Grade 10 · 🧬 Biology · Respiration
Blow up a balloon with yeast
Feed yeast sugar in a bottle and capture the carbon dioxide it releases during respiration — the same process that makes injera batter bubble.
Why this matters
Every bubble in injera, every rise of a loaf of dabo and every bottle of tella and tej depends on yeast respiration. Yeast are tiny living fungi that break down sugar for energy and release carbon dioxide as a by-product — just like your own cells do.
Watch the demo
Video coming soon
We're recording a demo for this experiment. Until then, follow the step-by-step guide below.
The science
Respiration releases energy
Living cells break down glucose to release energy. With oxygen (aerobic): glucose + oxygen → carbon dioxide + water + energy. Without oxygen, yeast carry out fermentation: glucose → ethanol + carbon dioxide + energy.
Temperature and enzymes
Respiration is driven by enzymes. They work fastest at a warm optimum temperature and are slowed by cold or destroyed by very hot water.
You'll need
- 1 packet (about 7 g) of dry baker's yeast↳ Sold in supermarkets and bakeries
- 2 spoons of sugar
- Warm water (comfortably warm to touch, not hot)
- A small plastic bottle
- A balloon
- Optional: a second bottle and balloon for a cold-water comparison
Safety first
- Use warm, not boiling, water — very hot water kills the yeast and could burn you.
- Don't drink the mixture.
Step by step
0/5 doneWhat you should see
Foam forms in the bottle and the balloon slowly inflates over 20–30 minutes. The cold-water bottle inflates much more slowly, or hardly at all.
Why it happens
The yeast use the sugar for respiration and release carbon dioxide gas. The gas has nowhere to go but into the balloon, so it inflates. In cold water the yeast's enzymes work slowly, so less carbon dioxide is produced.
Didn't work? Try this
Nothing happened after 30 minutes.
Your water may have been too hot and killed the yeast, or the yeast is old. Try again with fresh yeast and water just warm to the touch.
The balloon inflated a little and then stopped.
Check the balloon is sealed tightly on the bottle neck — gas may be leaking out.
Still stuck? Ask the lab assistant
Check your understanding
1. Which gas inflates the balloon?
2. Why did the cold-water bottle inflate more slowly?
Your lab notebook
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