Is Yeast Alive?
Adapted from "Is Yeast Alive?" by Penny Bernstein at Kent State University, Stark Campus
copyright 2011 by Dr. Ingrid Waldron and Jennifer Doherty, University of Pennsylvania Biology Department[1]
You can buy yeast to make bread in the grocery store. This yeast consists of little brown grains. Do you think that these little brown grains of yeast are alive? Why or why not?
To find out whether yeast is alive, we first need to think about what makes something alive. What are some characteristics of living organisms?
To begin to answer the question, "Is yeast alive?", you will test whether the grains of yeast have a chief characteristic of living things -- the ability to use energy (referred to as metabolism).
Scientific Experiment to Test for Metabolism
We will carry out an indirect test for metabolism. In other words, we will be indirectly testing if yeast carries on metabolism, which is one of the characteristics of living organisms.
When yeast, humans, and other living organisms use energy, they break down high-energy molecules like sugar to get the energy they need and give off carbon dioxide as a by-product of this reaction.
We will test whether yeast can metabolize sugar and produce a gas which we will presume is carbon dioxide. Specifically, we will test whether yeast produces a gas when it has sugar available as a food vs. when no sugar is available.
Research Question:
Does yeast metabolize sugar and produce a gas?
Predictions:
Do you expect yeast to produce a gas when sugar is available? ______
Do you expect yeast to produce a gas when no sugar or other food is available? _____
Do you expect yeast to produce a gas when only corn starch is present? _______
Do you expect yeast with sugar and also bleach to produce a gas? _______
Do you expect salt to affect the production of a gas? _______
Explain the reasons for your predictions.
What you will need for this experiment:
5 small (~ 1 cup) narrow mouth bottles (narrow enough for a balloon to fit over it). These bottles must be the same size.
5 small balloons, identical in size
Yeast, fresh, one package
Bleach, sugar, salt, corn starch
Procedure to Test Your Predictions
Set up five small bottles and number 1 - 5.
In another container, mix 1 packet of yeast with exactly 1 2/3 cups of warm tap water. Add the dry yeast a little bit at a time, mixing the yeast in thoroughly before adding more. Mix thoroughly. When you add this mixture to the bottles (below), try to keep the suspension mixed up, so the amount in each bottle is the same.
Label each bottle with a number, 1-5. Fill them as follows:
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Bottle
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Yeast suspension, 1/3 cup
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sugar, ½ tsp.
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corn starch, ½ tsp.
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Bleach, 1 tbsp.
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salt, 1 tsp.
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1
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x
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2
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x
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x
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3
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x
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x
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4
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x
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x
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x
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5
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x
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x
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x
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Mix the contents of each bottle thoroughly.
Cover the opening of each bottle with a balloon to catch any gas that is formed. Set the bottles in a room temperature location
Observe the bottles and balloons and record your observations carefully in the table on the next page (the table will expand as you enter your findings). Then, every 5 minutes for 25 minutes (the time necessary to see results may vary, so alter the times as necessary, but change the table below to show this), observe what occurs in the bottles and any changes in the balloons that cover each bottle, and record your observations. Be careful not to let the balloons slip off, or to expand to the popping point.
If the yeast grains are capable of metabolism, it will take some time to produce enough carbon dioxide to see the change in the balloons.
Observations of Yeast Bottles
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Bottle
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0 minutes
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5 minutes
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10 minutes
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15 minutes
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20 minutes
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25 minutes
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1
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2
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3
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4
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5
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How do you interpret your results? Do your results match your predictions?
What is the point of having a bottle with just yeast and water?
When you make bread, if you just mix flour, sugar and water, the dough does not rise, and the bread will be flat and hard. If you include yeast in the bread dough, then the dough rises and the bread is bigger and fluffier. Can you explain how the yeast helps the bread dough to rise?
Can you think of other similar experiments to investigate what might affect the CO2 production by the metabolism of sugar by yeast (e.g. temperature, light, sound, etc)?
[1] Teachers are encouraged to copy this student handout for classroom use. A Word file (which can be used to prepare a modified version if desired), Teacher Preparation Notes, comments, and other hands-on activities are available at http://serendip.brynmawr.edu/sci_edu/waldron/. Additional biology activities are available at http://serendip.brynmawr.edu/exchange/bioactivities.