Who took Bob’s iPod?
A Biological Molecule Mystery
Bob is a freshmen in college who works at a local café on the weekends. While on break, Bob was
studying for his biology test and listening to his new iPod. There were four other workers taking a
break at the same time, each having something different for lunch.
Bob‘s girlfriend stopped by near the end of his break, and he rushed out to see her and forgot his
iPod and biology book in the break room. When he realized, he hurried back and found only his
biology book and some spilled food. His iPod was gone!
At first Bob freaked out, but he calmed down when he realized he could use his knowledge of
organic compounds to figure out which of his coworkers left the spilled food on his biology book
while taking his iPod.
What are organic compounds?
Almost all of the food we eat comes from plants and animals. Plants and animals contain mainly
water and organic compounds, which are molecules made by living organisms such as plants or
animals.
Today you will be testing the substances listed in the following table. Predict whether each
substance is an organic compound and if so, what type.
Substance
Do you think this substance is a starch (carbohydrate),
lipid, monosaccharide (simple sugar), protein, or none of
these?
Vegetable oil
Table sugar
Starch from corn or
potatoes
Powdered egg whites
Water
What are indicators?
An indicator is a substance that changes color in the presence of a particular type of molecule.
Today you will learn how to use several indicators to test for the presence of starch (carbohydrate),
lipids, monosaccharides (simple sugar), and proteins. In this laboratory period, you will use these
tests to analyze the evidence left at the scene of the crime and find out who spilled the food on
Bob’s textbook.
For the initial part of this lab you will be testing five known samples: vegetable oil, table sugar
solution, mashed potato solution, powdered egg white solution, and pure water for the presence of
biological molecules. You will subject each know sample to each test.
1
Testing for lipids: Sudan IV Test
Lipids are fat molecules that are insoluble in water (hydrophobic). Like lipids, the chemical Sudan
IV is not soluble in water. it is, however, soluble in lipids. Therefore to test for the presence of
lipids in a solution you will use a Sudan IV Test. In this test dark red Sudan IV is added to a
solution along with ethanol to dissolve any possible lipids. If lipids are present the Sudan IV will
stain them reddish-orange, giving a positive test.
1. Add 1ml of each known sample to individual labeled cell well of a cell plate (one
sample per cell well)
2. Add 3 drops of a prepared Sudan IV solution to each known sample
3. Mix together known sample and Sudan IV solution by swirling the cell well plate
4. Add 2ml of water to each known sample
If fats or oils are present in your known samples they will appear as surface floating red droplets or
as a floating red layer colored by the Sudan IV dye
4. Record the outcomes of the reactions occurring in each of your known samples
Substance
Vegetable oil
Table sugar solution
Mashed potato solution
Powered egg white solution
Pure water
Sudan IV Test Results
Lipid or Not Lipid
Testing for Monosaccharides (simple sugar): Benedict’s Test
To test for the presence of monosaccharides (simple sugar) you will use the Benedict’s Test.
Benedict’s reagent is clear blue (from the presence of cupric copper ions) but when combined and
heated to boiling with a substance containing monosaccharides, the cupric ions are reduced to a
cuprous form and then oxidized to form copper oxide. Copper oxide is a brownish-orange
substance that is insoluble in water. Therefore, a positive reaction in a Benedict’s Test is the
change of the clear light blue solution to an opaque orange-brown solution in a a boiling water
bath. This color change indicates the presence of monosaccharides in a given solution.
1. Place 2ml of each known sample in a series of glass test tubes (one known
sample per test tube)
2. Add 2ml of Benedict's reagent to each of your tube; gently mix the test tube contents and record
the initial color
3. Heat the test tubes in a boiling water bath for 3 minutes
4. Take out your test tubes and place them in a metal rack
5. Record the color changes in each test tube
2
Substance
Color Prior to Heat
Color After Heat
Monosaccharide or
No Monosaccharide
Vegetable oil
Table sugar solution
Mashed potato
solution
Powdered egg white
solution
Pure water
Testing for Starches (carbohydrates): Lugol’s Reagent
To test for the presence of starch, the Lugol’s reagent is used. This is a simple test in which iodine
is added to a given solution. If a polysaccharide such as starch is present then the iodine ion will
lodge itself in the polysaccharide chain and give it a black-blue color. If iodine added to a solution
turns black-blue than starch is present. If the solution remains the color of iodine, reddish-orange,
there is no starch present.
