BIOMOLECULES

1. What are biomolecules ? Give examples.

Ans : Biomolecules are those molecules which interact with each other and constitute the molecular logic of life processes.
Carbohydrates, proteins, nucleic acids, lipids, vitamins etc. Study of biomolecules is called biochemistry.

2. Most of the carbohydrates have a general formula Cx(H2O)y and were considered as hydrates of carbon from where the
name carbohydrate was derived. Comment on the statement.
Ans: Most of the carbohydrates like glucose , fructose , galactose having formula C6H12O6 fit into this general formula but
there are certain compounds like acetic acid (CH3COOH ) and formaldehyde (HCHO) which are not carbohydrates still fit into
this formula. Rhamnose( C6H12O5) is a carbohydrate but does not fit into this definition of carbohydrates.

3. Define carbohydrates.
Ans: Chemically the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds
which produce such units on hydrolysis. Carbohydrates are also called saccharides. (In Greek sakchron means sugar)

4. Classify carbohydrates on the basis of physical properties.

Sugars Non sugars
Crystalline in nature Amorphous in nature
Soluble in water Insoluble in water
Sweet in taste Tasteless
Glucose, fructose, Starch, cellulose, glycogen
galactose, sucrose,
lactose, maltose
Mono saccharides and Polysaccharides are non
disaccharides are sugars sugars

5. Classify carbohydrates on the basis of their behavior on hydrolysis.

Ans : a) Monosaccharides: These are simplest sugars and cannot be hydrolyzed further into smaller units. About twenty
monosaccharides occur in nature. Example glucose galactose ribose, fructose
b) Oligosaccharides: These get hydrolyzed to give two to 10 monosaccharide units. They are further classified as
disaccharides, trisaccharides, tetrasaccharides depending upon the number of monosaccharide units they give on hydrolysis.
Sucrose ,maltose and lactose are examples of disaccharides.
c) Polysaccharides: These get hydrolyzed to give a large number of monosaccharide units. All polysaccharides are non sugars.
Some common examples are starch, cellulose, glycogen, gums etc.

6. Classify carbohydrates on the basis of reducing nature.

Reducing sugars Non reducing sugars
These carbohydrates reduce tollen’s reagent These carbohydrates do not reduce tollen’s
and give a silver mirror reagent and do not give a silver mirror
These carbohydrates reduce Fehling’s These carbohydrates do not reduce Fehling’s
reagent and give a brick red ppt of Cu2O reagent and do not give a brick red ppt of
Cu2O
They have at least one anomeric carbon They do not have any anomeric carbon atom
atom free that is not involved in glycosidic free. The anomeric carbon atoms are
bond formation. They have a free functional involved in glycosidic bond formation. They
group. do not have free functional group.
All monosaccharides are reducing sugars All polysaccharides are non reducing sugars
Among disaccharides maltose and lactose Among disaccharides sucrose is non reducing
are reducing in nature in nature.
7. Differentiate glucose and fructose.
D-(+)- Glucose D-(-)-Fructose
Contains aldehyde functional group so it is also called Contains ketone functional group so it is also called keto
Aldo hexose hexose
Dextro rotatory with a dextro rotation of + 52.5 degree Laevorotatory with a laevorotation of -92.4 degree.
First carbon atom is anomeric Second carbon atom is anomeric
Six membered ring structure resembling pyran ring Five membered ring structure resembling furan ring

8. What are the similarities between glucose and fructose.

Ans : Both glucose and fructose are monosaccharides, Reducing sugars that are soluble in water and sweet in taste. Both are
crystalline and have a molecular formula C6H12O6 . Glucose and fructose are functional group isomers of each other

9. Classify monosaccharides on the basis of functional group present in them.

Carbon atoms General term Aldehyde Ketone
3 Triose Aldo triose (Glyceraldehyde) Keto triose (Dihydroxyacetone)
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose (Ribose) Ketopentose
6 Hexose Aldohexose(Glucose) Ketohexose(Fructose)
7 Heptose Aldoheptose ketoheptose

