Lab 4 – Chemistry of Life
Objectives
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
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Compare pH of acids and bases
Perform chemical reactions to identify presence of starch, simple sugars, proteins, and lipids
Perform chemical reactions on an unknown solution to check for presence of starch, simple
sugars, proteins, and lipids
Introduction
We have all seen the science fiction horror films in which the evil scientist gets a solution
thrown in his face by the fleeing victim. Immediately, he clutches his face and screams in pain
while his face begins to smoke and burn. This is not just fiction. This is a chemical reaction.
All things that you can see, touch, smell, and taste are made of chemicals. This includes our own
bodies. The chemicals in the solution reacted with the chemicals in the mad scientist’s skin and
produced a gas. Hence, the emission of smoke occurred. As the chemical reaction continued,
nerve endings in the skin were damaged, sending a signal (via chemical reactions in the body) to
the brain that is translated into what we call pain.
A delicate balance between many different chemicals maintains life on Earth. For
instance, we maintain a balance between acids and bases in our bodies. Either one of these
chemicals in the wrong concentration could have serious consequences. Acids are defined as
compounds that release hydrogen ions (H+) in solution. Aqueous solutions of acids have sour
tastes due to the presence of hydrogen ions. Acids are what give grapefruits and rhubarbs their
tartness. Acids can range from weak (release a small amount of H+ ions) to strong (release a
large amount of H+ ions). Bases are defined as compounds that release hydroxide ions (OH-) in
solution. Solutions containing even a relatively small concentration of hydroxide ions have a
bitter taste. They also have a slippery, soapy feel. Like acids, bases can range from weak to
strong depending on the amount of OH- ions released.
The unit of measure used to represent the strength of an acid or a base is pH. The pH
scale ranges from 0 – 14. Acids have a range below 7 and bases have a range above 7. When a
solution has neither acidic nor basic properties, it is said to be neutral. This is not to say that
hydrogen ions and hydroxide ions are completely absent. Both ions are present in any aqueous
solution, but if the solution is neutral, they are present in equal numbers. Pure water is neutral
and has a pH of 7. The pH scale is a logarithmic scale. This means that an acid with a pH of 5 is
10X more acidic than one with a pH of 6.
Living cells are very sensitive to the pH of their environment. Your cells, for example,
are bathed in a fluid that has a pH of 7.2, slightly basic. If the pH of the fluid changes by only
two tenths of a point in either direction, cells may die.
Organic compounds contain the element carbon. The only exception is carbon dioxide,
which is considered to be an inorganic compound. Carbon readily combines with many elements
to produce a bewildering array of organic compounds. The six common elements found in living
things include carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. These six bulk
elements make up about 96% of our bodies’ composition. By combining these elements in
different ratios, the four main categories of biomolecules are formed. These are carbohydrates,
lipids, proteins, and nucleic acids.
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Lab 4 – Chemistry of Life
Carbohydrates provide energy and support. They consist of carbon, hydrogen, and
oxygen in the ratio 1:2:1. Carbohydrates range in size from the monosaccharides, like glucose, to
huge polysaccharides such as starch, glycogen, and cellulose. Glucose is the primary energy
source for cells. Starch and glycogen are used to store energy in plants and animals,
respectively. Cellulose is used for structural support in plants.
Lipids are a diverse group of organic compounds that store energy, waterproof the
outsides of organisms, cushion organs, and preserve body heat. Lipids, which include fats, oils,
and waxes, do not dissolve in water and contain carbon, hydrogen, and oxygen. A small amount
of carbohydrates are needed to ensure proper metabolism of lipids and prevent the release of
ketones into the blood.
Proteins are macromolecules built of chains of amino acids. Proteins have many
functions and great diversity of structures. A protein’s conformation, or three-dimensional
shape, is vital to its function and is determined by the unique sequence of amino acids. The first
protein scientists were able to determine the amino acid sequence for was insulin.
