A Level Biology Practice Exam

In a biuret test, the color change indicates the presence of what?

1. Fatty acids

2. Simple sugars

3. Proteins

4. Nucleic acids

The correct answer is: Proteins

The biuret test is specifically designed to detect the presence of proteins in a sample. When proteins are present, the biuret reagent, which contains copper(II) sulfate, reacts with the peptide bonds in the proteins, resulting in a color change typically from blue to purple. This reaction occurs because the copper ions form a complex with the nitrogen atoms in the peptide bonds. The other options do not have this specific reaction with the biuret reagent. Fatty acids, simple sugars, and nucleic acids do not contain peptide bonds, so they would not cause any color change in the biuret test. Thus, a positive result indicating the presence of proteins is what makes the biuret test a useful tool for assessing protein content in biological samples.