Lab #3 Organic Compounds
(Taken from General Biology:
Investigations into Life’s Phenomena)
Name _______________________
Date ___________
Period ___________
Group Data, Individual answers
Objectives
1. Distinguish among monosaccharides, disaccharides, and polysaccharides, giving examples
of each.
2. Describe the functions and chemical structure of proteins.
3. Describe the chemical structure of nucleotides and nucleic acids and explain the importance
of these compounds in living organisms.
4. Name the specific lab test used to identify (a) sugar, (b) starch, (c) fat, (d) protein, and (e)
nucleic acid.
Introduction
The various molecules found in all life forms are called organic compounds because they are produced
by organisms. Organic molecules are carbon based and range from small molecules to ones that are
enormous in size. These larger molecules are not as complex as they first seem because they are
composed of simpler, smaller molecules called monomers which link together into larger, longer chains
called polymers.
There are four major classes of biologically important organic compounds. Within a living cell, each of
these four major classes has specific important functions. The table on the following page describes each
of these classes, and lists examples. Study this table, and Section 2.3 of your text carefully.
Most of the tests you will be performing today will lead to a color change as an indication of a chemical
reaction. The chemical reaction will occur if the substance you are testing is present.
Table 1 Four classes of biologically important organic compounds
CARBOHYDRATES
Carbohydrates are energy sources and provide
structural support as in the cell wall of plants.
_____________, ______________ and
________________ are the elements found in
carbohydrates. Carbohydrates may be classified as
monosaccharides, disaccharides, or
polysaccharides.
1. Monosaccharides are _______________.
2. Disaccharides are _____ monosaccharides
bonded together.
3. Polysaccharides are three or more
monosaccharides bonded together. ______,
___________, and __________ are
important polysaccharides.
a. Starch:
LIPIDS
Lipids are a varied group of molecules that are
mostly _______________ in water. Like
carbohydrates, carbon, hydrogen, and oxygen are
the principal elements of lipids, although the
oxygen content is _____________. Lipids are
essential components of ________________, a
good means of ______________, and some lipids
are _____________________.
Neutral fats
Phospholipids
Steroids
b. Glycogen:
c. Cellulose:
Waxes
PROTEINS
NUCLEIC ACID
Proteins have several important roles in the living
organism. The elements in proteins are carbon,
hydrogen, oxygen, nitrogen, and sometimes sulfur.
A protein is made of these elements arranged into
subunits called ________________. Long chains
of amino acids are linked together with ________
_____________ which make the chain coil and
fold into complex shapes that determine the
function of the resulting protein molecule.
Proteins form many structural features such as
_____________, ______________ and
______________.
Many proteins called _______________ help
control the chemical reactions that keep the cells
alive.
Nucleic acids are extremely large and complex
molecules that have variety of important biological
functions. The elements of nucleic acids are
carbon, hydrogen, oxygen, nitrogen, and
phosphorus.
1. One general type of nucleic acid, called
______________________________ is
coded genetic information.
2. A second type, called ______________ is
what turns the genetic code into a protein.
In other words, it is involved in the
expression of the genetic code.
3. Nucleotides are the units, or the monomers,
of nucleic acid polymers
4. _____ is a nucleotide that stays in monomer
form. It is responsible for supplying
__________ for the chemical reactions of
the cell
Pre-Lab Questions
These questions will help you prepare for the lab. The pre-lab questions are due before your group can
start the lab.
1. Define organic compound ____________________________________________________________
____________________________________________________________________________________
2. Define monomer and polymer
______________________________________________________
____________________________________________________________________________________
3. Define reagent
__________________________________________________________________
____________________________________________________________________________________
5. Define chemical reaction ____________________________________________________________
____________________________________________________________________________________
6. Define polar and non-polar ____________________________________________________________
____________________________________________________________________________________
7. Define soluble
__________________________________________________________________
____________________________________________________________________________________
8. What elements make up carbohydrates?
________________________________________________
____________________________________________________________________________________
9. What are three categories of carbohydrates and what are their functions? ________________________
____________________________________________________________________________________
____________________________________________________________________________________
10. Why do plants have starch? Why do they have cellulose?
______________________________
____________________________________________________________________________________
11. What does a positive test for simple sugar look like? Protein? Starch? Lipid? __________________
____________________________________________________________________________________
____________________________________________________________________________________
12. What are two types of nucleic acids? What are their functions? ______________________________
____________________________________________________________________________________
Part 1: Testing for simple sugar = Benedict’s Test
Background
Some simple sugars (monosaccharides) have an aldehyde group ( H–C=O) in their molecular structure.
