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Rate of Fermentation (How Yeast Get Energy)

Purpose: To study how the rate of fermentation by yeast cells is affected by the concentration of molasses. You will be using yeast cells (microscopic organisms) and molasses (a product of photosynthesis in sugar cane). You will measure the rate of fermentation by measuring how much carbon dioxide (CO2) is given off by the yeast cells over a certain period of time.

Materials

Goggles
6 test tubes (18mm x 150 mm, for example), rimless is best
6 test tubes (10mm x 75 mm, for example), rimless is best
50 mL graduated cylinder
Medium sized beaker (for waste)
Test tube rack
Metric rule
6 little squares of aluminum foil (about 4 cm by 4 cm)
100 mL beaker with 50 mL of 25% molasses solution
20 mL of yeast suspension
Dropper
Marking pen
Masking tape

Introduction to the Student

Even cells as small as yeast cells need to obtain the energy to carry out life processes. Because yeast cells are so small, they do not require as much energy from their food as large multicellular organisms do. Yeast use a process called fermentation. What is fermentation? Fermentation is a way for cells to get energy without using oxygen. Small organisms can break down complex organic substances such as sugar into simpler ones and release the energy that is in the carbon-carbon bonds. The waste products of this process are molecules such as ethyl alcohol and lactic acid, as well as other.

Human beings have known about fermentation for a long time. Food can be spoiled by fermentation, food can be made by fermentation, and muscle cells use fermentation to give us quick bursts of energy. Louis Pasteur in 19th century helped us understand that fermentation is the result of the action of small organisms such as yeast and bacteria. In this lab we will be using yeast cells. Yeast cells break sugar down into ethyl alcohol and carbon dioxide, releasing a small amount of energy to be used by the cells for other processes.

Procedure

1. Number the 6 large test tubes 1-6. Put your lab group name on some masking tape and place the tape on your test tube rack.

2. Fill test tube one almost to the top with 25% molasses solution.

3. Measure 25 mL of molasses solution in the graduated cylinder. Add 25 mL of water; mix well.

4. Fill test tube two almost to the top with the solution from step 3.

5. Pour off all of the solution in the graduated cylinder except for 25 mL.

6. Add 25 mL of water to this remaining solution; mix well.

7. Fill test tube three with the solution from step 6.

8. Continue with this serial dilution until you have filled test tubes 1-5 with molasses solutions.

9. Fill test tube 6 almost to the top with plain water.

10. Shake the yeast suspension very well and add 10 drops of yeast to each of the 6 test tubes. Shake the yeast suspension each time before you add it to a test tube.

11. Mix the yeast and molasses solutions in each test tube. You can do this by holding your thumb over the mouth of the test tube and inverting a few times.

12. This step is the tricky (and sticky!) part. Working with each tube of solution one at a time, you will do the following: Take one of the small test tubes. Fill the small test tube up to the top with some of the solution that is in large test tube #1. Then holding the large tube in one hand and the small tube in the other hand; tip them slightly so that they are aimed toward each other. Then very quickly invert the small tube into the large tube. If you get bubbles in your small tube (and you probably will!) remove the bubbles of air by placing your thumb over the top of the large tube (trapping the small tube inside) and then tilting the tubes upside down. Very slowly return the tubes to the upright position and let any bubbles slide between the two tubes and up to the top of the solution.

NOTE: If you end up with a little bubble of air at the top of the small tube, you can measure the length of this column of air, record it in the data chart, and then subtract that amount from the data that you collect the next day.

13. Lightly cover each test tube pair with a piece of aluminum foil and place the tubes in the test tube rack. Put the rack in a warm place.

14. The next day, measure the length of the column of gas in each small test tube and record these amounts.

Hypothesis, Data, Conclusion:

1. Hypothesis: Before you record your data, state your hypothesis based on the introduction to this lab.

2. The “independent variable” in an experiment is the factor that you control, while the “dependent variable” changes depending on the conditions of the experiment. a. What is the dependent variable in this lab?

b. What is the independent variable in this lab?

3. How is the activity (rate of fermentation) of the yeast measured?

4. The molasses solution used in test tube # 1 is 25%. Based on the method you used to produce your diluted solution, what are the percentages of molasses in each of the other test tubes? Put your answers in the following data chart. Also record your measurements.

|TUBE |1 |2 |3 |4 |5 |6 |
|% molasses | | | | | | |
|Length of gas (mm) | | | | | | |
|day two | | | | | | |
|Bubble length (mm) | | | | | | |
|day 1 | | | | | | |
|Difference | | | | | | |
|Class Average | | | | | | |

5. Graph your data on a piece of graph paper. Put the independent variable on the “X” axis and the dependent variable on the “Y” axis.

6. Graph the class data on the same piece of graph paper using a different color.

7. What was the purpose of test tube 6?
8. Why was it important to shake the yeast suspension just before adding drops to the test tube?

9. The gas is actually filling the volume of the small test tube. Why is it acceptable to measure length of the gas column instead of volume?

10. Why is it important to look at data from the whole class?

11. Does your data support your hypothesis? EXPLAIN.

12. How could you verify your data?

13. What were some of the factors that could affect the activity of the yeast that you kept constant?

14. In the introduction, you were told that muscle cells also carry out a type of fermentation called “lactic acid fermentation.” What are the advantages and disadvantages of this process to human muscles and functioning?

15. Think of some other variable that might affect the rate of fermentation in this experiment and design a new experiment to test the affect of changing this variable.

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