Chapter 10 Biomolecules Intext Questions NCERT Solutions Class 12 Chemistry- PDF Download
Page No. 290
10.1. 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.
Or,
The solubility of a solute in a given solvent follows the rule ‘like dissolves like’. Glucose contains five and sucrose contains eight -OH groups. These -OH groups form H-bonds with water. As a result of this extensive intermolecular H-bonding, glucose and sucrose are soluble in water. On the other hand, benzene and cyclohexane do not contain -OH bonds and hence do not form H-bonds with water. Moreover, they are non-polar molecules and hence do not dissolve in polar water molecules.
10.2. What are the expected products of hydrolysis of lactose?
Solution
Lactose is composed of β-D galactose and β-D glucose. Thus, on hydrolysis, it gives β-D galactose and β-D glucose.
10.3. How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?
Solution
D-glucose reacts with hydroxylamine (NH2OH) 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 NH2OH to give an oxime.
But pentaacetate of D-glucose does not react with NH2OH. This is because pentaacetate does not form an open chain structure.
Page No. 295
10.4. 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 behaviour.
For this reason, the melting points and the solubility of amino acids in water is higher than those of the corresponding halo-acids.
10.5. 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.
Page No. 301
10.6. Why Vitamin C cannot be stored in our body?
Solution
Vitamin C is water-soluble in nature. It cannot be stored in the human body as it repeatedly gets eliminated through urine.
10.7. What products would be formed when a nucleotide from DNA containing thymine is hydrolysed?
Solution
When a nucleotide from the DNA containing thymine is hydrolyzed, thymine β-D-2-deoxyribose and phosphoric acid are obtained as products.
10.8. When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA?
Solution
A DNA molecule is double-stranded in which the pairing of bases occurs. Adenine always pairs with thymine, while cytosine always pairs with guanine. Therefore, on hydrolysis of DNA, the quantity of adenine produced is equal to that of thymine and similarly, the quantity of cytosine is equal to that of guanine.
But when RNA is hydrolyzed, there is no relationship among the quantities of the different bases obtained. Hence, RNA is single-stranded.