Aldehyde reacts with Benedict’s solution when heated. The reaction produces a color change in the
Benedict’s reagent from an original blue color to an orange/red. A color change indicates a positive test
for sugar. You will test three different types of carbohydrates to see which one is a simple sugar. You
will also test water, as a control. You will need to see if/how Benedict’s solution changes when heated,
so that you can compare the normal change to a change that is caused by the reaction with a simple
sugar.
Figure 1.
Structural formula of ribose. Try to identify the aldehyde functional group.
Materials
4 test tubes
test tube rack
test tube holder
boiling water bath
ruler
starch solution
Benedict’s Reagent
glucose solution
sucrose solution
distilled water
Procedure
1. Arrange your tables into stations, and obtain materials located in your station container. Note the
location of the boiling water bath(s) that your station will be using.
2. Use a ruler and wax pencil to make 2 marks on each test tube. Make one mark at 1cm from the
bottom, and another at 3cm from the bottom. Label the tubes 1-4.
3. Fill your test tubes according to Table 2.1.
Table 1 Results of Benedict’s Test
Tube Tube Contents
Tube Contents
#
Add up to 1cm mark Add up to 3cm mark
1
water
Benedict’s reagent
2
Glucose solution
Benedict’s reagent
3
Sucrose solution
Benedict’s reagent
4
Starch solution
Benedict’s reagent
Observations
4. Place all four test tubes in a boiling water bath and heat for three to five minutes.
5. Remove the test tubes from the hot water bath with the test tube holder, and place them in the test
tube rack. Record your observations of the final results in Table 2.1
6. Thoroughly rinse and dry your test tubes in preparation for Lab 2.2
Results/Analysis
1. Which carbohydrate solution contains reducing sugars? _____________________________________
2. What is the functional group in this particular carbohydrate molecule that produces a positive reaction
with Benedict’s reagent?
____________________________________________________________
____________________________________________________________________________________
3. If onion juice mixed with Benedict’s reagent gives a positive test, what sugar would you conclude is
present in onions?
__________________________________________________________________
____________________________________________________________________________________
4. If you were given an unknown solution, what test would you do to determine the presence of simple
sugars?______________________________________________________________________________
Part 2 Testing for Starch = Iodine Test
Background
Iodine reacts with starch resulting in a dark, blue/black color that is a positive test for starch. The
reaction occurs because the starch binds with the iodine molecules and thus produces a color change.
Starch is stored in the seeds, roots, and tubers of plants as an energy reserve. The starch is converted to
simple sugars and is used by the plant during times of growth or when low levels of photosynthesis may
not meet its energy requirements.
Materials
Ruler
Wax Pencil
4 test tubes
test tube rack
potato solution
sucrose solution
water
potato slice
razor blade
microscope
Procedure
1. Use the ruler and wax pencil to mark 1cm from the bottom of each test tube. Label the test tubes 1-4.
2. Fill your test tubes according to Table 2.2
Table 2 Results of Iodine Test for Starch
Tube Tube Contents
Tube Contents
#
Add up to 1cm mark Add 3 drops,
swirl to mix
1
Water
Iodine solution
2
Carrot solution
Iodine solution
3
Sucrose solution
Iodine solution
4
Potato solution
Iodine solution
Observations
4. Record your observations in Table 2.2
5. Thoroughly rinse and dry your test tubes.
Your instructor will demonstrate the following procedure. You will make observations.
6. Use a razor blade to cut a small, extremely thin slice of fresh potato. The potato slice should be thin
enough to be nearly transparent.
7. Place the potato slice on a clean microscope slide. Add a drop of water to the potato, and cover with a
cover slip.
8. Observe the potato slice under low power. Then switch to medium power. In detail, draw your field
of view.
9. Carefully remove the cover slip, and add 2 drops of iodine solution. Replace the cover slip, and wait 2
minutes. Observe the potato under low power. Then switch to medium power. In detail, draw your field
of view.
Results/Analysis
1. Which solutions contained starch? _____________________________________________________
2. Starch and cellulose are both polymers of glucose molecules. Make a prediction for how they would
react in a Benedict’s test.
