Anaerobic Cell Respiration by Yeast
BACKGROUND: 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. Complex
sugar molecules then break down into monosaccharides 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 break 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 flour.
Yeast is a facultative anaerobe, meaning that it can participate in aerobic respiration when
possible, but when this is impossible, it respires anaerobically. When using yeast in making
dough, the yeast will use the initial oxygen up very quickly and then start to respire
anaerobically. ATP will then be made via glycolysis, which requires no oxygen. Without
oxygen present, the yeast cells will quickly run out of NAD+ molecules which are vital to
the process of glycolysis. To regenerate the NAD+, the yeast will undergo alcoholic
fermentation, which converts pyruvic acid into CO2.as well as ethyl alcohol, with the NADH
being oxidized in the process. Overall, the final equation for glycolysis plus fermentation
would be:
C6H12O6  2CO2 + 2C2H5OH, with 2 ATP also produced.
For the yeast cell, this chemical reaction is necessary to produce the energy for life. The
alcohol and the carbon dioxide are waste products produced by the yeast. It is these
waste products that we take advantage of. 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.
Yeast Cellular Respiration Investigation
Explore:
Some cells are able to obtain energy from food without using oxygen. These organisms
obtain their energy through fermentation, an energy releasing process that does not
require oxygen. There are about 160 known species of yeast. The yeast we are using is
baker’s yeast or Saccharomyces cerevisiae. Yeast are single celled organisms that cannot
produce their own food either through photosynthesis or chemosynthesis. Their single cell
contain mitochondria.
Today we are going to perform an investigation that will test whether yeast breaks down
sugar to release the energy and determine the products of the reaction.
Materials for each group: 1 empty pint size plastic water bottles (16.9 fl. oz. or 500ml), 3
t. table sugar (sucrose), 1 transparent (11” when inflated) balloons, packet of active dry
yeast, very warm water (1050- 1150 F), Bromthymol blue solution (make a 50% solution),
medicine dropper or 10ml graduate cylinder, measuring spoon and funnel or sheet of paper.
Procedure:
1. Stretch out the balloon by blowing it up repeatedly (a pump may be used), then put
it to the side.
2. Fill the 2 empty small plastic water bottles about a 1/3 full with warm water. (Try
to make the water level in each bottle as even as possible)
3. Add 3 teaspoonfuls of sugar to one of the water bottles. (You can use a funnel or
fold a sheet of paper into a cone.) Place the lid on the water bottle and shake to
dissolve the sugar. Remove the lid and let the water settle. Label this bottle S for
sugar.
4. Add 1/2 dropper or 1 ml of a 50 % solution of Bromthymol blue to the balloon. Have
one person in the lab group hold onto the balloon.
5. One student adds 1 packet of active dry yeast to the bottle, another student
quickly places the mouth of the balloon over the top of the bottle being careful not
to let any Bromthymol blue into the bottle. Do not shake or swirl at this point.
6. Check to make sure that the balloon is tight over the top of the bottle. Gently swirl
the contents. Place the bottle in a warm place or a warm water bath.
Hypothesis: What will happen to the balloon? Why do you predict this?
7. As the balloon starts to fill, another student
holds the body of the balloon horizontal so that
the Bromthymol blue does not drop into the
bottle Continue holding the balloon to the side
until you can no longer or are told to let go (tape
may be used). Every so often, gently rotate the
balloon with your fingers so that the Bromthymol
blue swishes a bit without dropping into the bottle.
8. Observe what happens. After you observe what is happening in the balloon, you do
not have to hold the balloon any longer.
Observations:
Bottle with Sugar, Water and Yeast
Observations of what is happening in the bottle:
Observations of what is happening in the balloon:
Explain:
1.
2.
3.
4.
5.
What are the products of fermentation? Is there evidence for each product?
How can we determine in this lab whether fermentation occurred?
If you were asked to set up controls for this investigation, what would you do?
