Class 12 Chemistry Chapter 14 Biomolecules Notes

Class 12 Chemistry Study Notes Chapter 1 - The Solid State

Biomolecules may be defined as complex lifeless chemical substances which form the basis of life, i.e., they not only buildup living systems (creatures) but are also responsible for their growth, maintenance and their ability to reproduce.

The various biomolecules are carbohydrates, proteins, enzymes, nucleic acids, lipids, hormones and compounds for storage and exchange of energy such as adenosine triphosphate (ATP).

Carbohydrates:

These include compounds like sugars, starches, glycogen, cellulose, dextrins and gums.

They are formed in plants by a process known as photosynthesis and make up about 70% of the solid plant material.

Old definition of carbohydrate: (meaning hydrate of carbon)

A class of compounds containing only carbon, hydrogen and oxygen; the hydrogen and oxygen being present in the same ratio as in water, represented by the C_x(H₂O)_y.

Limitations of the old definition:

(i) A number of compounds such as rhamnose, (C₆H₁₂O₅) and deoxyribose (C₅H₁₀O₄) are known which are carbohydrates by their chemical behaviour but cannot be represented as hydrates of carbon.

(ii) There are other substances like formaldehyde (HCHO, CH₂O) and acetic acid [CH₃COOH, C₂(H₂O)₂] which do not behave like carbohydrates but can be represented by the general formula, C_x(H₂O)_y.

(iii) Carbon is not known to form hydrates.

Modern definition of carbohydrate:

Carbohydrates are defined as optically active poly hydroxy aldehydes or polyhydroxy ketones or substances which give these on hydrolysis.

Note that aldehydic and ketonic groups in carbohydrates are not present as such but usually exist in combination with one of the hydroxyl groups of the molecule in the form of hemiacetals and hemiketals respectively.

Classification of Carbohydrates

Classification of Monosaccharides

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

Different types of monosaccharides are listed 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

1. By boiling sucrose with dilute HCl or H₂SO₄ in alcoholic solution

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

Open Chain Structure of Glucose

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

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.

Hence it was proposed that 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

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 (Non-reducing sugar)

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.

Maltose (Reducing sugar)

Structure:

Lactose (Reducing sugar)

Commonly known as milk 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 table given below.

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. Isolecucine		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− 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.

Non-essential amino acids:

Amino acids that can be synthesised in the body

Example − Glycine, alanine, glutamic acid

Essential amino acids:

Amino acids that cannot be synthesised in the body, and must be obtained through diet

Example − Valine, leucine, isolecuine

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 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.

Globular Proteins

Polypeptide chains coil around, giving a spherical shape. 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

β-pleated sheet structure

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, each containing two or more polypeptide chains

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

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’.

They 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)

Fat-soluble vitamins: Vitamins A, D, E and K

Some vitamins with their functions, sources and the diseases caused by their deficiency:

Vitamin	Function	Source	Deficiency Diseases
Vitamin A	Maintenance of normal vision and healthy epithelial tissue	Fish liver oil, carrots, butter and milk	Xerophthalmia and night blindness
Vitamin B₁	Synthesis of ATP in the cell	Yeast, milk, green vegetables and cereals	Beriberi
Vitamin B₂	Conversion of carbohydrates into glucose 1. Protection of the cells and DNA from free radicals	Milk, egg-white, liver, kidney	Cheilosis, digestive disorders and burning sensation of the skin
Vitamin B₆	1. Synthesis of antibodies and haemoglobin 2. Maintenance of normal nerve function 3. Breakdown of proteins 4. Regulation of blood sugar	Yeast, milk, egg yolk, cereals and gram	Convulsions
Vitamin B₁₂	Formation of RBC and maintenance of CNS	Meat, fish, egg and curd	Pernicious anaemia
Vitamin C	1. Maintenance of teeth and gums 2. Repairing of tissues in the body 3. Inhibition of histamine 4. Improvement of body defence mechanism	Citrus fruits, amla and green leafy vegetables	Scurvy
Vitamin D	Absorption of calcium required for the growth of bones	Exposure to sunlight, fish and egg yolk	Rickets and osteomalacia
Vitamin E	1. Improvement of the immune system 2. Formation of RBC 3. To prevent the blood clotting inside blood vessels	Vegetable oils like wheat germ oil, sunflower oil	Increased fragility of RBC and muscular weakness
Vitamin K	Required for normal blood clotting and synthesis of proteins found in plasma	Green leafy vegetables	Delay of blood clotting

Nucleic Acids

Two types: (i) Deoxyribonucleic acid (DNA) (ii) Ribonucleic acid (RNA)

Chemical composition of nucleic acids:

Nucleic acid contains a pentose sugar, phosphoric acid and a 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 and 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
1.	RNA is not responsible for heredity.	DNA is the chemical basis of heredity.
2.	Proteins are synthesised by RNA molecules in the cells.	DNA molecules do not synthesise proteins, but transfer coded messages for the synthesis of proteins in the cells.

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