LITTLE MISS MUFFIT LAB
INTRODUCTION:
All living things are made up of cells, and cells in turn contain many kinds of molecules. In this lab we
will be looking at several different macromolecules: proteins, carbohydrates, and lipids (specifically
fats). Various chemicals will be used in this laboratory to test for the presence of these molecules.
Most often, you will be looking for a particular color change. If the change is observed, the test is said
to be positive because it indicates that a particular molecule is present. If the color change is not
observed, the test is said to be negative because it indicates that a particular molecule is not present.
You will be using these tests to determine which of the macromolecules are present in the various
food samples.
In all the procedures, you will need to include a distilled water sample as a control. Usually, a control
goes through all the steps of the experiment but lacks one essential factor (the experimental
variable). This missing factor allows you to observe the difference between a positive result and a
negative result. If the control sample tests positive, you know your test is invalid. Some tests may
also contain other controls to make sure certain additives are not contaminated with the substance
for which you are testing.
You will be testing a variety of foods and writing a lab report (see end of this handout)
PROTEINS
Protein molecules are long chains of amino acids joined by peptide bonds.
Biuret Reagent which is a blue color, contains a strong solution of sodium or potassium hydroxide
(NaOH or KOH) and a very small amount of dilute copper sulfate (CuSO4) solution. The reagent
changes color in the presence of proteins or peptides because the amino group (H2N-) of the protein
or peptide chemically combines with the copper ions in Biuret Reagent.
CARBOHYDRATES
Carbohydrates include sugars and molecules that are chains of sugars. Glucose is a simple sugar, also
known as a monosaccharide. Sucrose, or table sugar is a disaccharide; it has two sugar units bonded
together. Starch is a polysaccharide, a long chain of glucose units.
Benedict’s Reagent reacts with many sugars (monosaccharides) after being heated in a boiling water
bath. Increasing concentrations of sugar give a continuum of colored products ranging from green
through yellow and orange to orange red.
Iodine solution reacts with starch to produce colors ranging from a brownish to blue black.
LIPIDS
Lipids are hydrophobic molecules which are insoluble in water and soluble in solvents such as
alcohol and ether. Lipids include fats, oils, and cholesterol.
Lipids do not evaporate from brown paper, instead leaving an oily spot. Lipids also do not mix with
water, forming a separate layer, usually on top of the water. However, some molecules mix with both
water and lipids, and can be used to mix the two. These molecules are known as emulsifiers. The
liver produces bile salts which act as emulsifiers in the digestive tract. Soaps and detergents also act
as emulsifiers.
Summary of tests:
Biuret Reagent
Reacts with proteins
or peptides, turns
purple (protein) or
pink (peptides)
Benedict’s Reagent
Reacts with
monosaccharides,
turns green through
yellow to orange
(green, less sugar,
orange, more sugar)
Iodine Solution
Brown Paper
Reacts with starch,
turns dark brown to
black or blue-black
Lipids leave oily spot
PROCEDURE
TAKE SOME TIME TO PLAN WITH YOUR LAB PARTNERS WHAT TEST YOU WILL DO, IN WHAT
ORDER BEFORE BEGINNING THE PROCEDURE!
There are available in the lab a variety of different types of common food. Choose three foods and
test each for the presence of protein, carbohydrate (both starch, disaccharides, and
monosaccharides), and fats.
Form a hypothesis for each sample you have chosen to test.
Samples will need to be suspended in water for most tests. Break each piece to be tested into small
pieces and suspend the pieces in a small amount of distilled water using a test tube.
Also available are samples of each of the types of molecules for which you will be testing. Use these
samples to try out the tests so that you will know what a positive result looks like.
Be sure to include a blank control (distilled water) with each test so you will know what a negative
result looks like.
The procedures for testing for each type of molecule are given below.
Proteins:
1.
2.
3.
4.
5.
Use a separate test tube for each sample to be tested, as well as one for a control.
Label each test tube
Place 1 mL of each sample (and control) in its test tube.
Add 5 drops of Biuret solution to each tube and mix
Record the tube contents and final color in a data table
Starch:
1.
2.
3.
4.
5.
Use a separate test tube for each sample to be tested, as well as one for a control.
Label each test tube.
Place about 1 mL of each sample in its test tube.
Add 5 drops of iodine solution to each tube and mix
Record the tube contents and final color in a data table
Sugar
1.
2.
3.
4.
5.
6.
Use a separate test tube for each sample to be tested, as well as one for a control
Label each test tube.
Place about 1 mL of each sample (and control) in its test tube.
Add about 2 mL of Benedict’s Reagent to each tube and mix.
Heat the tubes in a boiling water bath for 5 – 10 minutes.
Record the tube contents and final color in a data table.
Lipids:
1. Draw 2 squares on brown paper, label one water and one sample
2. Place a small sample of the material to be tested on a square of brown paper in the square
marked “sample”. If your sample is a solid rub it into the brown paper.
3. Place a small drop of water on the square of brown paper in the square marker “water”.
4. Compare the drop of water to the sample drop.
5. Wait at least 5 minutes. Evaluate which substances impregnates the paper and which is
subject to evaporation. Record your results.
Analysis Questions
1. Why do experimental procedures include control samples?
2. How would you test an unknown solution for each of the following?
a. sugars
c. starch
b. fat
d. protein
3. Assume that you have tested an unknown sample with both Biuret Reagent and Benedict’s
Reagent and that both tests result in a blue color. What have you learned?
4. What purpose is served when a test is done using water instead of a sample substance?
5. Why did we not test for nucleic acids?
Lab Report must include the following:
1. Title for the lab
2. Purpose: What was studied in this lab & why did we study it
3. Procedure: a brief description of each type of test, what constitutes a positive test and what
constitutes a negative test
4. All data tables (numbered & named)
5. For each food sample, state your hypothesis and your conclusions.
6. Answers to
a. What worked in this lab & what did not work and possible explanations why?
b. How did this lab enhance your understanding of what you had already learned about
organic compounds?
c. Are there any areas on this topic that you are still confused about or would like to
know more about?