CBSE Class 12 Chemistry Revision Notes Chapter 10 Biomolecules

Biomolecules are organic compounds present in living systems and are needed for life processes. In CBSE Class 12 Chemistry, this chapter explains carbohydrates, proteins, enzymes, vitamins, nucleic acids, DNA and RNA.

Class 12 Chemistry Chapter 10 Biomolecules connects chemistry with living systems. The chapter explains how non-living atoms and molecules form complex substances that support life. Carbohydrates, proteins, nucleic acids and vitamins are some important biomolecules studied here.

This chapter is mostly concept-based, but it also has important structures, classifications and differences. Students should revise glucose, fructose, sucrose, starch, amino acids, peptide bonds, protein structure, denaturation, enzymes, vitamins and nucleic acids carefully. Many exam questions are asked from definitions, tables, differences and reasoning-based points.

Key Takeaways

• Carbohydrates: Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.
• Proteins: Proteins are polymers of α-amino acids joined by peptide bonds.
• Enzymes: Enzymes are biological catalysts that help reactions occur under mild conditions in the body.
• Nucleic Acids: DNA and RNA are polynucleotides responsible for heredity and protein synthesis.

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Access 30 Minutes Class 12 Chemistry Chapter 10 Biomolecules Notes

Students can use these CBSE Class 12 Chemistry Revision Notes Chapter 10 for quick exam revision. Biomolecules is a scoring chapter if definitions and differences are revised properly.

First revise carbohydrates and their classification. Then study glucose, fructose, disaccharides and polysaccharides. After that, move to amino acids, proteins, enzymes, vitamins and nucleic acids. For board exams, make sure you can write clear differences between DNA and RNA, reducing and non-reducing sugars, fibrous and globular proteins, and essential and non-essential amino acids.

Biomolecules Class 12 Chemistry Chapter 10 Overview

Biomolecules are complex organic molecules present in living systems. They help in growth, repair, energy storage, heredity and body functions.

Important biomolecules include:

• Carbohydrates
• Proteins
• Nucleic acids
• Vitamins
• Enzymes
• Hormones
• Lipids

In this chapter, the main focus is on carbohydrates, proteins, enzymes, vitamins and nucleic acids.

Carbohydrates in Biomolecules

Carbohydrates are mainly produced by plants. Common examples include glucose, cane sugar, starch and cellulose.

Earlier, carbohydrates were considered hydrates of carbon because many of them fit the general formula Cx(H₂O)y. But this definition is not correct for all carbohydrates.

Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or ketones, or compounds which produce such units on hydrolysis.

Carbohydrates are also called saccharides.

Classification of Carbohydrates

Carbohydrates are classified on the basis of their behaviour on hydrolysis.

Monosaccharides

Monosaccharides are the simplest carbohydrates. They cannot be hydrolysed further to give simpler polyhydroxy aldehydes or ketones.

Examples:

• Glucose
• Fructose
• Ribose

Monosaccharides are further classified according to the number of carbon atoms and the functional group present.

If the monosaccharide contains an aldehyde group, it is called an aldose. If it contains a keto group, it is called a ketose.

Reducing and Non-Reducing Sugars

Carbohydrates can also be classified as reducing and non-reducing sugars.

Reducing sugars reduce Fehling’s solution and Tollens’ reagent. All monosaccharides are reducing sugars.

Non-reducing sugars do not reduce Fehling’s solution or Tollens’ reagent because their reducing groups are involved in glycosidic bond formation.

Glucose in Class 12 Chemistry Chapter 10

Glucose is an aldohexose. It is also known as dextrose. It occurs freely in nature and is present in sweet fruits and honey.

Glucose is also found in ripe grapes in large amounts. It is the monomer of many larger carbohydrates such as starch and cellulose.

The molecular formula of glucose is C₆H₁₂O₆.

Preparation of Glucose

Glucose can be prepared from sucrose and starch.

From Sucrose

When sucrose is boiled with dilute hydrochloric acid or sulphuric acid in alcoholic solution, glucose and fructose are formed in equal amounts.

Sucrose + Water → Glucose + Fructose

From Starch

Commercially, glucose is obtained by hydrolysis of starch. Starch is boiled with dilute sulphuric acid at 393 K under pressure.

Starch + Water → Glucose

Structure of Glucose

Glucose is an aldohexose. It has six carbon atoms and one aldehyde group.

Important evidence for the structure of glucose:

• Its molecular formula is C₆H₁₂O₆.
• On prolonged heating with HI, it forms n-hexane, showing that all six carbon atoms are linked in a straight chain.
• It reacts with hydroxylamine to form oxime, showing the presence of a carbonyl group.
• It reacts with HCN to form cyanohydrin, again confirming the carbonyl group.
• It is oxidised by bromine water to gluconic acid, showing the presence of an aldehyde group.
• Acetylation gives glucose pentaacetate, showing the presence of five -OH groups.
• Oxidation with nitric acid gives saccharic acid, showing the presence of a primary alcohol group.

