Cell Lab 6
Yeast Fermentation
Problem: What are the optimum (best) conditions for yeast fermentation?
Background Reading:
Yeast are tiny single-celled (unicellular) fungi. The organisms in the Kingdom Fungi are not
capable of making their own food. Fungi, like any other organism, need food for energy. They rely on
sugar found in their environment to provide them with this energy so that they can grow and reproduce.
Yeast, like bacteria grow in or on their food source. They produce and release digestive proteins
(enzymes) into their environment where the sugar molecules are found. Sugar molecules then break down
into smaller molecules that can be absorbed by the yeast and used for food (energy).
There are many species of yeast, and each has a particular food source.
• Certain yeast feed on a variety of natural sources of sugar such as fruits, nectar from plants, and
molasses from the plant crop called sorghum.
• Others break down wood and corn stalks. In doing this, a compound called ethanol is produced.
This compound can be used in our cars like gasoline.
• Another species breaks down sugar from grain into alcohol. Others break down fruits into wine,
which is another type of alcohol.
• Bread recipes rely on yeast to break down sugar in our dough.
All of these processes are called fermentation, a type of anaerobic respiration (respiration without
oxygen). The formula for the yeast fermentation reaction is:
Reactant
Products
C6H12O6
>>>>>>>
2CH3CH2OH + 2CO2 + energy for 2 ATP molecules
Glucose breaks down into ethyl alcohol + carbon dioxide + energy for 2 ATP molecules.
For the yeast cell, this chemical reaction is necessary to produce the energy for life. The alcohol
and the carbon dioxide carbon dioxide are waste products produced by the yeast. It is these waste
products that we take advantage of. We use the alcohol and the carbon dioxide makes a cake rise when
baking.
The chemical reaction, known as fermentation, can be watched and measured by the amount of
carbon dioxide gas that is produced from the break down of glucose.
Do you think that the volume of carbon dioxide produced during fermentation would be affected by using
different masses of sugar????
In this exercise you will add the same amounts of yeast and water to different amounts of sugar in
water bottles and cap them with a balloon to see how much carbon dioxide gas is produced. The more the
fermentation, the more that carbon dioxide will be produced, and the more the balloon will expand.
Pre-lab Questions: Look at the reactions for both aerobic and anaerobic respiration:
Reactants
Aerobic
Respiration
C6H12O6 + 6O2
Anaerobic
Respiration
C6H12O6
>>>>>>>>>>>>>>>>>>
>>>>>
>>>>>
Products
6CO2 + 6H2O + 18 ATP (energy)
2CH3CH2OH + 2CO2 + 2 ATP (energy)
1. List 3 differences between the 2 reactions.
2. Why is aerobic respiration a “better way” to get energy out of food?
3. Optional: In our experiment, anaerobic respiration (fermentation) will occur because
there will be no oxygen able to get into the bottle. Why would the balloon not expand if
oxygen were in the mixture?
Materials (per group):
Dry yeast
Balance scale
25 mL Graduated Cylinder
100 mL Graduated Cylinder
Sugar
Warm water
Water bottle
Rubber band
Balloon
Procedure:
1. Label a water bottle with your assigned group letter: A, B, C, D, or E.
2. For each bottle:
o
A (Experimental Group) - 5 mL sugar, 3 grams of dry yeast
o
B (Experimental Group) - 10 mL sugar, 3 grams of dry yeast
o
C (Experimental Group) - 15 mL sugar, 3 grams of dry yeast
o
D (Control Group) - 3 grams of dry yeast
o
E (Control Group) - 15mL sugar,
3. Fill the bottle with 100mL of warm water
4. Place balloon over top of bottle and tighten with a rubber band.
5. Swirl bottle to mix contents.
6. Record observations in Table 1.
7. Measure the width and height of the balloon (from the top of the bottle to the top of the
balloon), and record in Table 1.
8. Graph Sugar Quantity vs. Balloon Height in an X-Y Scatterplot.
Data:
Table 1: Observations and Measurements of Balloon height in cm
Bottle
Observations
Height
Width
A
B
C
D
E
Questions:
1. Compare what happened to each of the balloons for flasks A through E.
2. What bottle(s) had the most CO2 production? Least? How do you know? Be sure to
describe WHY!
3. Describe what happened in this reaction using the following terms: yeast, warm water,
sugar, anaerobic respiration, and carbon dioxide.
Conclusion: (Use R-A-C-E format.)