Chapter 14 Biomolecules Solutions
Question - 11 : - Write two main functions of carbohydrates in plants.
Answer - 11 : -
Two main functions of carbohydrates in plants are:
(i) Polysaccharidessuch as starch serve as storage molecules.
(ii) Cellulose,a polysaccharide, is used to build the cell wall.
Question - 12 : - Classify the following into monosaccharides anddisaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose andlactose
Answer - 12 : -
Monosaccharides:
Ribose, 2-deoxyribose, galactose, fructose
Disaccharides:
Maltose, lactose
Question - 13 : - What do you understand by the term glycosidic linkage?
Answer - 13 : -
Glycosidic linkage refers to the linkage formed betweentwo monosaccharide units through an oxygen atom by the loss of a watermolecule.
For example, in a sucrose molecule, two monosaccharideunits, ∝-glucose and β-fructose, are joined together by aglycosidic linkage.
Question - 14 : - What is glycogen? How is it different from starch?
Answer - 14 : -
Glycogen is a carbohydrate (polysaccharide). In animals,carbohydrates are stored as glycogen.
Starch is a carbohydrate consisting of two components −amylose (15 − 20%) and amylopectin (80 − 85%).
However, glycogen consists of only one component whosestructure is similar to amylopectin. Also, glycogen is more branched than amylopectin.
Question - 15 : - What are the hydrolysis products of (i) sucroseand (ii) lactose?
Answer - 15 : -
(i) Onhydrolysis, sucrose gives one molecule of ∝-D glucoseand one molecule of β- D-fructose.
(ii) Thehydrolysis of lactose gives β-D-galactose and β-D-glucose.
Question - 16 : - What is the basic structural difference between starch andcellulose?
Answer - 16 : -
Starch consists of two components − amylose andamylopectin. Amylose is a long linear chain of ∝−D−(+)−glucoseunits joined by C1−C4 glycosidic linkage (∝-link).
Amylopectin is a branched-chain polymer of ∝-D-glucose units, in which the chain is formed by C1−C4glycosidic linkage and the branching occurs by C1−C6 glycosidic linkage.
On the other hand, cellulose is a straight-chainpolysaccharide of β-D-glucose units joined by C1−C4 glycosidic linkage(β-link).
Question - 17 : - What happens when D-glucose is treated with the followingreagents?
(i) HI (ii) Brominewater (iii) HNO3
Answer - 17 : -
(i) WhenD-glucose is heated with HI for a long time, n-hexane is formed.
(ii) WhenD-glucose is treated with Br2 water, D-gluconic acid is produced.
(iii) Onbeing treated with HNO3, D-glucose get oxidised to give saccharic acid.
Question - 18 : - Enumerate the reactions of D-glucose which cannot beexplained by its open chain structure.
Answer - 18 : -
(1) Aldehydesgive 2, 4-DNP test, Schiff’s test, and react with NaHSO4 toform the hydrogen sulphite addition product. However, glucose does not undergothese reactions.
(2) Thepentaacetate of glucose does not react with hydroxylamine. This indicates thata free −CHO group is absent from glucose.
(3) Glucoseexists in two crystalline forms − ∝andβ. The ∝-form (m.p. = 419 K) crystallises from a concentratedsolution of glucose at 303 K and the β-form (m.p = 423 K) crystallises from ahot and saturated aqueous solution at 371 K. This behaviour cannot be explainedby the open chain structure of glucose.
Question - 19 : - Define the following as related to proteins
(i) Peptidelinkage (ii) Primary structure (iii) Denaturation.
Answer - 19 : -
(i) Peptide linkage:
The amide formed between −COOH group of onemolecule of an amino acid and −NH2 group of anothermolecule of the amino acid by the elimination of a water molecule is called apeptide linkage.
(ii) Primary structure:
The primary structure of protein refers to the specificsequence in which various amino acids are present in it, i.e., the sequence oflinkages between amino acids in a polypeptide chain. The sequence in whichamino acids are arranged is different in each protein. A change in the sequencecreates a different protein.
(iii) Denaturation:
In a biological system, a protein is found to have aunique 3-dimensional structure and a unique biological activity. In such asituation, the protein is called native protein. However, when the nativeprotein is subjected to physical changes such as change in temperature orchemical changes such as change in pH, its H-bonds are disturbed. Thisdisturbance unfolds the globules and uncoils the helix. As a result, theprotein loses its biological activity. This loss of biological activity by theprotein is called denaturation. During denaturation, the secondary and thetertiary structures of the protein get destroyed, but the primary structureremains unaltered.
One of the examples of denaturation of proteins is thecoagulation of egg white when an egg is boiled.
Question - 20 : - What are the common types of secondary structure of proteins?
Answer - 20 : -
There are two common types of secondary structure ofproteins:
(i) ∝-helixstructure
(ii) β-pleatedsheet structure
∝– Helix structure:
In this structure, the −NH group of an amino acidresidue forms H-bond with the 
groupof the adjacent turn of the right-handed screw (∝-helix).
β-pleated sheet structure:
This structure is called so because it looks like thepleated folds of drapery. In this structure, all the peptide chains arestretched out to nearly the maximum extension and then laid side by side. Thesepeptide chains are held together by intermolecular hydrogen bonds.