Yeast Balloons

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Each group of 4 students will need:

  • Clean, empty plastic water bottles
  • 1 Cup of warm water (~42 °C / 108 °F)
  • 1 Balloon
  • 1 Packet of active dry yeast
  • 2-3 Tablespoons of sugar
  • Paper towels
  • 1 sheet of scratch paper
For the accompanying slideshow, click here.

Background :

Living things make changes to their environments. Some changes such as pollution, habitat destruction, and climate change are negative. But other changes are a natural part of organisms’ life functions. All living things undergo a process called metabolism. Metabolism is a process in which living things carry out many different chemical reactions in order to live, grow, and reproduce. An organism’s metabolism uses up different chemicals from the environment, and also releases different chemicals into the environment. This science activity allows students to explore the metabolic activity of yeast and observe some of the changes that yeast cells make in their environment.


Expected time: 10-15 minutes

  • Identify this lesson’s theme: Living things cause changes to their environments
  • Discuss some examples of living things making changes to their environments. Are they positive, negative, or both?
  • Define metabolism as the different chemical reactions that organisms conduct in order to survive, grow, and reproduce.
  • Review previous list of examples of organisms making changes to their environments. Which examples can be related to an organism’s metabolism? What are some other examples of metabolic activities? What are some examples of metabolic byproducts (metabolites)?
  • Discuss gas production as an important part of metabolism, and list examples (animals producing carbon dioxide, plants producing oxygen, gut bacteria producing fart).
  • Hypothesize: what will happen when yeast is added to warm, sugary water? If a balloon is placed over a bottle containing the yeast mixture (a yeast culture), what will happen to the balloon?


Expected time: 10-15 minutes

  1. Before the activity starts, prepare very warm water (~42 °C). Fill plastic bottles with 1 cup of warm water (1 per group).
  2. Divide the class into groups of four. Each group will receive a plastic bottle with warm water, a packet of active dry yeast, 2-3 tablespoons of sugar, paper towels, and one balloon.
  3. Assign roles to each person in the group.
    • Person 1 – Blow up the balloon and then release the air. Repeat this three times to loosen the balloon so it will be easier to inflate.
    • Person 2 – Add the sugar to the bottle using the paper funnel. Swirl to mix well.
    • Person 3 – Add the yeast to the bottle using the paper funnel. Swirl to mix well.
    • Person 4 – After everything is added and mixed, place the balloon over the neck of the bottle. Make sure it is snug!
  4. Observe the yeast culture and the balloon. During observation, begin the group discussion.
  5. Clean up:
    • Throw entire set-up (bottle, balloon, yeast culture) in the trash.
    • Use paper towels to wipe off any spills.


Expected time: 10 – 15 minutes

Group discussion questions:

  • What do you see in your bottle? What do you think is happening?
  • What kind of gas is being produced? (Hint: Many other organisms, including humans, also produce this gas!)
  • Why did we add sugar to the bottle?
  • What is happening to the sugar in the bottle? After a long time, will there be more sugar or less sugar left inside the bottle?
  • This experiment is about an organism (yeast) and its environment. Describe the yeast’s environment in this experiment.
  • What kinds of changes are the yeast cells making in their environment?

Challenge questions:

  • Define the term “metabolism” in your own words.
  • Antibiotics are chemicals that stop germs from growing. What might happen if we added an antibiotic to the bottle at the beginning of the experiment? Why?
  • Vitamins are chemicals that help organisms grow. What might happen if we added a vitamin that would help speed up the metabolism of the yeast cells? Why?

In your lab notebooks:

  • Draw a picture of your experiment set up: bottle, balloon, yeast mixture. Be sure to label all components of the yeast mixture!
  • In a few sentences, describe 1. what you did, and 2. what you observed.

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