Question

Which one consists of essential amino acids?
(a) Tryptophan and glutamic acid
(b) lysine and phenylalanine
(c) Leucine and glycine
(d) Valine and histidine

Answer 1

Amino acids are classified into essential, non-essential, and semi-essential amino acids. Essential amino acids are those which cannot be synthesized within our body so they must essentially be obtained from food. They are also termed as indispensable amino acids.

Complete step by step answer:
There are 20 major or standard amino acids. Of the 20 standard amino acids, some are interconvertible. Hence they can be produced in the human body in sufficient quantities. So they need not essentially be obtained from food. They are termed as non- essential amino acids or dispensable amino acids. Some others cannot be synthesized within our body so they must be obtained essentially from our food. They termed essential amino acids.Examples of essential amino acids are:Valine,lysine,leucine,histidine,phenylalanine,isoleucine,threonine,tryptophan etc.
Examples of non- essential amino acids are Alanine, proline, hydroxyproline, aspartic acid, glutamic acid, etc.
So Lysine and phenylalanine fall under the category of essential amino acids.
So, the correct answer is ‘Lysine and phenylalanine’.

Additional information:
- More than 300 kinds of amino acids have already been identified. But not all of them serve as constituents of proteins. So amino acids fall under two major categories, proteinogenic and non- proteinogenic
- Proteinogenic amino acids enter into the formation of proteins, but non- proteinogenic are not incorporated with proteins.
- Proteinogenic amino acids are in turn classified into major and rare.
- Major amino acids are 20 in number. They are generally known as standard, primary or normal amino acids.
- Major amino acids are classified into essential, non- essential, and semi- essential amino acids.
- Those amino acids which are partly obtained from food are known as semi- essential amino acids.
- Examples of semi- essential amino acids are Arginine, cysteine, glycine, tyrosine, serine, etc.

Note:
- Amino acids are organic carboxylic acids in which at least one carbon atom of the hydrocarbon chain is replaced by an amino group.
- They are the monomeric structural unit or building blocks of protein.
- Each protein has its own structural features and functional properties, determined by nature, number, and order of amino acids.