This is due to presence of carboxylic acid as well as amino groups. Amino acids which contain two -COOH group and one —NH2 group are called acidic amino acid, e.g., aspartic acid.
Amino acids which contain two —NH2 group and one -COOH group are called basic amino acids, e.g., lysine.
Amino acids which contain one -COOH and one —NH2 group are called neutral amino acids, e.g., glycine.
Amino acids that cannot be synthesized by the body and must be supplied in the diet are called essential amino acids, e.g., lysine, valine, leucine, etc.
The amino acids which are synthesized by our body are called non-essential amino acids, e.g. alanine, glycine, etc.