10. Which reactions of glucose cannot be explained by the open chain structure of glucose?
Ans : a) In spite of having aldehyde group glucose does not give Schiff’s test and it does not form the hydrogen sulfite
addition product with NaHSO3.
b) The Penta acetate of glucose does not react with hydroxylamine indicating the absence of free aldehyde group.
c) Glucose exists in two different crystalline forms which are α and β forms. The α Form ( melting point 419 K) is obtained by
crystallization from concentrated solution of glucose at 303 K . The β form ( melting point 423 K ) is obtained by crystallization
from hot and saturated aqueous solution at 371 K.
d) Mutarotation : The spontaneous change in the optical rotation of α and β forms of glucose to an equilibrium value of 52.5
degrees is called muta rotation.

11. How is glucose prepared?

Ans : a) From Sucrose ( Cane sugar ): C12H22O11 + H2O ---------→ C6H12O6 + C6H12O6
Glucose Fructose
b) From starch : (C6H10O5)n + nH2O ---------→ nC6H12O6
Starch 393 K 2-3 atm
12. How do you prove that
a) Glucose contains six carbon atoms linked in a straight chain: One prolonged heating with a strong reducing agent like
HI ,it forms n-hexane

b) Glucose contains carbonyl group: Glucose gives cynohydrin on reaction with hydrogen cyanide. Glucose gives an
oxime on reaction with hydroxylamine.
 c) Glucose contains aldehyde group: On reaction with mild oxidizing agent like bromine water glucose gets oxidized too
gluconic acid.

d) Glucose contains five alcohol groups: On reaction with acetic anhydride glucose gives Penta acetate.

e) Glucose contains only one primary alcohol group: On reaction with a strong oxidizing agent like nitric acid glucose as
well as gluconic acid both give saccharic acid.

13. Differentiate d/l and D/L forms

d/l forms D/L forms
d and l Forms signify the specific rotation of Optically D and L forms Signify the relative configuration with
active isomers. d means dextrorotatory and l means respect to the last asymmetric carbon atom
laevorotatory.
Dextro rotatory substances rotate the plane of polarized All naturally occurring carbohydrates have D configuration
light towards right direction whereas laevo rotatory whereas all naturally occurring amino acids have L
substances rotate the plane of polarized light towards left configuration
direction

14. Draw the structures of D(+) glyceraldehyde , D(+) Glucose , D(+) Gluconic acid , D(+)Saccharic acid

15. What are Anomers ? Draw the structures of anomers of Glucose and label the anomeric carbon atom.
Ans: Anomers are the isomers which differ from each other only at the configuration of one carbon atom. Anomeric carbon
atom is the carbon atom which carries the functional group before cyclization. After cyclization, when hemiacetal structure is
formed , the configuration around Anomeric carbon atom decides the alpha and beta isomers. Number 1 carbon atom
Labeled below is anomeric.
16. Draw the Haworth structures of D(+) Glucose . Why are these called pyranose structures ?
Ans : The Haworth structures of glucose are called pyranose structures Because of their resemblance with Pyran ring
.

17. Draw the Haworth structures of D(-) Fructose . Why are these called furanose structures ?
Ans : The Haworth structures of Fructose are called furanose structures Because of their resemblance with Furan ring

18. Draw the open chain structure of Fructose ? In spite of having ketone group why does it reduce Tollen and Fehling’s
reagents?
Ans : Fructose is an α- hydroxy ketone that tautomerises to an aldehyde easily so it reduce
Tollen and Fehling’s reagents
19. Draw the structure of Sucrose ( Cane Sugar ) ? Why is it non reducing ?
Ans : The structure of Sucrose involves a glycosidic linkage between C1 of α -D-(+) glucopyranose and C2 of β-D-(-)
fructofuranose. Sucrose does not reduce Tollen and Fehling’s reagents because Of the absence of free functional groups.
The anomeric carbon atoms of both the units of monosaccharides are involved in glycosidic bond formation.

