Advanced Placement (AP) Biology Practice Exam

Which component is NOT part of an amino acid structure?

1. Alpha carbon

2. Amino group

3. Phosphate group

4. Carboxyl group

The correct answer is: Phosphate group

The structure of an amino acid is characterized by several key components, which include the alpha carbon, an amino group, a carboxyl group, and a variable side chain or R group. The alpha carbon is the central carbon atom to which these groups are attached. The amino group, consisting of one nitrogen atom and two hydrogen atoms (-NH2), is a defining feature of amino acids, making them the building blocks of proteins. The carboxyl group (-COOH) also plays a critical role in the acid-base properties of amino acids, contributing to their behavior in biochemical reactions. In this context, the phosphate group is not part of the structure of an amino acid. Phosphate groups are typically found in molecules such as nucleotides and phospholipids, which are important for cellular energy transfer and membrane structure, respectively. This distinguishes them clearly from the basic structure of amino acids. Hence, the correct answer indicates that the phosphate group is an unrelated component and does not belong in the context of amino acid structure.