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Biology 611
Name: __________________________ Block:________
Date:____/___/_____
Partners Names: __________________ ________________ _________________
Biology 611—What’s in Your Food?
Identifying Biologically Important Organic Compounds
(adopted from Biology Laboratory Manual “Identifying Organic Compounds”)
Background Information:
All living things are composed of biomolecules. The most common organic
compounds found in living organisms are lipids, carbohydrates, proteins, and nucleic
acids. Common foods, which often consist of plant materials or substances derived from
animals, are also combinations of these organic compounds. 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 lipids,
carbohydrates, and proteins in various foods.
In this lab, you will perform simple chemical tests to detect carbohydate, protein, and
lipids in selected foods.
Question:
What are the major types of organic compounds in some common foods?
Pre-Lab Discussion:
Read the entire investigation. Then, work with a partner to answer the following
questions.
1. What is an indicator? How are indicators used in this experiment?
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2. What is the purpose of using distilled water as one of your test substances?
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3. What is the controlled variable in Part C?
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4. What is the purpose of washing the test tubes thoroughly?
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Part A—Test for Lipids
Grease Spot Test:
An easy method to test for lipid is based on the ability of lipids to produce a translucent
spot on paper.
Procedure:
1. Divide a piece of a brown paper bag into 5 equal sections. In each section, write the
name of one test substance, as shown in Figure 1.
1. Corn
2. Milk
3.
Vegetable
Oil
Figure 1.
4. Water
5. Food
Test Item:
2. In each section, place a small drop or rub the masher up piece of the identified food
onto the brown paper. With a paper towel, wipe off any excess piece of food that
may stick to the paper. Set the paper aside until the spots appear dry—about 10-15
minutes.
3. Hold the piece of brown paper up to a bright light or window. You will notice that
some foods leave a translucent spot on the brown paper. The translucent spot
indicates the presence of lipids.
4. In the Data Table, record any of your observation and place a check mark next to
those substances testing positive for lipid.
Grease Spot Test for Lipids Table 1
Spot #
Contents
1
Corn
2
Milk
3
Vegetable Oil
4
Water
5
Food Test Item 1:
Observations
Part B—Test for Carbohydrates
Test B.1. Benedict's Test for Simple Sugars
There are many kinds of carbohydrate molecules, all of which are made up of various
arrangements of carbon, hydrogen, and oxygen atoms. The simplest carbohydrates are
monosaccharides, such as glucose and fructose. Disaccharides consist of two bonded
monosaccarides. Examples of disaccharides include lactose, the sugar contained in
milk, maltose, used in brewing, and sucrose, common table sugar. All monosaccharides
and the disaccharides lactose and maltose can be detected using the Benedict's Test.
Sucrose cannot be detected with this test unless the sucrose is first treated with
hydrochloric acid (HCl).
Benedict's solution contains blue copper (II) ions (Cu++). When heated, the copper (II)
ions (Cu++) join with certain sugars and become insoluble, changing to red copper (I)
ions (Cu+). The test solution changes from blue to green to reddish brown as more
copper (I) ions are formed. Sucrose does not react with Benedict's reagent because the
functional groups are not free to bind with the copper (II) ions..
Benedict’s Test Procedure:
1. Number five (5) clean test tubes 1-5 (use grease pencil or marking tape).
2. To tube #1 add a small piece of mashed corn and a dropper full of Benedict's
solution.
3. To tube #2 add a dropper full (about 1 ml) of milk and a dropper full of Benedict's
solution.
4. To tube #3 add a dropper full of glucose solution and a dropper full of Benedict's
solution.
5. To tube #4 add a dropper full of distilled water and a dropper full of Benedict's
solution.
6. To tube #5 ad a dropper full of your chosen food item (ground up and mixed with
water) and a dropper full of Benedict’s solution.
7. Place all the test tubes in a hot water bath for 3 to 5 minutes. Do not rush this step!!!
8. After 3 to 5 minutes, remove the tubes, and record your results in the table below.
Benedicts Test Results Table 2
Tube Contents
Observations (include color, texture, smell, phases, etc)
#
1
Corn + Benedicts
2
Milk + Benedicts
3
Glucose + Benedicts
4
Water + Benedicts
5.
