Testing for Nutrient Macromolecules Molecules
I. Test for Proteins
Biuret reagent is a chemical used to test for the presence of protein molecules. In the presence of
proteins, the blue-colored Biuret reagent will turn purple. If proteins are absent, the Biuret reagent
will NOT turn purple.
Caution! Exercise care in using Biuret reagent, and follow you instructor’s directions for disposing
of the chemical. If any Biuret reagent should come in contact with your skin, rinse immediately with
water.
Materials and Method…
1. With a metric ruler and a marker, mark three clean test tubes at the 2 cm and 4 cm levels.
Number the tubes 1 through 3.
2. Tube 1: Fill to the first mark with water and to the second mark with Biuret reagent.
Gently swirl the tube to mix. Record final color in Table 1.
3. Tube 2: Fill to the first mark with albumin solution and to the second mark with Biuret
reagent. Gently swirl the tube to mix. Record final color in Table 1.
4. Tube 3: Fill to the first mark with the starch solution and to the second mark with biuret
reagent. Gently swirl to the tube to mix. Record final color in Table 1.
Table 1: Biuret Test for Proteins
Tube
1
Contents
Final Color
2
3
Questions:
1. Which of the three tubes was the control group?
2. What is the independent variable in this experiment?
3. What is the dependent variable in this experiment?
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Conclusion (+ or – for protein)
II. Test for Starch
Iodine is a chemical used to test for the presence of starch molecules. In the presence of starch, the
yellowish-brown colored iodine solution forms a blue-black color. If starch is absent, the iodine
solution will NOT turn blue-black.
Materials and Method 1…
1. With a metric ruler and marker, mark five clean test tubes at the 1 cm level.
2. Tube 1: Fill to the 1 cm mark with water, and add five drops of iodine solution. Note the
final color and record your results in Table 2.
3. Tube 2: Shake well before taking your sample, and then fill to the 1 cm mark with starch
solution. Add five drops of iodine solution. Note the final color and record your results in
Table 2.
4. Tube 3: Fill to the 1 cm mark with onion juice. Obtain the onion juice by adding water
and crushing a small piece of onion with a mortar and pestle. Clean mortar and pestle
after using. Add five drops of iodine solution. Note the final color and record your results
in Table 2.
5. Tube 4: Fill to the 1 cm mark with potato juice. Obtain the potato juice by adding water
and crushing a small piece of potato with a mortar and pestle. Clean mortar and pestle
after using. Add five drops of iodine solution. Note the final color and record your results
in Table 2.
6. Tube 5: Fill to the 1 cm mark with glucose solution. Add five drops of iodine solution.
Note the color change and record your results in Table 2.
Table 2: Iodine Test for Starch
Tube
1
2
3
4
5
Contents
Final Color
Conclusion (+ or – for starch)
Materials and Method 2…
Potato…
1. With a razor blade, slice a very thin piece of potato. Place the potato slice on a
microscope slide and add a drop of water and a coverslip.
2. With the microscope, observe the potato cells. Identify the cell walls (the large geometric
compartments) and the granules (numerous clear, oval-shaped objects).
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3. Remove the coverslip and place two drops of iodine solution onto the potato slice.
Replace the coverslip.
4. Microscopically observe the potato slice again where the iodine was applied, particularly
at the edge of the slice which is the thinnest region.
Questions:
1. What is the color of the small oval granules? ___________________
2. What molecule do these small oval granules contain? _____________
Onion… Repeat steps 1 through 4 with a small slice of onion.
1. What is the color of the onion cells?__________________________
2. Does the onion store its energy in the form of starch?____________
III. Test for Sugars
Sugars react with Benedict’s reagent after being heated in a boiling water bath. Increasing
concentrations of sugar (both monosaccharides and disaccharides) give a continuum of colored
products (see table 3).
Table 3: Typical Reactions with Benedict’s Test
After Heating with Benedict’s
Blue (no change)
Varies with concentration:
Very low – green
Low – yellow
Moderate – yellow-orange
High – orange
Very high - red
Varies with concentration:
(see glucose above)
Blue (no change)
Chemical
Chemical Category
Water
Inorganic molecule
Glucose Monosaccharide (carbohydrate)
Maltose
Disaccharide (carbohydrate)
Starch
Polysaccaride (carbohydrate)
Materials and Method…
1. Mark five clean test tubes at the 1 cm and 3 cm levels. Label the tubes 1-5.
2. Tube 1: Fill to the 1 cm mark with water, and then add Benedict’s reagent to the 3 cm mark.
Note the color before heating in table 4.
3. Tube 2: Fill to the 1 cm mark with glucose solution, and then add Benedict’s reagent to the 3
cm mark. Note the color before heating in table 4.
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4. Tube 3: Fill to the 1 cm mark with onion juice, and then add Benedict’s reagent to the 3 cm
mark. Note the color before heating in table 4.
5. Tube 4: Fill to the 1 cm mark with potato juice, and then add Benedict’s reagent to the 3 cm
mark. Note the color before heating in table 4.
6. Tube 5: Fill to the 1 cm mark with starch solution, and then add Benedict’s reagent to the 3
cm mark. Note the color before heating in table 4.
7. Place the five labeled test tubes into a boiling water bath for 5 to 10 minutes. Note any color
change and record in Table 4.
Table 4: Benedict’s test for Sugars
Tube
Contents
Color (before heating)Color (after
heating)
Conclusion (+ or – for
sugar)
1
2
3
4
5
Questions:
1. Enter your conclusions in Table 4.
2. Compare Tables 2 and 4. In what molecule do onion cells store their
energy?__________________
3. Do all organisms store their energy in the same molecule? Explain.
IV. Test for Lipids
Materials and method…
1. Place a drop of water on a square of brown paper. Describe the immediate effect.
_________________________________________________
2. Place a small drop of vegetable oil (a lipid) on a square of brown paper. Describe the
immediate effect._______________________________
3. Wait at least 5 minutes. Evaluate which substance penetrates the paper and which
substance is subject to evaporation. Record your observations.
Turn in the preceding pages (One copy per group).
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V. Analyzing the Glucose Content of an Unknown…
Materials and Method…
1. Obtain one of the numbered samples. Record the number.
2. Perform a glucose analysis on the sample and determine the relative glucose content based on
the color chart.
3. Write your value on the board.
4. Using combined class data and the background provided by the instructor, turn in a standard
report for the analysis.
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