The Sensational Single Cell

Yeast Dough Rising

​

Initiate a class discussion prior to beginning the Science Scene.

Paraphrase:

Have you ever baked bread before? What kind of bread? What kinds of ingredients were in your recipe?

Did your recipe use yeast? Many bakers use yeast to make the dough light and fluffy. Do you know why?

Show the time-lapse video of a yeast dough rising.

Prompt students to tell you what they see and take notes in their notebooks. Ask:

• What do you observe?
• Why do you think the dough falls as it rises?

Notice that their answers need not be scientifically correct nor do you need to answer the questions. The prompts are intended to have the students observe closely. Possible answers:

• I see bread dough that starts in a small lump that grows in size. The dough stretches and holes appear. Sometimes it gets a little bit smaller.
• The dough stretches so much that it pulls apart, and the air that is filling it up escapes.

Note: Video is used with permission. © Micaël Reynaud.

Optional extensions:

• Ideally, try making a batch of dough in your classroom for students to observe as it rises.

Some teachers have several groups of students read at the same time. Other teachers select a few students to "perform" in front of the class.

You may want to read the entire passage out loud to the class once before assigning roles to students or dividing up into groups.

While most of the Focus Words appear in this script, there are a couple of terms that appear in other activities in this unit but aren't included in here:

• exponential: you can mention to students that doubling a population as yeast does when it buds is exponential growth
• nucleus: this term is also explored in L7 and the M units

​

The Script:

Setting: In his mother’s panaderia or bakery, Trey is showing his friends Vanessa, Sarah, and Tim how to bake bread.

Trey: You can put in all kinds of things in bread: eggs, milk, honey, herbs, nuts, raisins, whatever. But this time we’ll use a really basic recipe. Three cups of flour, two teaspoons of salt, two teaspoons of baker’s yeast, and a little over one cup of water. You mix it all together like this.

Sarah: This dough is really sticky.

Trey: Yeah, let’s add a little more flour to make it easier to handle. There, that’s good. Okay, yeast, up and at ’em!

Tim: What is yeast, anyway?

Vanessa: It’s the magic ingredient that makes the bread dough come alive and rise, or puff up.

Sarah: No way! There’s no such thing as magic. But it’s true that the yeast comes alive.

Trey: Yes, the packet of yeast we put in the bread dough has millions of living individuals. They’re each microscopic—too small to see without a microscope.

Tim: Hold on. We’re putting animals in the dough and then baking it? But I’m a vegetarian!

Trey: No, yeast isn’t an animal. It’s an organism—a living thing. But organisms include other things besides animals. Plants and bacteria, for instance. Yeast is a fungus, like mushrooms and mold.

Vanessa: Ew!

Sarah: Like all those things, yeast is just a bunch of little, living cells.

Vanessa: Cells?

Sarah: Yeah, cells. Each human being is made of trillions and trillions of cells, but some microscopic organisms are made of only one cell. Yeast is like that. Each separate yeast cell just lives its own separate little yeasty life as an individual.

Trey: Right. The yeast cells in the bread dough use the flour as a food source. As they eat, they’re able to grow and make more of themselves. They multiply. That's called reproduction.

Sarah: But they also need to get rid of waste that is left over from the food they eat — carbon dioxide gas. And that has a lot to do with how yeast makes the dough rise.

Trey: You’ll see. Now, we’ve got lots to do. Let’s divide up the work. Tim, you grease the pans. Vanessa, you knead.

Vanessa: (long pause) What do I need? Another ingredient?

Trey: What? Oh, no, not “need.” Knead: K, N, E, A, D. It’s got a silent K. It means squish, press, and fold the dough, to make it the right texture. See, you sprinkle a little flour on the countertop to keep the dough from sticking, then you put the dough there and press it like this… We’ll all take turns.

Vanessa: My hands are already tired. Why do we have to knead the dough for so long?

 

Trey: Kneading makes the dough more flexible and stretchy, which makes it better at trapping the tiny bubbles of carbon dioxide gas that the yeast cells make. Those bubbles are what will give the bread its spongy texture once it’s baked.

Sarah: Okay, it’s been ten minutes, can we bake the dough yet?

Trey: No. Now we have to let it rise. The yeast works best when it’s in a warm place.

Trey puts the dough in a bowl, covers it with a clean kitchen towel, and puts it in a warm spot on the kitchen counter. Then the four friends leave the bakery and go for a walk. An hour and a half later, they come back to check on the dough.

Vanessa: Wow! The dough has gotten twice as big.

Tim: How is that possible?

Trey: Like I said, the yeast makes the dough puff up. The yeast cells all keep puffing up the dough with bubbles of carbon dioxide gas. That’s what it means for bread dough to rise. Given time, they reproduce or multiply—they make more of themselves—making more yeast cells that make more bubbles.

​

Vanessa: That’s amazing! It really does seem like magic. Could we let it keep going, for days, until the dough grew large enough to feed thousands of people? With enough time, yeast could solve the problem of world hunger!

