Testing for Presence of Biological Molecules - Carbohydrates, Lipids, Protein, and Nucleic Acids

CARBOHYDRATE TESTS:

1) Benedict's Reagent - ONLY tests for "reducing sugars" like glucose, galactose, and fructose. Will NOT detect non-reducing sugars like sucrose. This reagent must be mixed with the solution you are testing and then HEATED in a hot water bath for about 3 minutes--enough time to produce a visible color change IF the solution contains even a small concentration of reducing sugar(s).

Your NEGATIVE CONTROLS (-) should be: (a) DI water and/or (b) sucrose solution. DI water can be your negative control for any of the following tests, since we know pure water does not contain carbohydrates, lipids, protein, or nucleic acids (DNA, RNA). However, a BETTER negative control for Benedict's Reagent would be a sucrose solution, as sucrose is NOT a reducing sugar, and thus should NOT produce a positive indication (a color change) when the Benedict's Reagent is heated.

Your POSITIVE CONTROL (+) should be a pure glucose solution (or a solution containing a different reducing sugar).

COLOR CHANGE INDICATION (when the solution tests "positive" for reducing sugars): green (low content), orange (moderate content), red (high content), brown (very high content).


2) Iodine (i.e. IKI, I2KI, or Lugol's Iodine. Gram's Iodine will also work, but it tends to be a more expensive reagent) - ONLY detects starch (strings of glucose monomers joined together). Iodine will NOT detect individual glucose monomers.

Your NEGATIVE CONTROLS (-) should be: (a) DI water (deionized water) and/or (b) a pure glucose solution.

Your POSITIVE CONTROL (+) should be a pure starch solution.

COLOR CHANGE INDICATION (when the solution tests "positive" for starch): brown.



PROTEIN TEST:

Biuret Reagent - ONLY detects protein (amino acid chains). The Biuret Reagent WILL NOT detect individual amino acids (monomers of proteins/polypeptides).

Your NEGATIVE CONTROLS (-) should be: (a) DI water and/or (b) a pure amino acid solution.

Your POSITIVE CONTROL (+) should be a pure protein solution.

COLOR CHANGE INDICATION (when the solution tests "positive" for protein): purple or black.



LIPID TEST:

Sudan III or Sudan IV - ONLY detects lipids (i.e. fats, oils, triglycerides).

Your NEGATIVE CONTROLS (-) should be: (a) DI water

Your POSITIVE CONTROLS (+) should be: (a) pure lipid/fat/oil MIXED WITH a little bit of soap/detergent (this helps to prevent separation of any water content in the Sudan III/IV Reagent from the lipid/fat/oil), (b) whole or reduced-fat milk (emulsified fat).

COLOR CHANGE INDICATION (when the solution tests "positive" for lipids/fats/oils): red.



NUCLEIC ACID TEST:

Dische diphenylamine test - ONLY detects DNA.Will NOT detect RNA. Requires heating/boiling then cooling (ice bath).

Your NEGATIVE CONTROLS (-) should be: (a) DI water and/or (b) pure RNA solution.

Your POSITIVE CONTROL (+) should be a pure DNA solution.

 COLOR CHANGE INDICATION (when the solution tests "positive" for DNA): blue.


For photographic examples of these tests, go here.