So basically, this carbon forms a bond with this nitrogen shown in green, and that is our peptide bond between these two amino acids A and B. Now, whenever we react a certain type of reactants, and whenever a chemical reaction takes place, we know that the number of atoms is always conserved because we have the conservation of mass. Now, let's take a look at this carbon and then compare to this carbon. In this particular case, the carbon contains a single oxygen atom. But in this particular case, the carbon contains one two oxygen atoms. So somewhere in this reaction, there was a loss of an oxygen atom.