BIOL 102: Lab 7

Yeast Fermentation


Students are expected to read pages 1-2 before coming to the lab to complete the experiments.

Print this entire lab packet and bring it to the laboratory.

Please provide a FULL lab report for this experiment following the “Lab Report Guidelines”.

Please note that this lab report WILL include a HYPOTHESIS.


· Observe yeast fermentation

· Determine the optimum conditions for yeast fermentation


All fungi are eukaryotes. Although they vary in size and shape, fungi share key characteristics including their way of obtaining nutrients for growth and energy. Fungi are heterotrophs and they depend on preformed carbon molecules produced by other organisms. However, fungi do not ingest food and then digest it using enzymes; instead they invade -think of a moldy piece of bread-a food source and secrete digestive enzymes onto it. The digestion occurs outside the body. When the polymers are broken down into monomers, the fungi absorb the predigested food into its body.

Yeast are microscopic, unicellular organisms in the Kingdom Fungi. Like other fungi, yeast are incapable of making their own food, but like any other organism, need food for energy. They rely on carbohydrates (usually sugars) found in their environment to provide them with this energy so that they can grow and reproduce. There are many species of yeast, and each has a particular food source.

Regardless of the food source, yeast perform fermentation which does not utilize oxygen. In fermentation, the only energy extraction pathway is glycolysis, with one or two extra reactions tacked on at the end, but no electron transport chain. Therefore, only 2 ATPs are formed per glucose.

Fermentation and cellular respiration begin the same way, with glycolysis. In fermentation, however, the pyruvate made in glycolysis is not completely oxidized because it does not continue through the citric acid cycle and the electron transport chain does not run. Because the electron transport chain is not functional, the NADH cannot drop its electrons off to the electron transport chain, and thus very few ATP molecules are synthesized because the ATP synthase is not running.

Based on the end products, fermentation can be of two types: ALCOHOLIC fermentation (the subject of this lab) and LACTIC ACID fermentation.


Regardless of the type of fermentation, the purpose of the extra reactions in fermentation, is to regenerate (recycle) the electron carrier NAD+ from the NADH produced in glycolysis. The extra reactions accomplish this by letting NADH drop its electrons off with an organic molecule such as acetaldehyde to produce ethanol (alcoholic fermentation), or pyruvate to produce lactic acid (lactic acid fermentation). This “drop-off” of electrons allows glycolysis to keep running by ensuring a steady supply of NAD+.

Going from pyruvate to ethanol is a two-step process. In the first step, a carboxyl group is removed from pyruvate and released as carbon dioxide, producing a two-carbon molecule called acetaldehyde. In the second step, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.

Yeast breaks down glucose into ethanol, 2 carbon dioxide molecules, and 2 ATP molecules. The formula for the yeast fermentation reaction is:

Reactant Products

C6H12O6 >>>>>>> 2CH3CH2OH + 2CO2 + 2 ATP molecules

For the yeast cell, this chemical reaction is necessary to produce the energy for life. The ethanol and the carbon dioxide are waste products. It is these waste products that we take advantage of: we use the ethanol in alcoholic beverages and the carbon dioxide makes bread rise when baking.

Alcoholic fermentation, can be observed and measured by using the amount of carbon dioxide gas that is produced from the breakdown of glucose. In this exercise, you will observe alcoholic fermentation by yeast. To do so you will add the same amounts of yeast and water to different amounts of sugar in Erlenmeyer flasks and cap them with a balloon to see how much carbon dioxide gas is produced. You will also use water at two different temperatures and determine how much carbon dioxide is produced. The more fermentation that occurs, the more carbon dioxide will be produced, and the more the balloon will expand.

Information adapted from:

Solomon, Eldra P. et al. Biology. 10th ed. Cengage, 2015.


Determine the optimum conditions for yeast fermentation.

Think Scientifically:

Please explain your rationale to which flask or test variable will produce the most CO2. Look at the various bottles below and state whether bottle A-F will produce the most CO2 and explain why.



Dry yeast

Warm water

Ice cold water

Balance scale

Measuring spoons

100 mL Graduated Cylinder

6 Erlenmeyer flasks

6 Rubber bands

6 Balloons



1. Obtain 6 labeled Erlenmeyer flasks.

2. Fill each flask accordingly:

· Bottle A – 5 mL sugar, 3 grams of dry yeast

· Bottle B – 10 mL sugar, 3 grams of dry yeast

· Bottle C – 15 mL sugar, 3 grams of dry yeast

· Bottle D – 5 mL sugar, 3 grams of dry yeast

· Bottle E – 3 grams of dry yeast

· Bottle F – 15 mL sugar

3. Fill all flasks except D with 100 mL of warm water. Fill flask D with 100 mL of ice cold water.

4. Place a balloon over the top of each flask and tighten it with a rubber band.

5. Swirl flask to mix contents. Wait 20-30 minutes.

6. Record observations in Table 1.

7. Measure the width and height of the balloon (from the top of the flask to the top of the balloon) with a ruler, and record it in Table 1.

8. Graph the Sugar Quantity vs. Balloon Height in an X-Y Scatterplot. Insert DIGITAL scatterplot only. Written graphs and/or pictures of written graphs will not be accepted.

Table 1: Observations and Measurements of Balloon height in cm






1st to rise




3rd to rise




2nd to rise




Did not rise




Did not rise




Did not rise




Be sure to address the following:

· How did your original rationale compare to the data collected? If your rationale was incorrect, why do you think it did not produce the most CO2?

· Describe what happened in this reaction using the following terms: yeast, warm water, cold water, sugar, anaerobic respiration, and carbon dioxide.

· Compare what happened to each of the balloons for flasks A through F. Which flask had the most CO2 production? Least? How do you know? Be sure to describe WHY!

· There were four experimental flasks and two control flasks in this exercise. Which flasks were the experimental and which were the control flasks? Explain how each determination was made.

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