade organism; do not culture on agar without aseptic technique

Note

Limewater and methylene blue: RiskAssess (https://www.riskassess.com.au/) and the NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8 BIO12-12 BIO11/12-3 BIO11/12-4
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 12 focus area Biodiversity; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12-03
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-04
Biology 11–12 Syllabus (2025), Year 12 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12WS-03 BI-12WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities