classes of biological molecules thesis

Human Biology Lab 2 Name___________________


Pre-Lab Questions

Question 1:

The four main classes of biological molecules fill in the monomer.

Molecule Name Monomer, if applicable
Proteins (polypeptides)  
Nucleic Acids  


Question 2:

Based on what you know about these foods, which biological compound(s) (of the ones we are testing for) do you hypothesize will be present (lipids, starch, sugar, and/or protein)?



White grape juice?

Egg white?



During the following experiment you will be conducting two different types of tests, testing for lipids and a type of carbohydrates, starch. These tests will determine the compounds present in four different unknowns. You will be selecting these unknowns from your own pantry or fridge. You test or will be given the results of a known positive control and a negative control. A positive control is a known strong positive result for the macromolecule being tested. If the unknown solutions look like the positive control result, then that unknown solution is positive for that macromolecule. A negative control is a known negative result for the macromolecule being tested. If the unknown solution looks like the negative control result, then the unknown solution is negative for that macromolecule.


When selecting the foods or drinks you will test, keep in mind the first test, Testing for Lipids, you need the solutions to be liquid and light in color, if too dark it will be hard to tell the results. Some ideas for solutions to test are, juice (from the bottle or you could juice from the fruit), milk, milk alternative, coffee creamer, light colored soup broth, beaten up egg white, or another item you have on hand. For the Testing for Starch test you can use the same solutions used in the other test or you could select solid food options as well. Some ideas for the second test other than listed above are, bread, cereal, crackers, fruit, vegetables, or another food source you have on hand. You will be looking for a color change for when testing for starch so make sure the selected is light in color to easily detect the color change.


Note you will be using Iodine in this experiment. Iodine can stain so be careful!!!



*You need to go buy these materials if you do not have them at home.

Iodine Solution (Povidone Iodine)

Paper Towels

Cooking oil (Vegetable, olive oil, canola, or real butter melted.)

Water (Tap or Bottled)

4 – 8 selected foods and/or drinks to test

Plates and/or cups


Testing for Lipids:

1. Label six sections of a paper towel with the following: “water,” “oil,” and label spots for your selected 4 solutions. The negative control for this test is water, and the positive control is cooking oil. The selected solutions HAVE to be liquids!

2. Place a few drops of each liquid to be tested on the paper towel next to the corresponding labels. Trying your best to add the same amount of each solution to its corresponding spot.

3. Place the paper towel aside to let dry for 30 minutes. Can continue onto the next experiment while waiting.

4. After the spots have dried, hold the paper towel up to a light to see if you spot a translucent spot where the drops of solution were.

5. Analyze and record the results. Label your table with your selected solutions. A translucent spot indicates the presence of lipids. Dispose the paper towel in the regular trash.

Testing for Starch:

1. See the picture below to see the results from the positive and negative controls for the starch test. The negative control is water and the positive control was a starch solution. Note that Iodine will be used in this part of the lab this solution can stain so be careful and use disposable or old plates/cups to avoid staining sadness.

2. You will select 4 foods or drinks to test to see if they have a presence of starch. These DON’T have to be the same as in the lipid test but can be.

3. If testing solid foods, place a small chunk of food on a plate (YOU CAN NOT PLACE THE FOOD ON PAPER TOWEL IT WILL MESS WITH THE RESULTS because starch is present in paper towels). If testing a liquid, cover the bottom of a cup with the liquid solution.

4. Place a few drops of Iodine on or in the substance you are testing. If the substance turns black in color once the iodine is added that is a positive result for starch.

5. Analyze and record the results. Label your selected substance on the table. For Disposal: Pour the liquid down the drain and rinse with water. Throw out the solid food in trash. Make sure to wash hands after dealing with Iodine.


For each test you will prepare negative and positive controls in order to compare your unknown substance results and check to make sure the tests are working properly. The negative control for each test will be distilled water, while the positive will vary depending on the test that you run. It is advisable that you run an entire test at once, so that you do not wait too long between running a test and comparing your results, which could result in degradation of the result. Be sure to label each test tube before running your experiments and shake the extracts before each test.

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