Lab: Analysis of Biological Molecules in Food In this activity, you will use common chemical indicators used to test for the presence of the various macromolecules in food samples. IDENTIFICATION OF PROTEINS Polypeptides (proteins) are formed when amino acids are joined together by peptide bonds. The biuret reagent reacts with peptide bonds, and therefore reacts with proteins, but NOT free amino acids. The biuret reagent is light blue, but in the presence of proteins containing at least two peptide bonds, turns violet. A. Label 5 clean test tubes. Add 1 2 ml of each sample below to the respective tube. B. Add approx. 1 ml (~30 drops) of biuret reagent (or until a noticeable color is present in the solution.) C. Swirl well to mix. Determine the color and record your observations below. Table 1. Data Table for Biuret Test for Protein 2 Egg White 3 Starch conclusions in Table 1. 2. Tube 1 is considered a negative control sample? Why? 3. Tube 2 is the positive control sample? Why? 4. Why do experimental procedures include control samples? 5. Did test tubes 3 and 4 give a positive or a negative result? Page 1
IDENTIFICATION OF CARBOHYDRATES: BENDICTS TEST FOR MONOSACCHARIDES When Benedict s reagent is heated with a monosaccharide such as glucose, the color of the reagent changes from blue to green to red-orange, depending on the amount of monosaccharide present. It should not show a positive test in the presence of starch. Benedicts Test Procedure: A. Label 5 clean test tubes and add approx. 2 ml of the each sample below to the appropriate tube. B. Add approx 1 ml of Benedict s reagent to each tube (or until a noticeable blue color is present). C. Heat tubes in water bath for approx. 2 minutes or until glucose sample turns orange. HEAT ALL THE TUBES AT THE SAME TIME!! D. Determine the color and record your observations below. Table 2. Data Table for Benedict s Test for Monosaccharides 2 Starch 3 Onion Juice conclusions in Table 2. 2. Which of the tubes is the negative control sample? Why? 3. Which of the tubes is the positive control sample? Why? Page 2
IDENTIFICATION OF CARBOHYDRATES: IKI TEST FOR STARCH IKI (Lugol s solution) changes from yellow-brown to black in the presence of polysaccharides. A negative IKI test indicates either a mono- or disaccharide. IKI Test Procedure: A. In a spot plate and add approx. 0.5 ml of the each sample below to a separate well. B. Add 2 drops of IKI and mix by gently swirling. C. Determine the color and record your observations below. Table 3. Data Table for IKI Test for Starch 2 Potato Starch 3 Onion Juice conclusions in Table 3. 2. Which of the tubes is the negative control sample? Why? 3. Which of the tubes is the positive control sample? Why? 4. Compare Table 2 with Table 3. Sugars are an immediate energy source in cells. In plant cells, glucose is often stored in the form of starch. Is glucose stored as starch in the potato? Is glucose stored as starch in the onion? Page 3
LIPIDS 1. In a petri dish half-filled with tap water, place one drop of vegetable oil with a pipette. Stir the water with a glass rod. Observe what happens at the surface of the water and record your observations below. 2. Interpret your observations in terms of the structure of the vegetable oil molecule and its inability to form hydrogen bonds with water. 3. What can you say about the relative densities of oil and water? 4. In light of your answer to the previous question, what function other than energy storage does fatty blubber serve in a whale or walrus? 5. Explain why we can assume the vegetable oil you are using contains a large proportion of unsaturated fatty acids. How would the properties of the fat be different if it contained mainly saturated fatty acids? Page 4
PRE-LAB QUESTIONS Biuret Test for Proteins IKI Test for Polysaccharides Benedict s Test for Monosaccharides You test several unknowns and obtain the following results: Solution Results of IKI Results of Benedicts Results of Biuret 1 Yellow Blue Violet 2 Yellow Orange Blue 3 Black Blue Blue 4 Black Blue Violet 5 Yellow Orange Violet a. Which solution(s) contains starch? b. Which solution(s) contains glucose? c. Which solution(s) contains protein? Page 5