What defines an amino acid?

An amino acid is defined as a compound that contains both an amino group (-NH2) and a carboxyl group (-COOH). This dual presence of functional groups is crucial because it allows amino acids to serve as the building blocks of proteins. Each amino acid links together in a specific sequence through peptide bonds, forming polypeptides and ultimately proteins that perform a multitude of functions within living organisms.

The amino group is responsible for the basic characteristics of the amino acid, while the carboxyl group contributes to the acidic nature of these compounds. The unique side chains (R groups) of amino acids determine their specific properties, influencing how proteins fold and function.

In contrast, carbohydrates are organic molecules made primarily of carbon, hydrogen, and oxygen in specific ratios, while large organic molecules refer to macromolecules like nucleic acids or polysaccharides, which do not share the specific structure and characteristics of amino acids. Simple sugar molecules, or monosaccharides, consist of single units of carbohydrates, again contrasting with the structure of amino acids that specifically contain amino and carboxyl functional groups. Thus, the definition encapsulated in the correct option describes the fundamental chemical structure that distinguishes amino acids from other biomolecules.