Biomolecules TEXTBOOK Solution

Question 1.

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


Glucose or sucrose contain several hydroxyl groups in their molecules which form hydrogen bonding with water molecules due to which they dissolve in water. On the other hand compounds like benzene or cyclohexane cannot form hydrogen bonds with water molecules, so they are insoluble in water.

Question 2.

What products are expected when lactose is hydrolysed ?


Lactose (C12H22O11) on hydrolysis with dilute acid yields an equimolar mixture of D-glucose and D-galactose.

NCERT Solutions for Class 12 Chemistry Chapter 14 Biomolecules 1

Question 3.

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


The cyclic hemiacetal form of glucose contains an OH group at C-1 which gets hydrolysed in the aqueous solution to produce the open chain aldehydic form which then reacts with NH2OH to form the corresponding oxime.

Therefore, glucose contains an aldehydic group. On the other hand, when glucose is reacted with acetic anhydride, the OH group at C-1, along with the four other OH groups at C-2, C-3, C-4 and C-6 form a pentaacetate.

As the pentaacetate of glucose does not contain a free OH group at C-1, it cannot get hydrolysed in aqueous solution to produce the open chain aldehydic form and thus glucose pentaacetate does not react with NH2OH to form glucose oxime. Hence, glucose pentaacetate does not contain the aldehdye group.

Question 4.

The melting points and solubility in water of amino acids are higher than those of the corresponding haloacids. Explain.


The amino acids exist as zwitter ions, H3N+ OHR – COO. Because of this dipolar salt like character they have strong dipole- dipole attractions. So, their melting points are higher than halo acids which do not have sail like character.

Moreover, due to this salt like character, they interact strongly with H2O. Thus, solubility of amino acids in water is higher than that of the corresponding halo acids which do not have salt like character.

Question 5.

Where does the water present in the egg go after boiling the egg.


The boiling of an egg is a common example of denaturation of proteins present in the white portion of an egg.

The albumin present in the white of an egg gets coagulated when the egg is boiled hard. The soluble globular protein present in it is denatured resulting in the formation of insoluble fibrous protein.

Question 6.

Why vitamin C cannot be stored in our body?


Vitamin C is a water-soluble vitamin. Water-soluble vitamins when supplied regularly in the diet cannot be stored in out body because they are readily excreted in urine.

Question 7.

What products would be formed when a nucleotide from DNA containing thymine is hydrolysed ?


When a nucleotide from DNA containing thymine is completely hydrolysed, the products obtained are :

  1. 2-deoxy-D(-)ribose.
  2. two pyrimidine i.e., guanine (G) and adenine (A).
  3. two purines, i.e., thymine (T) and cytosine (C) and
  4. phosphoric acid.

Question 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 ?


A DNA molecule has two strands in which the four complementary bases pair each other, viz. cytosine (C) always pairs with guanine (G) while thymine (T) always pairs with adenine (A).

Therefore, when a DNA molecule is hydrolysed the molar amount of cytosine is always equal to that of guanine and that of adenine is always equal to that of thymine RNA also contains four bases, the first three are same as in DNA but the fourth one is uracil (U).

As in RNA there is no relationship between the quantities of four bases (C, G, A and U) obtained, therefore, the base-pairing principle, viz., (A) pairs with (U) and (C) pairs with (G) is not followed. So, unlike DNA, RNA has a single strand.