(a) The tertiary structure of proteins represents overall folding of the polypeptide chains i.e., further folding of the secondary structure.

It gives rise to two major molecular shapes viz. fibrous and globular. The main forces which stabilise the 2° and 3° structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction.

(b) Essential amino acid - The amino acids which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids. Example: Isoleucine, Arginine, Lysine, etc.

(c) Disaccharides - Carbohydrates that yield two monosaccharide units, on hydrolysis, are called Disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage.

For example, sucrose on hydrolysis gives one molecule each of glucose and fructose whereas maltose gives two molecules of glucose only.