20. Draw the structure of Maltose ( Malt Sugar ) ? Why is it reducing ?

Ans : The str. of Maltose involves a glycosidic linkage between C1 of α -D-(+) glucopyranose and C4 of another α -D-(+)
glucopyranose. Maltose reduces Tollen and Fehling’s reagents because Of the presence of free functional group. The
anomeric carbon atom of second unit of monosaccharide is not involved in glycosidic bond formation.

21. Draw the structure of Lactose ( Milk Sugar ) ? Why is it reducing ?

Ans : The str. of Lactose involves a glycosidic linkage between C1 of β -D-(+) galactopyranose and C4 of β -D-(+) glucopyranose
Lactose reduces Tollen and Fehling’s reagents because Of the presence of free functional group. The anomeric carbon atom
of second unit of monosaccharide is not involved in glycosidic bond formation.

22. Why is Sucrose called Invert Sugar ?

Ans : Sucrose is dextro rotatory but after hydrolysis gives dextro rotrotary glucose and levo rotatory fructose. Since the laevo
rotation of fructose( -92.40 ) is more than dextro rotation of glucose (+ 52.5o ) , the mixture is laevo rotatory. Thus the
hydrolysis of sucrose brings about a change in the sign of rotation from dextro to laevo so sucrose is called invert sugar.
23. What are the two components of starch? State differences between them.
• Starch is the stored polysaccharide in plants, mainly found in cereals [ formula (C6H10O5)n ]
• The two components of starch are Amylose and Amylopectin
Amylose Amylopectin
Linear polymer of α-D(+) glucose units. Branched polymer of α-D(+) glucose units.
Soluble in water because of linear structure Insoluble in water because of branched structure
Constitutes about 15% to 20% of starch Constitutes about 80% to 85% of starch
Contains C1- C4 glycosidic linkage Contains C1- C4 as well as C1- C6 glycosidic linkage

24. Differentiate starch and cellulose

Starch Cellulose
α links are present ie. α- D glucose units are joined to β links are present ie. β- D glucose units are joined to
each other by glycosidic bonds each other by glycosidic bonds
Amylose is linear whereas Amylopectin is branched Linear chain structure
Can be digested by human beings and forms an Cannot be digested by human beings and constitutes
important dietary source the cell wall of plant cells
Both starch and Cellulose have the same molecular formula, both are polysaccharides non reducing Carbohydrates

25. Glucose or sucrose are soluble in water but cyclohexane or benzene are insoluble in water. Explain.
Ans: Glucose or sucrose contains many OH groups which makes them polar. They are soluble in water because of
intermolecular hydrogen bonding with water molecules. Cyclohexane or benzene are typically non polar so they are insoluble
in water in accordance with the principle of like dissolves like.

26. How do you explain the absence of aldehyde group in the Penta acetate of D glucose ?
Ans : Due to the formation of hemiacetal six membered ring , the Penta acetate of glucose shows the absence of aldehyde
group. The aldehyde group and the OH group at C-5 get involved in ring formation

27. What are the expected products of hydrolysis of the following disaccharides ( C12H22O11 ) ?
Ans : a) Sucrose : α -D-(+) glucopyranose and β-D-(-) fructofuranose
b) Maltose: α -D-(+) glucopyranose and α -D-(+) glucopyranose
c) Lactose : β -D-(+) galactopyranose and β -D-(+) glucopyranose

28. 29. What are the expected products of hydrolysis of the following polysaccharides (C6H10O5)n ?
Ans : a) Starch : many units of : α -D-(+) glucopyranose
b) Glycogen: many units of : α -D-(+) glucopyranose
c) Cellulose: many units of : β-D-(+) glucopyranose

29. What is glycogen?

• Glycogen is the stored polysaccharide in animal body having formula (C6H10O5)n
• The structure of glycogen resembles that of amylopectin but it is more highly branched so glycogen is called
animal starch.
• Glycogen is present in liver ,muscles and brain. During starvation, enzymes breakdown glycogen into glucose
and the body gets energy.
• Glycogen is also found in yeast and fungi

30. Draw the structure of Amylose and Amylopectin .

31. Draw the structure of Cellulose .

32. Explain the significance of carbohydrates.

• Carbohydrates are energy giving foods
• Starch is the stored food material in plants and glycogen is a stored food material in animals
• Aldopentoses namely β- D- Ribose and 2-deoxy- β- D- Ribose All present in RNA and DNA respectively
• Honey and fructose are natural sweeteners
• Cellulose is a structural component of cell wall of plants.