Cyclic Structure of Glucose

The open-chain structure of glucose cannot explain all its properties.

For example:

• Glucose does not give Schiff’s test.
• Glucose does not form a hydrogensulphite addition product with NaHSO₃.
• Glucose exists in two crystalline forms, α and β.

These points show that glucose mainly exists in a cyclic hemiacetal form.

In cyclic glucose, the -OH group at C-5 adds to the aldehyde group at C-1. This forms a six-membered ring.

The α and β forms differ in the position of the -OH group at C-1. This carbon is called the anomeric carbon.

Fructose

Fructose is an important ketohexose. It is found in fruits, honey and vegetables.

It has the molecular formula C₆H₁₂O₆, same as glucose. But fructose contains a ketonic group at carbon number 2.

Fructose belongs to the D-series and is laevorotatory. It is written as D-(-)-fructose.

Fructose also forms cyclic structures. It forms a five-membered ring called furanose.

Disaccharides

Disaccharides give two monosaccharide units on hydrolysis.

The two monosaccharide units are joined by a glycosidic linkage. A glycosidic linkage is formed by the loss of a water molecule between two monosaccharide units.

Important disaccharides are:

• Sucrose
• Maltose
• Lactose

Sucrose

Sucrose is common table sugar. On hydrolysis, it gives one molecule of glucose and one molecule of fructose.

Sucrose + Water → Glucose + Fructose

In sucrose, glucose and fructose are joined through a glycosidic linkage between C-1 of α-D-glucose and C-2 of β-D-fructose.

Both reducing groups are involved in bond formation. So sucrose is a non-reducing sugar.

Invert Sugar

Sucrose is dextrorotatory. On hydrolysis, it gives glucose and fructose.

Glucose is dextrorotatory, while fructose is laevorotatory. Since the laevorotation of fructose is greater than the dextrorotation of glucose, the final mixture becomes laevorotatory.

This mixture is called invert sugar.

Maltose

Maltose is made of two α-D-glucose units.

The glycosidic linkage is between C-1 of one glucose unit and C-4 of another glucose unit.

Maltose is a reducing sugar because one aldehyde group can be produced in solution.

Lactose

Lactose is also known as milk sugar. It is found in milk.

It is made of β-D-galactose and β-D-glucose.

The linkage is between C-1 of galactose and C-4 of glucose. Lactose is a reducing sugar because a free aldehyde group can be produced at C-1 of the glucose unit.

Polysaccharides

Polysaccharides contain a large number of monosaccharide units joined by glycosidic linkages.

They are generally not sweet in taste. They mainly act as food storage materials or structural materials.

Important polysaccharides are:

• Starch
• Cellulose
• Glycogen

Starch

Starch is the main storage polysaccharide in plants. It is an important dietary source for human beings.

Starch is a polymer of α-glucose. It has two components:

• Amylose
• Amylopectin

Amylose is water-soluble and forms about 15-20% of starch. It is a long unbranched chain of α-D-glucose units joined by C1-C4 glycosidic linkages.

Amylopectin is water-insoluble and forms about 80-85% of starch. It is branched. The main chain has C1-C4 linkages, while branching occurs through C1-C6 linkages.

Cellulose

Cellulose occurs only in plants. It is the most abundant organic substance in the plant kingdom.

It forms the main structural component of plant cell walls.

Cellulose is a straight-chain polysaccharide made of β-D-glucose units. The units are joined by glycosidic linkage between C-1 of one glucose unit and C-4 of the next glucose unit.

Glycogen

Glycogen is the storage carbohydrate in animals. It is also called animal starch.

It is similar to amylopectin but more highly branched.

Glycogen is present in the liver, muscles and brain. When the body needs glucose, enzymes break glycogen into glucose.

Importance of Carbohydrates

Carbohydrates are essential for life. They form a major part of our food and provide energy.

Important functions of carbohydrates:

• Starch stores food in plants.
• Glycogen stores food in animals.
• Cellulose forms plant cell walls.
• Ribose and deoxyribose are present in nucleic acids.
• Carbohydrates are used in textile, paper, lacquer and brewery industries.

Proteins in Biomolecules

Proteins are the most abundant biomolecules in living systems. They occur in every part of the body.

Important sources of proteins include milk, cheese, pulses, peanuts, fish and meat.

Proteins are needed for growth, repair and maintenance of the body.

All proteins are polymers of α-amino acids.

Amino Acids

Amino acids contain both amino group (-NH₂) and carboxyl group (-COOH).

Only α-amino acids are obtained on hydrolysis of proteins.

The general structure of α-amino acid is:

R-CH(NH₂)-COOH

Here, R is the side chain.

Classification of Amino Acids

Amino acids are classified as acidic, basic or neutral depending on the number of amino and carboxyl groups.

Essential and Non-Essential Amino Acids

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

Essential amino acids cannot be synthesised by the body. They must be obtained through diet.

Non-essential amino acids can be synthesised in the body.

