4.1 amino acids are small organic molecules containing an amine and a carbonyl group

An amino acid is a small, organic molecule consisting of an amine group, a carboxylic acid and a side chain specific to each individual amino acid. The structure of an amino acid is shown below.

There are twenty naturally occurring (L) amino acids that generally make up the primary structure of a protein. These vary in structure by the side chain R. The exact nature of side chain R—structure, $p{K}_a$ , hydrophobicity etc.—has an enormous impact on the way the peptide chain folds into the three-dimensional protein.

Amino acids are zwitterions, with a positive charge on the amine group and a negative charge on the carbonyl group

deeper: protein primary structure for how amino acids join together

see: 4.1a A zwitterion contains both a positive and a negative charge for more on zwitterions

see: amino acid categories for how amino acids are generally classified