Amino Acid
Definition
The building blocks of proteins: small molecules with both an amino group and an acid group. Twenty types combine to make every protein in your body.
Technical definition
An amino acid is an organic molecule containing both an amino (-NH2) and a carboxyl (-COOH) group, plus a variable side chain (R). The 20 standard amino acids link via peptide bonds into proteins, and their sequence determines protein structure and function.
Example
Glycine (simplest, with R = H) and tryptophan (largest, with an indole ring) are both amino acids: same backbone, wildly different side chains.
Common confusion
Amino acids are not acids in the everyday sense. At neutral pH they exist as zwitterions, carrying both positive and negative charges that cancel out.