Trey: Wait a minute—

Tim: Isn’t it dangerous to just let it keep going and going? If it overflows its bowl, it could cover the counter, then grow across the kitchen floor, and burst out into the street. What’s to stop the dough from covering the entire world like some giant, angry blob in a horror movie?

Sarah: Hold on, hold on, I don’t think this bread dough can grow enough to solve world hunger or take over the planet.

Tim: Why not? Trey said the yeast multiplies in the dough, making more of itself. There’s more and more yeast, making the dough rise and rise. What limit is there to stop the yeast multiplying and the dough rising forever, getting bigger and bigger?

Vanessa: I’m with Tim. If something can make more of itself, what’s to stop it taking over?

Tim: Imagine if instead of tiny yeast cells you were dealing with big human-sized robots that made more and more of themselves. One robot copies itself, so you’ve got two… Then the two robots copy themselves, so you’ve got four… Then the four robots copy themselves, so you’ve got eight… Then the eight robots—

Trey: Okay, Tim, we get it.

Tim: The more robots there were, the faster their numbers would grow. Faster and faster, more and more...

Trey: Tim, stop. I don’t know where this crazy yeast-bot fantasy is going, but just to make sure our bread dough doesn’t attack us, let’s do this.

Trey takes the dough from its bowl, puts it on the flour-dusted counter, and punches it down until it’s back to its original size.

Vanessa: Huh! Where did all the extra go?

Trey: I squished the carbon dioxide gas bubbles out of the dough, like squeezing the air out of a balloon. Now we let it rise again, this time in a bread pan. Letting it rise twice improves the texture of the bread. After the second rise, we’ll bake it in the oven. Then: fresh bread!

Tim: Good! That ought to stop those yeast cells from taking over. I wonder if baking self-copying robots would be a good way of destroying them.

When finished with the Science Scene, ask students to answer these four questions.

You may opt to assign single questions in the third section, rather than having all the students answer all the questions. Students can discuss with their partners and try to reach an agreement about their answers to the questions. This activity is an opportunity for students to practice using scientific language as they discuss the questions.

Possible answers:

1. The organisms Trey listed were animals, plants, bacteria, and fungi. He also listed three kinds of fungus: yeast, mushrooms, and mold.
2. The word "individual" is another way of saying "separate."
3. The word "reproduction" is another way of saying "multiplying."
4. Trey said that the yeast in bread produce carbon dioxide gas.
5. Yeast cells need flour or "food" to produce more of themselves, plus enough time to reproduce. Warm conditions are best suited to yeast.

Extension: The four categories of organisms Trey lists fall into different kingdoms, so the first question is a good opportunity to discuss taxonomy. You can make this topic a quick aside; no need to delve deeply into the tree of life. If you want to take this digression, you can paraphrase the following:

American biology textbooks often list six kingdoms: Animalia, Plantae, Fungi, Protista, Archaea (bacteria), and (Eu)bacteria.

Texts in many other countries classify animals into five kingdoms: Animalia, Plantae, Fungi, Protista and Monera.

Comprehension Questions

Respond in writing to the questions, then compare
and discuss your answers with someone else.

​ Trey listed four kinds of organisms. What were they?

 

​ What is a different scientific word that is another way of saying "separate"?

 

​ What is a different scientific word that is another way of saying "multiplying"?

 

​ What gas did Trey say that the yeast in bread dough produce?

 

​ What did Trey say the yeast cells needed in order to produce more of themselves? Under what conditions does yeast work best?

 

Assign students to one or more of the questions to discuss in a group. Have students discuss with their partners and try to reach an agreement. This activity is an opportunity for students to practice using scientific language as they discuss the questions.

Here are some sample responses for the discussion questions. Your students will likely have much simpler answers or no answer at all to some of these questions. These questions will be answered by later activities in this unit.  Allow students to state their misconceptions, make note of the misconceptions, and actively address them later as you continue your exploration of cells. Misconceptions often persist even after students have “learned” that they are not correct. They know what to put on the quiz or test, but they don’t always see how their new understanding undermines their misconception.

Possible answers:

• When yeast cells multiply in bread dough, the matter that makes up the new cells comes from the flour that the yeast eats as food.
• The limit to how large the population of yeast cells can grow is the amount of matter in the food the yeast eats.
• Yeast need food, time, and a warm environment.

As you continue to explore cells in this unit, students will learn that all cells need instructions, ways to get energy and building materials, and a method to get rid of waste in order to live and reproduce.

Discussion Questions

You never hear about bakery emergencies in which runaway bread dough threatens to take over the world. There must be some reason why yeast doesn’t pose this kind of threat. Think about it…

When yeast cells multiply in bread dough, where do you think the matter that makes up the new cells comes from?

 

Do you think there’s any limit to how large the population of yeast cells can grow in a blob of bread dough? What, if anything, do you think is a limiting factor, the thing that keeps it from growing more?

 

What are some things yeast must be able to do in order to live and reproduce itself?

Be ready to share what you talked about in your discussion with the rest of the class.