Which of the following is the smallest of the twenty amino acids?

Glycine is the smallest of the twenty amino acids due to its unique structure. It has a simple side chain that consists of just a single hydrogen atom, making it the only amino acid that is not chiral (it does not have a stereocenter). This minimal side chain allows glycine to have the smallest molecular size among all amino acids, with a molecular weight of approximately 75 daltons. In contrast, the other amino acids listed have more complex side chains, which contribute to their larger molecular sizes. For instance, alanine has a methyl side chain, serine has a hydroxymethyl side chain, and cysteine contains a thiol group, all of which increase their overall size and complexity compared to glycine.