Biomolecules

14.8

A DNA is a double-stranded molecule (two polynucleotide chains whose nitrogenous bases are connected by hydrogen bonds).

In this molecule pairing or connection of bases occurs. Adenine always pairs with thymine, while cytosine always pairs with guanine.

So when DNA is hydrolyzed the amount of adenine produced is exactly equal to the amount of thymine produced, similarly, the amount of cytosine produced is equal to that of guanine.

But as given when RNA is hydrolyzed, there is no relationship between the quantities of different bases obtained. This suggests that RNA is a single-stranded molecule.

14.7

When a nucleotide from DNA containing thymine is hydrolyzed β-D-2 deoxyribose and phosphoric acid are obtained.

14.6

Vitamin C (ascorbic acid due to extensive –H bonding with water due to the presence of –OH group) is a water-soluble vitamin in contrast to vitamin A, D, E and K which are fat soluble. Also, humans cannot synthesize it due to the lack of specific enzyme and it is rapidly absorbed from the intestine.

Because it is water-soluble, it is not stored in our body to a significant amount but is readily excreted in the urine.

14.5

Eggs contain protein and proteins are a polymer in amino acids. When an egg is boiled the protein present inside the egg is denatured (the process of loss of biological activity of protein like their ability of –H bonding when subjected to a physical change like a change in temperature, pH) and coagulated (the process of liquid changing to a solid or semi-solid state). Due to this, the water present in the egg is absorbed by the coagulated protein by –H bonding.

14.3 

D-glucose reacts with hydroxylamine (NH₂OH) to form oxime due to the presence of the aldehyde functional group (-CHO). This is due to the cyclic structure of glucose which forms an open chain structure in an aqueous medium, which then reacts to give an oxime.

But in case of pentaacetate of D-glucose, it does not form open chain structure in an aqueous medium so it does not react with NH₂OH.

14.2

Lactose is a disaccharide carbohydrate (made up of two monosaccharide units) composed of β-D-galactose and β-D-glucose units. Hydrolysis breaks the glycosidic bond converting sucrose into β-D- galactose and β-D-glucose.

NOTE: But however, this reaction is so slow that it takes years for the solution of sucrose to undergo negligible change. Hence an enzyme called sucrase is added to proceed rapidly.

14.1 

Glucose and sucrose are carbohydrates (optically active polyhydroxy aldehydes or ketones). 
Structure of glucose:

Structure of sucrose:

As you can see both the compounds have five –OH and eight –OH groups respectively. These –OH groups are responsible for the extensive hydrogen bonding with water. This –H bonding is responsible for the solubility of glucose and sucrose in water.

In case of cyclohexane or benzene (simple six-membered ring compounds), they do not contain any – OH groups. Hence, they cannot undergo –H bonding with water and are insoluble.