1

### 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.
2

### 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.
3

### 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.
4

### AIPMT 2004

A sequence of how many nucleotides in messenger RNA makes a codon for an amino acid?
A
Three
B
Four
C
One
D
Two

## Explanation

There are four bases in mRNA, i.e., adenine, cytosine, guanine and uracil. They form triplets and each triplet behave as a code for the synthesis of a particular amino acid.