Macromolecules Virtual Lab

Macromolecules Virtual Lab

Carbohydrates, proteins, fats, vitamins, and other nutrients provide your body with energy and provide the raw materials necessary to carry on life activities. These compounds are present in the plants and animals you use as food. In this lab, you will examine the different tests used for determining the presence of macromolecules. This lab is worth 30 points.

Testing for sugars: The Benedict’s test will identify the presence of reducing sugars (monosaccharides and some disaccharides). This test is used in medicine to test for the presence of glucose in urine. The color change in this test is due to the oxidation of the reducing sugar by the copper ion in the Benedict’s reagent. Benedict’s solution has a bright blue color. No change in color indicates no reducing sugars are present. The positive results can give a range of colors, indicating an increase in the amount of simple sugars present (Fig. 1). This test is performed by adding the solution you are testing to a test tube with the Benedict’s solution and then heating it for 3-5 minutes in a hot water bath. The color change can happen rather quick.

Figure 1: Amounts of reducing sugars in a Benedict’s test. A bright blue color indicates no reducing sugars. As the amount of reducing sugars increases the colors will change from green to red. Image from This video walks you through the test. Water is a great negative control for all of these tests.

Another test that we can do to test for sugars is the Iodine test for starch. Remember that starch is a polysaccharide. Iodine has a color of light orange-brown. When added to a substance with starch it will turn blue-black. This is due to iodine interacting with amylose. Remember that starch is found only in plants. Only plants and plant-based foods will test positive for starch. This test cannot be performed on very dark solids are liquids. It will give inconclusive results as the color change cannot be observed. This video will walk you through the test. Testing for proteins: Proteins are made up of amino acids. Recall from lecture that there are 20 different amino acids and the 4 different levels of protein structure. A chain of two or more amino acids is called a peptide. Each amino acid is joined by a peptide bond. Biuret reagent is used to detect the presence of peptide bonds. This solution has a light blue color. Biuret solution will change color in the presence of peptide bonds as they chemically combine with the copper ions present in the reagent. The color change is faint and is easiest seen when held up against a white sheet of paper. A light blue color indicates no proteins are present. A violet color indicates that proteins are present (Fig. 2).



Figure 2: Positive and negative test result for Biurets test for protein. Image from: This video will walk you through the test. The color is really vibrant on this test because the Biurets reagent was not diluted down like we normally do for lab tests.

Testing for lipids: Lipids are compounds that are insoluble in water and soluble in solvents, such as alcohol and ether. This group includes fats, oils, phospholipids, steroids and cholesterol. There are a few tests used to determine the presence of lipids. One you might be familiar with is the grease spot test. If you ever get fast food, you have already performed this test. I call it the Five Guys test. What you do for this test is place a drop of the solution you are testing on a brown paper bag. When held against a light source if the bag is translucent it is because the fats get absorbed by the paper. In contrast, water will evaporate, and the paper will first appear wet and will eventually evaporate and the bag will not be translucent when held up to light (Fig. 3 is also a video on the test).

Figure 3: Grease spot test results. Fats will be absorbed and appear translucent when held up to a light source. Water will not be absorbed by the paper bag. Image from:

A color analysis test used for lipids is the Sudan test. Sudan is not soluble in water but is in lipids. In this test Sudan is added to a solution with ethanol to dissolve any possible lipids. Sudan has a light orange color. When added to water it will appear that way. When lipids are present the Sudan reagent will bind causing a color change to a dark red (Fig. 4). This video will walk you through the test:

Figure 4: Sudan test results for lipids. A positive test will show the Sudan reagent interacting with the lipid layer. It will appear bright red and sometimes you see it form little spheres. A negative result will be a pale orange. Image from:

Finally, the graded stuff! I will show some images from tests that were described below and ask some questions on them. Please type in your answers and either submit as a PDF or Word Doc. Refer to the figures to answer the questions. The figure number is located below the image.



Figure 5: Results from reducing sugar test.

Figure 6. Results from a reducing sugar test.

Figure 7. Results from a reducing sugar test.



Figure 8: Results from a protein test.

Figure 9: Results from a lipids test. Use Figure 5 for questions 1-3. 1. What reagent was used in this test? 2. What are results for each tube? Be specific. Remember positive results cover a range. A: B: C: 3. What causes the color change in the reaction above? Use Figure 6 for question 4.



4. Why did sucrose test negative? Be specific and you may have to look up the chemical structure of sucrose to help describe your answer. Use Figure 7 for question 5. 5. Why did the test tube containing starch not change colors? Be specific. Use Figure 8 for questions 6 – 8. 6. What is the reagent used in this test? 7. What are the results for each tube? (Positive or negative) A: B: 8. Tube A is a solution of single amino acids and Tube B is from a protein powder solution. Why did Tube A not test positive even though it has the components that make up proteins? Use Figure 9 for questions 9-10. 9. What is the reagent used in this test? 10. Is this result positive or negative?

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