Nucleic acids are specialized molecules that dictate the sequence of amino acids in a
protein. Both DNA and RNA are composed of subunits called nucleotides. Each nucleotide
consists of a phosphate group, pentose sugar, and one of four different nitrogenous bases.
In today’s lab, you will become familiar with the characteristics of acids and bases. In
addition, you will learn how to identify some common organic molecules by their characteristic
chemical reactions. You will then use this information to identify which biological molecules
are in an unknown organic compound.
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Lab 4 – Chemistry of Life
4.1 Observe petri plates from last lab
1.
2.
Obtain your petri plates from the incubator. Follow the instructions in the last lab.
Important reminder: after you’re done observing petri plates, put the plates in the
BIOHAZARD BAG.
4.2 pH of common acids and bases
1.
2.
From the common compounds available (which may include coffee, soda, shampoo,
lemon juice, antacids, and vinegar), test the pH of five compounds using the pH strips.
Record your results in Table 1 on QUESTIONS page.
4.3 Positive test for proteins
Biuret reagent (which is blue color) reacts with the amino group in peptides (compounds with
few amino acids) and polypeptides (compounds with large numbers of amino acids). If peptides
are present, the blue color changes to a pinkish color. If polypeptides are present, the blue color
changes to a darker violet. If no peptides or polypeptides are present, the blue color remains
blue.
Blue (no amino acids)  pink (few amino acids)  darker violet (lots amino acids)
1.
2.
3.
4.
5.
With a wax pencil, mark 3 test tubes at 2 cm from the bottom.
Fill one test tube with water to the 2 cm mark; fill another test tube with albumin (or egg
white) solution to the 2 cm mark; and fill the third test tube with pepsin solution to the 2
cm mark.
Add 3 drops of Biuret reagent to each test tube and shake each test tube back and forth
gently.
Record your results in Table 2 on QUESTIONS page.
To think about: why does one test tube have water?
4.4 Positive test for starch (a type of polysaccharide)
IKI (iodine in potassium iodide) is orange-yellow in color. When IKI is added to a compound
containing starch, the orange-yellow color will change to a brownish color or to a blue-black
color.
1.
2.
3.
4.
5.
Obtain 2 test tubes. Mark both tubes at 2 cm from the bottom.
Fill one test tube with water to the 2 cm level and fill the other test tube with starch
solution to the 2 cm level.
Add 5 drops of IKI solution to each test tube.
Place a small chunk of potato on a petri dish. Place 5 drops of IKI solution on the potato.
Record your results in Table 3 on QUESTIONS page.
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Lab 4 – Chemistry of Life
4.5 Positive test for monosaccharides and some disaccharides
Benedict’s reagent is a blue colored reagent that tests for the presence of monosaccharides (such
as glucose and fructose) and some disaccharides (such as maltose or lactose but not sucrose). If
very little sugar is present, the blue color will change to green. Low amounts of sugars result in
a yellow color. Moderate amounts of sugars result in a yellow-orange color. High amounts of
sugars result in an orange color. Very high amounts of sugars result in an orange-red color.
Blue (no sugars)  green  yellow  yellow-orange  orange  orange-red
1.
2.
3.
4.
Obtain 5 test tubes. Using a wax pencil, mark the test tubes at 1 cm and 3 cm from the
bottom.
Fill one test tube with water to the 1 cm mark, one with glucose solution to the 1 cm
mark, one with milk to the 1 cm mark, one with honey to the 1 cm mark, and one with
sucrose solution to the 1 cm mark.
Add Benedict’s reagent to the 3 cm mark to each test tube AND boil all tubes for 5
minutes.
Record your results in Table 4 on QUESTIONS page.
4.6 Positive test for lipids
Lipids can dissolve in non-polar solvents such as chloroform, ether, acetone, or fat-soluble dyes
such as Sudan IV, but not in water. If a lipid is in a solution with Sudan IV dye and water, the
lipid will dissolve in the Sudan IV and separate from the water.
