Download Macromolecules Lab

Macromolecules Lab
Bench #:
Lab Partner Name:
Name:
Period:
Purpose
To test several common food items and identify the macromolecules that are present in each of these foods.
Introduction
Living organisms are composed of molecules that come in diverse shapes and sizes and serve a variety of purposes. Some
molecules form the structure of an organisms body- for example, the cellulose that makes up the cell wall in plants, the
proteins and phospholipids that comprise cell membranes, and the fibers that make up the muscles of animals.
There is also a wide array of molecules that perform all the functions of life. For example, enzymes catalyze (speed up)
the chemical reactions necessary for biological processes, neurotransmitters convey information from one brain cell to
another, and visual pigments absorb light so that you can read the words on this page.
In this laboratory you will study three classes of the largest biological molecules, the macromolecules: carbohydrates,
lipids, and proteins. A fourth class of macromolecules, nucleic acids will not be investigated in this lab. In fact
macromolecules are so important that scientists have devised tests to determine if they are present in a sample. All of
these tests rely on chemical indicators that change color only if a certain molecule is present in the sample. In order to
understand if a certain color indicates a positive result or not, we will run positive controls and negative controls for each
tests. The positive control will show us what a positive result with a certain indicator should look lie. A negative control
will tell us what a negative result should look like.
Before beginning any of the testing, complete the table below by placing a “+” in a cell of the table if you believe the food
will contain that macromolecule and place a “ – ” in the cell of food items that you do not think will contain that
macromolecule.
Hypothesis: For each of the food items provided for testing, predict which types of macromolecules will be present
(carbohydrates, lipids, proteins). There may be more than one type of macromolecule present in each food item.
Do you expect the food item to contain:
Glucose?
Starch?
Lipid?
Protein?
Bread
Yogurt
Potato
Cheese
Salami
Peach Jelly
Use the data table below to record the results of all of the tests that you will run in the following sections. When
recording your results, record the color that you observed followed by a + or – to indicate if you believe this color means
the molecule is present or absent. Positive controls for each test are listed at the bottom of each column. Water is the
negative control for each.
Food Item
Benedicts
(monosaccharides)
Macromolecule Testing Results
Lugol’s
(polysaccharides)
Paper
(lipids)
Biuret’s
(proteins)
Bread
Yogurt
Potato
Cheese
Salami
Peach Jelly
Unknown
Water
Glucose Solution:
Positive Controls
Starch Solution:
Vegetable Oil:
Protein Solution:
Carbohydrates
Most carbohydrates contain only carbon, oxygen, and hydrogen, usually in the ratio of 1 Carbon : 2 Hydrogen : 1 Oxygen.
The simplest forms of carbohydrate molecules are the monosaccharides (single sugars). One of the most important
monosaccharides is glucose (C6H12O6), the end product of photosynthesis in plants. It is also the molecule that is
metabolized to produce cellular energy.
Carbohydrates are also found in the form of polysaccharides (many sugars), which are long chains of monosaccharides
linked together. Starch, a polysaccharide composed of only glucose monomers, is an especially abundant component of
plants. Starch is the main way plants store excess glucose created by photosynthesis. Most of the carbohydrates we eat
are derived from plants.
Animals store glucose as glycogen, which is another polysaccharide. Although starch and glycogen are both composed of
glucose subunits, the glucose molecules are bonded together in different ways so these polysaccharides are not identical.
In this part of the lab you will use Benedict’s reagent as a general test for small sugars. When this reagent is mixed with a
solution containing monosaccharides or disaccharides (double sugars) and then heated, a colored precipitate (solid
material) forms. The precipitate may be yellow, green, orange or red. If no monosaccharides or disaccharides are present
then the solution remains clear. Benedicts reagent does not react with all small sugars however. For example, sucrose, a
disaccharide, gives a negative result in the Benedict’s test.
You will also test for a large polysaccharide, starch, using Lugol’s Iodine Solution. In this test a dark blue color indicates
the presence of polysaccharides such as starch, glycogen, or cellulose. Potatoes are rich in starch and will be used as a
positive control for the presence of polysaccharides.
Procedure: Benedicts Test for Glucose
1. Fill a 250 mL beaker with 150 mL, place it on your hot plate and bring it to a gentle boil. This will be your water
bath for heating solutions today.
2. Crush small amounts of any solid food items in a paper towel and add a small amount of each food to separate test
tubes Label each test tube. Mix with a small amount of distilled water (DH2O).
3. Add any liquid food items to additional, separate test tubes. Label the test tubes.
4. Add 10 drops of Glucose to an additional test tube. Label this test tube.
5. Add 10 drops of water to an additional test tube. Label this test tube.
6. Add 10 drops of benedicts reagent to each of the test tubes from steps 2-5.
7. Place all test tubes in the gently boiling water bath and heat for 3-4 minutes.
8. Remove the beaker (water bath) from the hot plat and allow to cool. Carefully remove the test tubes and place
them in the test tube rack. Caution: the water and test tubes may still be HOT!
