Amino acids are small, organic molecules with an average molecular weight of 135 daltons. They are the fundamental building blocks of proteins and enzymes. Through dehydration synthesis, amino acids bind via peptide bonds to form the primary structure of proteins. The twenty standard amino acids share the common characteristic of an amino group and a carboxylic acid group bound to a central carbon, however are uniquely identified by the nature of their side chains. Small molecules are organic, biologically active molecules that can be neutral or artificial. The majority of drugs are small molecules because most proteins cannot pass through cellular membranes and are degraded upon oral ingestion.