AAMC FL Practice Exam 2025 – Complete Prep Test

A fatty acid is structurally characterized by having a long hydrocarbon chain that culminates in a carboxyl group (-COOH) at one end. This configuration is important because the carboxyl group is polar and hydrophilic, while the hydrocarbon chain is nonpolar and hydrophobic, giving fatty acids their amphipathic properties. This dual nature is critical for their role in forming lipid bilayers in cell membranes and for energy storage in the form of triglycerides.

The other options describe different molecules: a long chain of amino acids is indicative of proteins, a three-carbon chain with hydroxyl groups refers to a sugar alcohol or a carbohydrate, and a glycerol molecule with phosphate denotes phospholipids or other lipids, but none of these options represent the defining structure of a fatty acid. Thus, the characterization of a fatty acid is accurately captured by the presence of a hydrocarbon chain connected to a carboxyl group at one end.