TEACHER GUIDE: What substances contain proteins?

In this investigation, you will determine what types of organisms and materials contain proteins. You will combine different materials with Biuret solution. If a material changes the color of the Biuret solution from blue to purple, that material contains protein.

Objectives

-Students will learn that all living organisms, or organisms that were once alive, contain proteins.
-Students will conduct a Biuret assay to determine what types of substances contain protein.

Materials/Equipment Needed

For each student group:

- test tube rack
- 6 test tubes
- 30 ml Biuret solution
- Samples: kiwifruit, liver, oil, plant leaves, potato, sugar

Some questions to get you thinking about today’s lab:

Which of the samples do you expect to contain protein?

Students may come up with a variety of appropriate answers.

Why do you think so?

Students often misunderstand that only materials made of muscle contain proteins. In actuality, anything that contains nucleic acids will contain proteins, because nucleic acids are the instructions for making proteins.

What do proteins have that nucleic acids, carbohydrates, and fats don't have that might turn the Biuret solution from blue to purple?

Students may come up with a variety of appropriate answers. This question is most relevant to ask if you are discussing the structure of macromolecules with your students. Proteins (amino acids) contain an amino group (NH2), unlike nucleic acids, carbohydrates, and fats. When Biuret solution interacts with this amino group, its color changes from blue to purple.

Procedure

1. Label test tubes with the names of the samples and place them in the test tube rack.

2. Place a small amount (the size of your fingernail) of each sample into its test tube.

3. Add 5 ml Biuret solution to each of the test tubes. If the solution is spilled, wash it off with water.

The Biuret solution contains copper sulfate and sodium hydroxide (NaOH). Although the concentration of NaOH is low, it is still a base and should be rinsed off with water immediately if spilled.

4. Observe the tubes. For which samples does the Biuret solution turn purple? Record your findings in the data table below.

Protein Data Table: Which samples contain protein?

Analysis

1. Based on your results, what types of substances contain protein?

Students should be able to observe a color change with any living (or once living) material. The color change may be more or less drastic depending on the material, but this doesn't correlate with the amount of protein. Other protein assays are more quantitative than the Biuret assay.

2. What substances do NOT contain protein?

Student should not be able to observe a color change for materials made solely from carbohydrate, fat, or nucleic acid, including sugar and oil.

3. Did any samples you didn't expect to contain proteins actually have them?

Student may change their thinking about what materials contain protein. Likewise, student may be able to confirm they're thinking with their observations.

4. Why do you think it's important to know what substances contain protein?

Students may come up with a variety of appropriate answers. For example, knowing what foods contain protein can help someone plan a balanced diet, especially if he/she is vegetarian.

5. Where do you think proteins come from? How do you think they are made?

Student may come up with a variety of answers depending on their previous experiences. This question can be used to segue into the next two activities: What substances contain DNA? and Protein Fingerprinting of Different Species.

6. Draw and label an amino acid. Draw an arrow where the Biuret solution is most likely interacting with the amino acid.

This question is most appropriate if you are discussing the structures of macromolecules with your students. The generic structure of an amino acid is:

Recipes and Background Information

Biuret solution

For one liter:
1.5 g cupric sulfate pentahydrate (CuSO4 5 H2O)
6.0 g sodium potassium tartrate tetrahydrate (NaKC4H4O6 4 H2O).
Dissolve these in 500 ml of H2O. Add 300 ml of 10% NaOH. Add water to make total volume of 1 liter. Store in a plastic bottle protected from light.

BIOTECH Project Department of Molecular and Cellular Biology The University of Arizona August 3, 1998 Designed by: Ken Kubo, Ph.D. Nadja Anderson, Ph.D. nadja@email.arizona.edu   http://biotech.biology.arizona.edu