Biological (Organic) Molecules of Life
PA Standards –
PA S.T. & E.:
3.1.10A2. Explain cell processes in terms of chemical reactions and
energy changes.
3.1.10.A7. Describe the relationship between the structure of organic
molecules and the function they serve in living organisms.
3.1.B.A7. Compare and contrast the functions and structures of proteins,
lipids, carbohydrates, and nucleic acids.
Living organisms are composed of a variety of organic molecules
including carbohydrates, fats, proteins and nucleic acids. All
organic molecules in our bodies come directly or indirectly from
our food. However, different foods contain different amounts of
the various organic molecules. Grains and potatoes are rich in
starch, while meats are typically rich in protein and fat. It is often
necessary to determine the types of organic molecules present in
particular foods.
The purpose of today’s lab exercise is to introduce you to tests
used to detect several different classes of organic molecules. It is
possible to identify these organic molecules because each class of
molecule has unique properties. In this lab you will investigate
carbohydrates, fats, and proteins. Nucleic acids will be
investigated using other lab activities. You will also work with
lab partners to practice collaboration skills.
Carbohydrates
Fats
Proteins
Nucleic Acids
Particular classes of organic molecules can often be detected by
colorimetric tests. A chemical called a reagent is added to a
solution. If the organic molecule is present, the reagent will react
with it to form a colored product, whereas if it is not present there
will be no color change.
Tests which detect the presence of a molecule are called qualitative tests. Tests which allow you
to determine how much of a particular molecule is present are called quantitative tests.
Colorimetric tests can be quantitative tests when the intensity of color formation is proportional
to the amount of substance being tested present.
Guiding Questions:
1. What are four important organic, biological molecules found in living organisms?
2. Why are these same molecules also found in the foods we eat?
3. What are four basic tests for biological molecules?
Lab Safety: 1. Wear safety goggles when using chemicals in the lab.
Revised 2010. 1
2. Wear gloves when handling chemicals that are corrosive or that may stain your
hands.
3. Follow directions on lab sheets and suggestions given by your teacher.
4. Do not smell or taste any chemicals in the lab.
5. Use a hot pad or test tube holder when doing experiments with hot water.
6. Dispose of chemicals and other materials as directed by your teacher in specially
labeled waste containers for Benedict’s, Biuret’s and Iodine experiments. The
Sudan III will be reused.
7. Inform your teacher in the event of accidents or chemical spillage.
8. Always wash your hands at the end of each lab activity when using chemicals.
Vocabulary:
1. Carbohydrates are a class of organic molecules made of carbon,
hydrogen, and oxygen; a major source of energy for the humans and
animals.
2. A colorimetric assay is a qualitative measurement of a specified
substance in a sample that changes color due to a chemical reaction. It
may be quantitative if the darkness of the color change is directly
proportional to the amount of substance in the sample.
3. Lipids (fats) are a class of organic macromolecules that made mainly
from carbon and hydrogen atoms; they include fats, oils, and waxes, and
are stored energy source for humans and animals.
4. Proteins are a class of organic macromolecules that contain carbon,
hydrogen, oxygen, and nitrogen; needed by the body for growth and repair
and to make up enzymes.
5. Starches are types of organic macromolecules formed from simple sugars
that are a storage form of sugar; a polysaccharide.
6. Sugars are types of organic molecules called monosaccharides that are a
main energy source in humans and animals.
A. Colorimetric Assays
a. Benedict's Test for Reducing Sugars
A reducing sugar has an aldehyde or ketone group, which both have carboxyl groups (-CHO). A
reducing agent brings about reduction by itself being oxidized and acts as an electron donor in a
chemical reaction. Glucose and all other monosaccharides (simple sugars) like fructose can
reduces the blue copper (II) ion in Benedict’s Reagent to form a brick red precipitate of copper
(I) oxide. Although some disaccharides are reducing sugars, sucrose (table sugar) is a nonreducing sugar.
Revised 2010. 2
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
Set up a boiling water bath using a hot plate and 600ml beaker, and obtain a test tube rack.
Obtain five test tubes and number 1 to 5.
Add 2 mls apple juice to tube #1,
2 mls potato juice to #2,
2 mls glucose solution to #3,
2 mls starch solution to #4,
2 mls water to #5.
Add 4 mls Benedict's reagent to each tube, and leave 3 minutes in the boiling water bath.
Carefully remove tubes from the water bath with tongs or a hot pad after cooling a few
minutes, then note color of each tube in the table of the results section of your data sheet.
9. Report your experiments on your Data Sheet. Give 4 sentences of Introduction in which you
explain: 1) what reducing sugars are, 2) why they are important, 3) the specific objectives of
these experiments, and 4) your hypotheses for each substance. Under Materials and
Methods describe your methods using past tense. Under Results you should complete the
Table. Finally, your Discussion should interpret your results, explaining why the solutions
turned the colors that they did and which substances contained sugar.
b. Iodine Test for Starch
Starch is a polymer of -glucose made by plants to store energy. Humans and other animals eat
starch for its stored energy. Starch forms coils and can therefore be detected by a solution of
molecular iodine, because the individual iodine molecules of iodine are just the right size and
shape to fit within the loops of the starch coil. When that happens, a dark blue color develops.
1.
2.
3.
4.
5.
6.
7.
Obtain five test tubes and number 1 to 5.
Add 2 mls apple juice to tube #1,
Add 2 mls potato juice to #2,
Add 2 mls glucose solution to #3,
Add 2 mls starch solution to #4 (mix well before adding)
Add 2 mls water to #5.
