Lab 3: Building Blocks of living Things
Objectives:
Understand chemicals and chemical bonds
Understand the concept of polymer and monomer
Know the biomolecules and subunits that make up living things
Introduction:
Even though over 100 chemical elements (atoms) are known, only four (carbon,
hydrogen, oxygen and nitrogen) make up over 96% of living things. These four elements
are arranged in different ways to make up bigger, more complex molecules that are the
building blocks of life.
These molecules are proteins, lipids, carbohydrates and nucleic acids. Three of them
(proteins, carbohydrates and nucleic acids) are long polymers made of smaller repeating
units called monomers. The monomers of proteins are called amino acids, the
monomers of carbohydrates are sugars, and the monomers of nucleic acids are
nucleotides. Only the lipids are not polymers, but are sometimes made of glycerol and
fatty acid subunits.
In addition to using biomolecules as building blocks for tissues, living things also obtain
their energy from these complex molecules that they take in as food. Large complex
biomolecules cannot be directly absorbed in the digestive tract because of their size.
Enzymes along the digestive tract break down the large biomolecules into smaller
subunits and these subunits are then absorbed across the intestinal lining into the blood
stream where they are used to build complex molecules or utilized for energy.
In this lab you will examine chemicals and their bonding. You will also learn what kinds
of biomolecules are present in various foods.
Indicator Reagents:
Different biomolecules react with different chemical s or reagents to produce a specific
color reaction. We can determine the biomolecule present in a sample by adding the
test reagent specific for that molecule. If a specific color appears, we have a B test for
that molecule. When no color is present or when the color is different from the
specified test, we have a negative test because that molecule is not present.
Below is a table of reagents, molecule tested, and presence or absence of color. When
no color was produced the test was negative because the molecule was not present. A
positive test showed color because the molecule was present.
Table 1: Color Test for Molecules Using Test Reagents
Molecule\Reagent
Amino acid
Benedicts
Biuret
Lugols
Ninhydrin
Purple/violet
Fat/lipid
Red layer
Glucose/sugar
Red orange
Protein
violet
Enzyme treated
protein
Starch/carbohydrate
Enzyme treated
starch
Purple/violet
Blue/black
Red orange
Methods:
Part I: Chemical bonding
Review the types of chemical bonds
Diagram Hydrogen bonding between 3 water molecules
Part II: Biomolecules in Foods
Materials:
Sudan IV
food sample
test tubes (4 per food sample)
water in squeeze bottle
razor blade
mortar and pestle
Methods: Part A
1. Obtain your food sample (s).
2. For solid foods, prepare a slurry (liquefied version) of the food using the razor
blade, mortar and pestle and distilled water at your lab bench. The food must be
finely ground (ie no chunks) so that the cells are disrupted.
3. Add about 1 ml (thumbnail height) of each food sample to four different test
tubes.
Methods: Part B
Materials:
food sample (prepared)
Lugol’s reagent
Benedict’s reagent
Biuret’s reagent
filter paper
Transfer pipets
Sudan IV reagent
1. Run each of the 5 assays as described below for each food sample. Record your
results on the summary table. Share results with the class.
2. When you have finished, dispose of your food samples properly. Wash out your
test tubes with soap and water and invert them (turn upside down) in a test
tube rack to dry. BE SURE TO CLEAN UP THE LAB.
Assays (Tests)
Lugol’s (to detect Starch)
Lugol’s reagent changes from a yellow-brown color to blue-black only when starch is
present. Lugol’s reagent is a solution of iodine-potassium iodide (I2KI).
To test for starch, add 2 drops of Lugol’s reagent to one sample tube. If the color
doesn’t change, add 2 more drops.
Benedict’s to detect glucose and other simple sugars
When Benedict’s reagent is heated in the presence of simple sugars, it changes from a
blue/green color to yellow or reddish-orange. The color produced is dependent on the
amount of sugar present in the sample. Red-orange (sometimes with a precipitate)
indicates the highest amount of simple sugar.
To test for simple sugar, add enough Benedict’s reagent to just turn your food
solution blue (light blue). Mix well, place a marble on top of the tube and record
the color. Place the tube in a boiling water bath for three minutes and record
the color observed at the end of three minutes. Carefully remove tubes from
that water bath as they will be very hot.
Sudan IV to detect fats (lipids)
When several drops of Sudan IV are added to a sample that contains lipids, a floating
red droplet or floating red layer will form. If lipid is not present or is a low levels, the
red color will disperse throughout the sample.
To detect lipids add 5-10 drops of Sudan IV reagent to your sample and mix by
shaking.
Why does the red layer or drop float rather than sink to the bottom?
Sudan IV is messy and will get into everything, wash out your tubes with hot soapy
water when you are finished.
Biuret’s Reagent to detect protein
The Biuret reagent is light blue in color, but in the presence of proteins it turns violet.
Other types of chemicals may cause other changes, but only the violet color indicates
the presence of proteins.
To test for protein, add about 2 mls of Biuret’s reagent to your food sample
solution. Wait 2 minutes for the reaction to occur. The appearance of a violet
color indicates protein is present. If a violet color does not appear, add 2 mls
reagent again and check for a color change. If a violet color is absent, the sample
does not contain protein.
Ninhydrin to detect amino acids
DO not use the Ninhydrin reagent yourself!! The fumes of the Ninhydrin reagent are very
poisonous!
Ninhydrin turns purple to violet in the presence of the free amino groups of amino acids.
To test for free amino acids, obtain a piece of filter paper. Place one drop of
your foods sample solutions to be tested at different locations on the filter paper
(label your samples and your group). Use a pencil (pens will bleed and run).
Allow the spots to dry. This should be set up early to allow drying time. Your
instructor will apply the Ninhydrin reagent to your samples in a fume hood. The
reaction takes about 25 minutes to fully develop.
Results
Results Part I: Use the space provided to illustrate the three types of chemical bonds and
draw three water molecules in hydrogen bonding ( be sure to indicate and label all bonds,
atoms and charges.