A theme that we have seen and studied in this class is that a particular
form or
structure of a molecule plays a significant role in the function of that
molecule. Since we
are interested in the function of molecules. it helps to study their
structure. One of the
major classes of organic compounds found in cells are carbohydrates. These
are made of carbon, hydrogen, and oxygen in a ratio of 1:2:1 respectively
with a general
formula of X(CH2O)n. When the carbohydrates consists of one unit of sugar,
X=1, it is
called monosaccharide. If it consists of 2 units, X=2, the carbohydrate is
disaccharide. Carbohydrates made up of more than two units, X>2, are called
polysaccharides. Carbohydrates can also be branched or unbranched depending
on the
type of linkage. Those with alpha 1:4 linkages are linear or unbranched,
while those with
alpha 1:6 linkages are branched. Carbohydrates are necessary biomolecules
because they
play a role in energy metabolism as a source of potential chemical energy,
also they are
important building blocks for other biomolecules.
The word carbohydrate is very general, so in order to understand these
more precisely, we need to be able to identify more specific classifications.
experiments try to accomplish this using three common bioassay tests. The
first, the
Benedict test, will test various compound for reducing sugars. All
six-carbon hexose
sugars are reducing carbohydrates, as are most disaccharide. Sucrose is the
Most polsaccharides are not reducing. Secondly, we have the Barfoed test
which is
designed to test for monosaccharides. The third and final Iodine test is used
to test for
polysaccharides that are either branched or unbranched. By combining these
tests we
were able to make accurate predictions about the carbohydrate contents of a
given sample.
Now, let's take a closer look at how these bioassays do work. The Benedict
the Barfoed tests are based on the reaction of cupric ions with aldehyde or
ketone groups.
In the presence of a reactive group, the blue cupric ions are reduced to red
cuprous ions.
The Benedict test is a basic solution and upon heating turns green, yellow,
orange or brick
red which indicates a positive reaction. The final color is dependent on the
number of
reactive sites available; green indicates few sites, yellow more, and red
denotes many sites.
The Barfoed solution is acidic and only free aldehyde or ketone groups of
monosaccharides can reduce the blue ions to red ions. The color change to
red will occur
immediately. The lack of a change indicates only that the solution is not a
monosaccharide. The iodine test is used for polysaccharides. Iodine
combines with any
existing alpha helices. The more coiled the sample the darker the iodine
will turn. The
color change can range from deep black-blue with a sample of many coils to a
rust red
violet with fewer coils and more branchings. When there are no coils, there
is no color
change. Mono and disaccharides give negative results.
In summary, this lab attempts to investigate several different samples by
means of
series of tests, and based on the combined results of all three tests we can
attempt to
understand the carbohydrate composition of unknown samples. We hope to be
able to
predict the results of three bioassays for an unknown solution if given its
saccharide type
and reducing property. We should also be able to predict the saccharide type
and reducing
capability of an unknown solution if given the results of the three
Like any other experiment, this experiment needs some specific materials
including, beaker, graduated cylinder, hot plate, 11 test tubes, test tube
holder, wax pencil,
liquid soap, and test tube brush. Also, we used the Barfoed reagent,
Benedict reagent,
and iodine reagent.
Our eleven samples were distilled water (control), glucose, fructose,
lactose, sucrose, glycogen, starch, potato soup, and dilute honey.
First, we marked our test tubes with the wax pencil to keep track on the
then we place the eleven samples in the corresponding tubes. The first test
that we
performed was Benedict, followed by Barfoed, ending with iodine test. When
needed the
samples were heated and our results were immediately recorded in the
following tables. In
all three cases distilled water was used as a control.
*The details of the materials and the methods can be obtained from the lab
Experiments in Biology, From chemistry to sex by Linda