1. Add 1ml of each known sample to individual labeled cell well of a cell plate (one sample per cell
well)
2. Add several 10 drops of Lugol's reagent (I2KI solution) to each tube, mix and immediately record
your color changes in the corresponding cell wells
Substance
Vegetable oil
Table sugar solution
Mashed potato solution
Powered egg white solution
Pure water
Lugol’s Reagent Results
Starch or No Starch
Testing for Proteins: Biuret Test
The last macromolecule you will explore in this lab is protein. In this lab you will test for the
presence of protein using the Biuret Test. Like the Benedict’s Reagent, Biuret Reagent contains
copper ions. These copper ions reflect off closely clustered amide groups of proteins casting a
violet color to a solution with proteins. This violet color is a positive reaction in a Biuret Test.
1. Add 1ml of each known sample to individual labeled cell well of a cell plate (one
sample per cell well)
2. Add about 1 ml of water to each container. Stir the contents of each container to mix the
substance with the water.
3. Add 10 drops of biuret reagent to each container. Stir the contents of each container.
CAUTION: Biuret reagent contains sodium hydroxide, a strong base. Be very careful not
to splash or spill any. If you splash any reagent on yourself, wash it off immediately with
water. Call your instructor for assistance.
3
Substance
Vegetable oil
Table sugar solution
Mashed potato solution
Powered egg white solution
Pure water
Biuret Test Results
Protein or No Protein
Solving the Mystery
Today you and your classmates will solve the mystery of who stole Bob's iPod by figuring out
which coworker's lunch had the same organic compounds as the evidence Bob found at the crime
scene.
Begin by predicting which types of compounds you expect to find in each type of food.
Food
Do you expect this food to contain
Monosaccharide
Starch
Protein
Lipid
Pretzel
Butter
Jelly
Fat-free yogurt
Beans
In addition to these known samples, your group will be assigned a evidence sample to test.
Record your test results in the data table below.
Testing for lipids: Sudan IV Test
1. Add 1ml of each known sample to individual labeled cell well of a cell plate (one
sample per cell well)
2. Add 3 drops of a prepared Sudan IV solution to each known sample
3. Mix together known sample and Sudan IV solution by swirling the cell well plate
4. Add 2ml of water to each known sample
If fats or oils are present in your known samples they will appear as surface floating red droplets or
as a floating red layer colored by the Sudan IV dye
4. Record the outcomes of the reactions occurring in each of your known samples
Substance
Pretzel
Butter
Jelly
Fat-free yogurt
Beans
Sudan IV Test Results
Lipid or Not Lipid
4
Testing for Monosaccharides (simple sugar): Benedict’s Test for Reducing Sugars
1. Place 2ml of each known sample in a series of glass test tubes (one known
sample per test tube)
2. Add 2ml of Benedict's reagent to each of your tube; gently mix the test tube contents and record
the initial color
3. Heat the test tubes in a boiling water bath for 3 minutes
4. Take out your test tubes and place them in a metal rack
5. Record the color changes in each test tube
Substance
Color Prior to Heat
Color After Heat
Monosaccharide or
No Monosaccharide
Pretzel
Butter
Jelly
Fat-free yogurt
Beans
Testing for Starches (carbohydrates): Lugol’s Reagent
1. Add 1ml of each known sample to individual labeled cell well of a cell plate (one
sample per cell well)
2. Add 10 drops of Lugol's reagent (I2KI solution) to each tube, mix and immediately record
your color changes in the corresponding cell wells
Substance
Pretzel
Butter
Jelly
Fat-free yogurt
Beans
Lugol’s Reagent Results
Starch or No Starch
Testing for Proteins: Biuret Test
1. Add 1ml of each known sample to individual labeled cell well of a cell plate (one
sample per cell well)
2. Add about 1 ml of water to each container. Stir the contents of each container to mix the
substance with the water.
3. Add 10 drops of biuret reagent to each container. Stir the contents of each container.
CAUTION: Biuret reagent contains sodium hydroxide, a strong base. Be very careful not
to splash or spill any. If you splash any reagent on yourself, wash it off immediately with
water. Call your instructor for assistance.
5
Substance
Pretzels
Butter
Jelly
Fat-free yogurt
Beans
Biuret Test Results
Protein or No Protein
Who took Bob’s iPod?
The table shows what each worker in the break room was eating while Bob was studying. Use the
information from the table on the bottom of page 5 to complete this table to show which types of
organic compounds would be in each lunch and which types of organic compounds were found in
the evidence.
Worker in
break
room
Mark
Ashley
Bruce
Lisa
Thief
Lunch he or she was eating
Glucose
Starch
Protein
Lipid
Bean burrito with cheese
Fat-Free Yogurt
Toast with butter and jelly
Pretzel
Collected Evidence
Complete the following table to summarize the evidence and your interpretation of the evidence.
Worker
in break
room
Mark
Did he/she
take Bob’s
ipod?
How do you know?
Describe the evidence that supports your conclusion.
Ashley
Bruce
Lisa
6