BIOL 1101 Practical Notes on Food Tests Biological Molecules Lab Notebook
*Lab is not needed to complete this assignment* You are simply making notes based on facts as in what will happen in the situations. Answer all questions and make notes for all statements.BackgroundAll living organisms are composed of the same four biological molecules. These molecules are sugars, fats or lipids, proteins, and nucleotides. All meats, dairy products, and plant materials such as fruits or vegetables contain at least some amount of each of these biological molecules.
Any food you consume is broken down and the molecules are absorbed by your body. Your body then recycles these molecules to produce sugars, lipids, proteins, and nucleic acids. So yes, you are what you eat.Biological MoleculesAll organic molecules are made up of the basic elements — carbon, oxygen, hydrogen, phosphorous, sulfur, and nitrogen. When these elements are arranged in different ways, they group together to form four general types of biological molecules with unique characteristics. The four general types of molecules are carbohydrates, lipids, proteins, and nucleic acids.Carbohydrates: Sugars and StarchesBenedict's solution (Benedict’s reagent) is a solution made up of copper(II) ions dissolved in an alkaline solution. When mixed with Benedict's solution and heated, the carbonyl functional group (carbon double bonded to oxygen) in reducing sugars reduces the copper(II) ions to copper(I), which precipitates as copper(I) oxide (Cu2O). This reduction reaction causes the Benedict’s reagent to change color, varying from blue to green to yellow to orange to red, depending on the quantity of reducing sugars present in the solution (Figure 1).Two monosaccharides can bind together to form a disaccharide. For example, maltose contains two glucose molecules bound together. Some disaccharides have reducing carbonyl groups, such as maltose and lactose, but some do not, such as sucrose. Because the bond between two monosaccharides frequently contains the free carbonyl group that acts as a reducing agent in simple sugars, many disaccharides do not test positive with Benedict's solution. Despite a few other differences, monosaccharides and disaccharides are chemically similar and are both classified as sugars.Monosaccharides can also bind together in long chains to form polysaccharides. Polysaccharides have very few carbonyl groups and do not react with Benedict’s solution.Starches, a subclass of polysaccharides, are chemically distinct from simple sugars. The presence of starch can be detected by adding a solution of Lugol’s iodine. Lugol’s iodine contains iodine and potassium iodide (I2KI). I2KI by itself has a reddish brown color. However, when it interacts with the amylose molecules in starch, the solution changes to a deep bluish black color. Glycogen also reacts with Lugol’s iodine to give a brownish blue color. Other polysaccharides, such as cellulose, do not react with Lugol’s iodine.LipidsLipids are compounds composed of long chains of hydrocarbons. They are insoluble in water, but soluble in organic solvents such as alcohols and ethers. Lipids include fats, oils, phospholipids, steroids, and cholesterol. Cellular membranes are rich in lipids. A type of fat that you may be familiar with is a triglyceride. The tri- in triglyceride represents the three fatty acids that bind to one molecule of glycerol. Lipids do not form polymers, whereas carbohydrates and proteins do. A polymer is a compound made of many repeating smaller units. For example, a polysaccharide is a carbohydrate polymer.Sudan III is a dye that is not soluble in water, but is soluble in lipids. The Sudan III test checks for the presence of lipids in a solution. A red color indicates the presence of lipids.ProteinsProteins are the most plentiful biological molecules in living cells. They have numerous functions in organisms, especially as enzymes, which act as catalysts. Enzymes assist almost every chemical reaction that takes place in your body. Pepsin is a digestive enzyme active in the stomach that helps to speed up the digestion of food proteins.Amino acids are the building blocks of proteins. About 20 different amino acids are found in cells. All amino acids have an acidic carboxyl group (–COOH) and a basic amino group (H2N–). Amino acids differ from each other by the R group attached to the carbon atom. R groups vary in size, shape, and chemical activity. For example, the R group of glycine is a single hydrogen atom, whereas the R group for cysteine contains a thiol group.Just as a long chain of monosaccharides forms a polysaccharide, a long chain of amino acids forms a polypeptide. The amino acids in a polypeptide are held together by peptide bonds that are formed between the carboxyl group of one amino acid and the amino group of another amino acid.The three-dimensional shape of a protein determines its function. This shape depends on the amino acids present in the protein as well as on the intermolecular interactions between the amino acids in the protein.Biuret solution (biuret reagent) is used to test for the presence of peptide bonds. It is a blue solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and a very small amount of copper(II) sulfate (CuSO4). This reagent changes color in the presence of proteins because the nitrogen atom in the peptide bond forms a violet-colored complex with the copper ions in the biuret reagent. To notice a color change, the peptide has to contain at least four peptide bonds. The more peptide bonds that are present, the more intense the color change. In the presence of protein, the reagent turns violet. If proteins have been broken down into short peptide chains, the reagent turns pink.About