Amino acids are building blocks or monomers that make up proteins. They consist of a central alpha carbon atom bonded covalently to an amino group, a carboxyl group, a hydrogen atom and a variable side chain, also called the R group. These are basic monomeric constituents of proteins found in varying amounts depending upon the type of protein. They are classified based on the properties of their side chains or R groups that varies in size, structure and charge. The polarity of the side chains is one of the main basis for this classification.

Amino acids having non-polar, aliphatic side chains include glycine, alanine, proline, valine, leucine, isoleucine and methionine.  Essential amino acids are those that cannot be synthesized de novo in the organism and therefore must be included in the diet. Non-essential amino acids on the other hand, can be synthesized from various precursors. Serine, threonine, aspargine, glutamine & cysteine consist of polar but uncharged side chains. Lysine, Arginine and Histidine- amino acids have positively charged side chains. Aspartic acid and glutamic acid are polar and negatively charged amino acids. Tryptophan, tyrosine and phenylalanine are all essential amino acids having an aromatic side chain.