What defines amino acids?

Amino acids are defined primarily as simple organic compounds that contain both a carboxyl group (-COOH) and an amino group (-NH2). This dual functionality is fundamental to their structure and classification, allowing them to serve as the building blocks of proteins. The presence of these two functional groups is essential for amino acids' role in forming peptide bonds, which link them together in specific sequences to create proteins.

While other options reference important biochemical components, they do not correctly define amino acids. For instance, nucleic acids are made up of nucleotides, not amino acids. Energy transfer in cells primarily involves ATP and other molecules, rather than amino acids directly. Complex carbohydrates are polysaccharides, which consist of sugar molecules and do not pertain to the definition of amino acids. Thus, the defining chemical structure and the functional roles of amino acids align clearly with the first choice.