An Amino Acid is any organic molecule made up of an amine group (2 molecules of hydrogen and one of nitrogen and a carbolic acid functional group (two atoms of oxygen with one each of carbon and hydrogen).  There are about 500 such molecules.  Of these, 22 are used by life forms on Earth, including humans, as the constiutent building blocks of proteins.

In the cell, DNA in the nucleus creates strands of RNA that code for protein.  Each group of three base pairs on an RNA strand binds to a particular amino acid.  In the ribosomes of the cell, the RNA is matched up with the amino acids, then are joined together to make the proteins.  

Amino acids also play other important roles in metabolism, acting as transmitters that allow cellular communication.

Of the 22 amino acids needed for human life, most can be produced through other processes in the body.  However, a large group must be obtained through the diet.  Animal meat contains all the necessary amino acids.  All amino acids can be obtained from plant sources, but this requires a fairly varied diet as no one plant source contains all the necessary amino acids.  For example, rice and legumes both are lacking necessary amino acids, but together they can provide all the necessary ones.  

Amino acid at Wikipedia

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