Name:__________________________________ Cohort: __________________ Date:____________________
Is Yeast Alive?
Objective: Conduct a controlled experiment demonstrating understanding and importance of the experimental
design. Draw a conclusion regarding data and identifying possible sources of experimental error.
Humans use yeast every day to make bread rise and to brew beer, among other uses. This yeast consists of little
brown grains. But are these tiny grains alive? Before answering this question we need to think about what
makes something alive. What are the characteristics of life?
Scientific Experiment to Test for Metabolism
To begin to answer the question, "Is yeast alive?” you will test whether the grains of yeast display the ability to
use energy (referred to as metabolism). We will carry out an indirect test for metabolism by testing whether
yeast can use energy, which is one of the characteristics of living organisms.
When living organisms use energy, they break down high-energy molecules like sugar to get the energy they
need, 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. Specifically, we will test whether yeast produces a gas when it has sugar
available as a food vs. when no sugar is available.
PRE-LAB QUESTIONS:
1. What is the scientific question that this lab will answer?
2. What is your hypothesis?
3. Read the procedure below and DRAW and LABEL the experimental set up.
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 10 ml of hot water to each test tube.
4. Add 1 tsp of yeast to each test tube.
5. Add one packet of sugar to test tubes #1 and #2.
6. Fill each test tube ¾ full with hot tap water.
7. Carefully place your thumb over the opening of each test tube and gently shake to mix the solution. If
the yeast has clumped, use a stir stick to mix it.
8. Cover the opening of each test tube with a balloon (to catch any gas that is formed) using the balloon to
seal the end of the tests tube.
9. Make an initial observation of each test tube and record your observations in the table on the next page.
10. Every 5 minutes (for a total of 25 minutes) observe what occurs in the test tubes and any changes in the
balloons which cover each test tube and record your observations.
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.
•
EXPERIMENTAL SETUP:
Observations:
Test Tube:
1
2
3
4
0
5
Time (minutes)
10
15
20
25
POST LAB QUESTIONS:
1. What was the control group in this experiment?
2. What was the independent variable in this experiment?
3. What was the dependent variable in this experiment?
4. List 3 constants existing in this lab experiment.
5. After analyzing this data, what conclusion(s) can you make about yeast?
6. 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 solution?
7. What possible sources of experimental error could there be in this experiment?
8. 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?
Teacher Instructions for Is Yeast Alive? LAB
Standards:
IE a – Select and use appropriate tools and technology to perform test, collect data, analyze relationships, and
display data.
IE b – Identify and communicate sources of unavoidable experimental error.
IE c – Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions.
Objective: Student will conduct a controlled experiment demonstrating their understanding and importance of
the experimental design. Students will also draw a conclusion regarding data and identifying possible sources
of experimental error.
Duration:
2.5 days
LAB:
• Prior to LAB gather materials.
Materials Needed per Group:
§ Safety goggles
§ Test tube rack
§ 4 test tubes
§ 4 small balloons (need to fit over top of test tubes)
§ Yeast, sugar, and warm/hot water
§ Stirring rod
§ Measuring devices (10mL graduated cylinder, ¼ teaspoon)
•
Day 5 – Pre-Lab discussion (0.5 day)
o Hand out student lab sheet
o Have students read and answer the pre-lab questions.
o Check Q#3 prior to allowing students to conduct the lab. This is a great check to make sure they will
follow proper procedure AND have enough time to conduct the entire lab in one class period.
o You could have students MAKE their own data tables to assess/demonstrate their understanding of
organizing data.
•
Day 6 – Conduct LAB
o Give any last minute instructions, reminding them of proper lab safety.
o Some balloons may pop off of the test tube (hence the safety goggles).
•
Day 7 – Data Analysis
o Discuss class results. Probing students as to what conclusion they can make from their data and the class
results (if different.)
o Allow students time to answer post-lab questions – and finish as homework if needed.