LAB 2: Organic Compounds
PRE-LAB QUESTIONS
Identifying Organic Compounds
1. What are the four types of macromolecules that are vital for survival?
2. What are indicators? What are they used for in this experiment?
3. What is the purpose of conducting this lab investigation?
4. Is it possible for some foods to test positive for more than one test? Explain your answer.
LAB 2: Organic Compounds
Introduction –
The most common organic compounds found in living organisms are carbohydrates, lipids, and
proteins, and nucleic acids.
In Part A of this experiment, you will be using indicators to test common foods for the presence of
carbohydrates, lipids, and proteins. Substances called indicators can be used to test for the presence
of organic compounds. An indicator is a substance that changes color in the presence of a particular
compound. Four indicators will be used in this lab investigation.
Carbohydrates:
Carbohydrates are “sugars” which are important sources of energy for living things. Most
carbohydrates contain only carbon©, hydrogen (H), and oxygen (O) in a 1:2:1 ratio. (i.e. glucose =
C6H12O6) . The monomers for carbohydrates are called monosaccharides or “simple sugars”. Some
common monosaccharides are glucose, fructose, galactose, and ribose. Disaccharides are “double
sugars” which contain two simple monosaccharides together. Examples include lactose and sucrose.
The indicator Benedict’s solution tests for some small sugars (monosaccharides or disaccharides).
Benedicts solution changes from light green to yellow to orange depending on the amount of simple
sugars detected. If none are detected, the reaction mixture remains clear. Long chains of
monosaccharides subunits linked together are known as polysaccharides. Two common
polysaccharides found in plants are starch and cellulose. Animals store sugars as glycogen. Iodine
causes starch (complex carbohydrates) to turn blue-black in color.
Lipids:
Lipids are considered “fats” or “oils” which are hydrophobic or repel water. Lipids are used for longterm energy and are important in the structure of cell membranes. Lipids provide organisms with
energy, twice as many calories per gram as carbohydrates or proteins. The primary component of fats
or lipids is the fatty acids. Fatty acids are most often stored and transported as triglycerides, three
fatty acids bonded to a glycerol molecule. Lipids can be indicated by a Sudan fat stain test. Sudan IV
is a dye that causes lipids to turn a scarlet red color.
Proteins:
Proteins are large molecules essential to the survival of organisms. They make up many structures of
the body, such as hair, fur, nails, skin, and muscles. Proteins are complex organic compounds formed
from long chains of monomers, called amino acids, chemically linked together. Twenty different
amino acids are known to occur in nature. Any and all of these amino acids may occur in any number,
sequence, and configuration to form a vast number of proteins. A Biuret test turns pink or purple in
the presence of proteins, and remains blue when proteins are not detected.
LAB 2: Organic Compounds
Overview- In this experiment, students will use indicators to test common foods for the organic
compounds – carbohydrates, lipids, and proteins.
Identifying Organic Compounds in Common Foods
Materials for the lab group of 4 students:
7 test tubes
Test-tube rack
egg whites
gloves
Biuret reagent
Test-tube holder
egg yolks
Sudan stain
iodine solution
Masking tape
vegetable oil
sweet potato
Hot water bath
cranberry juice
Benedict’s solution
Paper towels
distilled water
milk
LAB 2: Organic Compounds
Procedure –
1. Make masking-tape labels for each of the test tube by placing a piece of masking tape just
below the upper lip of the test tube. Write the name of the different food samples, listed in the
materials section, on each masking-tape label.
2. Simple Carbohydrate Test (Glucose)
a. Place 2mL of the material indicated on the masking-tape label.
b. Add 5 drops of Benedict’s solution to each test tube. Gently mix the contents of each
test tube. The contents of the test tube should turn blue.
c. Place the test tubes in the hot water bath. Heat the test tubes for two to three minutes.
d. Carefully remove the test tubes from the hot water bath. Use the test tube holder to
avoid burning your fingers. After heating, Benedict’s solution reacts with most sugars
producing a yellow to orange color.
e. Observe the color of the contents of each test tube and record the color in the Results
Data Table.
f. Dispose the content of the test tubes into Benedict’s solution waste container.
g. Wash the test tubes thoroughly and reuse for the next experiment.
3. Complex Carbohydrate Test (Starch)
a. Place 4 mL of the material indicated on the masking-tape label.
b. Add 4 drops of iodine solution to each test tube and gently mix. Iodine reacts with starch
producing a blue-black color. CAUTION: Avoid getting iodine on your hands or clothes.
c. Record the color of each test tube in the Results Data Table.
d. Dispose the content of the test tubes into Iodine solution waste container.
e. Wash the test tubes thoroughly and reuse for the next experiment.
4. Lipid Test
a. Fill each test tube with 2 mL of the material indicated on the masking-tape label.
b. Add 3 mL of distilled water and 3 drops of Sudan stain to each test tube. Sudan stain
diffuses through the material, staining the lipids red.
c. Gently mix the contents of each test tube. Record the color of each test tube in the
Results Data Table.
d. Dispose the content of the test tubes into Sudan waste container.
e. Wash the test tubes thoroughly and reuse for the next experiment.
5. Protein Procedure –
a. Fill each test tube with 2mL of the material indicated on the masking-tape label.
b. Add 5 drops of Biuret solution to each test tube. Biuret solution, normally blue, turns
purple (or violet) in the presence of proteins. CAUTION: Avoid getting biuret solution
on your hands. It contains a strong base solution and will burn.
c. Gently mix the contents of each test tube. Record the color of each test tube in the
Results Data Table.
d. Dispose the content into Biuret reagent waste container.
e. Wash the test tubes thoroughly and leave on the rack to dry.
LAB 2: Organic Compounds
Data Collection –
Table 1: Results of Carbohydrate, Lipid, and Protein Test on various organic molecules
Substance
Carbohydrate Test
Benedict’s
Color
Egg White
Vegetable
Oil
Egg Yolk
Sweet
Potato
Cranberry
Juice
Distilled
Water
Milk
Is
Glucose
Present?
(Y/N)
Iodine
Color
Is Starch
present?
(Y/N)
Lipid Test
Sudan
Color
Are
Lipids
Present?
(Y/N)
Protein Test
Biuret
Color
Are
Proteins
Present?
(Y/N)
LAB 2: Organic Compounds
Name: _________________________
Date: ________________________
POST-LAB QUESTIONS
Part A: Identifying Organic Compounds in Common Foods
1. Proteins are generally the building blocks of animal tissues while starches are generally
the building blocks in plant tissues. Does data from this laboratory support this
statement? Explain your answer using data from your lab investigation.
2. The main energy-storing molecule in animals is a lipid molecule (fat) while starch is the
main energy-storing molecule in plants. Does data from this laboratory support this
statement? Explain your answer using data from your lab investigation.
3. Why do you need to test distilled water for the presence of organic molecules in this
experiment? What might any positive result with the distilled water indicate?
4. A very thin slice of a Brazilian nut was treated with several drops of Sudan stain. Next,
several drops of iodine solution were put on the Brazil nut slice. The slice was then
examined under the microscope. Patches of red and patches of blue-black color were
visible. What conclusion can you draw from these results?
LAB 2: Organic Compounds