Chapter : Biomolecules

Top Concepts:

1. Definition of carbohydrates: Polyhydroxy aldehydes or polyhydroxy

ketones or compounds which give these on hydrolysis.

2. Classification of carbohydrates:

a. Monosaccharides

 Simplest carbohydrates

 Cannot be hydrolysed into simpler compounds

 E.g. Glucose, mannose

b. Oligosaccharides

 Carbohydrates which give 2 to 10 monosaccharide units on hydrolysis

 E.g. Sucrose, Lactose, Maltose

c. Polysaccharides

 Carbohydrates which on hydrolysis give large number of monosaccharide

units.

 E.g. Cellulose, starch

3. Anomers: Such pairs of optical isomers which differ in configuration only

around C1 atom are called anomers. E.g. -D-glucopyranose and -D-
glucopyranose

4. Epimers: Such pairs of optical isomers which differ in configuration

around any other C atom other than C1 atom are called epimers. E.g. D-
glucose and D- mannose are C2 epimers.
5. Preparation of glucose (also called dextrose, grape sugar):

a. From sucrose:

H
C12H22O11  H2O  C6H12O6  C6H12O6

Sucrose Glu cos e Fructose

b. From starch:

C6H10O5  n  nH2O 
H
393K ;2 3 atm
 nC6H12O6

Sucrose Glu cos e

or
cellulose

6. Structure elucidation of glucose:

 COOH
|
HNO3
Glucose 
(C HOH)4 
 Thi s in dicates the presence of
|
COOH primary alcoholic group in glu cos e

Saccharic acid

CHO
|
5 (CH3CO)2 O
Glu cos e 
ZnCl
 (C HOCOCH3 )4  5 CH3COOH 
 Shows the presence
2 |
CH2OCOCH3 of 5  OH groups

Penta acetyl glu cos e

Unusual reactions of glucose: (Presence of ring structure)

Glucose does not give Schiff’s test and does not react with sodium bisulphite
and NH3.

Pentaacetyl glucose does not react with hydroxyl amine. This shows the
absence of –CHO group and hence the presence of ring structure.

7. Cyclic structure of glucose:

8. Cyclic structure of fructose:

9. Reaction of Glucose with Phenyl hydrazine: Formation of Osazone:

Glucose and fructose gives the same osazone because the reaction takes
place at C1 and C2 only.
10. Lobry De Bruyn Van Ekenstein Rearrangement:

CHO CH2OH CHO H  C  OH

| | | ||
NaOH NaOH
H  C OH   C|  O
       HO  C|  H
         C OH
| |
(CHOH)3 (CHOH)3 (CHOH)3 (CHOH)3
| | | |
C H2OH C H2OH C H2OH C H2OH

D-Glucose D-Fructose D-Mannose ene diol

 Same results are obtained when fructose or mannose is reacted with an

alkali.

 It is because of form of ene - diol intermediate.

 It explains why fructose acts as a reducing sugar.

11. Mutarotation: The spontaneous change in specific rotation of an

optically active compound is called mutarotation. E.g. -D-glucose (m.p. =
146°C) with specific rotation []D = +111° and -D-glucose (m.p. = 150°C)
with specific rotation []D =+19.2° When either form is dissolved in water &
allowed to stand, the specific rotation of the solution changes slowly and
reaches or constant value of +52.5°.

-D-glucose 
    open chain form     -D-glucose
sp. rotⁿ = 111° sp. rotⁿ = +52.5° sp. rotⁿ = +19°

12. Haworth projection: Representation for ring structure:

6 membered ring is called pyranose ring.

5 membered ring is called furanose ring.

How to write the projection: Groups projected to the right in the Fischer
projection are written below the plane and those to the left are written above
the plane.

Haworth projection of Glucose:

Haworth projection for Fructose:
13. Glycosidic linkage: The oxide linkage formed by the loss of a water
molecule when two monosaccharides are joined together through oxygen
atom is called glycosidic linkage.

14. Sucrose (invert sugar): Sucrose is dextrorotatory but on hydrolysis it

gives dextrorotatory & laevorotatory and the mixture is laevorotatory.

H
C12H22O11 + H2O  C6H12O6 + C6H12O6

Sucrose D – glucose D – fructose

[]D = + 66.5° []D = +52.5° []D = -92.4°

Sucrose is a non-reducing sugar because the two monosaccharide units are

held together by a glycosidic linkage between C1 of -glucose and C2 of -
fructose. Since the reducing groups of glucose and fructose are involved in
glycosidic bond formation, sucrose is a non-reducing sugar.

