Biological Molecules of Life

Using specific reagents in each food sample, this laboratory test was performed to identify the organic molecules present in various substances. Each type of organic compound present will show a distinct color change as a result. A color change would indicate that the macromolecule had been identified as positive. Benedict’s solution was used as a reagent for sugar reduction. When the solution was heated, a blue precipitate formed, indicating that the sugars had been removed. Iodine solution was used as a reagent for starch. Organic compounds containing starch, according to the theory, will change color from blue to black. The starch solution, small potato piece, bread, and cracker all yielded positive results. Also, biuret test was used to test for proteins. For this sample test, the only hypothesis was that sucrose solution would not change color. The results showed a color change for the protein solution, milk, and gelatin. For Nucleic Acids, Dische Diphenylamine was used as the reagent. The hypothesis was that among the five samples only those with DNA were expected to show color changes. The results concluded that deoxyribose sugar in DNA reacts with diphenylamine reagent when heated. For lipid test, Sudan IV and Grease spot test reagents were used. The hypothesis was that a mixture of lipids with Sudan IV forms a red color which makes the fat more visible than other substances. The Grease spot test was expected to show a translucent spot with milk and vegetable oil.


Organic compounds are in all forms of life because of their association with living organisms. These organic compounds are made up of carbon, hydrogen and often oxygen or nitrogen. The major categories of an organic compound found in living things include proteins, carbohydrates, lipids, and nucleic acids. In this lab, with the use of different reagents, various food samples were analyzed. Detection was based on observing a color change. There are several methods for identifying organic compounds in living organisms. Typically, these tests have used the type of macromolecules present in an unknown sample. During the experiment, known compounds are used for comparison with the results obtained. Controls play a significant role in determining the specificity of a particular test. For example, if glucose and distilled water react the same in a given reaction, another test could be used to the organic compounds present to distinguish glucose solution from the water. In this instance, distilled water acts as a control experiment to demonstrate the negative results. In this experiment, different reagents were used to determine the presence of protein, carbohydrates, lipid, and DNA.

Carbohydrates are made up of carbon, hydrogen, and oxygen elements. Carbohydrates are classified into monosaccharide which includes glucose, fructose, and galactose while an example of a disaccharide is sucrose. The polysaccharides include starch, glycogen, or cellulose. Monosaccharides are referred as simple sugars because they contain one unit. Benedict’s test can be used to determine the presence of simple sugars. The experiments aim is for students to become familiar with enzyme-catalyzed reactions and reducing or non-reducing nature of carbohydrates. Benedict’s solution can reduce copper II ions to copper II oxide to identify the reducing sugars.

In this test, a blue solution indicates an absence of reducing sugars, and amber color and brown color showed the presence of reducing sugars.

Iodine solution was used to distinguish starch from monosaccharide, disaccharides and other polysaccharides. Starch contains a coiled polymer of glucose that reacts with iodine to form a blue-black color. Iodine remains yellowish brown when reacted with carbohydrates. Therefore, the change of color to blue-black confirms the presence of starch while a yellowish-brown color indicates the absence of starch.

Protein molecules are found in all life-forms. They contain building blocks known as amino acids, which are linked together by peptide bonds. The peptide bond forms the site of action for the Biuret reagent to test for protein. In this test, the peptide bond forms a complex molecule with copper II ions present in Biuret reagent. A purple color observed indicates the positive test for protein. Therefore, the intensity of the color formed depends on the amount of peptide bonds that reacted with Biuret reagent. Thus, the presence of numerous peptide bonds produces a positive reaction.

Lipids are macromolecules that are soluble in non-polar solvents such as acetone but insoluble in water. Fatty acids are the major building blocks for lips. Fatty acids are saturated or unsaturated carboxylic acid with a long hydrocarbon chain. Three fatty acids bonded together to form triglycerides for storage and transport in the body. An ester linkage bond is formed from a dehydration synthesis. By using Sudan IV, the absorption of fat-soluble dyes can be used to determine the presence of lipids. The Grease-spot Test was also used to identify the ability of soluble fats to form translucent grease marks on a filter paper.

Nucleotides link together to form long strands of nucleic acids. For example, DNA is composed of a deoxyribose sugar, which chemically reacts with Dische diphenylamine reagent when heated. The formation a blue complex solution is due to the acidic conditions by the reaction of Dische diphenylamine reagent with a molecule from deoxyribose sugar.

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