Biomolecule Tests

The fundamental chemical components of cells may be grouped into four basic categories—carbohydrates, proteins, lipids, and nucleic acids. Other chemicals such as water, salts and minerals are also present and vital to living cells. Characteristic tests to identify each of the four major classes of biological molecules are described below.

Test for Complex Carbohydrates (Polysaccharides)

Carbohydrates such as starch are detected by mixing a sample with 2–4 drops of iodine or Lugol's solution. The solution will turn from a yellow-brown color to a dark purple if starch is present. The color change is due a chemical reaction bewteen the large carbohydrate molecule and the iodine ions. It is thought that the charged iodine ions are attracted to slight charges on the starch (polysaccharide) molecules and this causes the color change that we observe.

Iodine test results: positive result - test tube on the left, negative result - test tube on the right

Test for (Reducing) Simple Sugars (Monosaccharides)

The presence of smaller carbohydrates like glucose can be determined by using Benedict's solution. Benedict's solution has copper ions (Cu²⁺) that have a light blue color. When this solution is heated in the presence of sugars like glucose the copper ions change and turn anywhere from a light green to rusty orange-brown color.

Test for Lipids

The presence of lipids can be determined using Sudan III indicator. Sudan Indicators are fat-loving molecules that are colored. When testing a solution for lipids there are two results you should be looking for:

1. Do you get a separation of layers (water & lipid)?,

and

2. Does the dye migrate toward one of the layers?

If the mixtures are all water soluble (no lipids present), then the Sudan IV will form small micelles/droplets and disperse throughout the solution. A positive result will see the lipid layer sitting on top of the water layer and the lipids layer will have a red-orange color

Test for Nucleic Acids

The (Dische) Diphenylamine Test is used for determining the presence of nucleic acids. The presence of DNA will turn a clear solution blue. The more DNA present the darker the color. Another nucleic acid, RNA, will turn green.