Advanced Placement (AP) Biology Practice Exam

Amino acids are categorized based on which of the following?

1. Their length

2. Their side chain (R group)

3. Their electrical charge

4. Their ability to form hydrogen bonds

The correct answer is: Their side chain (R group)

Amino acids are categorized based on their side chain, also known as the R group. The R group determines the unique characteristics of each amino acid, influencing its properties, polarity, solubility, and reactivity. There are 20 standard amino acids, and their R groups can be classified into various categories such as nonpolar, polar, acidic, and basic. This classification is critical because the specific properties of the side chains affect how amino acids interact with each other and fold into proteins, ultimately impacting protein structure and function. While the electrical charge of an amino acid's side chain is relevant and can contribute to its categorization, it is still intrinsically linked to the nature of the R group itself. Similarly, the ability to form hydrogen bonds is also associated with specific side chain characteristics rather than serving as a primary basis for categorization. Length does not distinguish between amino acids effectively, as they all share a similar backbone structure, and the variation resides mainly in the side chains. Thus, the most appropriate basis for categorizing amino acids is indeed their side chain.