9. Record your observations of each test tube in your data table.
10. Clean your test tubes.
Procedure: Iodine Test for Starch
1. Refill each of your test tubes with 10 drops of the same food items used in the previous test and place the test
tubes in the test tube rack.
2. Add 10 drops of potato juice to another test tube, and place in the test tube rack.
3. Add 10 drops of water to another test tube, and place in the test tube rack.
4. Add 3-4 drops of Lugols solution to each of the test tubes.
5. Record your observations of each test tube in your data table.
6. Clean your test tubes.
Lipids
Lipids are compounds that contain mostly carbon and hydrogen. They are grouped together solely on the basis of their
insolubility in water. The lipids we consider in this lab are fats and oils, which are generally used as storage molecules in
plants and animals. You are probably familiar with the fact that the human body converts food into fat. This fat is stored
in your tissues until your food intake is lower than your metabolic needs, at which time the fat is converted to energy to
fuel cellular work. Plants too, can store lipids. Seeds are often surrounded by lipids that can be metabolized by the
developing embryo when germination time comes in order to provide energy for growth. Thus we obtain corn oil, peanut
oil, olive oil and others by pressing seeds.
To test for lipids in various food items, you will use the paper test. Although this test is not exceptionally sophisticated it
is convenient, quick and reliable.
Procedure: Test for Lipids
1. Get a small square of brown paper for each food item you are testing. Label each one with the name of the food
item. Label one of the additional papers “water” and the other one “vegetable oil”
2. If the food is available as a solid, rub a small amount of it on the appropriate paper. If the food is a liquid, place a
small drop on the appropriate paper and rub it in. Use a different finger for each substance to avoid crosscontaminating the paper samples.
3. Allow each of the squares to dry and hold them up to the light.
4. Record your observations as translucent or non - translucent in your data table.
5. Throw away the paper squares.
Proteins
A proteins structure is determined by the amino acid subunits that make up the molecule. Although there are only 20
different naturally occurring amino acids, each protein has a unique sequence. The amino acids are linked by fairly tight
bonds, and the functional groups that are part of the amino acids also interact with each other to help shape the molecule.
Proteins have a greater diversity of functions than either carbohydrates or lipids. The shape of a protein is key to it
function. Proteins work by selectively binding to other molecules. Their shape helps determine which molecules they can
attach to.
In this lab you will use Biuret’s solution as a test for proteins. This reagent, which is blue, reacts with proteins to give a
light violet or lavender color. Egg white is rich in proteins and will be used as a positive control for protein.
Procedure: Biuret’s Test for Proteins
1. Add food items to separate test tubes. If they are liquids add them directly to the test tubes. If they are solids
crush them and add a small amount to the test tube and mix with a small amount of distilled water.
2. Add 10 drops of the egg white solution to an additional test tube.
3. Add 10 drops of water to an additional test tube.
4. Add 10 drops of Biuret’s solution to each test tube.
5. Record your observations in your data table.
6. Clean the test tubes and place them upside down to drain in the clean empty 250 mL beaker you used for the
water bath in the sugars test.
Conclusions and Questions
1. Does pure water contain any macromolecules? Why did we test pure water for each of the macromolecules?
2. Why did we test each of the following items with the solutions that we did: Glucose solution, Starch Solution,
Vegetable Oil, and Protein solution?
3. Describe the results of a positive test for each of the following reagents and the class of molecules that would
result in the positive test: Benedict’s solution, Lugol’s Solution, Paper bag test, Biuret’s solution.
4. Which items contained carbohydrates? Did they contain monosaccharides, polysaccharides or both?
5. What is the relationship between glucose and starch? Why might a fruit, contain both glucose and starch?
6. Several tests are performed on a sample of whole milk. The results are shown in the table below. Based on these
results which macromolecules were present in the milk sample?
Indicator
Result
Lugol’s Solution
Orange / brown
Benedict’s Solution
Orange
Biuret’s Solution
Purple
Paper Bag
Translucent / clear
7. A student conducted several tests on a piece of bacon. The results of his testing are shown in the table below.
Based on the results, which molecules were present in the bacon?
Indicator
Result
Lugol’s Solution
Dark purple / Black
Benedict’s Solution
Light Blue
Biuret’s Solution
Purple
Paper Bag
Translucent / clear
8. Which of the results from the student’s bacon experiment are most likely to make someone question his results?
Explain.
9. What class of macromolecule does not consist of molecules made from monomers? For each of the other three
classes of macromolecules, name the polymer and the corresponding monomer.
10. A molecule is tested and found to have the chemical formula C7H14O7. What class of macromolecule is this most
likely to be a monomer of? Explain.