Proteins are made up of polypeptide chains which are formed by the polymerization of amino acid molecules, linked together by peptide bonds.
The amino acids can be broadly classified into non-polar and polar.
The non-polar amino acids can be defined as those amino acids whose side chains possess hydrocarbon molecules only. They do not possess any charge and are incapable of forming hydrogen bonds with water molecules. They tend to avoid water and so they tend to remain buried into the interior of a protein. Examples can be illustrated as Leucine, Valine, Isoleucine, Phenylalanine, etc.
The polar amino acids can be defined as those amino acids whose side chains either possess a charge (positive or negative) or a hydroxyl group which is capable of forming hydrogen bonds with water molecules. As they tend to form hydrogen bonds with water molecules they are usually found on the surface of the proteins. Examples can be illustrated as Lysine, Arginine, Serine, Asparagine, etc.

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