Question

The presence of what group differentiates most amino acids from each other?

Answer 1

 Amino acids are the basic structural unit of all proteins. A 'free' amino acid (a single amino acid) always has: 
an amino group -NH2, 
a carboxyl group -COOH 
a hydrogen -H 
a chemical group or side chain -"R". 
Since the amino acids (except glycine) have 4 different groups attached to the a-carbon, they are optically active stereoisomers (recall 23.101 Organic Chemistry) although only L-isomers are found in proteins. 
At a neutral pH, both the amino and the carboxyl groups are ionised, giving what is termed the zwitterionic