Haworth Projection of Sucrose:

15. Maltose: Maltose is composed of two -D-glucose units in which C1 of
one glucose is linked to C4 of another glucose unit. The free aldehyde group
can be produced at C1 of second glucose in solution and it shows reducing
properties so it is a reducing sugar.
H O
Maltose: Maltose 2 
 Glucose + Glucose

C1  C1 

Haworth projection of maltose:

16. Lactose (Milk sugar): It is composed of β-D-galactose and
β-D-glucose. The linkage is between C1 of galactose and C4 of

glucose. Hence it is also a reducing sugar.

H O
Lactose 2 
 Glucose + Glucose

C4 β C1 β

Haworth projection of lactose:

17. Starch: It is a polymer of -glucose and consists of two components —
Amylose and Amylopectin.

Amylose Amylopectin

Water soluble component Water insoluble component

It is a long unbranched chain polymer It is a branched chain polymer

Contains 200 – 1000 -D-(+)- It is a branched chain polymer of -

glucose units held by  - glycosidic D-glucose units in which chain is
linkages involving C1 – C4 glycosidic formed by C1–C4 glycosidic linkage
linkage whereas branching occurs by C1–C6
glycosidic linkage.

Constitutes about 15-20% of starch Constitutes about 80- 85% of starch

18. Amino acids:

R  CH  COOH
| Where R – Any side chain
NH2

Most naturally occurring amino acids have L – Config.

COOH COOH

H NH2 [D] H2N H [L]

R R

19. Essential amino acids: Those amino acids which cannot be synthesised
in the body and must be obtained through diet, are known as essential amino
acids. Example: Valine, Leucine

20. Non- essential amino acids: The amino acids, which can be
synthesised in the body, are known as non-essential amino acids. Example:
Glycine, Alanine
21. Zwitter ion form of amino acids: Amino acids behave like salts rather
than simple amines or carboxylic acids. This behaviour is due to the presence
of both acidic (carboxyl group) and basic (amino group) groups in the same
molecule.
In aqueous solution, the carboxyl group can lose a proton and amino group
can accept a proton, giving rise to a dipolar ion known as zwitter ion. This is
neutral but contains both positive and negative charges.
In zwitter ionic form, amino acids show amphoteric behaviour as they react
both with acids and bases.
 
OH  OH 
R  C H  COOH     
 R  C H  COO     
 R  C H  COO
| H | H |
 
NH3 NH3 NH2

zwitter ion

22. Isoelectronic point: The pH at which the dipolar ion exists as neutral
ion and does not migrate to either electrode cathode or anode is called
isoelectronic point.

23. Proteins: Proteins are the polymers of  -amino acids and they are
connected to each other by peptide bond or peptide linkage. A polypeptide
with more than hundred amino acid residues, having molecular mass higher
than 10,000u is called a protein.

24. Peptide linkage: Peptide linkage is an amide linkage formed by

condensation reaction between –COOH group of one amino acid and –NH2
group of another amino acid.

25. Primary structure of proteins: The sequence of amino acids is said to

be the primary structure of a protein.

26. Secondary structure of proteins: It refers to the shape in which long

polypeptide chain can exist. Two different types of structures:

a.  – Helix:

 Given by Linus Pauling in 1951

 It exists when R- group is large.

 Right handed screw with the NH group of each amino acid residue H –
bonded to – C = O of adjacent turn of the helix.
  Also known as 3.613 helix since each turn of the helix has
approximately 3.6 amino acids and a 13 – membered ring is formed by
H – bonding.

 C = O and N – H group of the peptide bonds are trans to each other.

 Ramchandran angles ( and ) -  angle which C makes with N – H

and  angle which C makes with C = O.

b.  – pleated sheet:

 Exists when R group is small.

 In this conformation all peptide chains are stretched out to nearly

maximum extension and then laid side by side which are held
together by hydrogen bonds.

27. Tertiary structure of proteins: It represents the overall folding of the

polypeptide chain i.e., further folding of the 2° structure.

Types of bonding which stablise the 3° structure –

i. Disulphide bridge (-S – S-)

ii. H – bonding – (C = O … H – N)

iii. Salt bridge (COO– … +NH3)

iv. Hydrophobic interactions

v. van der Waals forces

Two shapes are possible:

Fibrous proteins Globular proteins

When the polypeptide chains run This structure results when the
parallel and are held together by chains of polypeptides coil around to
hydrogen and disulphide bonds, then give a spherical shape.
fibre– like structure is formed.

