Biomolecules

Classification of Carbohydrates & Glucose - Preparation and Structure

• Carbohydrates are called saccharides.

• Classification

Classification of Monosaccharides

• Monosaccharides are classified based on the number of carbon atoms and the functional group
present in them.

• Different types of monosaccharides arelisted in the given table.

Carbon atoms General term Aldehyde Ketone

3 Triose Aldotriose Ketotriose

4 Tetrose Aldotetrose Ketotetrose

 5 Pentose Aldopentose Ketopentose

6 Hexose Aldohexose Ketohexose

7 Heptose Aldoheptose Ketoheptose

Glucose

• Preparation of glucose

• By boiling sucrose with dilute HCl or H2SO4 in alcoholic solution

• By boiling starch with dilute H2SO4, at 393 K, under pressure

• Structure

• Glucose has been assigned the above structure based on the following evidences.

(i) Molecular formula − C6H12O6

(ii) Suggestion of straight chain

 (iii) Confirmation of carbonyl (> C = O) group

(iv) Confirmation of the presence of carbonyl group as aldehydic group

(v) Confirmation of the presence of five −OH groups

(vi) Indication of the presence of a primary alcohol

• The correct configuration of glucose is given by

• Glucose is correctly named as D (+) − Glucose

• To understand the concept of configuration further, let us go through the following puzzle.

Cyclic Structure of Glucose

• The following reactions of glucose cannot be explained by its open-chain structure.

• Aldehydes give 2, 4-DNP test, Schiff’s test, and react with NaHSO3 to form the hydrogen sulphite
addition product. However, glucose does not undergo these reactions.

• The penta-acetate of glucose does not react with hydroxylamine. This indicates that a free −CHO
group is absent from glucose.

• Glucose exists in two crystalline forms, α and β.

The α-form (m.p = 419 K) crystallises from a concentrated solution of glucose at 303 K and the β-
form (m.p = 423 K) crystallises from a hot and saturated aqueous solution at 371 K. This behaviour
cannot be explained by the open-chain structure of glucose.

• Glucose exists in two cyclic forms, which exist in equilibrium with the open- chain structure.

• Representation of the cyclic structure of glucose by Haworth structure:

 Structure of Fructose, Disaccharides & Polysaccharides

Structure of Fructose

• Open-chain structure:

• Cyclic structure:

• Representation of the structure of fructose by Haworth structures

Disaccharides
 Glycosidic linkage − Linkage between two monosaccharide units through oxygen atom

• Sucrose

• Hydrolysis of sucrose:

• Structure:

• The product formed on the hydrolysis of sucrose is called invert sugar as the sign of rotation
changes from dextro (+) of sucrose to laevo (−) of the product.

• Non-reducing sugar

• Maltose

• Structure:

• Reducing sugar

• Lactose

• Commonly known as milk sugar

• Structure:
• Reducing sugar

Polysaccharides

They mainly act as food storage or structural materials.

• Starch

• Main storage-polysaccharide of plants

• Polymer of α-glucose; consists of two components − amylase and amylopectin

• Cellulose

• Predominant constituent of the cell wall of plant cells.

• Straight-chain polysaccharide, composed of only β-D-Glucose

• Glycogen

• Storage-polysaccharide in animal body

• Also known as animal starch because its structure is similar to amylopectin.

Proteins

• Proteins are polymers of α − amino acids.

 Amino Acids

• Some amino acids with their symbols are listed in the given table.

Name Side chain, R Three-letter symbol One-letter code

1. Glycine H Gly G

2. Alanine −CH3 Ala A

3. Valine (H3C)2CH− Val V

4. Leucine (H3C)2CH−CH2− Leu L

5. Isoleucine Ile I

6. Lysine H2N−(CH2)4− Lys K

7. Glutamic acid HOOC−CH2−CH2− Glu E

8. Aspartic acid HOOC−CH2− Asp D

 9. Cysteine HS−CH2− Cys C

10. Methionine H3C−S−CH2−CH2− Met M

11. Phenylalanine C6H5−CH2− Phe F

12. Tryptophan Trp W

Classification of Amino Acids

• Based on the relative number of amino and carboxyl groups, they are classified as acidic, basic and
neutral.

• Amino acids are also classified as essential and non-essential amino acids.

• Non-essential amino acids: Amino acids that can be synthesised in the body. Example − Glycine,
alanine, glutamic acid etc.

• Essential amino acids: Amino acids that cannot be synthesised in the body, and must be obtained
through diet. Example − Valine, leucine, isolecuine etc.

Properties of Amino Acids

• Colourless and crystalline solids

• Exist as dipolar ions, known as zwitter ions, in aqueous solution

• In zwitter form, amino acids show amphoteric behaviour.

• All naturally occurring α-amino acids (except glycine) are optically active.

Structure of Proteins

• Proteins are polymers of α-amino acids, joined to each other by peptide linkage or peptide bond.

• Peptide linkage: Amide formed between −COOH group and −NH2 group of two amino acid
molecules.

• Dipeptide − Contains two amino acid molecules

Tripeptide − Contains three amino acid molecules

Polypeptide − Contains more than ten amino acid molecules

• Based on the molecular shape, proteins are classified into two types −

• Fibrous proteins
• Globular proteins

• Fibrous Proteins: In fibrous proteins, polypeptide chains run parallel and are held together by
hydrogen and disulphide bonds. Example: keratin and myosin.