Your food item +
Benedicts
Test B.2. Lugol’s Iodine Test for Starch
Organisms store simple sugars in the form of large macromolecules comprised of
hundreds or thousands of linked monosaccharides. Some of these polysaccharides are
broken down as needed for energy. Other polysaccharides serve a structural function
(the most abundant organic compound on earth is cellulose, a polysaccharide that is the
main component in plant cell walls.) Plants store sugars for later energy needs in the
form of a polysaccharide called starch.
In this exercise, you will use the Lugol’s iodine test to detect the presence of starch.
Starch interacts with iodine to produce a bluish-black color. (A yellowish-brown color
indicates that no starch is present.) Monosaccharides do not react with Lugol’s Iodine.
Procedure:
1. Wash your test tubes thoroughly and if necessary, renumber them 1 - 5.
2. To tube #1 add a small piece of mashed corn and 3-5 drops of Lugol’s iodine.
3. To tube #2 add a dropper full of milk and 3-5 drops of Lugol’s iodine.
4. To tube #3 add a dropper full of starch solution and 3-5 drops of Lugol’s iodine.
5. To tube #4 add a dropper full of distilled water and 3-5 drops of Lugol’s iodine.
6. To tube #5 add a small amount of your food item and 3-5 drops of Lugol’s iodine.
7. Observe and record your results in the table below.
Lugol’s Iodine Test for Starch Table. 2
Tube Contents
Observations
#
1
Corn + Lugol’s Iodine
2
Milk + Lugol’s Iodine
3
Starch + Lugol’s Iodine
4
Water + Lugol’s Iodine
5
Food Item +
Part C—Test for Proteins
Biuret Test for Proteins
Proteins consist of linked subunits called amino acids, which are in turn composed of
carbon, hydrogen, oxygen, and nitrogen. The Biuret Test detects the presence of a
particular type of chemical bond in amino acids, called the peptide bond. (O=C-NH)
When copper sulfate (CuSO4) reacts with peptide bonds in the presence of sodium
hydroxide (NaOH), a violet color results. Only proteins with four to six linked amino
acids will react in this way, thus free amino acids will not be detected. The intensity of
the color is related to the number of amino acids linked together.
Procedure:
1. Wash your test tubes and if necessary, label 1 - 5.
2. To tube #1 add a piece of mashed corn.
3. To tube #2 add a dropper full of milk.
4. To tube #3 add a dropper full of gelatin solution.
5. To tube #4 add a dropper full of distilled water.
6. To tube #5 add your food item.
7. Carefully add one dropper full of 10% NaOH to each tube. NaOH is a strong base
and can burn your skin. Use extreme care with this solution.
8. Add 4 drops of 0.5% CuSO4 to each tube. Mix gently and let stand five minutes for
the color to develop.
9. Record your results in the table below.
Biuret Test Results for Proteins Table 3
Tube # Contents
Observations
1
Corn + NaOH +
CuSO4
2
Milk + NaOH +
CuSO4
3
Gelatin + NaOH +
CuSO4
4
Water + NaOH +
CuSO4
5
Food Item +
NaOH + CuSO4
Summary Data Table:
Test Item
Simple Sugar
Corn
Starch
Protein
Lipid
Milk
Your Food
Item
Glucose
Starch
Gelatin
Vegetable Oil
Analysis Questions : Answer the following questions in complete sentences. Make
sure you rephrase the question in your answer so I DO NOT have to reread the
questions to know what you are answering.
1. Which test substances contain lipids?
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2. Which test substances contain starch?
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3. Which test substances contain protein?
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4. What was your result for the control (distilled water)?
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5. 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?
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6. Your brown lunch bag has a large, translucent spot on the bottom. What explanation
could you give for this occurrence?
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7. What conclusion could you make if a positive test for any of the organic compounds
occurred in the test tube containing only distilled water?
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8. A very thin slice is removed from a peanut. You add a drop of biuret reagent to the
peanut slice. When you observe the peanut slice under a microscope, patches of red
and blue-violet are visible. Also, when you rubbed the slice on a piece of brown
paper bag, you find a large translucent spot. What conclusions can you draw from
your examination?
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