The Relationship Between Food Concentration, and Respiratory Activity

In this experiment different concentrations of sucrose were tested to determine
which leads to the most respiratory activity in yeast. Yeast is a heterotrophic
anaerobic fungus which lacks chlorophyll. Yeast is used commercially to
ferment the sugars of wheat, barley, and corn to produce alcohol, and in the
baking industry to raise or expand dough. Yeast or alcoholic fermentation is
the anaerobic process of respiration by which sugars, such as glucose and
sucrose, are converted into ethanol and carbon dioxide (CO2 ). This process is
illustrated in the following equation:
yeast C12H22O11 + H2O
---> 4 CH3CH2OH + 4 CO2
sucrose + water (yields) ethanol + carbon dioxide

In order to determine what concentration of sucrose and water leads to
the most respiratory activity, ten large test tubes were set with different
concentrations by the process of serial dilution. The first test tube was
filled with 40 ml of 60% sucrose solution. Then, the nine remaining test tubes
were serially diluted, so that the sucrose concentration ranged from 30% to
The hypothesis in this expriment was that the most respiratory activity
would take place with 60% sucrose concentration. Since yeast fermentation
requires sucrose and water, aproximately equal proportions of both would yield
to the most respiratory activity.

Once the sucrose concentration was serially cut to the desired level,
the experimenter added 5 ml of yeast suspension to each one of the ten test

Then, ten small test tubes were placed invertedly into each one of the
large test tubes, making sure no air bubbles remained within the small tibes.
The test tubes were left 24 hours, allowing for fermentation to take
place. But, no respiratory activity was detected.
In previous experimentation, it was found that yeast fermentation did
take place in different molasses concentrations. Since, molasses contains
large quantities of sucrose, it was assumed that different concentrations of
pure sucrose would yield similar results, when mixed with yeast. However, this
was not the case.

The probable explanation is that in order for fermentation to take place,
an enzyme is needed to break down sucrose --a disacharide-- into glucose and
fructose --monosacharides. This enzyme is present in molasses, but it is absent
in the sucrose solution.


1) Encarta Encyclopedia, CD-Rom Edition, Microsoft, 1994.

2) Biological Science, Prentice Hall, 1983.

3) Grolier Encyclopedia, CD-Rom Edition, Grolier Publishing, 1995.

Category: Science