Enriched Biology
Investigation: Are Yeast Alive?
Name____________________________________ Date_______
Introduction:
Dogs, cats, people, and trees are all living things. Rocks, water, and air are nonliving
things. Distinguishing between living and nonliving things seems simple, yet the differences
have been a source of argument among scientists for many years. But scientists agree that all
living things are made up of one or more cells, reproduce, grow and develop, obtain and use
energy, and respond to the environment.
Yeast are unicellular fungi. They cannot make their own food and must obtain it from the
environment, thus, yeast are referred to as heterotrophs. In this investigation, you must try to
prove that yeast are alive by using the life characteristic “requires energy”. Food is the source of
energy for an organism. Yeast need food like sugar. Sugar can be found in juice. When yeast
are given sugar, they can extract the energy out of the sugar by a chemical reaction. What is the
name of the chemical reaction? __________________________________Write the chemical reaction?
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In the Homeostasis: Investigating Breathing Rate lab, you learned that the blood becomes
acidic when you exercise. The carbon dioxide (CO2) from cellular respiration causes this to
happen. How? Water is found in many types of liquids. Blood is a liquid; coffee is a liquid; ice
tea is liquid. All of these are made with water. When CO2 mixes with water, an acid called
carbonic acid can form in the liquid and make it become acidic or more acidic. Knowing this will
help up prove that yeast is are living. If you are more interested in this, click on this: Making
Water Acidic with Carbon Dioxide
You Tube Demo About Bromthymol Blue: https://www.youtube.com/watch?v=fuOMKWBDAT0
In this You-tube video, let’s watch how bromothymol blue may be able to help us prove that yeast
require and use energy. Answer the following questions as you watch the video.
1. What color is bromothymol blue in the flask? ________________________
2. How do you know that carbon dioxide (CO2) is being dissolved in the bromthymol blue?
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3. What happens to the color of the bromothymol blue over time? ______________________
4. Why did this happen? __________________________________________________________
Carbon dioxide is a byproduct of a process called cellular respiration (organisms burn their food
and produce energy). What would happen if you blew into a beaker of bromthymol blue? _______
_________________________________________________________________________________________
Question: Are yeast organisms that require and use energy?
Learn about yeast. Click on the link: https://www.youtube.com/watch?v=Gs0XStzsbAs Now
read the investigation. As you read it, think of a hypothesis.
Materials:
3 small test tubes
3 Erlenmeyer flasks
Yeast mixture
Bromothymol blue
Test tube rack
One-hole rubber stopper
Glass tubing
Rubber hosing
pipette or graduated cylinder
label tape
marker
Procedure:
1. Obtain one Erlenmeyer flask and one small test tube. Label the flask yeast + water, yeast
+ juice OR water + juice using label tape.
2. Place 5 mL of bromothymol blue into the small test tube and place the test tube in the test
tube rack.
3. Place 30 mL of yeast + grape juice mixture or the yeast + water mixture or the water +
juice mixture OR water + juice into the flask.
4. Place a rubber stopper with a glass tubing and hosing firmly into the mouth of the large
flask.
5. Place the flask next to one of the small test tubes containing bromothymol blue as
demonstrated by your teacher.
6. Seal the small test tube with parafilm.
7. Record your initial observations in Data Table 2.
8. After ______ hours, again record your observations in data table 2.
9. Clean up all materials as per your teacher’s instruction.
Hypothesis: Write a hypothesis for this investigation. Remember, a hypothesis states what you
think will happen and why based on time –tested ideas (theories) or real observations.
Data Table 2
Initial Observation
Color of
Color
Presence of
Bromothymol
Of Mixture
Bubbles
Blue
Final Observation
Color of
Color
Presence of
Bromothymol
Of Mixture
Bubbles
Blue
Test Tube:
Yeast and
Juice
Test Tube:
Yeast and
Water
Test Tube:
Water and
Juice
Analysis Questions:
1. What does the production of carbon dioxide gas indicate about the yeast?
2. How do you know that the carbon dioxide gas was produced?
3. Why should the yeast with juice be more active than the yeast with only water?
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4. Why did you have a setup with juice and water? _______________________________________
__________________________________________________________________________________________
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Conclusion: Write a conclusion on the back of this paper.