Identifying Biological Molecules

Purpose: To discover the presence of carbohydrates, proteins, and lipids using a wide range of
tests and to find which biological molecules are in various different types of food sources.

Materials and Equipment:

Materials Equipment

Water (H2O) Safety goggles

Food samples Lab apron

Benedict’s solution Rubber gloves

Biuret solution Test tubes

Lugol’s solution (iodine) Test tube rack

Test tube tongs

Pipettes

500mL beaker

Glass plate

Hot plate

Brown paper bag

Light source

Mortar

Pestle

Procedure: Refer to BC Biology 12, activity 2-A pages 46-47. With the following exceptions:
-​ In Part III, chips was substituted with crackers, and sesame seeds was substituted with
pine nuts.

Safety:
 -​ Food samples and chemicals are not to be eaten
-​ Wear lab aprons, safety goggles, close toed shoes, and hair tied back
-​ Carefully handle iodine
-​ Biuret solution is corrosive. Only touch the eyedropper of the solution
-​ Food should be disposed of in the appropriate waste beakers
-​ Wash your hands when you are done

Observations:

Table I: Benedict Solution Test for Simple Sugars

Test Tube Test Sample Observations for Simple Sugars Test Presence of Sugars

1 Water (control) Clear, colourless liquid No

2 Cereal Dark orange liquid with chunks Yes

3 Potato Bright orange, clear liquid Yes

4 Corn Syrup Bright, dark orange liquid Yes

5 Cooked Pasta Yellow liquid on top of blue liquid No

6 Unknown (quinoa) Bright orange liquid with chunks Yes

Table II: Lugol’s Solution Test for Starch

Test Tube Test Sample Observations for Starch Test Presence of Starch

1 Water (control) Golden yellow liquid No

2 Potato Yellow liquid No

3 Bread Black, chunky liquid Yes

4 Apple Orange liquid No

5 Crackers Black, chunky liquid Yes

6 Cooked pasta Black, chunky liquid Yes

7 Unknown (quinoa) yellow/orange liquid No

Table III: Grease Spot Test for Lipids

Section Test Sample Observations for Lipids Test Presence of Lipids

1 Water (control) No stain No

2 Oil Circular stain Yes

3 Lard Small circular stain Yes

4 Crackers No stain No

5 Butter Light stain Yes

6 Pine nuts Dark stain Yes

7 Unknown (quinoa) Very light stain Yes

Table IV: Biuret Solution Test for Proteins

Test Tube Test Sample Observations for Proteins Test Presence of Proteins

1 Water (control) Transparent, light blue liquid No

2 Egg Lavender liquid with chunks Yes

3 Milk Dark purple liquid Yes

4 Tofu Very dark purple, chunky liquid Yes

5 Meat Dark, chunky purple liquid Yes

6 Yogurt Bright, clear purple liquid Yes

7 Cheese Dark purple liquid Yes

8 Unknown (quinoa) Clear, grey liquid No

Sources of Error:
1.​ In any part of the lab when chemicals or substances were being poured into beakers,
test tubes, etc, an incorrect amount could have been poured. This could result in data
being inaccurate.
2.​ If the water in Part I of the lab wasn’t heated up for long enough to reach a certain
temperature, the results may be slightly inaccurate.
3.​ If equipment such as the hot plate were to malfunction, the results of the lab could take
longer to record or the lab could be inaccurate.
4.​ If a step in the lab was read incorrectly, the recording of the lab results would be
inaccurate.

Results and Discussion Questions:

 1.​ - For the food samples in Part I, the food samples that had a presence of simple sugars
had polymers of disaccharides that were formed from simple sugars such as glucose,
fructose, or a galactose.
- For the food samples in Part II, the food samples that had a presence of starch had
polymers of starch that were formed from monomers of glucose.
- For the food samples in Part III, the food samples that had a presence of lipids had no
polymers, but monomers such as glycerol were present.
- For the food samples in Part IV, the food samples that had a presence of proteins had
polymers of protein that were formed from monomers of amino acids.

2.
Molecule Simple Sugar Starch Fats Protein

Function Energy storage Plant energy storage Fuel source Transport, support, etc

Food source Corn syrup Potatoes Oil Cheese

Name of chemical test Benedict Solution Lugol’s Solution Test Grease Spot Test Biuret Solution Test for
Test for Simple for Starch for Lipids Proteins
Sugars

Negative test result Clear, colourless Golden, yellow liquid No stain Transparent, light blue
(control) liquid liquid

Positive test result Orange solution Black, chunky solution Stain Purple colour solution

3. A control is necessary and important for each of these tests because controls (such as
water) allow a comparison between two substances. Comparing a positive result with a control
can show how accurate the results are. For example if two different experiment results that look
different from each other are compared to a control, the comparison can be used to tell which
result reacted more than the other.

Conclusion: During this lab, different food samples were tested to detect the presence of
carbohydrates, sugars, lipids, and proteins. In each of the parts of this lab, food samples were
tested and compared to a control (water). Some food samples did contain biological molecules
(ex. Starch in potatoes, simple sugars in corn syrup, etc) while others didn’t. This lab contributes
to the understanding of biological molecules and foods that contain these molecules.