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Lab 2 exercise : The Chemistry of Life
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UMUC Biology 102/103
Lab 2: The Chemistry of Life
INSTRUCTIONS:
Pre-Lab Questions
1. Nitrogen fixation is a natural process by which inert or unreactive forms of
nitrogen are transformed into usable nitrogen. Why is this process important to
life?
2. Given when you have learned about the hydrogen bonding shared between
nucleic acids in DNA, which pair is more stable under increasing heat: adenine
and thymine, or cytosine and guanine? Explain why.
3.
Which of the following is not an organic molecule; Methane (CH4), Fructose
(C6H12O6), Ethanol (C6H12O), or Ammonia (NH3)? How do you know?
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Experiment 1: Testing for Proteins
Materials
(2) 250 mL Beakers
25 Drops Biuret Solution,
H2NC(O)NHC(O)NH
(1) Knox Gelatin Packet
5 mL 1% Glucose Solution, C6H12O6
(1) 10 mL Graduated Cylinder
(1) 100 mL Graduated Cylinder
Permanent Marker
5 Pipettes
5 Test Tubes (Glass)
Test Tube Rack
5 mL Unknown Solution
Egg White
Hot Water
Tap Water
The protein molecules in many foods provide the amino acid building blocks required by
our own cells to produce new proteins. To determine whether a sample contains protein, a
reagent called Biuret solution is used. Biuret solution contains copper ions, similar to
Benedicts solution (another common reagent). However, the chemical state of the copper
ions in Biuret solution causes them to form a chemical complex with the peptide bonds
between amino acids (when present), changing the color of the solution. Biuret solution is
normally blue, but changes to pink when short peptides are present and to violet when
long polypeptides are present.
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Figure 6: Biuret solution only is located on
the far left side of the image (blue). Note the
transition from blue to violet as proteins are
added to the solution, causing the solution to
transition from blue to violet.
Procedure
1. Predicted results based on a previously learned set of information is sometimes
called an a priori prediction. Before you begin, take a moment to construct a
priori predictions stating whether or not there are proteins present in each of the
following solutions: Albumin (Egg White), Knox Gelatin, Glucose, and Water.
Record these predictions in Table 1.
2. Then, use your knowledge of Biuret solution chemistry (refer to the experimental
introduction) to predict the color of each of the four solutions when mixed with
Biuret solution. You must predict the initial color, as well as the final color (the
color after ). Record these predictions in Table 1.
3. You may now begin your experiment by using the permanent marker to label five
test tubes 1, 2, 3, 4 and 5.
4. Prepare your testing samples as follows:
a. Mix one egg white with 25 mL water in a 250 mL beaker to create an
albumin solution. Pipette 5 mL of this solution into Test Tube 1.
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b. Mix the packet of Knox gelatin with 50 mL hot water in a second 250
mL beaker. Stir until dissolved. Pipette 5 mL of this solution into Test
Tube 2.
5. Pipette 5 mL of the 1% glucose solution into Test Tube 3.
6. Use the 10 mL graduated cylinder to measure and pour 5 mL of water into Test
Tube 4.
7. Pipette 5 mL of the Unknown Solution into Test Tube 5.
8. Add five drops of Biuret solution to each test tube. Swirl each tube to mix.
9. Record the initial color of each sample in Table 2.
10. Record the final color in Table 2.
Note: Protein is present in the sample if a light purple color is observed.
Data Tables and Post-Lab Assessment
Table 1: A Priori Predictions
Table 1: A Priori Predictions
Sample
Will There
be Protein
Present?
Initial
Color
Final Color
1 – Albumin
Solution
2 – Gelatin
Solution
3 – Glucose
4 – Water
5 – Unknown
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Table 2: Testing for Proteins Results
Table 2: Testing for Proteins Results
Sample
Initial Color
Initial Color
Is Protein Present?
