(a) Essential amino acids

Definition : Amino acids that cannot be synthesised by the human body and must therefore be obtained from the diet are called essential amino acids.

Ten of the twenty amino acids found in proteins are essential.

Example : Valine (any of leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, methionine, histidine or arginine is equally acceptable)

(b) Zwitter ionic form of an amino acid

In aqueous solution the acidic $-COOH$ group transfers its proton to the basic $-NH_2$ group of the same molecule, producing a species that carries both a positive and a negative charge but is electrically neutral overall :

$$H_2N-\underset{|}{\overset{R}{C}}H-COOH \rightleftharpoons \overset{+}{H_3}N-\underset{|}{\overset{R}{C}}H-COO^-$$

This dipolar ion is the zwitter ion.

Its consequence : the zwitter ion is amphoteric — it reacts with acids through its $-COO^-$ group and with bases through its $-NH_3^+$ group. The high melting points and low solubility of amino acids in organic solvents also follow from their existing as these salt-like ions.

(c) Fibrous vs globular proteins

(Any two differences were required.)

OR

(c) Denaturation of protein

Definition : Denaturation is the process in which a protein loses its native three-dimensional shape, and with it its biological activity, when subjected to a change in physical or chemical conditions such as heating or a change in pH.

Structures destroyed : the secondary and tertiary structures are destroyed.

The weak forces holding these levels together — mainly hydrogen bonds, along with disulphide and other interactions — are broken, so the globular protein uncoils into a random, fibrous form.

Structure that survives : the primary structure remains intact, because the strong covalent peptide bonds are not broken. The sequence of amino acids is therefore unchanged.

Everyday examples : the coagulation of egg white on boiling, and the curdling of milk.