Name____________________________________Date_____________________________________
Biology Lab: Identifying Organic Compounds
Carbohydrates, Lipids & Proteins
Background: Four types of organic compounds are found in organisms. They are
carbohydrates, lipids, proteins and nucleic acids. Common foods, which often consist of plant or
animal substances, are combinations of these organic compounds. Simple chemical tests with
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. In this
investigation, you will use several indicators to test for the presence of carbohydrates, lipids and
proteins in various foods.
Purpose: How are indicators used to test for the presence of organic compounds in common
foods?
Materials:
12 test tubes
Well tray
Brown paper
Benedict’s solution
Corn oil
Melted butter
Distilled water
Unknown solution C
Test tube rack
Hot water bath
Iodine solution
Honey solution
Glucose solution
Starch and water
Unknown solution A
Test tube clamp
Paper towels
Biuret solution
Egg white & water
Gelatin & water
Apple juice & water
Unknown solution B
Hypothesis: Predict which type of organic compound will be found in each type of
food.
Procedure:
Part A: Testing for Carbohydrates: Monosaccharides
Monosaccharides can be identified in a substance using an indicator called Benedict’s
solution. When Benedict’s is heated in the presence of a monosaccharide, the color
changes from blue to green to yellow to reddish-orange, depending on the amount of
monosaccharide present.
1. Add 5mL of a sample to a test tube. Add 5 drops of Benedict’s solution to the
test tube.
2. Place the test tube in the hot-water bath. Heat the test tube for 1 to 2 minutes,
or until the indicator changes color, if it changes color.
3. With a test tube holder, remove the tube from the hot-water bath and place it
back in the test tube rack.
4. Record any changes in your data table.
Part B: Testing for Carbohydrates: Polysaccharides
Polysaccharides can be tested for using the indicator Iodine. Iodine will change color
from yellowbrown to blue-black in the presence of starch.
1. Add one dropper-full of a sample to a well in the well tray.
2. Add 2-3 drops of Iodine to the sample.
3. Record any changes in your data table.
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Part C: Testing for Lipids
Lipids can be tested for using the “sack lunch” method: the lipid is rubbed into a brown
bag. If the paper appears translucent (a grease spot appears), then a lipid is present in
the food.
1. Tear a small square from a piece of brown paper. Write the name of one test
substance. Add a few drops of each solution to the paper.
2. Rub the solution until a “wet” spot appears on the paper.
3. Set the paper aside until the spot appears to dry.
4. Hold the brown paper up to a bright light.
5. Record observations in data table.
Part D: Testing for Proteins
Proteins can be tested for using Biuret’s reagent, which reacts with the peptide bonds
between amino acids in the polypeptide. Biuret’s reagent turns from blue to purple-violet
if peptide bonds are present.
1. Add one dropper of a sample to a clean well in the well tray.
2. Add 3 drops of Biuret’s reagent to the sample.
3. Record any changes in color.
Data: Reproduce the following data table with your data in your lab report.
Monosaccharide Test
Sample
Honey
Egg White
Corn Oil
Glucose
Gelatin
Butter
Starch
Apple Juice
Distilled water
Unknown A
Unknown B
Unknown C
Benedicts color
+ or -
Iodine
Starch + or -
Polysaccharide Test
Sample
Honey
Egg White
Corn Oil
Glucose
Gelatin
Butter
Starch
Apple Juice
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Distilled water
Unknown A
Unknown B
Unknown C
Lipid Test
Sample
Honey
Egg White
Corn Oil
Glucose
Gelatin
Butter
Starch
Apple Juice
Distilled water
Unknown A
Unknown B
Unknown C
Translucent?
Lipid + or -
Biuret color
Protein + or -
Protein Test
Sample
Honey
Egg White
Corn Oil
Glucose
Gelatin
Butter
Starch
Apple Juice
Distilled water
Unknown A
Unknown B
Unknown C
Analysis Questions:
1. Did any of the food substances contain all of the organic compounds for which you
tested?
2. Which one had the most?
3. Which tests were positive for your unknown substances? What type of organic
compound do you think each is?
4. Which organic compound is most common in foods that come from plants?
5. Which organic compound is most common in foods that come from animals?
6. How might the original colors of the tested food substances affect the test results?
7. Does distilled water contain any of the organic compounds you tested? What is the
purpose of using distilled water as one of your test substances?
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8. What conclusion could you make if a positive test for any of the organic compounds
occurred in the distilled water?
9. People with diabetes are instructed to avoid foods that are rich in carbohydrates. How
could your observations in this investigation help you decide whether a food should be
served to a person with diabetes?
10. Biuret solution will turn skin brownish-purple. Explain why this occurs.
Conclusion: Were your hypotheses supported or refuted? Discuss the data that caused
you to decide whether your hypothesis was valid or not. What did you learn in this experiment?
Were there any sources of error?