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Biological Molecules At-home Lab

Read carefully. When you get to the questions, please answer them using red font.

Introduction
Biochemistry is the study of the chemistry of life. All living organisms consist of organic
compounds which are compounds that contain carbon. These molecules are made of
smaller subunits, called monomers which are also known as “building blocks”. These
building blocks link together to form larger molecules called polymers.

Objectives
Biological molecules are divided into four classes called macromolecules:
Lipids which include fats, waxes and oils
Carbohydrates which are comprised of sugars that form starch and cellulose
Proteins which are composed of amino acids
Nucleic acids which includes DNA and RNA.

You should be able to:

1) Recognize the four classes of biological molecules.
2) Describe the basic structure of the four classes of macromolecules
3) Name the indicators used to test for each macromolecule
4) Identify positive and negative tests for the macromolecules

(NOTE – this document describes the procedures normally used in the actual
lab setting. I have left the procedures in place so that you can study them
and become familiar with them, and have provided links to videos in an
attempt to replicate the lab experience.)
Lipids
Lipids include fats, oils, steroids and waxes. Lipids are the main components of the
plasma membrane of all living cells. The lipids, found in plants are the oils and those
found in animals are the fats. Oils are liquids and fats are solids at room temperature.
Lipids are non-polar, hydrophobic. They do not dissolve in water.

Here is the normal lab procedure. Read it, then make a prediction below based on
what you know from lecture about lipids: Add one full pipette of water and one
pipette full of vegetable oil to the same test tube. Shake the contents of the test tube,
using your gloved thumb as a stopper. Put the tube in the test tube rack. You will use this
tube to answer the following questions. You will also use it for the following experiment.

Would the oil dissolve in water?

 What property of lipids explains the result you obtained?

Is there anyway you can think of that can dissolve the oil in water?

Here is the normal procedure. Read it, then watch the video which shows you the
result: Sudan IV is used to identify the presence of a fat or oil. Using the tube from
above, add 9 drops of Sudan IV to the test tube. Shake tube and observe the color of both
layers.
Watch this video, then answer the questions:
https://www.youtube.com/watch?v=pn8ZN3QAVtE
(copy and paste into your browser if the hyperlinks don’t work)

What color is the lipid layer?

What color is the water layer?

Sudan IV indicates the presence of which macromolecule?

What property does Sudan IV have in order to react with this macromolecule?

Carbohydrates
Carbohydrates can be simple sugars or polymers made up of many sugar units.
Carbohydrates are organic molecules made of carbon, hydrogen and oxygen. The
simplest sugars are the monosaccharides which are comprised of one sugar unit. The
most common monosaccharide is glucose which serves as the immediate source of
energy. Glucose is the primary form of sugar stored in the human body for energy.
Fructose is the main sugar found in most fruits. Both glucose and fructose have the same
molecular formula (C6H12O6) however they have different structures or different
structural formulas as seen below (note: the carbon atoms that sit in the 'corners' of the
rings are not labeled)

Observe the diagram of the monosaccharides: glucose and fructose:

 Glucose Fructose

What is the difference between glucose and fructose?

How many carbon atoms do glucose and fructose have?

What is the molecular formula for glucose?

What is the molecular formula for fructose?

Monosaccharides can be bonded together to form disaccharides which contain two

sugar units linked by a glycosidic bond. Sucrose (table sugar), maltose (in seeds), and
lactose (milk sugar) are some examples of disaccharides. Common table sugar sucrose is
a disaccharide that consists of a glucose unit bonded to a fructose unit.

Observe the diagram of sucrose:

Sucrose is made of what two monosaccharides?

What is the molecular formula of sucrose?

 What molecule must be lost in order to produce sucrose?

The joining of two monosaccharides to produce a disaccharides occurs through a

reaction.

Test for monosaccharides

Benedict’s solution is an indicator for monosaccharides.
Here is the normal procedure. Read it, then watch the videos which show you the
results: Take 100 ml of tap water in a beaker and place in on a hot plate to bring the
water to a rolling- boil. Pipet 2 ml of glucose solution to the test tube. Add 2 ml of
Benedict’s solution to the test tube and shake from side to side. Repeat the procedure for
a sucrose solution (disaccharide).