1.
2.
3.
4.
5.
Obtain 2 test tubes. Using a wax pencil, mark the test tubes at 1 cm and 2 cm from the
bottom.
Fill both test tubes with water to the 1 cm mark.
Fill one test tube to the 2 cm mark with water and fill the other test tube to the 2 cm mark
with an oil solution.
Add 5-6 drops of Sudan IV to both test tubes. Swirl the test tubes gently to mix the
solution as best as possible. Look for any layers.
Record your results in Table 5 on QUESTIONS page.
4.7 Testing an unknown to see if unknown contains proteins, carbohydrates, or lipids
1.
2.
3.
4.
5.
6.
Obtain an unknown sample from your instructor and four empty test tubes.
Following the procedures for using Biuret reagent, check to see if your unknown contains
any proteins.
Following the procedures for using IKI, check to see if your unknown contains any
starch.
Following the procedures for using Benedict’s reagent, check to see if your unknown
contains any monosaccharides or the appropriate disaccharides.
Following the procedures for using Sudan IV, check to see if your unknown contains any
lipids.
Record your results in Table 6 on QUESTIONS page.
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Lab 4 – Chemistry of Life
4.2 QUESTIONS
Complete the table below for your pH results.
Table 1. pH of common compounds.
Compound
1.
pH
Suppose you had tested lemon juice and found the pH to be 3. Suppose you also tested
antacid solution and found the pH to be 8. How many more hydrogen ions (H+) does the
lemon juice have compared to the antacid? Therefore, would lemon juice be considered
acidic or basic?
4.3 QUESTIONS
Complete the table below for your protein results.
Table 2. Biuret reagent test for proteins.
Sample
Final color result
Conclusions
Water
Albumin
Pepsin
2.
If you ate a meat product with lots of proteins, your digestive enzymes would break the
proteins into smaller subunits called __________ __________ that would be absorbed
into the bloodstream from the small intestine.
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Lab 4 – Chemistry of Life
4.4 QUESTIONS
Complete the table below for your starch results.
Table 3. IKI test for starch.
Sample
Final color result
Conclusions
Water
Starch solution
Potato
3.
If you ate a starchy product such as potato, your digestive enzymes would break the
starch into smaller subunits called __________ that would be absorbed into the
bloodstream from the small intestine.
4.
Suppose an individual’s body cells and tissues could not use the above subunits for
energy production because of either a lack of insulin or because the cells and tissues no
longer respond to insulin, what disease results?
4.5 QUESTIONS
Complete the table below for your sugar results.
Table 4. Benedict reagent test for monosaccharides and some disaccharides.
Sample
Final color result
Conclusions
Water
Glucose solution
Milk
Honey
Sucrose solution
Why does the color remain blue?
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Lab 4 – Chemistry of Life
5.
Suppose you obtained the following results:
Sample
Final color result
Water
yellow-orange
Glucose solution
orange-red
Milk
blue
Honey
orange
Sucrose solution
blue
Discuss what your conclusions would be based on the above results.
4.6 QUESTIONS
Complete the table below for your lipid results.
Table 5. Sudan IV test for lipids.
Sample
Final color result
Conclusions
Water
Oil
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Lab 4 – Chemistry of Life
4.7 QUESTIONS
Complete the table below for your unknown results.
Table 6. Unknown solution.
Write the letter or number of your unknown here:
Test done on unknown
Biuret reagent
Final color result
Conclusions
IKI
Benedict’s reagent
Sudan IV
6.
Suppose you obtained the following results:
Test done on unknown
Biuret reagent
Final color result
Solution turned dark violet
IKI
Solution turned pink
Benedict’s reagent
Solution stayed blue
Sudan IV
Two layers formed, and the top layer was pink
Discuss what your conclusions would be about each test based on the above results.
7.
Explain why you used water in procedures 4.3-4.6.
HAND IN QUESTIONS PAGES (5-8).
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