• Globular Proteins: Polypeptide chains coil around, giving a spherical shape. Example: Insulin.

• Structures and shapes of proteins are studied at four different levels: primary, secondary, tertiary
and quaternary.

• Primary structure of proteins: Contains one or more polypeptide chains, and each chain has amino
acids linked with each other in a specific sequence. This sequence of amino acids represents the
primary structure of proteins.
• Secondary structure of proteins: Shape in which a long polypeptide chain can exist; two types of
secondary structures: α-helix, β-pleated sheet

• α-helix structure of protein is as follows:

• β-pleated sheet structure of proteins is as follows:

• Tertiary structure of proteins: Overall folding of the polypeptide chains; results in fibrous and
globular proteins; secondary and tertiary structures of proteins are stabilised by hydrogen bonds,
disulphide linkages, van der Waals forces and electrostatic forces.

• Quaternary structure of proteins: Spatial arrangement of subunits (two or more polypeptide chains
forming some proteins) with respect to each other is known as quaternary structure.

• The diagrammatic representations of the four structures of proteins are given below
 Denaturation of Proteins

• Loss of biological activity of proteins due to the unfolding of globules and uncoiling of helix.

• Example − Coagulation of egg white on boiling, curdling of milk

• Enzymes, Vitamins & Nucleic Acids

Enzymes

• Enzymes are biocatalysts.

• Specific for a particular reaction and for a particular substrate

• For example, maltase catalyses hydrolysis of maltose

• The name of an enzyme ends with ‘−ase’.

• Reduce the magnitude of activation energy

Vitamins

• Organic compounds required in the diet in small amounts to maintain normal health, growth and
nutrition

• Classified into groups −

• Water-soluble vitamins: Vitamin C, B-group vitamins (B1, B2, B6, B12 etc)
 • Fat-soluble vitamins: Vitamins A, D, E and K

Some vitamins with their functions, sources and the diseases caused by their deficiency are given
in the following table.

Name of Deficiency
Function Source
Vitamin Diseases
Maintenance of normal vision
Fish liver oil, Xerophthalmia
and healthy epithelial tissue
Vitamin A carrots, butter and
and milk night blindness

Yeast, milk,
It plays a key role in the
Vitamin B1 green vegetables Beriberi
production of energy
and cereals
1. Conversion of carbohydrates
into glucose Cheilosis,
2. Protection of the cells and digestive
DNA from free radicals Milk, egg-white, disorders and
Vitamin B2
liver, kidney burning
sensation of the
skin

1. Synthesis of antibodies and

haemoglobin
2. Maintenance of normal nerve Yeast, milk, egg
Vitamin B6 function yolk, cereals and Convulsions
3. Breakdown of proteins gram
4. Regulation of blood sugar

Formation of RBC and Meat, fish, egg Pernicious

Vitamin B12
maintenance of CNS and curd anaemia

1. Maintenance of teeth and

gums
2. Repairing of tissues in the Citrus
body fruits, amla and
Vitamin C Scurvy
3. Inhibition of histamine green leafy
4. Improvement of body vegetables
defence mechanism

Absorption of calcium Rickets and

Vitamin D
required for the growth of Exposure to osteomalacia
 bones sunlight, fish and
egg yolk

1. Improvement of the immune

system Increased
Vegetable oils
2. Formation of RBC fragility of RBC
Vitamin E like wheat germ
3. To prevent the blood clotting and muscular
oil, sunflower oil
inside blood vessels weakness

Required for normal blood

Green leafy Delay of blood
Vitamin K clotting and synthesis of
vegetables clotting
proteins found in plasma

Nucleic Acids

• Two types:

• Deoxyribonucleic acid (DNA)

• Ribonucleic acid (RNA)

• Nucleic acids are polymers of nucleotides. Hence, they are also known as polynucleotides.

• Each nucleic acid contains a pentose sugar, phosphate moeity and a nitrogenous base (heterocyclic
compound containing nitrogen).

• In DNA, sugar is β-D-2-deoxyribose; in RNA, sugar is β-D-ribose

• Bases in DNA: Adenine (A), guanine (G), cytosine (C) and thymine (T)
• Bases in RNA: Adenine (A), guanine (G), cytosine (C) and uracil (U)

• Structure of nucleic acids

• Structure of a nucleoside:

• Structure of a nucleotide:

• Formation of a di-nucleotide:
•

• In secondary structure, the helices of DNA are double-stranded while those of RNA are single-
stranded.

• The two strands of DNA are complementary to each other.

Reason: H-bonds are formed between specific pairs of bases.

• Double-strand helix structure of DNA:

• Types of RNA:

• Messenger RNA (m-RNA)

• Ribosomal RNA (r-RNA)

• Transfer RNA (t-RNA)

• Functional differences between RNA and DNA:

- RNA DNA
 RNA is not responsible for
1. DNA is the chemical basis of heredity.
heredity.

Proteins are synthesised by RNA DNA molecules do not synthesise proteins, but transfer
2.
molecules in the cells. coded messages for the synthesis of proteins in the cells.