This video explains what is happening:

https://www.youtube.com/watch?v=gsAqUwHwRrc

This video shows the result for glucose:

https://www.youtube.com/watch?v=Lt7RCIfudYQ

This video shows the result for sucrose:

https://www.youtube.com/watch?v=bx1z9sdR9lo

What color change do you observe in the glucose test tube?

What color change do you observe in the sucrose test tube?

What color is the positive indicator for monosaccharaides?

What does this result tell you about Benedict’s solution as an indicator?

Polysaccharide
If more than two sugar units are joined by a glycosidic bond, the carbohydrate is called
a polysaccharide. Some examples of polysaccharides are glycogen, starch and
cellulose. Glycogen is a polysaccharide found in animals. Starch is a polysaccharide
found only in plants and cellulose is a polysaccharide only found in the cell walls of
plants.
 Starch is the principal polysaccharide used by plants to store glucose for later use as
energy. Plants often store starch in seeds or other specialized organs, for example,
common sources of starch include rice, beans, wheat, corn, potatoes, etc. When humans
eat starch, an enzyme that occurs in saliva and in the intestines called amylase breaks the
bonds via hydrolysis reactions between the repeating glucose units thus allowing the
sugar to be absorbed into the bloodstream. Once absorbed into the bloodstream, the
human body distributes glucose to the areas where it is needed for energy or stores it as
its own special polymer - glycogen. Glycogen, another polymer of glucose, is the
polysaccharide used by animals to store energy. Excess glucose is bonded together to
form glycogen molecules, which the animal stores in the liver and muscle tissue as an
"instant" source of energy. Both starch and glycogen are polymers of glucose.

Examine the diagram of starch:

n is the number of repeating glucose units and ranges in the

1,000’s

What structural differences do you observe between starch and sucrose?

What monomer units do you observe in starch?

Test for polysaccharides

Read the normal procedure, then refer to the video below for the results: The most
common test for a polysaccharide is Lugol’s test. Lugol’s is an amber colored solution.
It tests for the presence of starch by turning a blue-black color. Place 3 ml of starch in
one test tube and 3 ml of water in another test tube. Add 9 drops of Lugol’s to each test
tube.

https://www.youtube.com/watch?v=J5eaJc854uQ

What color do you observe in the test tube containing water?

 What color do you observe in the test tube containing starch?

Proteins
Proteins are macromolecules with a diverse range of functions. They exist as enzymes,
antibodies, hormones and transport proteins. Proteins are made up of amino acids linked
by peptide bonds. There are 20 amino acids that occur in nature. Proteins are also
known as polypeptides. Observe the following diagram indicating the formation of a
dipeptide. It is this process that is repeated many times to form a polypeptide.

FORMATION OF A DIPEPTIDE

The Biuret test is a positive indicator of the number of peptide bonds in proteins. It turns
a pink to purple color in the presence of proteins. The color darkens as the number of
peptide bonds increase in the polypeptide.

Why are proteins known as polypeptides?

The lab procedure asks you to place 10 mL of Biuret reagent in a test tube with 3 mL of water,
and another test tube with 3 mL of 1% albumin (egg protein). This video shows a positive
result for protein (even though the narrator says it’s an unknown):

https://www.youtube.com/watch?v=L4Rjpp8x9-A

What color does the Biuret solution turn in the presence of a great number of peptide bonds?

Nucleic Acids
Nucleic acids comprise of DNA and RNA which is the genetic material of all life.
Observe the diagram of DNA
DIAGRAM OF DNA

What is the shape of the DNA molecule?

Why is DNA referred to as a double-stranded molecule?

How are the two strands held together?

DNA is comprised of smaller subunits called nucleotides

What are the components of the nucleotides?

Fill in the Table:

Macromolecule Describe Atoms found in Indicator used Color change
Structure this class to test for this
category

Lipids

Monosaccharides
Glucose

NA NA
Disaccharides
Sucrose

Polysaccharides
Starch

Proteins

NA NA
Nucleic Acids