Macromolecule Lab – Proteins and Starches

Macromolecule Lab – Proteins and Starches


Today we will perform several experiments that will test for some of the organic macromolecules you read about in chapter 5, including proteins, starch, sugar, and fat. Before we begin, I would like to walk you through the materials we will use to perform our experiments today. A Test Tube Rack, 4 to 5 Test Tubes, Transfer Pipet, Protective Glasses, 3 Pieces of Paper Bag, DI Water, Evaporated Milk, 50% Eggs White Solution, 1% Sucrose Solution, 1% Starch Solution, Onion Juice, 10% Sucrose Solution, Vegetable Oil, Biuret Reagent, Iodine, Benedicts Reagent. To perform the test for proteins, starch, and sugar, the protocol is relatively similar. To test for these molecules, we will add 3mls of whatever substance we are testing to a test tube using a transfer pipet. We will then add 5 drops the reagent , swirl to mix the contents, and then observe and record the final color of the solution. For each test we will perform today, there is a specific reagent with a known positive and negative result. We will discuss these tests in more detail, but for now, we will begin with the test for protein. To perform the test for protein, we will use Biuret reagent and will test di water, milk solution, 50% egg white solution, and 1% sucrose solution. Before we get started, you may want to compose a results table that includes a column for the contents of each test tube, a column for the test results, and a strong title. I would label this Table 1 in your work, and an example of a strong title is “The test for protein”. The results you will input will be the final color of the substance when mixed with Biuret reagent. Please take a moment to make an educated guess, called a hypothesis, as to which of the substances we will test today will test positive or negative for protein. For example, do you expect the di water to test positive for proteins? Why or why not? The answer is no, we do not expect this substance to test positive for protein but there is a specific reason why di water will be included in our test. Di water will serve as our negative control. Let’s begin our experiment. I will first label my test tubes with a wax pencil. Then I will add 3 mls of each substance starting with di water to the test tubes. We will now add 5 drops of the reagent, to each of the test tubes, swirl to mix, and record the results. As you can see, Biuret is blue in color alone. If Biuret is added to the substance and that mixture remains blue in color, this will indicate a negative test result for protein. However, if Biuret is mixed with the substance and the mixture is purple or pinkish-purple in color, this will indicate a positive result, and the presence of protein. Let’s work through the results of our experiment. Test tube 1, contains di water. Clearly, the contents of this tube are BLUE, which indicates a negative test for protein. In table 1, under the result column for test tube 1, you would record blue. Note, in your table you will only include the actual results. You will want to save any conclusions for the conclusion section of your lab report. In this case, your conclusion for this test would be that di water tested negative for protein, since it remained blue when Biuret was added. Please take a moment to record the results in Table 1 and let’s continue reading our results. Test tube 2, which contains a milk solution is pinkish purple in color, indicating a positive result for protein. Test tube 3, which contains a 50% egg white solution is purple in color, also indicating a positive result for protein. Finally Test tube 4, which contains a 1% succrose is light blue in color, indicating a negative result for protein. Please record the final results on your table. Now that you have the results for this experiment, you will want to work through each test tube and make conclusions like we did together for the water sample. I would like you to answer the following questions in your conclusion section for each substance that we tested. 1. What were the results? In other words, what color was the substance when Biuret was added and the contents of the test tube mixed. 2. Were your results expected? Why or why not? And 3; If you find that your results were unexpected, please explain why, indicate possible reasons for this, and also include what you would do moving forward given this result. For example, if we test di water for protein, and found the di water tested positive for protein, this would be an unexpected result. What would we do? Would we throw out the results of the entire experiment? Why? Water is the negative control. We know water should not contain proteins and if it tested positive, we would start the experiment with fresh equiptment, reagent, and water samples to ensure accurate results. Next, we will work through the test for starch. This test will be performed in the same way as the test for protein, but with a different reagent. The reagent that tests for starch is Iodine. Since Iodine alone is yellowish-brown, any substance that remains yellowish-brown when Iodine is added will be negative for starch. If the solution turns purple to dark-purple/ black when Iodine is added, this will indicate a positive result for starch. In this experiment, we will test di water, 1% starch solution, onion juice, and 1% sucrose solution. The first step in this experiment will be to label 4 test tubes and set up our second table. Please set up table 2 for your lab report. I will now add 3mls of each substance to a test tube, and then 5 drops of Iodine, swirl and record the color in table 2. Let’s take a look at the results. Test tube 1, which contains di water is light yellow in color, indicating a negative result for starch. Test tube 2, which contains 1% starch solution is light purple in color, indicating a positive result for starch. Test tube 3, which contains onion juice is light yellow in color, indicating a negative result for starch. And Test tube 4, which contains 1% sucrose solution is light yellow as well, also indicating a negative result for starch. Now that we have completed our experiment, I would like you to work through the same questions you were asked for the first test. Please take a moment to think about your answers to the following questions for each test tube. 1. What were the results? 2. Were your results expected? Why or why not? And 3; if you find that your results were unexpected, please explain why, indicate possible reasons for this, and also include what you would do going forward given this result.