INTRODUCTION we were able to make accurate predictions

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

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
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
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
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
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
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
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 Van Thiel, page 13.

The actual results of the Benedict test are as follows: distilled water
blue, glucose turned a dark green, fructose blue-green, galactose was red,
maltose was
slightly red, lactose blue-green on the top of the test tube and red on the
bottom, sucrose,
glycogen, starch, and potato soup were all negative(blue). Finally, the
dilute honey sample
was dark orange.

The actual results of the Barfoed test are as follows: distilled water
formed no
precipitate, glucose, fructose and galactose did form red precipitate,
maltose, lactose,
sucrose, glycogen, starch, and potato soup did not form a precipitate,
honey did
form a precipitate.

The actual results of the iodine test are as follows: distilled water,
fructose, maltose, lactose, and sucrose all remained yellow or negative.

Glycogen turned
a rust color, starch was black-blue, potato soup was rust colored, and
sample dilute
honey remained yellow.

Combining the three tests we have the over all results as follows: for our
distilled water we can conclude that it is non-reducing, non-
and non-
polysaccharide; glucose, fructose, and galactose were all reducing,
non-polysaccharides. Maltose and lactose were both reducing,
non-polysaccharides. Sucrose was non-reducing, non-monosaccharide, non-
polysaccharide. Glycogen was a non-reducing, non-monosaccharide, and a
polysaccharide. Starch was a non-reducing, non-monosaccharide, and a
coiled polysaccharide. Potato soup was non-reducing, non-monosaccharide,
branched polysaccharide. Dilute honey was reducing, monosaccharide, and a

Let’s continue the discussion of this lab with a closer look at our

Based on our results we can conclude that glucose, fructose, galactose and
dilute honey
are the monosaccharides since they all formed a precipitate in the Barfoed
test. The
sample of dilute honey was of greatest interest to me since we did not know
prior to the
test whether it were a monosaccharide or not. I suspected that it was
reducing since the
honey was diluted. A non-reducing carbohydrate I do not believe we could
dilute since it
will not dissolve. Based on the precipitate formation of dilute honey in
Barfoed, it can
be concluded that it is comprised of monosaccharides.

Looking at our results I can reasonably conclude that the disaccharide
samples are
maltose, lactose, and sucrose since they all were negative for both the
Barfoed and iodine
tests. If we also look at the probable disaccharides, we see that none of
our tests used
were designed to specifically test positively for them. Since we know that
are comprised of two monosaccharides by way of dehydration reaction, we
test for
disaccharides by adding water to the possible disaccharide samples and may
heat them
so they will undergo a hydrolysis process, then run them through the
test again.

If the sample which before adding water was negative in the Barfoed test,
was positive
after adding water then we could conclude that the original sample was a

Our tested samples that we believe to be polysaccharides are glycogen,
starch, and
potato soup since they all had some color change in the presence of Lugol’s

Polysaccharides can be further classified by their overall structure, in
particular, whether
they are highly branched, highly coiled and unbranched, or both slightly
coiled and
branched. We learned that the starches can be coiled profusely or coiled
with no branches.

The iodine test will result in a different degree of color change based on
the amount of
coiling present. Namely, a highly coiled carbohydrate will turn a dark
blue-black color.

The particular highly coiled polysaccharide that we discussed in the class
amylose which
is an unbranched storage starch found in plants. Since our starch sample
turned black, it
may be compromised of amylose starch. The potato soup sample did not turn
dark, a
color indicating to me that the starch in this sample probably consisted of
smaller starch
units called dextrin. Dextrin have very short terminal ends that coil only
sightly so the
color change would not be so dramatic as in the presence of highly coiled
starch like
amylose. The potato soup was made from dehydrated buds. This dehydration
process of
the fresh potatoes does cause structural change in the starches. A fresh
potato sample I
predict to turn a dark black since its starches would be intact. Glycogen
turned a rust
color as we should expect since we know that glycogen is a slightly coiled

I did predict prior to the experiment that the color change in the presence
of the iodine
would be different for starch and glycogen since they have different

The data, in my opinion, did not conflict with our expected results. These
when used together allow us to make predictions about unknown samples with
confidence. I believe that the data provide sufficient information to
carbohydrates and how we can more precisely describe carbohydrates.


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