___________________________________________________________
___________________________________________________________________________________
3. Starch gives a positive iodine test, but cellulose doesn’t, even though they are both polymers of
glucose. What is a plausible explanation for this difference? ____________________________________
____________________________________________________________________________________
4. What molecule is located in the dark bodies of the potato slice after the iodine was added? _________
5. If you wanted to test if a mixture had starch in it, what test would you run? _____________________
Part 3: Testing for Proteins = Biuret Test
Background
Protein molecules consist of multiple amino acid molecules linked in long chains. The amino acids are
linked by bonds called peptide bonds. Peptide bonds react with a copper-containing molecule in the
Buiret reagent. This reaction causes a purple/violet color.
Materials
4 test tubes
test tube rack
ruler
Water
Honey solution
wax pencil
Egg white solution
Potato solution
Biuret reagent
Procedure
1. Mark four test tubes at 2cm and 4 cm from the bottom. Label the tubes 1-4.
2. Fill your test tubes according to Table 2.3 and record observations
Table 3 Results of Biuret Test for Protein
Tube Tube Contents
Tube Contents
#
Add up to 2cm mark Add up to 4cm mark
1
Water
Biuret reagent
2
Honey solution
Biuret reagent
3
Egg white solution
Biuret reagent
4
Potato solution
Biuret reagent
Observations
3. Thoroughly rinse and dry your test tubes.
Results/Analysis
1. Which solution contained protein? _____________________________________________________
2. What type of chemical bond causes the biuret reagent to react and give a violet color? ___________
___________________________________________________________________________________
3. Would you expect a solution containing an enzyme to yield a positive biuret test? Why or why not?
___________________________________________________________________________________
___________________________________________________________________________________
Part 4: Testing for Nucleic Acids = Dische Diphenylamine
This portion of the lab will be omitted. The reagents are highly caustic.
Part 5 Testing for Lipids = Sudan III Test
Background
Lipids are non-polar compounds composed of a glycerol molecule bonded to three fatty acid molecules.
Because lipids are non-polar, they do not dissolve in water or other polar liquids. Lipids are soluble in
non-polar substances such as ether. Therefore, solubility in Sudan III dye can be used as a positive test
for lipids. Non-lipid substances will not dissolve in Sudan III dye and will form two distinct layers in a
test tube.
Materials
4 test tubes
test tube rack
wax pencil
ruler
water
honey solution
egg white solution
vegetable oil
Sudan III dye
Procedure
1. Mark 4 test tubes at 2cm from the bottom. Label the tubes 1-4.
2. Fill your test tubes according to Table 2.5 and record your observations
Table 2.5 Results of Sudan III Test for Lipids
Tube Tube Contents
Tube Contents
#
Add up to 2cm mark Add 10 drops and
swirl to mix
1
Water
Sudan III dye
2
Honey solution
Sudan III dye
3
Egg white solution
Sudan III dye
4
Vegetable oil
Sudan III dye
Observations
3. Thoroughly rinse and dry your test tubes. Please wipe off any wax pencil residue.
Results/Analysis
1. Which compound(s) contained lipids?
________________________________________________
2. Which tube served as the control? ______________________________________________________
3. If you were given an unknown substance, which test would you run to see if the unknown substance
contained lipids?
__________________________________________________________________
Part 6 Testing an UNKNOWN SUBSTANCE
Your instructor will provide you with a substance of unknown composition.
1. What test will you run to find out if the unknown has simple sugars?
2. What test will you run to find out if the unknown has proteins?
3. What test will you run to find out if the unknown has starch?
4. What test will you run to find out if the unknown has lipids?
Results/Analysis
Which organic molecule(s) did your unknown substance contain? ______________________________
____________________________________________________________________________________
Post-lab questions
1. Using the pre-lab information and the information received from this investigation distinguish
between monosaccharides, disaccharides, and polysaccharides. Give an example of each type.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
2. Most of the most of the tests performed in this investigation lead to a color change in the presence of
the compound being tested for. What caused these color changes? ______________________________
____________________________________________________________________________________
3. A positive result for lipids in the Sudan III test was not a color change. What was considered a
positive result in this test? What does this tell us about the nature of lipids? _______________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
4. Contrary to popular belief the tests used in this investigation were not created to torture students.
Name a real life situation where it might be important to test for a specific type of organic compound.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
5. In your opinion is it important in the context of biology to understand organic compounds? Can
studying organic compounds lead to a greater understanding of life? Explain. _____________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________