Why out of 20 amino acids only nine amino acids are classified as essential amino acids ?

• Amino acids play central roles both as building blocks of proteins and as intermediates in metabolism. 
• The 20 amino acids that are found within proteins convey a vast array of chemical versatility. 
• The precise amino acid content, and the sequence of those amino acids, of a specific protein, is determined by the sequence of the bases in the gene that encodes that protein.
•  The chemical properties of the amino acids of proteins determine the biological activity of the protein.
•  Proteins not only catalyze all (or most) of the reactions in living cells, they control virtually all cellular processes.
•  Also, proteins contain within their amino acid sequences the necessary information to determine how that protein will fold into a three-dimensional structure, and the stability of the resulting structure. 
• The field of protein folding and stability has been a critically important area of research for years and remains today one of the great unsolved mysteries. It is, however, being actively investigated, and progress is being made every day.

As we learn about amino acids,

          It is important to keep in mind that one of the more important reasons to understand amino acid structure and properties is to be able to understand protein structure and properties. We will see that the vastly complex characteristics of even a small, relatively simple, protein are a composite of the properties of the amino acids which comprise the protein.

Essential amino acids

• Humans can produce 10 of the 20 amino acids.
•  The other amino acids must be supplied with food. 
• Failure to obtain enough of even 1 of the 10 essential amino acids, those that we cannot make, results in degradation of the body's proteins—muscle and so forth—to obtain the one amino acid that is needed.
•  Unlike fat and starch, the human body does not store excess amino acids for later use—the amino acids must be in the food every day.
• The 10 amino acids that we can produce are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.
•  Tyrosine is produced from phenylalanine, so if the diet is deficient in phenylalanine, tyrosine will be required as well. 
• The essential amino acids are arginine (required for the young, but not for adults), histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. 
• These amino acids are required in the diet. Plants, of course, must be able to make all the amino acids. 
• Humans, on the other hand, do not have all the enzymes required for the biosynthesis of all of the amino acids.
• You might have wondered I told you only nine amino acids are essential but it is applicable only for adults whereas the young does have ten essential amino acids including arginine. 

• The first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet.
• They isolated a compound in asparagus that was subsequently named asparagine, the first amino acid to be discovered.

All the best friends life is awaiting wonderful ideas and contributions from you.

Regards K.B.Dharun Krishna