1 – Albumin
Solution
2 – Gelatin Solution
3 – Glucose
4 – Water
5 – Unknown
Post-Lab Questions
1. How did your a priori predictions from Table 1 compare to your actual results in Table
2? If there were any inconsistencies, explain why this occurred.
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2. Identify the positive and negative controls used in this experiment. Explain how each
of these controls are used, and why they are necessary to validate the experimental
results.
3. Identify two regions which proteins are vital components in the human body. Why are
they important to these regions?
4. Diet and nutrition are closely linked to the study of biomolecules. Describe one
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method by which you could monitor your food intake to ensure the cells in your body
have the materials necessary to function.
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Experiment 2: Testing for Reducing Sugars
Many of the foods we eat contain carbohydrates. Monosaccharides and short chains
such as disaccharides taste sweet due to certain aspects of their chemical structure. A
structural characteristic of some sugars can be identified using a chemical solution
called Benedicts reagent. When heated, the copper ions in Benedicts solution react with
the free end of any reducing sugars, such as glucose molecules. Copper ions are
reduced by the sugars, producing an orange or red colored precipitate.
Materials
5 mL Benedicts Solution
5 mL 1% Glucose Solution, C6H12O6
10 mL Graduated Cylinder
Permanent Marker
3 Pipettes
Ruler
Spatula
5 Test Tubes (Glass)
Thermometer
5 mL Unknown Solution
Fork
Hot Water Bath (stovetop or microwave and a
deep, heat-safe bowl)
Knife
Onion
Potato
Stopwatch
Tap Water
Many of the foods we eat contain carbohydrates. Monosaccharides and short chains such as
disaccharides taste sweet due to certain aspects of their chemical structure. A structural
characteristic of some sugars can be identified using a chemical solution called Benedicts
reagent. When heated, the copper ions in Benedicts solution react with the free end of any
reducing sugars, such as glucose molecules. Copper ions are reduced by the sugars,
producing an orange or red colored precipitate.
Materials
5 mL Benedicts Solution
5 mL 1% Glucose Solution, C6H12O6
10 mL Graduated Cylinder
Permanent Marker
3 Pipettes
Ruler
Spatula
5 Test Tubes (Glass)
Thermometer
5 mL Unknown Solution
*Fork
*Hot Water Bath (stovetop or microwave
and a deep, heat-safe bowl)
*Knife
*Onion
*Potato
*Stopwatch
*Tap Water
*You Must Provide
Note: Use great caution when handling a knife and/or cutting. Ask for assistance if you need help
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or are uncomfortable with knife work.
Procedure
1. Label five test tubes as 1 – 5.
2. Prepare your testing samples as follows:
a. Cut a raw potato into a 1.0 cm x 1.0 cm x 1 cm cube. Cut this cube into smaller
pieces, and mash with a fork and approximately 5 – 10 drops of water. Place half of
the mashed raw potato into Test Tube 1. Use the 10 mL graduated cylinder to
measure and pour 5 mL of water into Test Tube 1.
b. Cut a raw onion into a 1.0 cm x 1.0 cm x 1.0 cm cube. Cut this cube into smaller
pieces, and finally mash with a clean or new fork. Place half of the mashed raw
onion into Test Tube 2. Use the 10 mL graduated cylinder to measure and pour 5
mL of water into Test Tube 2.
3. Pipette 5 mL of the 1% glucose solution into Test Tube 3.
4. Use the 10 mL graduated cylinder to measure and pour 5 mL of water into Test Tube 4.
5. Pipette 5 mL of the Unknown solution into Test Tube 5.
6. Record the initial color of each solution in Table 3.
7. Prepare a hot water bath using the following information:
a. Heat water to a temperature between 85 and 100 C (not boiling) using a stovetop
or microwave safe container. Be sure to confirm this temperature using the
thermometer just prior to use in Step 9. The hot water bath must be of appropriate
size and shape to fit five glass test tubes in a vertical orientation.
