THE YEAST OF OUR WORRIES
OVERALL PROBLEM
1. Do yeast demonstrate any of the characteristics of life?
EXPERIMENTAL PROBLEM:
1. Do yeast use energy when food (sugar) is available?
2. What evidences may be observed to indicate that cellular respiration
occurs?
MATERIALS PER GROUP:
Sugar
Warm water
7 test tubes
Test tube rack
Yeast
Bromothymol Blue
6 balloons
BACKGROUND INFORMATION
Living things need food. Food provides energy and building material to create more
living mass. Living things must be able to break down food in order to obtain
energy to carry out their activities. When food is broken down in a process called
cell respiration, carbon dioxide is made and released. The process of cell respiration
occurs in every cell. Carbon dioxide is a gas and thus will cause bubbles in solution.
These gas bubbles will escape from the solution into the atmosphere if not captured
somehow. A pH indicator known as Bromothymol Blue, tests for the presence of
carbon dioxide by turning color. In the presence of carbon dioxide Bromothymol
Blue will go from blue to green to yellow.
Yeast are tiny single-celled (unicellular) fungi. The organisms in the Kingdom
Fungi are not capable of making their own food. Fungi, like any other organism,
need food for energy. They rely on sugar found in their environment to provide
them with this energy so that they can grow and reproduce.
Yeast, like bacteria grow in or on their food source. They produce and release
digestive proteins (enzymes) into their environment where the sugar molecules are
found. Sugar molecules then break down into smaller molecules that can be
absorbed by the yeast and used for food (energy).
Yeast are facultative anaerobes, which means that they can respire or ferment
depending upon environmental conditions. In the presence of oxygen, respiration
takes place (aerobic respiration). Without oxygen present, fermentation occurs
(anaerobic respiration). Both processes require sugar to produce cellular energy
and both produce carbon dioxide as a waste product.
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PROCEDURE:
PART A:
1. Obtain six test tubes and label each test tube with your groups name as well as
the following:
a. Test tube 1
b. Test tube 2
c. Test tube 3
d. Test tube 4
e. Test tube 5
f. Test tube 6
2. Add to each of six test tubes 15 ml of warm water (4/5 full).
3. Slowly add 1.25 grams of yeast to test tubes 1-4, mixing the yeast in thoroughly
before adding more. Mix by putting your thumb over the top of the test tube and
shaking.
4. Add 1.25 grams of sugar to test tube 1, 2, 5 and test tube 6. Do not add sugar to
test tube 3 or 4.
5. Cover the opening of each of the six test tubes with a balloon to catch any gas
that is formed. Using the balloon to seal the end of the tests tube, hold a finger
over the end of each test tube, and shake it vigorously to thoroughly mix the
contents.
6. Make sure none of the materials remains clumped at the bottom.
7. Write time of initial setup _________________________
8. Record initial observations in data table below.
9. Record observations at 20- 30 minutes after completing setup record your
observations of the balloon and contents in the table below:
10. After 24 hours record your observations of the balloon and the contents in table
below:
PART B:
1. Wait until at least one balloon is inflated
2. Fill a 7th test tube with Bromothymol Blue
3. Carefully remove the balloon that has inflated the most making sure not to
release contents of balloon.
4. Slip balloon over mouth of test tube with Bromothymol Blue.
5. Squeeze contents out of balloon into test tube.
6. Gently mix test tube and observe and record any color changes.
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RESULTS:
THE EFFECT OF SUGAR ON YEAST
FROTH/BUBBLES
Test tube #
1
2
3
4
BALLOON INFLATION
5 6 1
2
3 4 5
6
BROMOTH
YMOL
BLUE
COLOR
CHANGE
7
X
INITIAL
TIME
20-30
MINUTES
24 HOURS
ANALYSIS-CONTENT
1. If you knew nothing about yeast except for the results of this
experiment, could you call yeast a living thing? Explain your
answer using evidence from this investigation.
2. What characteristic(s) of living things were demonstrated in this
investigation? Explain why.
3. Which test tubes did cell respiration take place? Explain
4. What substance served as food? What is the evidence for your
answers?
5. Was the food broken down and energy released? If so, what broke
down the food? What is the evidence for your answer?
6. What does the inflated balloon tell you?
7. What does the Bromothymol Blue changing colors from blue to
green or yellow tell you?
8. What does the presence of carbon dioxide indicate?
9. What is the name of the process that involves breaking down food
to extract energy for a living thing to use?
10. What are the reactants in the observed reaction given in # 9?
11. What are the products in the observed reaction in #9?
12. Why do organisms need food?
13. How did you know if carbon dioxide was being produced?
14. If bubbles appeared in test tube 3 and 4 or balloon inflated, how
did it compare to test tubes 1& 2 and how can you explain it?
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PROCESS OF SCIENCE BACKGROUND:
1. The control is the standard against which the researcher compares the
results from each treatment group (level) in the experiment. It is the same as
the treatment group(s) except it lacks the independent variable.
2. The independent variable is the variable that is intentionally changed in the
experiment by the researcher. The levels of the independent variable are
the different values of the independent variable, such as using water at 10°,
20°, 30°, 40°, and 50° C. The levels of the independent variable can also be
thought of as the experimental groups that are set up.
3. The dependent variable is the variable that responds to the changes in the
independent variable. It is what is being measured.
4. Constants (control variables) are the things that are kept the same in both
the control group and experimental group. As many outside factors as
possible should be kept constant in an experiment so that the researcher can
be sure that any changes that occur do so because of the independent
variable.
ANALYSIS-PROCESS OF SCIENCE
1. Which test tube(s) in experiment 1 would be considered a
control? Explain your answer.
2. In experiment 1, what is the independent variable?
3. In experiment 1, what is the dependent variable?
4. In experiment 1, list three constants or control variables.
5. Why is it better to have two test tubes with yeast, sugar, and
water, two test tubes with just yeast and water, and two test
tubes with sugar and water instead of only one test tube with
each type of mixture?
6. Why did testing the gas in the balloon with Bromothymol Blue
make this a better experiment?
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