NCERT Solutions for Class 12 Chemistry Chapter 14 - Biomolecules

Question 1:

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


Glucose and sucrose are polar in nature and hence are soluble in polar water. On the other hand, cyclohexane and benzene are non-polar and hence are not soluble in water.

Question 2:

What products are expected when lactose is hydrolysed?


Lactose on hydrolysis gives β-D-galactose and β-D-glucose.

Question 3:

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


The pentaacetate of glucose does not react with hydroxylamine (NH2OH) which shows that aldehyde (–CHO) group is not present in glucose pentaacetate.

Question 4:

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


This is due to dipolar or zwitter ion structure of amino acids

Therefore, like ionic compounds, amino acids have strong electrostatic attractions and hence have high melting points and are highly soluble in water.

Question 5:

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


When an egg is boiled in water, the water present in egg is used in denaturation of protein probably through H-bonding. In this process, the globular protein in egg changes to rubber like insoluble mass.

Question 6:

Why vitamin C cannot be stored in our body?


Vitamin C cannot be stored in our body because it is soluble in water and is readily excreted in urine and cannot be stored in our body.

Question 7:

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


2­-deoxy-D-ribose, thymine and 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?


This suggests that there are different types of RNA molecules which contain different quantities of bases.

Question 9:

What are monosaccharides?


Monosaccharides are polyhydric aldehydes and ketones which cannot be hydrolysed into simpler carbohydrates. The monosaccharides with aldehydic group

–CHO) are called aldoses while those with ketonic group ( == O) are called ketoses. For example, glucose, fructose, ribose, etc.

Question 10:

What are reducing sugars?


The sugars which can reduce Tollen’s reagent or Fehling’s solution are called reducing sugars. They contain a characteristic group either aldehydic group (–CHO) or α-ketol group (–CO–CH2OH). For example, glucose, galactose, fructose are reducing sugars.

Question 11:

Write two main functions of carbohydrates in plants.

  1. Carbohydrates (except cellulose) work as body fuels and act as the main source of energy.
  2. The polysaccharide cellulose acts as chief structural material in the cell walls of the plant cells.
Question 12:

Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.


Ribose : monosaccharide
2-Deoxyribose : monosaccharide
maltose : disaccharide
galactose : monosaccharide
fructose : monosaccharide
lactose : disaccharide

Question 13:

What do you understand by the term glycosidic linkage?


The condensation of hydroxyl groups of two monosaccharides to form a link between them is called glycosidic linkage.

Question 14:

What is glycogen? How is it different from starch?


Glycogen is a polysaccharide found in all animal cells mainly in muscles and liver. It serves as a reserve carbohydrate of animals and hence is known as animal starch. It is a polymer of about thousand glucose units arranged in the form of highly branched chain.
On the other hand, starch is a principal food storage of glucose energy. Starch is a polymer of α-D-glucose consisting of two components amylose and amylopectin.

Question 15:

What are the hydrolysis products of (i) sucrose and (ii) lactose?


(i) Glucose and fructose

Question 16:

What is the basic structural difference between starch and cellulose?


Question 17:

What happens when D-glucose is treated with the following reagents?
(i) HI (ii) Bromine water (iii) HNO3.


(i) When glucose is treated with HI, it forms n-hexane, suggesting that all the six carbon atoms are linked in a straight line.

Question 18:

Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.


Open structure of D-glucose could not explain the following reactions :

  1. Despite having the aldehyde group, glucose does not give Schiffs test and 2, 4-DNP test.
  2. Glucose does not react with sodium hydrogen sulphite to form addition product.
  3. The pentaacetate of glucose does not react with hydroxyl amine showing the absence of free –CHO group.
  4. When glucose is heated with methanol in the presence of dry HCl gas, it forms two isomeric monomethyl derivatives known as α-D-glucoside (m.p. = 165°C) and β-D-glucoside (m.p = 107°C). Since only one molecule of methanol is used for the formation of methyl glucoside, these must be hemiacetals.

These results show that glucose does not have open chain form structure.

Question 19:

What are essential and non-essential amino acids? Give two examples of each type.


The amino acids which can be made by our bodies and are not required in our diet are called non-essential amino acids. For example, glycerine and alanine.
The amino acids which cannot be made by our bodies and must be supplied in our diet are called essential amino acids. For example, valine and leucine.