How is fermentation different from aerobic respiration?
How is fermentation similar to aerobic respiration?
Conclusion: Was your hypothesis correct? Why or why not? What have you learned from
this investigation?
Model: Using diagram(s) and labels, show and explain what happened to the balloon and its
contents and to the contents of the bottle.
Elaborate: For further investigation.
1. Add a bit of yeast to a teaspoon of warm water, place a drop on a slide with a cover
slip and examine under a microscope at 430x. Make a large drawing of a single
yeast cell and label what you observe.
2. In a zip lock sandwich bag, add ½ cup flour, 1/4 pkg. active dry yeast that has been
added to 1/8 cup of warm water, and 1 teaspoon of sugar. Seal the bag carefully
making sure there is no air inside the bag. Rub the plastic bag with your fingers to
thoroughly mix the substances. Place the bag in a warm place and observe after 15
minutes or so.
3. Perform the Anaerobic Cell Respiration by Yeast investigation on the next page.
For further Investigation:
Anaerobic Cell Respiration by Yeast Investigation
Do you think that the rate of carbon dioxide production during fermentation would
be affected by the availability of simple sugars? Explain.
OBJECTIVE:
In this lab, we will observe the effect of food source on the process of cellular respiration
by yeast. You will attempt to determine whether a yeast “bread dough” contains only flour
or flour and sugar, based on the rate of CO2 production. You will assess CO2 production by
measuring how much the dough rises in a set period of time.
MATERIALS:
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Two plastic cups per group
Two plastic spoons per group
Marker
Several packages yeast
Warm Water
Table sugar
Flour
Plastic wrap
PROCEDURE:
Get two plastic cups and label them A and B.
2.
Place flour only in cup A and a mixture of flour and sugar in cup B.
3.
Add 45 ml of warm water-yeast solution into each cup.
4.
Mix each cup with a separate spoon until all the flour is moistened. Be sure to
check the bottom of the cup to make sure NO DRY flour remains.
5.
Continue to mix the dough for 2 to 3 minutes.
6.
Stop mixing when you see the mixture forming gluten “threads” as you pull it apart.
These gluten threads make the dough stretchy enough to capture bubbles of CO2,
resulting in puffy dough.
7.
Use the spoons to gently push the dough down to a relatively flat surface in each
cup.
8.
Mark the level of the dough on the side of the cups. Use a ruler to measure
the approximate height in cm of the dough from the bottom of the cup. Record the
starting height in the group data section.
9.
Write your names on the cup and cover it with plastic wrap. Wash the spoon and
then place the cup in the warm water bath. Wait at least 35 minutes for the dough to rise.
1.
After Rising
Mark the level of your dough now on the side of the cup and measure the change in
the height.
10.
Convert this number to a percentage of the starting height. To do this, divide the
change in height by the original height. An increase in height would be a positive number,
while a decrease would be negative. Record your data in the group data section, and put
your % change data in the class data table.
11.
RESULTS:
Group data:
Starting
height
Change in
height
% change
in height
Cup A
Cup B
Class Data: Percent Change in Height
Cup A
Cup B
Calculate the average % change for the class data for each cup.
Cup A average % change:
Cup B average % change:
Analysis Questions:
1. Sugar was added to either the cups marked A or the cups marked B. Given what you
know about anaerobic respiration, which cup had the added sugar? Explain you answer
in terms of the energy source(s) available to the yeast.
2. At what point will the dough stop growing? Why?
3. Was anything else produced during the fermentation process? If so, where is it?
4. Why was it necessary to knead the dough?
5. What was the purpose of the warm water bath?
6. Most recipes for bread have you add a small amount of sugar or molasses to the
mixture. Looking at the results, what purpose do you think this may serve?
7. If yeast utilized lactic acid fermentation, what would happen to the dough in contrast
to what we observed in class?
8. Outline another method that you might have used to accomplish the same information
about yeast respiration.