33. Define the following:

Ans : a) Glycosidic bond : It is an oxide linkage formed between two monosaccharide units joined together after the loss of a
water molecule. It is responsible for the formation of oligosaccharides and polysaccharides
b) Peptide bond: It is an amide linkage formed between two alpha amino acids joined together after the loss of a water
molecule . It is responsible for the formation of dipeptides, polypeptides and proteins etc.
c)Phosphodiester bond : It is a diester linkage formed between two nucleotides joined together after the loss of a water
molecule. It is responsible for the formation of polynucleotide chains of DNA and RNA.
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34. What are proteins ? Give their sources.

• All proteins are polymers of α amino acids
• Proteins are important biomolecules responsible for muscle building
• Proteins occur in milk, cheese, pulses, peanuts ,fish and meat.
• The word protein is derived from the Greek word “proteios” which means primary or of prime importance

35. What are α amino acids?

Ans : α amino acids contain amino group and carboxylic acid functional groups attached to the same carbon atom. Alpha
amino acids are the hydrolysis products of all proteins.
36. What are essential and nonessential amino acids. Give examples.
Ans: The amino acids which can be synthesized in the body are known as nonessential amino acids. For example glycine,
alanine, glutamic acid, aspartic acid, tyrosine.
The amino acids which cannot be synthesized in the body and must be obtained through diet are called essential amino acids.
For example Valine, Leucine, isoleucine, arginine, lysine, methionine phenylalanine.

37. Classify amino acids on the basis of the relative number of amino acid and carboxylic groups present in them
Ans : Equal number of amino and carboxylic groups makes an amino acid neutral. For example glycine, alanine and valine
More number of amino groups than carboxylic groups makes an amino acid basic. For example lysine and Arginine.
More number of carboxylic acid groups than amino groups makes an amino acid acidic. For example Aspartic acid and
glutamic acid.

38. What is meant by a native protein?

Ans : Protein found in a biological system with a unique three dimensional structure and biological activity is called native
protein.

39. How does denaturation of proteins occur? What is the effect of denaturation on the structure of proteins?
• Denaturation of protein occurs when proteins are subjected to physical change like changes in temperature or
chemical changes like changes in pH
• During denaturation, hydrogen bonds are broken and the protein loses its biological activity
• The secondary and tertiary structures are destroyed but primary structure remains intact
• The Globules unfold and the helix get uncoiled.
• The coagulation of egg white on boiling and curdling of milk caused due to the formation of lactic acid by
bacteria are examples of denaturation of proteins.

40. Where does the water present in the egg go after boiling the egg?
Ans: After boiling the egg the proteins get denatured and the water present in the egg gets vaporized. The vapors get
adsorbed on the coagulated proteins. The secondary and tertiary structures are destroyed but primary structure remains
intact. The Globules unfold and the helix get uncoiled. The hydrogen bonds are broken and the protein loses its biological
activity

41. Differentiate polypeptides and proteins

Ans : When More than 10 amino acid residues are joined to each other by peptide bonds, the products obtained are called
polypeptides. A polypeptide with more than 100 amino acid residues having molecular mass higher than 10,000 U is called a
protein. The distinction between a polypeptide and a protein is not very sharp. Polypeptides with fewer amino acids are
called proteins if they ordinarily have a well defined conformation of a protein such as insulin which has 51 amino acids.

42. What is meant by primary structure of a protein?

Ans : The number and specific sequence of α amino acids linked to each other in a polypeptide chain is called the primary
structure of a protein. Any change in the primary structure creates a different protein.

43. What is meant by a secondary structure of a protein?

Ans: There are two kinds of secondary structures in proteins:
a) α Helix : In this, a polypeptide chain forms all possible hydrogen bonds by twisting into a right handed screw or helix
with -NH- group of each amino acid residue hydrogen bonded to the >C=O of an adjacent turn of the helix
b) β pleated : In this, all polypeptide chains are stretched out to nearly maximum extension and then laid side by side
which are held together by intermolecular hydrogen bonds. The structure resembles the pleated folds of drapery.

