This experiment demonstrates yeast respiration by placing yeast, sugar, and warm water in bottles sealed with balloons. Under aerobic conditions, yeast uses oxygen to produce carbon dioxide rapidly, inflating the balloon more. Under anaerobic conditions, yeast ferments sugar into ethanol and carbon dioxide, inflating the balloon more slowly.

1. Prepare two clean bottles and label them “aerobic” and “anaerobic.”
2. Add warm water (not hot) and sugar to each bottle, stirring until dissolved.
3. Add an equal amount of yeast to each bottle and swirl gently.
4. Stretch a balloon over the mouth of each bottle to seal it.
5. For aerobic conditions, leave the setup as is so oxygen can dissolve into the solution.
6. For anaerobic conditions, limit oxygen by wrapping the bottle in plastic or using a balloon that fits tightly and prevents outside air from entering.
7. Leave the bottles in a warm place and observe over time.
8. Record the rate and size of balloon inflation for each condition.

Yeast Aerobic and Anaerobic Respiration - Animated Science:

The Yeast Respiration Experiment with S4's Biologist Ali - S4 Project:

• Test different sugars (glucose, sucrose, honey) to compare respiration rates.
• Vary the temperature of the water to see the effect on yeast activity.
• Try one setup with no sugar as a control.

• Do not drink or taste the yeast mixtures.
• Use bottles that can withstand slight pressure but avoid sealing too tightly to prevent bursting.
• Handle warm water carefully to avoid burns.
• Dispose of yeast mixtures in the sink with plenty of water; do not store sealed bottles for long periods.

• Why does the balloon inflate during yeast respiration? (CO2 gas is produced and collects in the balloon.)
• Why does the aerobic bottle inflate more quickly? (Oxygen allows yeast to fully respire sugar, releasing more energy and carbon dioxide.)
• What additional product is made in anaerobic respiration? (Ethanol, along with CO2.)
• Why is warm water used instead of hot water? (Hot water can kill yeast, while warm water activates it.)
• What does this experiment reveal about yeast’s role in bread-making or brewing? (CO2 makes bread rise, while ethanol is central to brewing alcohol.)