Question 20:

Define the following as related to proteins:

  1. Peptide linkage
  2. Primary structure
  3. Denaturation.
  1. Peptide linkage. Peptide bond is formed by the condensation of two or more same or different α-amino acids. The condensation occurs between amino acids with the elimination of water. In this case, the carboxyl group of one amino acid and amino group of another amino acid get condensed with the elimination of water molecule.

Question 21:

What are the common types of secondary structure of proteins?


Common types of secondary structures are:

  1. α-helix structure
  2. β-pleated structure
Question 22:

What type of bonding helps in stabilising the α-helix structure of proteins?


Hydrogen bonding between —NH and —C = O groups of peptide bonds stabilise the α-helix structure.

Question 23:

Differentiate between globular and fibrous proteins.


Question 24:

How do you explain the amphoteric behaviour of amino acids?


Due to dipolar or zwitter ion structure, amino acids are amphoteric in nature. The acidic character of the amino acids is due to the — NH3+ group and the basic character is due to the —COO_ group as shown below:

Question 25:

What are enzymes ?


The enzymes are biological catalysts produced by the living cells which catalyse biochemical reactions in living organisms. Chemically enzymes are naturally occurring simple or conjugate proteins. For example, maltose, amylose, invertase, etc.

Question 26:

What is the effect of denaturation on the structure of proteins?


During denaturation, the protein molecule uncoils from an ordered and specific conformation into a more random conformation. Denaturation does not change the primary structure of protein but results from a rearrangement of secondary and tertiary structures.

Question 27:

How are vitamins classified? Name the vitamin responsible for the coagulation of blood.


Vitamins are generally classified into two broad types based on the solubility :

  1. Water soluble vitamins. The vitamins which are soluble in water are called water soluble vitamins. For example, vitamins B group (B-complex), vitamin C, etc. The water soluble vitamins are stored in much lesser amounts in the cells. Water soluble vitamins must be supplied regularly in diet because they are readily excreted in urine and cannot be stored (except vitamin B12<.sub>) in our body.
  2. Fat soluble vitamins. The vitamins which are soluble in fats are called fat soluble vitamins. For example, vitamins, A, D, E and K. Liver cells are rich in fat soluble vitamins such as vitamin A and vitamin D.
    This group of hydrophobic, lipid soluble vitamins are not absorbed in the body unless fat digestion and absorption proceed normally. Their deficiency can cause malabsorptive disease. Excess intake of these vitamins may cause hypervitaminoses.
Question 28:

Why are vitamin A and vitamin C essential to us? Give their important sources.


Vitamin A is essential for us because its deficiency causes xerophthalmia (hardening of cornea of eye) and night blindness.
Vitamin C is essential for us because its deficiency causes scurvy (bleeding of gums) and pyorrhea (loosening of teeth and bleeding of gums).
Source of vitamin A : Fish liver oil, carrot, butter, milk, etc.
Source of vitamin C : Citrus fruits, amla, green vegetables, tomato, orange, etc.

Question 29:

What are nucleic acids? Mention their important functions.


Nucleic acids are biomolecules which are found in the nuclei of all living cells in the form of nucleoproteins or chromosomes (proteins containing nucleic acids as the prosthetic group).

Nucleic acids are of two types :

  1. Deoxyribonucleic acid (DNA)
  2. Ribonucleic acid (RNA).

The important functions of nucleic acids are :

  1. Replication. It is the property of a molecule to synthesise the other molecule.
  2. Protein synthesis. The genetic information coded in DNA in the form of specific base sequences has to be translated and expressed in the form of synthesis of specific proteins which perform various functions in the cells. This involves the steps transcription and translation.

Question 30:

What is the difference between a nucleoside and a nucleotide?


A nucleoside contains only two basic components of nucleic acids namely a pentose sugar and a nitrogenous base.

A nucleotide contains all the three basic components of nucleic acids namely a phosphoric acid group, a pentose sugar and a nitrogenous base.

Question 31:

The two strands in DNA are not identical but are complimentary. Explain.


The bases of one strand of DNA are paired with bases of other strand through hydrogen bonding. The hydrogen bonding is very specific because structures of heterocycles allow only one mode of pairing. The two strands of DNA are complementary to each other because the sequence of bases in one strand automatically determines that of the other.

Question 32:

Write the important structural and functional differences between DNA and RNA.


The important differences between DNA and RNA are :

Question 33:

What are different types of RNA found in the cell?


The different types of RNA found in the cells are:

  1. messenger RNA : m-RNA
  2. transfer RNA : t-RNA
  3. ribosomal RNA : r-RNA.