Detection of Biological Molecules


Introduction: Without carbon, nitrogen, hydrogen, sulfur, oxygen and phosphorus,
life wouldn\'t exist. These are the most abundant elements in living organisms.
These elements are held together by covalent bonds, ionic bonds, hydrogen bonds,
and disulfide bonds. Covalent bonds are especially strong, thus, are present in
monomers, the building blocks of life. These monomers combine to make polymers,
which is a long chain of monomers strung together. Biological molecules can be
distinguished by their functional groups. For example, an amino group is
present in amino acids, and a carboxyl group can always be found in fatty acids.
The groups can be separated into two more categories, the polar, hydrophilic,
and the nonpolar, hydrophobic. A fatty acid is nonpolar, hence it doesn\'t mix
with water. Molecules of a certain class have similar chemical properties
because they have the same functional groups. A chemical test that is sensitive
to these groups can be used to identify molecules that are in that class. This
lab is broken down into four different sections, the Benedict\'s test for
reducing sugars, the iodine test for the presence of starch, the Sudan III test
for fatty acids, and the Biuret test for amino groups present in proteins. The
last part of this lab takes an unknown substance and by the four tests,
determine what the substance is.

BENEDICT\'S TEST

Introduction: Monosaccharides and disaccharides can be detected because of
their free aldehyde groups, thus, testing positive for the Benedict\'s test.
Such sugars act as a reducing agent, and is called a reducing sugar. By mixing
the sugar solution with the Benedict\'s solution and adding heat, an oxidation-
reduction reaction will occur. The sugar will oxidize, gaining an oxygen, and
the Benedict\'s reagent will reduce, loosing an oxygen. If the resulting solution
is red orange, it tests positive, a change to green indicates a smaller amount
of reducing sugar, and if it remains blue, it tests negative.

Materials: onion juice 5 test tubes 1 beaker potato juice
ruler hot plate deionized water permanent
marker 5 tongs glucose solution labels starch solution 6 barrel
pipettes Benedict\'s reagent 5 toothpicks

Procedure: 1. Marked 5 test tubes at 1 cm and 3 cm from the bottom. Label test
tubes #1-#5. 2. Used 5 different barrel pipettes, added onion juice up
to the 1 cm mark of the first
test tube, potato juice to the 1 cm mark of the second, deionized water
up to the 1
cm mark of the third, glucose solution to the 1 cm mark of the fourth,
and the
starch solution to the 1 cm mark of the fifth test tube. 3. Used the
last barrel pipette, added Benedict\'s Reagent to the 3 cm mark of all 5
test tubes and mix with a toothpick. 4. Heated all 5 tubes for 3
minutes in a boiling water bath, using a beaker, water, and
a hot plate. 5. Removed the tubes using tongs. Recorded colors
on the following table. 6. Cleaned out the 5 test tubes with deionized
water.

Data:

Benedict\'s Test Results

Discussion: From the results, the Benedict\'s test was successful. Onion juice
contains glucose, and of course, glucose would test positive. Starch doesn\'t
have a free aldehyde group, and neither does potato juice, which contains starch.
Water doesn\'t have glucose monomers in it, and was tested to make sure the end
result would be negative, a blue color.

IODINE TEST

Introduction: The iodine test is used to distinguish starch from
monosaccharides, disaccharides, and other polysaccharides. Because of it\'s
unique coiled geometric configuration, it reacts with iodine to produce a blue-
black color and tests positive. A yellowish brown color indicates that the test
is negative.

Materials: 6 barrel pipettes potato juice starch solution 5 test
tubes water iodine solution onion juice
glucose solution 5 toothpicks

Procedure: 1. Used 5 barrel pipettes, filled test tube #1 with onion juice,
second with potato
juice, third with water, fourth with glucose solution, and fifth with
starch solution. 2. Added 3 drops of iodine solution with a barrel pipette,
to each test tube. Mixed
with 5 different toothpicks. 3. Observed reactions and recorded
in the table below. Cleaned out the 5 test tubes. Data:

Iodine Test Results

Discussion: The iodine test was successful. Potato juice and starch were
the only two substances containing starch. Again, glucose and onion juice
contains glucose, while water doesn\'t contain starch or glucose and was just
tested to make sure the test was done properly.

SUDAN III TEST

Introduction: Sudan III test detects the hydrocarbon groups that are remaining
in the molecule. Due to the fact that the hydrocarbon groups are nonpolar, and
stick tightly together with their polar surroundings, it is called a hydrophobic
interaction and this is the basis for the Sudan III test. If the end result is
a visible orange, it tests positive.

Material: scissors deionized water margarine Sudan
III solution petri dish starch ethyl alcohol
forceps lead pencil cream 5 barrel