Capstone: The Yeast Balloon
Feed living cells, capture their waste gas, and check the volume against a number you calculated in advance.
Builds on: 1.2 Respiration: Spending the Sugar0.3 Enzymes: Why Biology Happens at All6.2 Populations: Growth and Its Limits
Same idea, other course:⚗️ Catalyst 2.3 — the mole arithmetic this capstone runs on
What you are measuring
Yeast is a single-celled fungus. Give it sugar and no oxygen and it respires anaerobically — fermentation — producing ethanol and carbon dioxide. You are going to capture that CO₂ in a balloon and compare the volume against a prediction.
This is the reaction behind bread and beer, running in front of you, at a rate you set.
Predict before you measure
Do this arithmetic first — the point of the experiment is that you knew the answer beforehand. It is Catalyst’s stoichiometry, applied to a living organism.
- Glucose has a molar mass of 180 g/mol.
- 10 g of sugar is 10 ÷ 180 = 0.056 mol.
- Each glucose gives 2 CO₂, so 0.056 × 2 = 0.111 mol of CO₂.
- One mole of any gas occupies about 24 litres at room temperature, so 0.111 × 24 ≈ 2.7 litres.
That is the maximum, if every molecule of sugar is fermented. In practice you will get less, and the gap is itself informative: yeast uses some sugar for growth, some CO₂ dissolves in the water, and the reaction will not have finished.
What you need
- A sachet of dried yeast (about 7 g)
- 2 tbsp sugar (roughly 25 g — weigh it if you can)
- 250 ml warm water, around 35–40 °C
- A narrow-necked bottle (a 500 ml plastic one is ideal)
- A balloon, and a tape measure or string
Temperature is the variable to play with
Fermentation is run by enzymes, so Unit 0.3 applies directly: too cold and the yeast is sluggish, too hot and the enzymes denature and the cells die. Around 35 °C is close to optimal; above about 50 °C you will kill it outright. Running the same experiment at three temperatures turns this from a demonstration into an actual investigation — and if you do, keep everything else identical, or you will not know what caused the difference.
Do not seal the bottle with a cap — the balloon must be the only outlet, and pressure needs somewhere to go. Do not drink it: this is not a brewing kit, and what you have is unsanitised. Rinse everything afterwards.
Estimating the volume
Measure the balloon’s circumference C with string, then radius r = C ÷ 2π, and volume ≈ (4/3)πr³. A balloon 30 cm around has a radius of about 4.8 cm and holds roughly 0.46 litres. It is a rough estimate — balloons are not spheres — but good enough to compare against your prediction, and being explicit about that is part of doing it properly.
⚗️ Lab — Digital Twin: The Balloon
Model it before you run it — sugar, temperature and time.
- Set 10 g of sugar and read the maximum volume. That is the number to predict.
- Push the temperature to 55 °C. The yeast dies and the balloon stops growing.
- Compare 15 °C and 35 °C at the same time setting — that is the investigation worth doing for real.
Work it by hand — 0 / 3
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±3% count.