Is It Alive?
In small bowls at the front of the room you will find some light brown powder. You maybe
familiar with this powder, it is yeast and it’s used in baking all kinds of good stuff. It is your
mission in this lab to design and carry out an experiment to determine whether or not the
yeast is alive. You already know that living things:
Are orderly/organized (cells)
Grow and develop
Obtain and use energy
Respond to their environment
Adapt and evolve
In order to determine that, in fact, the yeast is alive you will have to devise means for
testing the powder for as many of the characteristics as possible.
Part One: Respiration: Can Yeast Use Energy?
Most living things will take in oxygen to help burn up their food and release carbon dioxide
as a result. The change is described by the following chemical reaction:
C6H12O6 + 6O2 6CO2 + 6H2O + Energy (As ATP)
This process is called aerobic respiration since it uses oxygen to breakdown glucose
(C6H12O6) into carbon dioxide (CO2) and water (H2O). The energy created by this reaction
(ATP) fuels all of the cellular processes that enable an organism to live. 93% of all of your
bodily functions require ATP. Without it, you wouldn’t exist. Carbon dioxide is a waste
product that living organisms must get rid of. It is poisonous to cells and the releasing of
CO2 is a good indicator of energy consumption. When CO 2 is dissolved in water an acid is
formed. You can use an indicator solution to test for the production of CO2. This is a
solution that will change color if CO2 is present in the water. The solution you have at your
disposal is bromthymol blue. Bromthymol blue will turn from blue to green to yellow in the
presence of an acid such as CO2.
To test for CO2 production you will need to use the following set up
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. Measure out 0.2 g of dry yeast on the balance. Put the yeast in a small cup.
4. Using a graduated cylinder measure out 60 ml of warm tap water and add it to the
yeast in the cup. Give it a stir to dissolve the yeast in the water.
5. Pour 15 ml of the yeast solution into each of the four test tubes.
6. Measure out 7 g of sugar and place it into test tube 1. Repeat for test tube 2.
7. 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, hold a finger over the end of
each test tube and shake it vigorously to thoroughly mix the contents.
8. Observe the test tubes and record your observations carefully in the table on the
next page. Then, every 5 minutes for 25 minutes, observe what occurs in the test
tubes and any changes in the balloons which cover each test tube, and record your
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. While you are waiting for
this change, set up part 3.
0 minutes 5 minutes 10 minutes 15 minutes 20 minutes 25 minutes
9. Discuss the results you obtained with your group. How do you interpret your
10. 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
11. 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?
Part Two: Testing for simple sugars
All living organisms breakdown their food into simple sugars. Enzymes in our bodies help to
take the cheeseburger and fries at lunch and make a simple sugar out of it. Indicator
solutions are available to help identify the presence of simple sugars. Benedict’s solution
will help do this. Simple sugars will turn orange/red in the presence of Benedict’s solution
after it is broken down.
1. Devise an experiment to test for the presence of simple sugars in three solutions,
glucose, sucrose, and starch.
2. First, run a practice Benedict’s test to see what the reaction should look like. This is
called a control, because you need to know what the results should be before you can
compare to the brown powder.
3. Fill a 250ml beaker about half full with water. Place it on a hot plate and bring it to
4. Put on your safety glasses/goggles.
5. Get three test tubes. Label one A, the other B and the third C.
6. Fill tube A ¼ full with glucose solution.
7. Fill tube B ¼ full with sucrose solution.
8. Fill tube C ¼ full with starch solution.
9. Add 20 drops of Benedicts solution to each tube. Each tube should turn pale blue in
color. Record your initial tube colors in the observations.
10. Place all three tubes into the hot water bath and wait a two or three minutes.
Record any color change you observe in any of the tubes. Yellow or orange indicates
a positive test for simple sugar.
11. Clean everything up. Wash the test tubes out with soap and water using a test
Part 3: Design Your own Experiment: Okay, now you know what to look for when testing
for simple sugars. Design an experiment with your lab partner to test if the yeast can
convert complex sugars such as starch and sucrose into simple sugars like glucose. Be sure
to include a control group and a variable.
THINK ABOUT THIS FOR A LITTLE WHILE, TALK TO YOUR PARTNER. DON’T
PANIC! YOU NEED TO REASON YOUR WAY THROUGH THIS PROBLEM!
Write down a procedure and then come to me so I can okay it before you begin. Make sure
you take good observations and present data in tables. Clean up when you’re done.
Part 4: Procedure to Test for Growth
Research Question: Can the little brown grains of yeast grow?
1. Obtain a Petri dish with yeast growth media, and label the bottom with your name,
teacher, and class period. (A Petri dish is a flat, covered dish used by scientists,
and the yeast growth media in the Petri dish contains a mixture of substances that
yeast requires to grow.)
2. Spread10-12 grains of yeast across your plate.
3. Add several drops of water on the grains of yeast.
4. Your plates will be incubated at 37° C until the next lab class. How warm or cold is
37° C is equivalent to _______° F.
5. At the next lab class, inspect your plate. Do you see any signs of growth on the
plate? Sketch what you see.
6. Take a sample of the growth and observe it under the microscope. Describe what
Based on your findings, do you think the little brown grains of yeast are alive? Explain why
or why not.
Is It Alive?: Lab Report
Title: 5 Points
Purpose: 5 Points
Background Information Questions: 40 points (4 points each)
1. List the seven characteristics of life.
2. What is a control and why are they important in biological experiments?
3. What is aerobic respiration and why do cells use it? (look this up)
4. Where is energy stored in chemical compounds like glucose?
6. Explain what the significance of the balloon in the respiration test.
7. What type of sugar would you give the yeast to use as an energy source? Explain
8. Describe how you performed the Benedict’s test.
9. Explain what the Benedict’s test proves.
10. What characteristics of life can you observe using a microscope?
Data and Observations: 40 Points (include tables, results for each test and
observations you made).
Analysis Questions: 50 Points (10 points each)
1. What do the results of the growth test on the Petri dish tell you about the yeast?
2. Which of the seven characteristics of life were you able to observe in this lab?
3. Part three of this lab is considered a control. Explain why.
NOTE: THE ANSWER TO THE PREVIOUS THREE QUESTIONS IS NOT THAT THE
POWDER IS ALIVE!
4. Can you make an inference about the growth and development of the cells? Explain
what you can infer from your results.
5. What do you suppose was in the “gel” that the Petri dishes were filled with? Base
this answer on your data and observations.
Conclusion: 30 Points
I. Based on your observations and answers to the analysis questions decide
whether the yeast is alive. Write an explanation how each investigation
provided you with proof for the presence or absence of each
characteristic of life. (15 points)
II. Why was it necessary to have a control to prove that the yeast was
responsible for the changes you observed in the flask? (15 Points).