Biological Molecules: Carbohydrates, Lipids, Proteins
By the end of this lab, the student should be able to:
Identify the functional groups for each of the biomolecules that react in the following biochemical tests: Benedict’s test, Iodine test, Brown Bag test, Sudan III/IV test, and the Biuret's Test.
Describe the mechanism of reaction for: Benedict’s test, Iodine test, Sudan III/IV test, and the Biuret's Test.
Interpret the results when presented with data for each of the biochemical tests.
Design experiments to identify biomolecules using biochemical tests. Introduction
Biological Molecules contain specific chemical structures called functional groups, which can be distinguished by biochemical tests, such as Benedict's Iodine, and Sudan III/IV. The Benedict's test identifies reducing sugars (monosaccharide's and some disaccharides), which have free ketone or aldehyde functional groups). The groups reduce CuSO4, a component of Benedicts, resulting in a color change in the benedicts solution from a turquoise to a brick to rusty-brown color. Another class of carbohydrates called starches, a polysaccharide, can be detected using the Iodine's test. Having a high molecular weight, iodine attaches to the helical structure of starches yielding a blue-black precipitate. The Biuret's test identifies proteins by oxidizing 4-6 peptide bonds of a protein using CuSO4 and NaOH resulting in a color change in the CuSO4 solution from purple to a darker purple color. Finally, the Sudan III/IV test is used to detect the hydrocarbon chains of lipids. Sudan is a red, non-polar, dye that forms hydrophobic interactions with the hydrocarbon chains of lipids. Alternatively, the Brown Bag test can also be used to identify lipids due to the oily nature of hydrocarbon chains. Carbohydrates: Reducing sugars and starches
Some sugars such as glucose are called reducing sugars because they are capable of transferring hydrogens (electrons) to other compounds, a process called reduction. When reducing sugars are mixed with Benedicts reagent and heated, a reduction reaction causes the Benedicts reagent to change color. The color varies from green to dark red (brick) or rusty-brown, depending on the amount of and type of sugar.
Increasing amounts of reducing sugar
green orange red brown
In a typical Benedicts Test (shown below), approximately 1 ml of sample is placed into a clean test tube along with10 drops of Benedict's reagent (CuSO4). The reactions are heated in a boiling water bath for 5 minutes. Below: The test solutions and Benedict's reagent are boiled in a water bath for five minutes.
Click on the image to view an enlargement. Press the "Back" button to return. Below: Results of several solutions tested with the Benedict's test
Click on the image to view an enlargement. Press the "Back" button to return. Activity #1: Draw a large chart containing four equally divided columns and seven rows. In the first row, label the first column "Unheated Reaction"; the second column"Color of Heated Reaction"; the third column "positive (+) reaction and negative (-) reaction"; and the fourth column "Structure of Reacting Functional Group". In the first column, label rows 1-7 with the appropriate samples below: Test tube #1: distilled water + Benedicts
Test tube #2: glucose solution + Benedicts
Test tube #3: sucrose solution + Benedicts
Test tube #4: starch solution + Benedicts
Test tube #5: onion solution + Benedicts
Test tube #6: Potato solution + Benedicts
In the second column labeled "Color of Heated Reactions", indicate the results of the Benedict's test after the reaction was heated. In the third column, indicate a (+) for positive benedict's reaction and a (-) for negative benedict's reaction. In the fourth column, draw the reacting functional group(s) of the samples that reacted with the benedict's reagent.
Iodine solution (IKI) reacts with starch to produce a dark...
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