Key concepts
Life
Food
Metabolism

From National Science Education Standards: Life cycles of organisms

Introduction
Have you ever looked closely at a piece of sandwich bread—really closely? Notice all of those tiny holes? They probably got there thanks to tiny living organisms called yeast. Even though these organisms are too small to see with the naked eye (each granule is a clump of single-celled yeasts), they are indeed alive just like plants, animals, insects and humans. In fact, we have some interesting things in common with these little creatures!

When you breathe out, part of what you are exhaling is a gas known as carbon dioxide. Yeast also releases carbon dioxide when it is active (although it's way too small and simple an organism to have lungs). Yeast are so small you can't see individual ones very well. So how can you tell if they are alive or not? You can enlist a whole bunch of them to blow up a balloon for you!

Background
When you buy a packet of baker's yeast at the store, the organisms inside are in a state of inactivity so they don't need to eat (keeping them cool and dry helps keep them preserved this way). But when you mix them into dough, they wake up and begin eating—and making carbon dioxide.

When you make yeast-based bread, you often have to wait for it to rise. During this step the dough might appear to be growing. But what is really happening is that you're giving the tiny yeast organisms time to eat and create small pockets of carbon dioxide inside the dough, which is what makes the dough seem to grow larger—and which leads to fluffy bread! (Bread products that don't have yeast rise during baking thanks to other ingredients, such as baking powder.)

Why do the yeast organisms "wake up" when you put them into a dough mixture? Like other living organisms, they need food and water. So by putting them in a moist environment with nutrients (such as sugar), they become "active."

Materials
•    Fresh packet of baker's yeast (check the expiration date)
•    Tablespoon of sugar
•    Clear plastic bottle with a small opening (such as a water bottle)
•    Funnel
•    Small balloon
•    Warm water

Preparation •    Carefully stretch out the balloon by blowing it up a few times (might as well give the tiny yeast a hand!).
•    Pour an inch or two of warm water into the clear plastic bottle.

Procedure
•    Pour the packet of yeast into the bottle and swirl it around.
•    Now add the sugar, and swirl the mixture around a little bit more.
•    Stretch the balloon opening over the top of the plastic bottle.
•    Look through the bottle—do you see any signs of life?
•    Leave the balloon-covered, yeast-filled bottle in a warm place for 15 or 20 minutes.
•    Any signs of life? Do you see any changes in the balloon?
•    Will the yeast keep making more and more carbon dioxide? Why might it stop?
•    Extra: If you have more yeast, try making a loaf of bread from scratch. You can find simple recipes—with the science behind them—on the Exploratorium's "Science of Cooking" website.

[To get the full effect for the time-lapse section in our video we used three tablespoons of yeast, three tablespoons of sugar, and we allowed the mixture to sit for 40 minutes.]

Read on for observations, results and more resources.