Q.Tryptophan is called essential amino acid because
a. It contains aromatic nucleus.
b. It is present in all the human proteins.
c. It cannot be synthesised by human body.
d. It is essential constituent of enzymes.

Amino acids are the monomer building blocks of proteins. Roughly 20 different amino acids occur naturally, and every one shares the same basic skeleton: a central (alpha-) carbon bonded to an amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and a variable side chain (the 'R group') that gives each amino acid its distinct chemical identity. Because an amino acid carries both an acidic group and a basic group, it is amphoteric - able to act as an acid or a base - and typically exists as a zwitterion (a dipolar ion with both a positive and a negative charge, and zero net charge) at its isoelectric point. Based on the chemical nature of the R group, amino acids are classified as acidic, basic, polar (uncharged) or non-polar; among the basic amino acids, arginine's guanidinium side chain gives it the highest s …