This Lab In this lab, you will test for the presence of biological molecules using some basic chemical test methods and techniques. You will use Benedict’s solution to test for the presence of reducing sugars and iodine to test for the presence of starch. To test for the presence of lipids, you will use Sudan III solution and to test for the presence of proteins, you will use biuret solution. After visualizing positive and negative results for all these tests, you will test food items for the presence of these four biological molecules. Samples of various foods ground into liquid suspensions are on the Materials shelf. You will test each for the presence of reducing sugars, starch, lipids, and protein.ExperimentsOpen the simulation by clicking on the virtual lab icon below. The simulation will launch in a new window.You may need to move or resize the window in order to view both the Procedure and the simulation at the same time.Follow the instructions in the Procedure to complete each part of the simulation. When instructed to record your observations, record data, or complete calculations, record them for your own records in order to use them later to complete the post-lab assignment. ProceduresExperiment 1: Test for Reducing SugarsTake a constant temperature bath from the Instruments shelf and place it onto the workbench.Set the constant temperature bath to 100 °C.Take a test tube from the Containers shelf and place it onto the workbench.Take the 5% glucose solution from the Materials shelf and add 6 mL to the test tube.Record the color of the solution to reference later.Take Benedict's solution from the Materials shelf and add 6 mL to the test tube. Record any color change.Move the test tube into the constant temperature bath. Wait a few moments to see if the color changes. The color change usually begins at temperatures greater than 60 °C, but will finish by the time the solution reaches 100 °C. Take a thermometer from the Materials shelf and attach it to the test tube to monitor its temperature. Record any color changes.Empty the test tube in the waste bin, then place the empty test tube in the sink.Repeat steps 3 – 8 two more times, replacing the 5% glucose solution in step 4 with:5% sucrosewater, as a negative controlClear your station by dragging all instruments back to the Instruments shelf and by emptying all containers in the waste bin and then placing the empty containers in the sink.Experiment 2: Test for StarchTake two test tubes and a 250 mL beaker from the Containers shelf and place them onto the workbench.Add 90 mL of water and 10 mL of Lugol’s iodine from the Materials shelf to the 250 mL beaker. Rename the beaker “diluted Lugol’s iodine” by double-clicking on it.Take the 2% starch solution from the Materials shelf and add 6 mL to one test tube.Take the water from the Materials shelf and add 6 mL to the other test tube.Record the color of the solutions in the test tubes.Add 6 mL of the diluted Lugol’s iodine solution in the 250 mL beaker to each test tube.Observe what happens in each test tube. Record your observations for reference later.Empty the two test tubes in the waste bin, then place the empty test tubes in the sink. Keep the beaker of diluted Lugol’s iodine on the workbench to use in Experiment 5.Experiment 3: Test for LipidsTake two test tubes from the Containers shelf and place them onto the workbench.Take the corn oil from the Materials shelf and add 6 mL to one test tube. Take the water from the Materials shelf and add 6 mL to the other test tube.Record the initial colors of the liquids in the test tubes.Take the Sudan III solution from the Materials shelf and add 6 mL to each test tube. Observe any potential color changes in the two test tubes. Record your observations.Empty the test tubes in the waste bin, then place the empty test tubes in the sink.Experiment 4: Test for ProteinsTake two test tubes from the Containers shelf and place them onto the workbench. Take the 35% egg albumin from the Materials shelf and add 6 mL to the first test tube. Take the water from the Materials shelf and add 6 mL to the second test tube.Record the colors of the solutions.Take the biuret solution from the Materials shelf and add 6 mL to each test tube.Observe any color changes in the two test tubes. Record your observations to reference later.Empty the test tubes in the waste bin, then place the empty test tubes in the sink.Experiment 5: Test the Contents of FoodsFor each food sample, do the following:Take four test tubes from the Containers shelf and place them onto the workbench. Based on steps 2 and 3, label each test tube with the material and test being performed. For example, label the test tube used to test onion juice for the presence of reducing sugars as “Onion Juice Benedict’s Solution”.Add the reagents to the test tubes as directed in the table below. Be sure to use the diluted Lugol’s iodine you made in Experiment 2.Test TubeReagent16 mL Benedict’s solution26 mL diluted Lugol’s iodine solution36 mL Sudan III solution46 mL biuret solution
Take the potato juice from the Materials shelf and add 6 mL to each of the four test tubes. Observe and record all color changes.Remember that the test tube with the Benedict's solution requires heating in a 100 °C constant temperature bath for the color change in the presence of reducing sugars to occur. Recall that you can use a thermometer to monitor the heating.Empty the test tubes in the waste bin, then place the empty test tubes in the sink.Repeat steps 1 – 4 for the onion juice, whole milk, and skim milk. Be sure to record all of your observations.Clear the bench of all materials, containers, and instruments, then return to your course page to complete any assignments for this lab.
