Name ________________________________________Date _____________
Investigation: Fermentation
Humans use yeast every day to make bread and alcohol. Yeast purchased from
the grocery store looks like small brown grains that, at first glance, do not
appear to be alive. This experiment will confirm that yeast is living through
testing how it generates energy from food.
When yeast, humans, and other living organisms use energy, they break down
high-energy molecules like sugar to get the energy they need and give off
carbon dioxide as a by-product of this reaction.
Fermentation is the process by which cells release energy without using
oxygen. In cellular respiration, the cell first breaks glucose into smaller
molecules. This releases a small amount of energy (ATP). Without oxygen,
cellular respiration cannot continue any further. Instead of entering the
mitochondria, the smaller molecules stay in the cytoplasm, where fermentation
occurs.
There are two main types of fermentation: alcoholic fermentation and lactic
acid fermentation. Both types of fermentation start with the small molecules
made from sugars at the beginning of cellular respiration. Without oxygen,
different reactions occur that produce either alcohol and carbon dioxide, or lactic
acid. In both cases, a small amount of energy (ATP) is still released.
Many everyday foods are produced using fermentation. The bread
products that you eat are a result of alcoholic fermentation. Flour,
egg and sugar mixed with yeast in warm water results in the yeast
running out of oxygen. The sugar is then converted into carbon
dioxide and alcohol. The bread rises due to the production of carbon
dioxide gas bubbles. The baking process evaporates the alcohol and
kills the yeast, giving bread its light, airy texture.
When lactose (found in milk) is the sugar present, lactic acid
fermentation occurs. Certain bacteria release energy this way,
producing lactic acid as a by-product. Lactic acid fermentation
changes milk into yogurts and cheeses. This type of fermentation
also occurs in your muscles during prolonged or hard exercise. If
you use up the available oxygen during this time, fermentation
occurs in your muscle cells, releasing lactic acid. Lactic acid burns,
resulting in pain and soreness in the affected muscles. The pain
goes away when the lactic acid is finally converted into carbon
dioxide and water.
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Pre-lab Questions:
1. What is the name of the process that allows cells to release energy
without oxygen?
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2. What are some common foods that include fermentation in their
production?
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Activity Investigation: How can you tell if fermentation releases carbon
dioxide?
Set up the following procedure:
1.
2.
3.
4.
Fill bottles A & B ¾ of the way with warm water and ½ teaspoon yeast
Add a pack of sugar to bottle A only
Carefully stir both bottles
Place a balloon securely around the mouth of both bottles and set aside
overnight
Day 2
After observing both bottles the next day, draw the positions of the balloons on
top of each bottle in the space below.
Bottle A
Bottle B
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1. What provided the energy for fermentation to occur in one bottle and not
the other?
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2. Where, in the cell, does cellular respiration occur?
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3. Where, in the cell, does fermentation occur?
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4. What do yeast produce that causes bread to rise during fermentation?
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5. What do you think is contained within Bottle A’s balloon?
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6. What type of sugar must be present for lactic acid to form?
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7. Muscle soreness goes away when lactic acid is finally converted into what?
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8. Based on your observations, would you conclude that yeast is alive or
not? Explain your answer
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