Answers 1. An amide linkage between the carboxyl group of one amino acid and the amine group of the other amino acid. 2. An ester is a compound formed from the reaction of a carboxylic acid and an alcohol. ( I apologise, I cant figure out number 3.. )
The other option that would make the most sense would be electrons, however the mass of an electron is so small that is basically negligible, so it's not included in the atomic mass.
