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					Lab 4 – Chemistry of Life


Objectives

   Compare pH of acids and bases
   Perform simple chemical reactions to identify presence of starch, simple sugars, proteins, and
    lipids
   Perform simple 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. I’m sure the mad
scientist would agree. 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


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     Lab 4 – Chemistry of Life


     different ratios, the four main categories of biomolecules are formed. These are carbohydrates,
     lipids, proteins, and nucleic acids.
              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 insure 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 an unknown organic
     compound.

     4.1 Observe petri plates from last lab

1.      Obtain your petri plates from the incubator. Follow the instructions in the last lab.
2.      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.      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.
2.      Record your results in Table 1.




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     Lab 4 – Chemistry of Life


     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.      With a wax pencil, mark 3 test tubes at 2 cm from the bottom.
2.      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.
3.      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. 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.      Obtain 2 test tubes. Mark both tubes at 2 cm from the bottom.
2.      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.
3.      Add 5 drops of IKI solution to each test tube. Record results in Table 3.
4.      Place a small chunk of potato on a petri dish. Place 5 drops of IKI solution on the potato.
        Record results in Table 3.

     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.      Obtain 5 test tubes. Using a wax pencil, mark the test tubes at 1 cm and 3 cm from the
        bottom.
2.      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.
3.      Add Benedict’s reagent to the 3 cm mark to each test tube AND boil all tubes for 5 minutes.
        Record results in Table 4.

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     Lab 4 – Chemistry of Life


     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. Two separate layers will be
     seen.

1.      Obtain 2 test tubes. Using a wax pencil, mark the test tubes at 1 cm and 2 cm from the
        bottom.
2.      Fill both test tubes with water to the 1 cm mark.
3.      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.
4.      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.
5.      Record results in Table 5.

     4.7 Testing an unknown to see if unknown contains proteins, carbohydrates, or lipids

1.      Obtain an unknown sample from your instructor and four empty test tubes.
2.      Following the procedures for using Biuret reagent, check to see if your unknown contains
        any proteins.
3.      Following the procedures for using IKI, check to see if your unknown contains any starch.
4.      Following the procedures for using Benedict’s reagent, check to see if your unknown
        contains any monosaccharides or the appropriate disaccharides.
5.      Following the procedures for using Sudan IV, check to see if your unknown contains any
        lipids.
6.      Record your results in Table 6.




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Lab 4 – Chemistry of Life


Table 1. pH of common compounds. (2 pts)

   Common Compound                           pH




Table 2. Biuret reagent test for proteins. (2 pts)

            Sample                       Final color result                Conclusions
Water

Albumin

Pepsin



Table 3. IKI test for starch. (2 pts)

            Sample                       Final color result                Conclusions
Water

Starch solution

Potato



Table 4. Benedict reagent test for monosaccharides and some disaccharides. (2 pts)

            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


     Table 5. Sudan IV test for lipids. (2 pts)

                  Sample                          Final color result              Conclusions
     Water

     Oil



     Table 6. Unknown solution. (4 pts)

     Write the letter of your unknown here:

        Test Done on unknown                      Final color result              Conclusions
     Biuret reagent

     IKI

     Benedict’s reagent

     Sudan IV

     In conclusion, does your unknown solution have proteins, starch, simple sugars, and/or
     lipids?


     GENERAL QUESTIONS

1.         Explain why you used water in procedures 4.3-4.6.




2.         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 consider acidic
           or basic?




3.         [Note: you may need a reference for this question. Your text is sufficient or Internet.] You
           performed a positive test on pepsin, which is an enzyme. (a) Which biological molecule (i.e.,
           carbs, fats, proteins, or nucleic acids) does pepsin break down? (b) Where in the body does
           pepsin break down the biological molecule? (c) As a result, under which pH level would
           pepsin perform best? (d) Name an enzyme that breaks down starch in the digestive tract.


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