The Sensational Single Cell

Use this 25-second video as a seed for the discussion prior to beginning the lab. Ask students to share their ideas about what they know about yeast so far. This lab assumes you have done other lessons in L5 The Sensational Single Cell. or have introduced the properties of cells in other ways.

Play the video once, then ask for observations (not inferences). Ask:

What does the life of a yeast cell look like at an individual level, through a microscope?

What do you see?

What does it remind you of?

You are not looking for any one correct answer, but rather waiting to see where students take the discussion. Steer them away from dead ends and toward fruitful finishes.

Play the video again with instructions to choose one spot in the field to watch. Again ask students for their observations.

Ask students to compare and contrast what they have observed with what they learned about yeast in the other lessons, or to the behaviors of Dr. Otto's robots. Take notes on the white board using a Venn diagram.

Play the video a third time. Ask the students to Turn and Talk: did they capture all the comparisons and contrasts? If not, give them another peek by playing the video a fourth time.

Paraphrase:

While we won’t be able to see a single sensational yeast cell as close up as Dr. Otto could see his robots, we can see evidence that the yeast cells use energy and materials, dispose of waste, and reproduce themselves.

Yeast Growing

 

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Credits for video: Yeast Budding video shown with permission from University of Massachusetts Amherst Technology Transfer Office. Distributed by the University of Massachusetts Amherst under a license agreement with the Education Development Center, Inc. Excerpted from the Developmental Biology Film Series episode 57; The Elementary Science Study Film loop ES-11. Produced under a grant from the National Science Foundation. Biologist: Clifford Anastasiou. Microcinematographer Joseph V. Durden. ©1965 Educational Services Inc. (now Educational Development Center, Inc.) All rights reserved.

Preparation: Before class, label two bottles "A" and "B." Pour the packet of yeast into "A." Pour two spoons (about 30 mL) of baking soda into the "B." You can pour it straight from the packet, or use a funnel if you have one, or paper bent into an open-tipped cone.

For bottle "A," you will mix to dissolve the outer layer of dead yeast cells on the granules, and in bottle "B" you will mix to dissolve the baking soda.

Run a demonstration at the start of the class. In this front-of-class demo, you will show the differences between yeast and baking soda as leavening agents. The students will not know which substance is in which bottle.

Leavening agents are the things that add the air pockets to baked goods. Sometimes we use yeast (as in yeast breads, like a baguette or a sliced sandwich loaf) and sometimes we use acid and bases that react (as in many "quick breads," such as a pumpkin loaf or banana muffins, which use baking soda and something sour like buttermilk).

Summarize your steps as you run the demo for your class. As students observe the changes happening in the bottles, it’s likely someone will say that one of them is baking soda and vinegar. This is an inference, and they should be made to back it up by stating observations.

Paraphrase:

In one of these bottles, I have a living sample of yeast, a fungus. In the other, I put baking soda. We'll add warm water to each.

Does anyone know what happens when you add water to yeast?

What happens when you add water to baking soda?

Does anyone know what happens when you add sugar to yeast? Are there any by-products of mixing the two?

What happens when you add vinegar to baking soda? Are there any by-products of mixing the two?

Possible answers:

Water "wakes up" the yeast by removing the granules' outer shells of dead cells.

Water dissolves the baking soda.

Yeast eat sugar. Yeast cells release carbon dioxide.

Baking soda and vinegar combine to make carbon dioxide.

Note that both mixtures yield carbon dioxide as an output, and that the balloons fill with this carbon dioxide.

Show the next slide with the chemical formulas written as an equation. Students don’t need to understand the formulas, but exposure is good. See if they can figure out which chemical formula is vinegar (CH3COOH) and which is baking soda (NaHCO3). They will likely guess rather than know the answers.

Paraphrase:

Baking soda and vinegar combine to produce carbon dioxide. It is a chemical, not a biological process.

Yeast produces carbon dioxide after eating sugar. Over time it will make more yeast.

Let's visit this setup again later.

Since yeast will continue to grow overnight, we will be able to make more gas in its bottle the next day. The baking soda won’t make more baking soda.

In this part, students change the conditions for the yeast and identify variables to change. If your students are unfamiliar with dependent and independent variables, see more about them here.

Having demonstrated what happens with one bottle of yeast, water and two cubes of sugar, and how it compares to a similar setup with baking soda, challenge your students to come up with ways to change the conditions for the yeast. As a class, you will run an experiment to find yeast's optimal growth conditions.

As a class, talk about what factors you can vary in the experimental setup. Each student should choose just one variable to change. Variables in this setup include:

• amount of yeast
• amount of sugar
• amount of water
• temperature of water
• temperature of environment

Keep track of what variables the students choose to change, and encourage the students to change their variable differently. Or, to keep things simple, you can also assign variables to groups. So, for example, no more than three students should increase the amount of sugar added to the bottle to 40 mL of sugar. If a fourth student wants to increase the sugar, encourage them to choose a different larger value. This variation will give you a better spread of data as a class. Not all the variables need to be tested. In fact, you could have the students only test the results with different water temperatures, or different amounts of sugar. If you have enough bottles, each student or lab pair/group can set up two or three bottles with different values for the variable.

You will set up a control similar to the yeast bottle in the class demo.

Before you run the experiment, the students should write a hypothesis. Ask the students:

What do you think will happen in your bottle, compared to the control setup? Why?

If time permits, students should write their own procedures. The more they write for themselves, the more thinking they do about how their actions will affect their results.

Keep the focus on objective description, with the mantra, "If you don’t know what you saw, then you don’t know what it means."

Students follow the procedure to set up their bottle, changing one aspect of the setup and keeping all others constant. They observe and record their immediate impressions and observe the results of what happened with the yeast lab setup overnight.

For students who used different temperatures of water in their experiment, comparing their results to this Exploratorium bread baking chart may be helpful.

The students measure and record their data after the yeast bottles have sat overnight. When finished with the activity, ask students:

• What did you observe in your setup?
• If there was another student with a similar change in their variable, how do their results compare to yours?
• Explain what you think is happening, using what you observe and what you know about baker’s yeast.

Graph the data as a class. Collect data from the different researchers who worked on the same variable.

Write up results. The main focus here is on objective, descriptive reporting, as opposed to interpretation. Both kinds of writing are important in science; it’s important to distinguish between them and practice both.

Distinguishing findings and speculations

Respond in writing to the questions, then compare and discuss your answers with someone else. What's the difference between findings and speculations? For example, which of these two statements is a finding? A speculation?

The yeast produced more gas to inflate the balloon
when the amount of sugar was increased.

vs.

The yeast must really like sugar!