The monomers or small molecules that are used to make proteins are called amino acids. All amino acids share a basic common structure. Off the central or "alpha" carbon will be 4 group, 3 of which are always the same: an amino group (-NH2), a hydrogen, and a carboxyl group (-COOH). *some books will write those as -NH3+ & -COO-, it's essentially the same thing* What makes one amino acid different from another is the 4th group called the "R" group. If there's a -H in that 4th position, for example, then the amino acid is glycine. -CH3 is in alanine, -CH2SH is cysteine, etc. I often make the analogy that an amino acid is like a train car- every train car has the same chassis with identical wheels, connectors, etc, (like the amino and carboxyl groups, etc) but what makes each train car different is what's on top of the chassis (like the R group)- put a large empty container= box car, put a compartment with chairs= passenger car, etc.