(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) Two types of secondary structure

1. $\alpha$-Helix — the polypeptide chain coils into a right-handed spiral, stabilised by hydrogen bonds between the $>C=O$ group of one amino acid residue and the $-NH$ group of the fourth residue along the chain.

Example : keratin in hair and wool.

2. $\beta$-Pleated sheet — the chains lie stretched out side by side and are held together by hydrogen bonds between neighbouring chains, giving a folded, sheet-like arrangement.

Example : fibroin in silk.

Both are held by hydrogen bonds, and both are destroyed on denaturation.

(c) Water soluble vitamins and vitamin C deficiency

Why they must be taken regularly :

The water-soluble vitamins — the B-complex group and vitamin C — dissolve in the aqueous body fluids. Any excess passes into the blood, is filtered out by the kidneys and is readily excreted in the urine.

Because they cannot be stored in the body, a fresh supply is needed every day. A gap in the diet soon produces deficiency symptoms.

Contrast : the fat-soluble vitamins A, D, E and K dissolve in fat and are stored in the liver and adipose tissue, so they need not be supplied daily.

Disease caused by deficiency of vitamin C : Scurvy

Its symptoms include bleeding and spongy gums, loosening of the teeth, slow healing of wounds and general weakness.

Sources of vitamin C : citrus fruits, amla (Indian gooseberry), green leafy vegetables.

OR

(c) Denaturation of protein

Definition : Denaturation is the process in which a protein loses its native three-dimensional structure, 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.

The hydrogen bonds and other weak forces — disulphide and hydrophobic interactions — that hold the folded shape are broken. The globular protein uncoils into a random, fibrous form and becomes biologically inactive.

Structure that survives : the primary structure.

It remains completely intact, because the strong covalent peptide bonds are not broken. The sequence of amino acids is unchanged.

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