Add 3 drops Lugol's iodine to each tube then describe the color of each tube in the table
in your results section.
8. Complete the Data Sheet for this experiment. Give 4 sentences of Introduction in which
you explain: 1) what starch is, 2) why it is important, 3) the specific objectives of these
experiments, and 4) your hypotheses for each substance. Under Materials and Methods,
describe your methods using past tense. Under Results you should complete the Table.
Finally, your Discussion should interpret your results, explaining why the solutions
turned the colors that they did and which substances contained starch.
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c. Sudan Test for Fats or Lipids
Fats are hydrophobic (not able to dissolve water) organic molecules that are rich in energy due to
their high density of C-H bonds. Sudan III stains fats and will color the spots on the filter paper
pink/red if those spots have fat. Specifically, it will bind to the hydrocarbon groups in the tails of
the fatty acids.
1. First trace the outline of the bottom of a Petri dish on a filter paper disc. Cut out the disc
cutting slightly inside the lines so that it will fit inside the bottom part of the Petri dish.
With a pencil, make a dot in the center of the filter paper and measure 3 cm in 5 equally
spaced directions and place a pencil dot at each spot.
2. Again with a pencil, label these circles “W”, “A”, “O”, “M”, and “S”.
3. To the first circle labeled “W”, add two drops of water. To the second, labeled A, add
two drops of apple juice, to the third, labeled O, add two drops of cooking oil. To the
fourth, labeled M, add two drops of whole milk. To the fifth, labeled S, add two drops of
skim milk.
4. Make sure that the droppers used to add the reagents are kept separate from each other to
avoid cross contamination and poor experimental results.
5. Allow the filter paper to dry completely; use a blow dryer if necessary. Next, soak the
filter paper in a Petri dish in SUDAN III solution for 3 minutes. Make sure it is
completely covered.
6. Using forceps, remove the paper from the stain container. Rinse the paper in a water bath
in a flat plastic dish for one minute. The filter paper itself will pick up some of the dye,
so the excess must be soaked off in order for you to see the spots. The entire paper may
turn a pale pink; you are looking for at least a slightly darker color. RINSE SOME
MORE until you see at least one spot.
7. Finally, examine the intensity of the staining of the five circles and rate the four foods as
0=no, + = faint red/pink color, ++ = definite red/pink color.
8. Report your experiment on your Data Sheet. Give 4 sentences of Introduction in which
you explain: 1) what lipids or fats are, 2) why they are important, 3) the specific objective
of this experiment, and 4) your hypotheses for each substance. Under Materials and
Methods describe your methods using past tense. Under Results you should complete the
Table. Finally, your Discussion should interpret your results, explaining why the
solutions turned the colors that they did and which substances contained fats.
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d. Biuret’s Reagent Test for Protein
Biuret’s Reagent is used to identify the presence of protein and is a blue solution that, when it
reacts with protein, will change color to pink-purple (darkness of color and amount of protein are
directly proportional). The Biuret technique for protein is based upon the coordination of cupric
ion (II) of the Biuret solution with unshared electron pairs of a protein's nitrogen atoms and
water's oxygen atoms. The peptide chain must have at least three amino acids to yield a color
complex.
1.
2.
3.
4.
5.
6.
7.
8.
Obtain five test tubes and number them 1-5.
Add 2 mls egg white solution to #1,
Add 2 mls apple juice to #2,
Add 2 mls potato juice to #3,
Add 2 mls glucose solution to #4,
Add 2 mls water to #5.
Then add 4 mls Biuret's Reagent to each and look for a pink or lavender color forming.
You may need to warm the solutions in test tubes in a warm water bath using a hot plate.
Do not exceed a temperature of 40 degrees C. This is the temperature at which many
proteins become denatured and will no longer react.
9. Report your experiment on your Data Sheet. Give 4 sentences of Introduction in which
you explain: 1) what starch is, 2) why it is important, 3) the specific objectives of this
experiment, and 4) your hypotheses for each substance. Under Materials and Methods
concisely describe your methods using past tense. Under Results you should complete the
Table. Finally, your Discussion should interpret your results, explaining why the
solutions turned the colors that they did and which substances contained proteins.
Revised 2010. 5
Student Data sheet: Name: ____________________ Section: _________Date:______
a. Benedict's test for reducing sugar
Introduction:
Materials and Methods:
Results:
Tube
1
Substance
Apple Juice
2
Potato Juice
3
Glucose
4
Starch
5
Water
Color
Discussion:
Revised 2010. 6
Data sheet
Name: ____________________ Section: _________Date:______
b. Iodine test for starch
Introduction:
Materials and Methods:
Results:
Tube
1
Substance
Apple Juice
2
Potato Juice
3
Glucose
4
Starch
5
Water
Color
Discussion:
Revised 2010. 7
Data sheet
c.
Name: ____________________ Section: _________Date:______
Sudan III test for lipids/fats
Introduction:
Materials and Methods:
Results:
Spot
1
Substance
Water
2
Apple juice
3
Cooking oil
4
Whole milk
5
Skim milk
Color
Discussion:
Revised 2010. 8
Data Sheet
Name: ____________________ Section: _________Date:______
d. Biuret’s test for protein
Introduction:
Materials and Methods:
Results:
Tube
1
Substance
Egg white
2
Apple juice
3
Potato juice
4
Glucose
5
Water
Color
Discussion:
Revised 2010. 9