Zwitter Ion

Amino acids behave like salts because they contain both acidic and basic groups.

In aqueous solution, the carboxyl group loses a proton and the amino group accepts a proton. This forms a dipolar ion called a zwitter ion.

In zwitter ionic form, amino acids show amphoteric behaviour. This means they react with both acids and bases.

Peptide Bond

Proteins are polymers of α-amino acids joined by peptide bonds.

A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another amino acid, with loss of water.

The peptide linkage is:

-CO-NH-

When two amino acids join, a dipeptide is formed. When three amino acids join, a tripeptide is formed. A long chain of amino acids is called a polypeptide.

A polypeptide with more than 100 amino acid residues and molecular mass above 10,000 u is generally called a protein.

Classification of Proteins Based on Shape

Proteins are classified into fibrous and globular proteins based on molecular shape.

Structure of Proteins

Protein structure is studied at four levels:

1. Primary structure
2. Secondary structure
3. Tertiary structure
4. Quaternary structure

Primary Structure of Proteins

The primary structure of a protein is the specific sequence of amino acids in its polypeptide chain.

Any change in the sequence changes the protein.

Secondary Structure of Proteins

The secondary structure refers to the shape in which the polypeptide chain exists.

Two common secondary structures are:

• α-helix
• β-pleated sheet

These structures are stabilised by hydrogen bonding between peptide linkages.

α-Helix Structure

In α-helix structure, the polypeptide chain twists into a right-handed helix.

The -NH group of one amino acid residue forms a hydrogen bond with the C=O group of another residue in the nearby turn.

β-Pleated Sheet Structure

In β-pleated sheet structure, peptide chains are stretched out and arranged side by side.

They are held together by intermolecular hydrogen bonds. The structure looks like pleated folds.

Tertiary Structure of Proteins

The tertiary structure represents the overall folding of the polypeptide chain.

It gives proteins their final three-dimensional shape.

Tertiary structure is stabilised by:

• Hydrogen bonds
• Disulphide linkages
• van der Waals forces
• Electrostatic forces

Quaternary Structure of Proteins

Some proteins have two or more polypeptide chains. These chains are called sub-units.

The spatial arrangement of these sub-units is called quaternary structure.

Haemoglobin is an example of a protein with quaternary structure.

Denaturation of Proteins

A protein in its natural active form is called a native protein.

When a protein is exposed to heat or change in pH, its hydrogen bonds get disturbed. The globules unfold and the helix gets uncoiled. The protein loses its biological activity.

This process is called denaturation.

During denaturation, secondary and tertiary structures are destroyed, but the primary structure remains intact.

Examples:

• Coagulation of egg white on boiling
• Curdling of milk due to lactic acid formation

Enzymes

Enzymes are biological catalysts. They help chemical reactions occur in living organisms under mild conditions.

Almost all enzymes are globular proteins.

Enzymes are very specific for a particular reaction and a particular substrate.

For example, maltase catalyses the hydrolysis of maltose into glucose.

Maltose → Glucose

Enzymes are usually named with the suffix -ase.

Mechanism of Enzyme Action

Enzymes reduce the activation energy of a reaction.

Only a small amount of enzyme is needed for a reaction.

For example, sucrose hydrolysis needs less activation energy when catalysed by sucrase than when done by acid hydrolysis.

This is why enzymes are highly efficient biological catalysts.

Vitamins

Vitamins are organic compounds needed in small amounts for normal growth and health.

Most vitamins cannot be synthesised in the human body. So they must be taken through diet.

Vitamins are important because their deficiency causes specific diseases.

The term vitamin came from “vital amine.” Later, it was found that many vitamins do not contain amino groups, so the final “e” was removed from vitamine.

Classification of Vitamins

Vitamins are classified into fat-soluble and water-soluble vitamins.

Water-soluble vitamins are easily excreted in urine and are not stored in the body, except vitamin B12.

Important Vitamins and Deficiency Diseases

Nucleic Acids

Nucleic acids are biomolecules responsible for heredity.

They are found in the nucleus of living cells. Chromosomes are made up of proteins and nucleic acids.

There are two main types of nucleic acids:

• DNA
• RNA

Nucleic acids are long-chain polymers of nucleotides. So they are also called polynucleotides.

Nucleotides and Nucleosides

A nucleoside contains a nitrogenous base and a sugar.

A nucleotide contains a nitrogenous base, a sugar and a phosphate group.

So:

Nucleoside = Base + Sugar

Nucleotide = Base + Sugar + Phosphate

DNA and RNA

DNA stands for deoxyribonucleic acid.

RNA stands for ribonucleic acid.

DNA stores genetic information and passes hereditary characters from one generation to another. RNA helps in protein synthesis.

Difference Between DNA and RNA

Hormones

Hormones are chemical messengers produced by endocrine glands.

They are released directly into the bloodstream and regulate many body functions.

Examples include insulin, adrenaline and thyroxine.

Hormones are needed only in small quantities, but their effect on body functions is very important.

Quick Revision Table for Biomolecules

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