44. What is meant by tertiary structure of a protein?

Ans : There are two kinds of tertiary structures in proteins:
a) Fibrous proteins: These proteins are formed when the polypeptide chains run parallel and are held together by
hydrogen and disulphide bonds. These proteins are insoluble in water. For example keratin present in hair,nails, horns
and wool. Fibroin present in silk and myosin present in muscles.
 b) Globular proteins: These proteins are formed when the polypeptide chains coil around each other to give a spherical
shape. These proteins are soluble in water. For example insulin, Haemoglobin and albumin. All enzymes are globular
proteins.

45. What is meant by a Quaternary structure of a protein ?

Ans : It is the most complex structure of a protein. The Spatial Arrangement of two or more polypeptide chains (called
Subunits) with respect to each other is called the Quaternary structure of a protein.

46. List the main forces that stabilize the secondary and tertiary structures of proteins
• Hydrogen bonds
• Disulfide linkages
• Vanderwaal forces of attraction
• Electrostatic forces of attraction

47. Which is the only amino acid that is optically inactive? Why is it so named?
Ans : Glycine is optically inactive. It is so named because of its sweet taste. In Greek glykos means sweet.

48. Define a dipeptide . Write the structure of dipeptide Gly-Ala. Label the peptide bond .
Ans : A dipeptide contains two amino acids linked to each other by a
peptide bond .

49. Why do amino acids show amphoteric behavior?

• The amphoteric behavior of amino acids is due to the
presence of both acidic carboxyl group and basic amino group
in the same molecule.
• In aq. solution the carboxylic group loses a proton and amino
group accepts a proton giving rise to a dipolar ion called Zwitter
ion.( Ampholyte)
• In Zwitter ionic form amino acids can react with both acids
and bases.

50. List the physical properties of amino acids. Account for this behavior of amino acids
Ans: a) Amino acids are usually colorless, crystalline solids .
b)These are water soluble, high melting solids and behave like salts rather than simple amines or carboxylic acids.
c) This behavior is due to the presence of both acidic and basic groups in the same molecule.
d) Amino acids exist as Zwitter ions, which is neutral but contains both positive and negative charges.
e) Amino acids show amphoteric behavior and can react with both acids and bases.

52. Why is tyrosine so named ?

Ans : The amino acid tyrosine was first obtained from cheese ( in Greek tyros means cheese)

53. The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids
Explain.
Ans: This behavior is due to the presence of both acidic and basic groups in the same molecule.
Amino acids exist as Zwitter ions, which is neutral but contains both positive and negative charges. Halo acids are typically
covalent and dissolve in water due to hydrogen bonding.
 Most of the naturally occurring amino acids
have L configuration. L- Amino acids are
represented by writing the NH2 group on left
hand side.

54. What are enzymes? Give their characteristic features

Ans : Enzymes are biochemical catalysts that lower the activation energy of biochemical reactions. For example the activation
energy for acid hydrolysis of Sucrose is 6.22 KJ/mol but the enzyme sucrase lowers this value to 2 .15 KJ/ mole
All enzymes are globular proteins
Enzymes are very specific for a particular reaction and for a particular substrate.
Enzymes work under optimum temperature and pH conditions.
Enzymes are needed only in very small quantities for the progress of a reaction
55. What are the criteria for naming enzymes?
Ans: 1) Enzymes are sometimes named after the reaction which they catalyze. For example the enzyme oxidoreductase
catalyzes the oxidation of one substrate with simultaneous reduction of another substrate.
2) Enzymes are generally named after the compound or the substrate upon which they work. For example the enzyme
maltase catalyzes the hydrolysis of mild toes into glucose.
3) The name of an enzyme often ends with -ase.

