1

### AIPMT 2005

Which functional group participates in disulphide bond formation in proteins?
A
Thioester
B
Thioether
C
Thiol
D
Thiolactone

## Explanation

Disulphide bond may be reduced to thiol by means of reagents i.e., NaBH4 , which shows the presence of thiol group in disulphide bond formation.
2

### AIPMT 2005

The cell membranes are mainly composed of
A
fats
B
proteins
C
phospholipids
D
carbohydrates

## Explanation

Cell membranes (Plasma membranes) constitutes bilayer of phospholipid with embedded proteins. In humans, lipids accounts for upto 79% of cell membrance.
3

### AIPMT 2004

Which of the following structures represents the peptide chain?
A
B
C
D

## Explanation

In peptide linkage i.e., (– CONH) group, the carboxyl group of one amino acid molecule forms an amide by combination with the amino group of the next amino acid molecule with the liberation of water molecule.
4

### AIPMT 2004

The helical structure of protein is stablised by
A
dipeptide bonds
B
hydrogen bonds
C
ether bonds
D
peptide bonds

## Explanation

The $\alpha$-helix structure is formed when the chain of $\alpha$-amino acids coils as a right handed screw (called $\alpha$-helix) because of the formation of hydrogen bonds between amide groups of the same peptide chain, i.e., NH group in one unit is linked to carbonyl oxygen of the third unit by hydrogen bonding. This hydrogen bonding between different units is responsible for holding helix in a position.