Name:___________________________________DATE:__________________________PERIOD:____
How Do We Know Yeast Is Alive?
We will test two characteristics of living things using yeast.
1. response to the environment - Yeast responds to the presence of food (sugar) by absorbing it. We
can’t see yeast absorbing its food, but we will know it has eaten when we collect its waste using a
balloon.
2. metabolism – After yeast absorbs the sugar, it breaks it down into ethyl alcohol, energy and
carbon dioxide. When we catch the carbon dioxide with a balloon, we know that yeast has a
metabolism.
Hypothesis:
If we add sugar to yeast, then it will produce
___________________________________________________.
Materials:
Two test tubes and rack
String
Yeast packet
Scissors
Very warm water (If you use HOT water you will kill the yeast!)
Ruler
Sugar packet
Newspaper
2 Balloons
Gloves
Experimental Design:
What is the Independent variable?___________________________________________________
What is the dependent variable?_____________________________________________________
How will you set up the control
group?_______________________________________________________________________________
_____________________________________________________________________________________
How will you set up the experimental group?
___________________________________________________________________________________
Procedures:
1. Put newspaper down on your table. This might get messy!
2. Label your test tubes 1 and 2 with tape.
3. Test tube 1 (your experimental group) will contain yeast, sugar and water. Test tube 2 (your
control group) will contain only yeast and water.
4. Fill both tubes about 1/2 full with warm water (NOT HOT OR YOU WILL KLL THE YEAST).
5. Make a tiny rip in the top of the yeast packet. SLOWLY, ADD A SMALL AMOUNT of yeast
to each tube. Put a glove on so you can place a finger over the opening of the tube and
shake it gently. ADD A SMALL AMOUNT AND SHAKE. ADD A SMALL AMOUNT AND
SHAKE. STOP WHEN THE TEST TUBE IS ABOUT ¾ full of yeast and water.
6. Make a tiny rip in the top of the yeast packet. SLOWLY, A SMALL AMOUNT OF SUGAR TO
TEST TUBE 1 ONLY. Put a glove on so you can place a finger over the opening of the tube
and shake it gently. ADD A SMALL AMOUNT AND SHAKE. ADD A SMALL AMOUNT
AND SHAKE. STOP WHEN THE TEST TUBE IS ALMOST FULL. You should notice bubbles
beginning to form at the top of the liquid in the tube.
7. QUICKLY, Put a balloon over the end of each tube.
8. Measure the circumference of the balloon in centimeters every five minutes for the next 25
minutes. DO NOT REMOVE THE BALLOON FROM THE TEST TUBE. Fill in the table
below. Measure the circumference by wrapping a string around the thickest part of the
balloon. Cut the string where it meets around the balloon. Then put the string onto a ruler
to obtain the measurement.
9. While you are waiting, please read “The History of Yeast.”
Test Tube
1
2
After 5 mins
After 10 mins
After 15 mins
After 20 mins
After 25 mins
10. If your balloon does not begin to inflate after 10 minutes, put both tubes into the same cup
of very warm water and take your first circumference five minutes later.
Analyze your Data:
1. What happened to the balloon over the test tube with the yeast, water and sugar?
2. What happened to the balloon over the test tube with the yeast and water?
3. What gas is in the balloon?
4. Where did it come from?
5. Was your hypothesis correct?
Formulate a Conclusion:
Yeast is alive because (write the two characteristics of living things you tested)
_____________________________________________________________________________________
______________________________________________________________________________
Research Questions (use page 224 in text to answer):
1. Yeast and flour are mixed together and baked to make bread. The yeast eats the flour and
turns it into ethyl alcohol and carbon dioxide gas. How does carbon dioxide make dough
rise?
2. Name three facts you did not know about yeast from the story “The History of Bread Yeast.”
3.
Imagine you are interested in testing this hypothesis.
Hypothesis: If I add the artificial sweetener to yeast, then it will produce carbon dioxide gas.
What is the Independent Variable:________________________________________________________
What is the Dependent Variable:_________________________________________________________
What is the Control Group:
_____________________________________________________________________________________
_____________________________________________________________________________________
What is the Experimental
Group:_______________________________________________________________________________
_____________________________________________________________________________________