A Level Biology Practice Exam

Which system does the iodine test specifically assess for the presence of?

1. Proteins

2. Carbohydrates

3. Nucleic acids

4. Starch

The correct answer is: Starch

The iodine test is specifically designed to determine the presence of starch. This test operates on the principle that iodine interacts with the helical structure of starch, forming a complex that produces a characteristic blue-black color. When iodine is added to a sample containing starch, if it turns blue-black, this indicates a positive result for starch presence. In contrast, the test does not assess for proteins, carbohydrates (other than polysaccharides like starch), or nucleic acids, as they do not produce the same reaction with iodine. For instance, proteins are typically tested using the biuret test, while reducing sugars are assessed through Benedict's test. Nucleic acids require different methods for analysis, such as spectrophotometry or specific staining techniques. Thus, the iodine test is clearly specific to starch, making it a vital tool in identifying this polysaccharide in various samples.