Such proteins are generally insoluble These are usually soluble in water.
in water

Examples: keratin (present in hair, Examples: Insulin and albumins

wool, silk) and myosin (present in
muscles), etc
28. Quaternary structure of proteins: Some of the proteins are composed
of two or more polypeptide chains referred to as sub-units. The spatial
arrangement of these subunits with respect to each other is known as
quaternary structure of proteins.

29. Denaturation of proteins:

The loss of biological activity of proteins when a protein in its native form, is
subjected to physical change like change in temperature or chemical change
like change in pH. This is called denaturation of protein. Example:
coagulation of egg white on boiling, curdling of milk

30. Nucleoside:
Base + sugar

31. Nucleotide:
Base + sugar + phosphate group

32. Nucleic acids (or polynucletides): Long chain polymers of

nucleotides. Nucleotides are joined by phosphodiester linkage between 5’ and
3’ C atoms of a pentose sugar.

33. Two types of nucleic acids:

Deoxyribonucleic Acid (DNA) Ribonucleic Acid (RNA)

It has a double stranded -helix It has a single stranded -helix

structure in which two strands are structure.
coiled spirally in opposite directions.
Sugar present is –D–2-deoxyribose Sugar present is –D–ribose
Bases: Bases:
Purine bases: Adenine (A) and Purine bases: Adenine (A) and
Guanine (G) Guanine (G)
Pyrimidine bases : Thymine (T) and Pyrimidine bases: Uracil (U) and
cytosine (C) cytosine (C)
It occurs mainly in the nucleus of the It occurs mainly in the cytoplasm of
cell. the cell.
It is responsible for transmission for It helps in protein synthesis.
heredity character.

34. Double helix structure of DNA:

 It is composed of 2 right handed helical polynucleotide chains coiled

spirally in opposite directions around the same central axis.

 Two strands are anti-parallel i.e. their phosphodiester linkage runs in

opposite directions.

 Bases are stacked inside the helix in planes  to the helical axis.

 Two strands are held together by H – bonds (A = T, G  C).

 The two strands are complementary to each other because the

hydrogen bonds are formed between specific pairs of bases. Adenine
forms hydrogen bonds with thymine whereas cytosine forms hydrogen
bonds with guanine.

 Diameter of double helix is 2 nm.

 Double helix repeats at intervals of 3.4 nm. (One complete turn)

 Total amount of purine (A + G) = Total amount of pyramidine (C + T)

35. Vitamins: 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.

36. Classification of vitamins: Vitamins are classified into two groups

depending upon their solubility in water or fat.

Fat soluble vitamins Water soluble vitamins

These vitamins are soluble in fat and These vitamins are soluble in water.
oils but insoluble in water.
They are stored in liver and adipose Water soluble vitamins must
(fat storing) tissues be supplied regularly in diet because
they are readily excreted in urine and
cannot be stored (except vitamin
B12) in our body.
Example: Vitamin A, D, E and K Example: Vitamin C, B group
vitamins
37. Important vitamins, their sources and their deficiency diseases:

Name of vitamin Source Deficiency diseases

Vitamin A Fish liver oil, carrots, erophthalmia
butter and milk (hardening of cornea of
eye)
Night blindness
Vitamin B1 Yeast, milk, green Beri beri (loss of
(Thiamine) vegetables and cereals appetite,
retarded growth)
Vitamin B2 Milk, egg white, liver, Cheilosis (fissuring at
(Riboflavin) kidney corners of mouth and
lips), digestive disorders
and burning sensation
of the skin.
Vitamin B6 Yeast, milk, egg yolk, Convulsions
(Pyridoxine) cereals and grams
Vitamin B12 Meat, fish, egg and Pernicious anaemia
curd (RBC deficient in
haemoglobin)
Vitamin C Citrus fruits, amla and Scurvy (bleeding gums)
(Ascorbic acid) green leafy vegetables
Vitamin D Exposure to sunlight, Rickets (bone
fish and egg yolk deformities
in children) and
osteomalacia
(soft bones and
joint pain in adults)
Vitamin E Vegetable oils like Increased fragility of
wheat RBCs and muscular
germ oil, sunflower oil, weakness
etc.
Vitamin K Green leafy vegetables Increased blood clotting
time