8. Pipette 10 drops of Benedicts Solution to each test tube. Swirl each tube gently to mix.
9. Place the five test tubes into the hot water bath and let sit for three minutes. Remove the
tubes from water and place them in test tube rack to cool for five minutes.
10. Record the final color in Table 3.
Note: A reducing sugar is present in the sample if a red, yellow or green precipitant forms.
Wash your test tubes immediately after recording results to prevent permanent staining
from the reaction products.
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Data Tables and Post-Lab Assessment
Table 3: Testing for Reducing Sugars Results
Table 3: Testing for Reducing Sugars Results
Sample
Initial Color
Final Color
Reducing Sugar
Present
1 – Potato
2 – Onion
3 – Glucose
Solution
4 – Water
5 – Unknown
Post-Lab Questions
1.
Write a statement to explain the molecular composition of the unknown
solution based on the results obtained during testing with each reagent.
2.
What can you conclude about the molecular make-up of potatoes and onions
based on the test you performed? Why might these foods contain these
substance(s)?
3.
What results would you expect if you tested ribose, a monosaccharide, with
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Benedicts solution? Biuret solution?
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Experiment 3: What Household Substances are
Acidic or Basic?
In the following experiment, you will be using pH test strips to determine the pH of
various household substances. pH stands for potential hydrogen and is broken into a
scale of 1 – 14 to indicate the acidity or basicity of a solution. Generally speaking, more
hydrogen ions in a solution correlates to lower pH values, and more acidic solutions.
Conversely, fewer hydrogen ions correlates to higher pH values, and more basic
solutions. 7 is located in the middle of this number scale, and represents neutral solutions.
Figure 7: Note that many strong acids and bases do not have a pH that is
indicated on this scale. For example, lead battery acid has a pH that is below one.
Refer to the color key provided in the module with your pH test strips to determine which
color corresponds to each pH value. In this way, pH paper allows scientists to determine
to what degree a substance is acidic or basic and can provide an approximate pH value.
Materials
5 mL 4.5% Acetic Acid (Vinegar), C2H4O2 4 Liquid, Household Solutions
Paper Towels
(3) 100 mL Beakers
Water Source (Jug or Sink)
(3) 250 mL Beakers
10 mL Graduated Cylinder
(10) pH Test Strips
5 mL Sodium Bicarbonate (Baking
Soda)Solution, NaHC)3
Procedure
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1. Find four household substances to test (ex: grape juice, lemon juice, dishwashing
liquid, milk, tomato juice, shampoo, corn starch solution, etc.). You will use the
vinegar (acidic) and sodium bicarbonate (basic) solution provided in your kit as
standards.
2. Predict the pH of each substance before testing with a pH strip. Record your
predictions in Table 4.
3. Use the permanent marker to label each of the beakers with the name of one of the
six solutions. It does not matter which size beaker is used for the different
solutions.
4. Use the graduated cylinder to measure and pour five mL of vinegar into the
beaker labeled Vinegar.
5. Thoroughly rinse the graduated cylinder with water to remove any remaining
vinegar. Use paper towels to dry the graduated cylinder and repeat Step 4 with
each of the five remaining solutions and beakers.
6. Measure the pH of each solution by dipping the pad of the pH strip into the
solution for 5 – 10 seconds and comparing it with the pH test strip key (located in
the lab module). Record your results in Table 4.
Data Tables and Post-Lab Assessment
Table 4: pH Values of Common Household Substances
Table 4: pH Values of Common Household Substances
Substance
pH Prediction
pH Test Strip Color
Acetic Acid (Vinegar)
Sodium Bicarbonate Solution (Baking
Soda)
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Post-Lab Questions
1. What is the purpose of determining the pH of the acetic acid and the sodium
bicarbonate solution before testing the other household substances?
2. Compare and contrast acids and bases in terms of their H+ ion and OH- ion
concentrations.
3. Name two acids and two bases you often use.
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