Amino acids are composed of a carboxyl group (-COO) and an amine group (NH2) and a functional group is attached to them. These functional groups are specific for every amino acid, and they impart different properties to them. Amino acids combine together to make peptide chains and proteins. They are organic compounds, and there exist 20 amino acids overall. These 20 amino acids are divided into two categories which are essential and non-essential amino acids. Among the 20 amino acids, nine of them are essential amino acids and eleven of them are non-essential amino acids. Figure.1. Structure of Amino AcidEssential amino acidsThe humans are unable to do the de novo production of these amino acids inside their body, so they are called as essential amino acids. These amino acids are also referred as proteinogenic amino acids. These amino acids should be taken by the food that one’s eat. The nine essential amino acids are as follows:Phenylalanine: The abbreviation of Phenylalanine is Phe or F and its chemical formula is C9H11NO2. It contains phenyl group which is placed at the terminal hydrogen of alanine. It is non-polar in nature.Valine: The abbreviation of Valine is Val or V. It is alpha-amino acid and is branch-chained. It is non-polar in nature.Threonine: The abbreviation of threonine is Thr or T. It has alcohol as the functional group and alpha-carboxylic acid group. It is polar in nature.Tryptophan: The abbreviation of tryptophan is Trp or W. Its functional group is alcohol, and it has both alpha amino and carboxylic acid group. It is non-polar and aromatic in nature.Methionine: Methionine contains sulfur because its functional group is S-methyl thioether side chain. It has both alpha carboxylic and amino groups. It is also written as Met or M. Leucine: Leucine has isobutyl side chain and has both alpha nature carboxylic and amino group. It is also written as L or Leu. The compound is non-polar in nature.Isoleucine: It contains both alpha carboxylic acid and an amino group. It is non-polar and aliphatic in nature. It is abbreviated as Ile or me.Lysine: Lysine is an alanine and has a lysyl side chain. It is polar in nature and is positively charged. It is also represented as K or Lys.Histidine: The functional group of histidine is imidazole. It is positively charged and alpha carboxyl and amino group. It is also represented as H or His.There are also amino acids which become essential amino acids under some circumstances, and these amino acids are called conditional essential amino acids. They are not synthesized by humans in case of some pathological condition and in some infants. These amino acids are as follows:Arginine: The abbreviation of arginine is Arg or R. It's functional group is a linear chain of 3 carbon atoms which has a cap of guanidinium. It is positively charged.Cysteine: Cysteine contains sulfur, and it has a thiol functional group. It acts as a nucleophile and is similar to serine. It is also written as Cys or CGlycine: Glycine is also written as Gly or G. It is the smallest amino acid having only a hydrogen as its functional group. It is ambivalent and achiral.Glutamine: Its side chain contains amide as a functional group and has an alpha amino group and a carboxyl group. It is polar in nature. It can also be represented as Gln or Q.Proline: Pyrrolidine is the sidechain of this amino acid. It is aliphatic and is nonpolar in nature. It is written as Pro or P.Tyrosine: The side chain of tyrosine is tyrosyl. It can be written as Tyr or Y. Tyrosine is polar in nature.All of the above-mentioned amino are used in processes in the human body and have important applications.