Laboratory #3 - Biologically Important Molecules

There are four major groups of organic compounds that make up the bulk of an organism. These four groups of "macromolecules" are carbohydrates, proteins, lipids, and nucleic acids. After completing this lab, you should be familiar with the basic structure and properties of each of the biologically important molecules. You should also be aware of the various chemical tests used to detect each class of molecules.

Carbohydrates are molecules composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio. Glucose (C6H12O6) is an example of a carbohydrate. Glucose is also an example of a monosaccharide or "single sugar". Sucrose is a disaccharide since it consists of two sugar subunits. Other carbohydrates, such as starch, are said to be polysaccharides since they are composed of many sugar subunits.

Benedict's reagent is commonly used to test for sugars. In particular, it tests for reducing sugars (glucose and fructose are examples). When heated, a solution containing Benedict's reagent and a reducing sugar will turn from blue to red/orange in color. The picture below shows a positive test for reducing sugar.

Besides tests for simple sugars, complex carbohydrates can be detected with iodine. The picture below shows the color change observed when starch was added to iodine. Notice the dark purple/brown color.


Proteins are fairly complex molecules composed of many amino acids linked in a long chain. The sequence of amino acids is determined by the sequence of DNA used to code for a particular protein. Additionally, it is the sequence of amino acids that confers specific properties of proteins. In lab, the Biuret test was used to determine the presence of proteins. A solution of proteins will change to a violet color when Biuret reagent is added. The reagent does not react if individual amino acids are present. The picture below shows a positive test for a protein using the Biuret test.


Lipids are insoluble in water and are a major component of cell membranes. Their chemical structure includes a 3-carbon backbone (glycerol) with fatty acids attached to each of the 3 carbons. In lab, the Sudan IV test was used to detect the presence of lipids. Lipids stain red when Sudan IV (a common stain) is added. Both of the test tubes below contain lipids.


DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are common examples of nucleic acids. In lab, the Dische diphenylamine test was used to detect the presence of DNA. A positive test for DNA is indicated by a blue color change. The picture below shows a positive test for DNA.

This concludes the review for Lab 3 - Biologically Important Molecules.

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