The general structure of amino acids, protein's building blocks, with the constituting groups highlighted: in green, the amino group; in yellow, the carboxyl group; in blue, the alpha carbon; and in red, the R group (a.k.a. side chain), which is the only thing that differentiates one amino acid from another. Drawn in Inkscape. 氨基酸 氨基 酸 氨基酸 (突出显示)