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To Yeast or Not to Yeast

Background: Dr. Art's Guide to Planet Earth, pages 62-65.

Utah State Earth Systems Core:
Standard 3600-01: Students will investigate biological systems and summarize relationships between systems.
Objective 03: Analyze the carbon cycle.
Objective 04: Evaluate the influence of people on the biosphere.

Purpose:
Respiration is an important part of the global carbon cycle. To get energy, organisms (plants, bacteria, animals, yeast, etc.) respire. That means they internally burn sugars back into carbon dioxide gas which is then released into the atmosphere. We can use yeast to experiment with and explore this process. Human activities that change conditions in the air, water and land affect other organisms. We can add different chemicals to the yeast and determine what effects they have by measuring any differences in the observed respiration.

Equipment and Supplies:
For each person or group: 6 test tubes; test tube rack; 100 mL beaker; styrofoam cup (not as tall as the test tubes that you are using); larger beaker or other container to steady the styrofoam cup; thermometer; balance that can weigh gram amounts; 25 mL graduated cylinder; a teaspoon of rapid rise yeast; sugar; water; at least ten balloons that fit tightly over the test tube mouth (smaller balloons sizes are better; long and thin is better than round); vinegar; ammonia; liquid plant fertilizer; liquid bleach; small cups or beakers; heat source; pipettes or eyedroppers.

Safety Issues:
If you are under 18 years of age, do not perform this experiment unless you are being directly supervised by a responsible adult (teacher, parent, other educator or guardian). Wear safety glasses and dispense chemicals in small quantities. Turn heat sources off when not in use. Do not get any chemicals in or on eyes, skin, mouth, body. Immediately wash any spills with lots of water. Read all labels on bottles, and strictly observe safety precautions.

Duration:
50 minutes

Procedures:

  1. This experiment should be done by working groups of 2 to 4 people. Read the directions and assign roles before beginning (materials manager; experimenter(s); recorder; timer).
  2. Raise the temperature of a water bath to 40 degrees Celsius and maintain close to that temperature. Amount of water should be at least 100 mL for each group doing the experiment.
  3. Label the test tubes 1 through 6 with a marker or tape. Using the Table below as a guide, add the appropriate substances to each of the test tubes. DO NOT ADD THE YEAST SOLUTION.

    Test Tube 1 Test Tube 2 Test Tube 3 Test Tube 4 Test Tube 5 Test Tube 6
    5 mL yeast solution 5 mL yeast solution 5 mL yeast solution 5 mL yeast solution 5 mL yeast solution 5 mL yeast solution
      1 g sugar 1 g sugar 1 g sugar 1 g sugar 1 g sugar
        25 drops ammonia 25 drops vinegar 25 drops bleach 25 drops fertilizer solution


  4. Each group makes a yeast solution by mixing 3 to 4 g of dry yeast (about one teaspoon) in 50 mL of warm water. Mix the solution so it is uniform. Add 5 mL of yeast to each test tube (use the graduated cylinder or a measuring pipette). Gently shake each tube to mix the substances.
  5. Carefully place the opening of balloon over the mouth of the test tube. Try to remove all the air from the balloon before you attach it over the mouth of the test tube. It may be easier if one person holds the test tube while another attaches the balloon. The balloon needs to fit tightly over the mouth of the tube.
  6. Fill the styrofoam cup one half to two thirds full of 40 degree celsius water. Put the test tubes in the styrofoam cup to help keep the solutions warm and at the same temperature. Make sure the styrofoam cup is steady and will not tip over. (Hint: you can put it in a larger beaker).
  7. Record the amount of gas in the balloon. Start from the very beginning and observe/record at 5, 10, 15 and 20 minutes. Write your observations by giving each tube a symbol for the amount of gas (0 for no gas; + for a little gas; ++ for a medium amount of gas; and +++ for a lot of gas). Record the results in a data table.
  8. Draw a picture of the balloon after 20 minutes.

Time 1 2 3 4 5 6
Contents:            
0            
5            
10            
15            
20            
drawing:            

Analysis:

  1. Which tube acted as the control and what did it prove? Is there another control tube that could have been done? (ANSWER)
  2. Which chemicals seemed to encourage yeast respiration?
  3. Which chemicals seemed to discourage yeast respiration?

Conclusions and Discussion:

  1. Trace the flow of carbon in the experiment. Start with photosynthesis producing the sugar. (ANSWER)
  2. Some industrial processes (such as burning coal to make electricity) can make rainwater more acidic. Does your experiment provide any evidence that changes in acidity can affect organisms?
  3. Fertilizers used in agriculture often get washed into groundwater, streams and lakes. Does your experiment provide any evidence that the use of fertilizers can affect ecosystems?

Appendix:

  1. Yeast are living organisms. You can examine them under the microscope. They reproduce by forming buds. People use yeast in baking (where the released carbon dioxide causes breads and pastries to rise). People also use the yeast in making wine and beer where conditions are adjusted so the yeast break the sugar down to alcohol and not to carbon dioxide.
  2. In general, a source of nitrogen such as fertilizer and ammonia causes increased activity in the yeast. Chemicals that make the solution more acidic (vinegar), basic (ammonia, chemical bases) or toxic (bleach) cause reduced activity. Depending on the amount, ammonia may increase or decrease gas production.
  3. Other chemicals can be substituted for the compounds suggested. Any solid substances such as fertilizer or borax need to be dissolved in water. It should be a concentrated solution. You can test many different safe chemicals from home, the store, or school laboratories. You can also experiment with the effect of temperature on yeast respiration.
  4. ADVANCED: You can actually test the gas that is produced and captured in the balloon to show that it is carbon dioxide. You need to bubble the gas from the balloon into a solution. There are two different solutions that you can use. One way is to show that the gas acidifies a water solution using bromthymol blue as the indicator dye. A more specific way is to show that the gas in the balloon forms a milky white precipitate when added to a saturated lime solution (calcium hydroxide). We will put these on the web at a later date. If you want to receive email updates when we add new features to the web site, click here to register.