The student conducted an experiment to determine the effect of different carbohydrate sources (table sugar, honey, maple syrup, and molasses) on yeast respiration rates. Over 20 minutes, the yeast grew the most and produced the highest level of foam when exposed to table sugar. More concentrated sources like honey and molasses inhibited yeast growth, as the high sugar concentrations decreased the water potential and slowed respiration. Table sugar provided the most ideal environment for yeast growth and respiration.

1. The Effects of Varied
Carbohydrate Sources on
Respiration Rates in Yeast
RiceX: AdvBIO.1x AP® Biology - Part 1: The Cell
Student Presentation (I hope this is correct)
Figure 1: Carbohydrate solutions after 20 minutes

2. Determine the optimal conditions
for yeast respiration.
Hypothesis:
Table sugar will result in the highest rate of yeast respiration. The source is
not too concentrated compared to the other carbohydrate sources. The
more sugar content a carbohydrate source contains the more active the
yeast will be, unless the source is a very strong sugar concentrate. If the
concentration of sugar is too strong in a source, then the yeast will not have
as much growth.
Questions: What carbohydrate sources are best for yeast growth?
Figure 2: The Chemistry of Everyday Things
(http://4sbccfaculty.sbcc.edu/lecture/90s/lectures/Gary_Carroll_print.html)

3. Materials & Procedure for Investigation
Materials Procedure
Water Label each 1 L jar with carbohydrate source
Funnel Measure out 4 teaspoons of each carbohydrate source
Thermometer Using a funnel, pour table sugar into its respective
bottle.
Ruler Measure 3 teaspoons of yeast and pour into jars.
Teaspoon Swirl the bottle with yeast and table sugar to mix
thoroughly.
1 L Jar (4) Heat 1.5 L of water to 40*C
Large glass
mixing cups
Using a funnel, add 250 ml of 40*C water to each of the
bottles. For the liquid carbohydrate sources, mix water
and carbohydrate source before pouring into their
respective bottles.
Carbohydrate
sources (table
sugar, honey,
maple syrup,
molasses)
Immediately start timer.
Yeast Make observations by measuring the foam that
develops on top of the water in each bottle in cm every
5 minutes.
Timer Stop measurements at 20 minutes
Computer
Figure 3: Measuring Maple Syrup Solution at start.

4. Data Collection
Start 00:05 00:10 00:15 00:20
0
2
4
6
8
10
Yeast Respiration Rates
height of foam in cm
Table Sugar
Honey
Maple Syrup
Molasses
Time (minutes)
Centimeters
Table
Sugar
Honey Maple
Syrup
Molasses
Start 3.5 3.5 3.5 3.5
5 4.5 4.5 4.5 4.5
10 5.75 5.75 5.75 5.75
15 7 7 7.75 7.5
20 9 8.5 8.75 8.25
Results
All of the carbohydrate
sources began at 3.5 cm and
remained consistent at each
5 minute interval in their
growth pattern. However, at
the 15 minute mark Source 3
(maple syrup) began to show
higher foam levels. By the
end of 20 minutes, Source 1
(table sugar) surpassed
Source 3 (maple sugar).
Figure 4 (top): data chart
Figure 5 (bottom) data source

5. Conclusion
I accept the hypothesis. The more concentrated carbohydrate sources inhibited growth of the yeast. The
water potential of the sugar solution decreased until its water potential became lower than the yeast cell,
so the yeast cell moved water out instead of in, which slowed the rate of growth. Excess sugar inhibited
the yeast.
The highly concentrated carbohydrate solutions (honey and molasses) slowed the reaction. The table sugar and
the molasses contained an ideal environment for the yeast to grow because of the accessibility to water. The
respiration of the yeast cell produced ample amounts of carbon dioxide in all of the carbohydrate sources so the
yeast did not die, but the most effective environment was the source with a sugar concentrate that was not too
strong.
The honey and molasses grew at similar rates throughout the investigation. Therefore, their sugar concentrate is
likely similar.
The maple syrup had a thinner consistency than both honey and molasses. I believe the water content was higher
and more favorable for the yeast than honey or molasses.
The table sugar was the most ideal environment for the yeast. The growth was steady and consistent throughout
the investigation. It raised just a bit higher than the other carbohydrate solutions in the last 5 minute interval. I
noticed that the air pockets of both maple syrup and table sugar were very tiny as opposed to the honey and
molasses which had larger air pockets near the water line.
The release of carbon dioxide from cell respiration was higher in table sugar than any other solution because the
water potential and available oxygen was the most ideal environment for the table sugar, as opposed to the other
carbohydrate concentrates where the osmotic concentrate was too great and inhibited the growth of the yeast.
I learned that highly concentrated sugars inhibit the growth of yeast. This is beneficial to me because now I know
that if I want my bread to rise, I should use sugar, not honey.
One possible error in this experiment was the temperature of the water used. My thermometer was not calibrated
prior to the experiment. Thankfully, no yeast were killed in this investigation.

6. Conclusion
I accept the hypothesis. The more concentrated carbohydrate sources inhibited growth of the yeast. The
water potential of the sugar solution decreased until its water potential became lower than the yeast cell,
so the yeast cell moved water out instead of in, which slowed the rate of growth. Excess sugar inhibited
the yeast.
The highly concentrated carbohydrate solutions (honey and molasses) slowed the reaction. The table sugar and
the molasses contained an ideal environment for the yeast to grow because of the accessibility to water. The
respiration of the yeast cell produced ample amounts of carbon dioxide in all of the carbohydrate sources so the
yeast did not die, but the most effective environment was the source with a sugar concentrate that was not too
strong.
The honey and molasses grew at similar rates throughout the investigation. Therefore, their sugar concentrate is
likely similar.
The maple syrup had a thinner consistency than both honey and molasses. I believe the water content was higher
and more favorable for the yeast than honey or molasses.
The table sugar was the most ideal environment for the yeast. The growth was steady and consistent throughout
the investigation. It raised just a bit higher than the other carbohydrate solutions in the last 5 minute interval. I
noticed that the air pockets of both maple syrup and table sugar were very tiny as opposed to the honey and
molasses which had larger air pockets near the water line.
The release of carbon dioxide from cell respiration was higher in table sugar than any other solution because the
water potential and available oxygen was the most ideal environment for the table sugar, as opposed to the other
carbohydrate concentrates where the osmotic concentrate was too great and inhibited the growth of the yeast.
I learned that highly concentrated sugars inhibit the growth of yeast. This is beneficial to me because now I know
that if I want my bread to rise, I should use sugar, not honey.
One possible error in this experiment was the temperature of the water used. My thermometer was not calibrated
prior to the experiment. Thankfully, no yeast were killed in this investigation.