Four Tests to Detect 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
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