Biomolecules

Chemistry

NCERT

1   Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.

Solution :

A glucose molecule contains five $-OH$ groups while a sucrose molecule contains eight $-OH$ groups. Thus, glucose and sucrose undergo extensive H-bonding with water.$\\$ Hence, these are soluble in water.$\\$ But cyclohexane and benzene do not contain $-OH$ groups. Hence, they cannot undergo $H-$ bonding with water and as a result, are insoluble in water.

2   What are the expected products of hydrolysis of lactose?

Solution :

Lactose is composed of $beta-D$ galactose and $\beta-D$ glucose. Thus, on hydrolysis, it gives $\beta-D$ galactose and $\beta-D$ glucose.$\\$ $\underset{\text{Lactose}}{C_{12}H_{22}O_{11}+H_2O} \to$$\underset{\text{D-(+)-Glucose}}{C_6H_{12}O_6}+$$\underset{\text{D-(+)-Galactose}}{C_6H_{12}O_6}$

3   How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?

Solution :

D-glucose reacts with hydroxylamine $(NH _2 OH)$ to form an oxime because of the presence of aldehydic $(-CHO)$ group or carbonyl carbon. This happens as the cyclic structure of glucose forms an open chain structure in an aqueous medium, which then reacts with $NH _2 OH$ to give an oxime.$\\$ But pentaacetate of D-glucose does not react with $NH _2 OH$. This is because pentaacetate doesnot form an open chain structure.

4   The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain.

Solution :

Both acidic (carboxyl) as well as basic (amino) groups are present in the same molecule of amino acids. In aqueous solutions, the carboxyl group can lose a proton and the amino group can accept a proton, thus giving rise to a dipolar ion known as a zwitter ion.$\\$ Due to this dipolar behaviour, they have strong electrostatic interactions within them and with water. But halo-acids do not exhibit such dipolar behavior.$\\$ For this reason, the melting points and the solubility of amino acids in water is higher than those of the corresponding halo-acids.

5   Where does the water present in the egg go after boiling the egg?

Solution :

When an egg is boiled, the proteins present inside the egg get denatured and coagulate. After boiling the egg, the water present in it is absorbed by the coagulated protein through H-bonding.