56. What are vitamins?

Ans: Vitamins are organic compounds required in the diet in small amounts to perform specific biological functions for normal
maintenance of optimum growth and health of the organism. Excess of vitamins is harmful and vitamin pills should not be
taken without the advice of doctor. The term “vitamine” was coined from the word vital + amine since most of the earlier
identified compounds had amino groups. But now it is known that many vitamins do not contain amino groups so the letter e
has been dropped and the term vitamin is used these days.

57. Classify vitamins on the basis of their solubility in water or fat

Ans : a) Fat soluble vitamins: Vitamins which are soluble in fat and oils but insoluble in water are called fat soluble vitamins
These are vitamins A, D ,E and K. They are stored in liver and adipose fat storing tissues.

b)Water soluble vitamins: These vitamins are soluble in water. For example vitamin B and C . These vitamins must be
supplied regularly in diet because they are readily excreted in urine and cannot be stored in our body. Vitamin B12
however can be stored in the body.

58. What are nucleic acids? Why are they called polynucleotides?
Ans : Nucleic acids are biomolecules present in the nucleus of the cell and are components of chromosomes. Since nucleic
acids are long chain polymers of nucleotides so they are called polynucleotides. Example DNA and RNA.

59. Differentiate nucleotide and nucleoside.

Nucleoside + phosphate group = Nucleotide
Nucleoside (a) Nucleotide (b)
It has two parts: It has three parts:
Heterocyclic nitrogenous base Heterocyclic nitrogenous base
Pentose sugar Pentose sugar
Phosphate moiety
It is formed by the attachment When nucleoside is linked to
of a Heterocyclic nitrogenous phosphoric acid at 5’ position of
base to 1’ position of pentose pentose sugar moiety, we get a
sugar nucleotide

60. State the structural and functional differences between DNA and RNA

DNA RNA
Structural Differences
The structure of DNA is a double stranded helix The structure of RNA is single stranded. Sometimes in the
secondary structure the RNA single stranded helix is folded
back on itself.
The Pentose sugar present in DNA is 2-deoxy- β- D- Ribose The pentose sugar present in RNA is β- D- Ribose.
The heterocyclic nitrogenous bases present are Adenine (A) The heterocyclic nitrogenous bases present are Adenine (A)
Guanine (G) , Cytosine (C) and Thymine(T) Guanine (G) , Cytosine (C) and Uracil(U)
Functional Differences
DNA is responsible for heredity ie. transmission of RNA is responsible for protein synthesis.
characters from one generation to another.
Another function of DNA is DNA replication

61. How are nucleotides joined together in a polynucleotide chain?

Ans : Nucleotides are joined together by
phosphodiester linkage between 5’ and 3’
carbon atoms of the pentose sugar.

62. Name the three types of RNA Found in the cell.

Ans : mRNA : Messenger RNA
tRNA : Transfer RNA
rRNA: Ribosomal RNA
63. What is meant by the primary structure of nucleic acids?
Ans : The number and specific sequence of nucleotides linked to each other in a
polynucleotide chain is called the primary structure of Nucleic acid.

64. The two strands of DNA are not identical but are complementary. Explain .
OR
Explain the secondary structure of DNA. ( Diagram →)
Ans : James Watson and Francis Crick gave a double stranded Helix structure for DNA.
• Two nucleic acid chains are wound about each other and held together by hydrogen
bonds between pairs of bases.
• The two strands are complementary to each other because the hydrogen bonds are
formed between specific pairs of bases in accordance with Ervin Chargoff
“complementary base pairing rule”.
• Adenine forms two hydrogen bonds with thymine whereas cytosine forms three
hydrogen bonds with guanine.
• Each turn of the DNA double helix contains 10 base pairs.
• The two polynucleotide strands run in anti parallel directions.

65. 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 is single stranded.

66. What products would be formed when a nucleotide from DNA containing thymine is
hydrolyzed ?
Ans : 1) Heterocyclic nitrogenous base : Thymine
2) Pentose sugar: 2-deoxy- β- D- Ribose
3) Phosphate moiety

67. What is meant by dinucleotide ?

Explain the formation of a dinucleotide and label the phosphodiester linkage.
Ans : Two nucleotides joined to each other by a phophodiester linkage gives a dinucleotide.