Advanced Placement (AP) Biology Practice Exam

What defines enantiomers?

1. They have the same structure but different properties

2. They are mirror images of each other

3. They have different molecular formulas

4. They differ in their covalent arrangements

The correct answer is: They are mirror images of each other

Enantiomers are defined as molecules that are non-superimposable mirror images of each other. This means that if you were to place one enantiomer in front of a mirror, the reflection would correspond to the other enantiomer, but the two cannot align perfectly. Enantiomers often have identical physical properties (such as melting and boiling points), but they can interact differently with other chiral substances, including biological molecules, which can lead to significantly different effects in biological systems. The other options describe different types of isomerism or molecular relationships that do not specifically pertain to the defining characteristic of enantiomers. For instance, having the same structure but different properties could apply to other types of isomers that are not necessarily enantiomers. Similarly, enantiomers do not have different molecular formulas; they share the same molecular formula but differ in spatial arrangement. Finally, differing in covalent arrangements typically refers to structural isomers, which is distinct from the concept of enantiomers.