Chapter 14 Biomolecules CBSE Class 12 Chemistry Notes

Chapter 14 Biomolecules Class 12 Chemistry Notes For CBSE Students

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

Ans: For any compound to be water soluble, it should develop dipoles (partial negative and partial positive charges) at the two ends of compound. The development of charges results in the formation of hydrogen bond between the water molecule and the compound. The development of charges at two ends is due to the difference in the electronegativity between two atoms

A glucose molecule contains five –OH groups while a sucrose molecule contains eight –OH groups . Thus sucrose and glucose undergo extensive H- bonding with water.

Hence, these are soluble in water.

Benzene (C6H6) and cyclohexane (C6H12) are hydrocarbons which don’t have any polar group. They, therefore, don’t dissolve in water since there is hardly any scope of hydrogen bonding in their molecules with those of H2O (water).

14.2 What are the expected products of hydrolysis of lactose?

Ans: The hydrolysis of lactose (disaccharide) can be done either with dilute HC1 or with enzyme emulsin. D-glucose and D-galactose are the products of hydrolysis. Both of them are monosaccharides with the molecular formula C6Hi206.

hydrolysis of lactose

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

Ans: Glucose, as we know is an aldohexose and it is expected to give the characteristic reactions of the aldehydic group e.g., action with NH2OH, HCN, Tollen’s reagent, Fehling reagent etc.  However, the pentadactyl glucose formed by the acylation of glucose with acetic anhydride does not give these reactions.

Thus, glucose contains an aldehydic group. However, when glucose is reacted with acetic anhydride, the -OH group at C-l along with the other -OH groups at C-2, C-3, C-4 and C-6 form a pentaacetate.

Since the penta acetate of1 glucose does not contain a free -OH group at C-l, it cannot get hydrolysed in aqueous solution to produce open chain aldehydic form and hence glucose pentaacetate does not react with NH2OH to form glucose oxime.


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

Ans:  a-amino acids as we all know, are dipolar in nature (N+H3-CHR-COO–) and have strong dipolar interactions.  On the contrary, the haloacids RCH(X)COOH are not dipolar like a-amino acids.

The amino acids exist as zwitter ions, H3N+ — CHR-COO-. Due to this dipolar salt like character, they have strong dipole-dipole attractions. Therefore, their melting points are higher than corresponding haloacids which do not have salt-like character.

Due to their salt-like character, amino acids interact strongly with water.

As a result, their solubility in water is higher than corresponding haloacids which do not have a salt-like character.

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

Ans: When egg is boiled, proteins first undergo denaturation.

Denaturation of proteins is a process that changes the physical and biological properties of proteins without affecting the chemical composition of protein.

When an egg is boiled in water, the globular proteins present in it change to a rubber like insoluble mass which absorbs all the water present in the egg by making hydrogen bond with it.

14.6 Why cannot vitamin C be stored in our body?

Ans: Vitamin C is mainly ascorbic acid which is water-soluble.

Humans cannot synthesize vitamin C due to the deficiency of a enzyme L- gulonolactone oxidase and also vitamin C is rapidly absorbed from the intestine. Because it is water soluble vitamin it is not stored in the body to a significant extent. As a result, it is readily excreted in the urine.

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

Ans: DNA polymer forms a duplex structure consisting of 2 strands of polynucleotide chains coiled around each other in the form of a double helix. The nucleotide form the backbone of DNA structure.

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

  • 2-deoxy-D(-)ribose.
  • To pyrimidine i.e., guanine (G) and adenine (A).
  • Two purines, i.e., thymine (T) and cytosine (C) and
  • Hosphoric acid.

14.8 When RNA is hydrolyzed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA?

Ans: RNA (Ribose nucleic acid) has a structure similar to DNA except that it is a single-strand structure.

A DNA molecule has two strands in which the four complementary bases pair each other, i.e., cytosine (C) always pair with guanine (G) while thymine (T) always pairs with adenine (A). Thus, when a DNA molecule is hydrolysed, the molar amounts of cytosine is always equal to that of guanine and that of adenine is always equal to thymine.

However, this is not seen when RNA is subjected to hydrolysis. This suggests that RNA has not a double-strand structure like DNA. It exists as a single strand.

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