Name _____________________
Period ________
Date ________
Is Yeast Alive?
1. Humans use yeast every day. What are some common uses of yeast?
2. To find out whether yeast is alive, we first need to think about what makes something alive. What are
four of the seven characteristics of a living organism?
To answer the question, "Is yeast alive?,” you will test whether the grains of yeast have one
characteristic of living things -- the ability to use energy (referred to as metabolism).
Instructions to Test for Metabolism
We will carry out an indirect test for metabolism. In other words, we will be indirectly testing whether
yeast can use energy in the same way as other living organisms.
When yeast, humans, and other living organisms use energy, they break down high-energy molecules
like sugar and give off a gas called 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.
1. Do you expect yeast to produce a gas when sugar is available? _______
2. Do you expect yeast to produce a gas when no sugar or other food is available? _______
3. Based on your answers above, are the little brown grains of yeast are alive? Write a hypothesis below
to this question below:
To test your predictions on metabolism, use the following procedure.
1. Set up four test tubes in a test tube rack.
2. Label each tube with a number, 1-4. Test tubes 1 and 2 will both have yeast, sugar and water. Test tubes 3
and 4 will both have only yeast and water, with no sugar.
3. Add 15mL of warm water to each test tube.
4. Add 1 gram of sugar to tubes 1 and 2. STIR! These tubes will be your experimental group. Do not add sugar
to tubes 3 and 4!
**BE READY TO COVER THE TEST TUBES WITH A BALLOON AS SOON AS YOU ADD THE YEAST AND
STIR IN STEP 5!
5. Now carefully add 0.7 grams of yeast to each tube (test tubes 1-4) and STIR!
6. Cover the opening of each test tube with a balloon to catch any gas that is formed.
7. Observe what occurs in the test tubes and any changes in the balloons, which cover each test tube every 3
minutes. Record your observations carefully in the table on the next page. Remember that observations
should include color, bubbles, size of the balloon, etc. Be as detailed as you can be. At the end of 15 minutes,
you will record the height in cm of the balloon.
***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. Draw a picture at 0, 5, 10, and 15 minutes and describe what
happened. Include the liquid in the test tubes!
DESCRIPTION:
DESCRIPTION:
DESCRIPTION:
DESCRIPTION:
0 Minutes
5 Minutes
10 Minutes
15 Minutes
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Final Height Measurements:
Test tube 1: ____________________
Test tube 2: ____________________
Test tube 3: ____________________
Test tube 4: ____________________
Questions: Testing for Metabolism
1. Which requirement for life were we testing for in this lab? __________________
2. What is the independent variable in this experiment? ______________________
3. What is the dependent variable in this experiment? _______________________
4. What is the control group in the experiment? ____________________________
5. What are some controlled variables in this experiment?
6. Using a sign test (table below), is there a significant difference in the test tubes that contained sugar
compared to the ones that did not?
Test Tubes with Sugar
Test Tubes with No Sugar
Sign (+, -, 0)
7. Did the yeast show metabolic activity? Does this support your hypothesis or reject your hypothesis.
8. Why is it better to have two test tubes with yeast, sugar, and water and two test tubes with just yeast
and water, instead of only one test tube with each type of mixture?