Biomolecules – Complete CBSE Class 12 Chemistry Notes

Unit 10 | 7 marks | CBSE 2026–27

1. Carbohydrates

Carbohydrates include polyhydroxy aldehydes, ketones and compounds that yield them on hydrolysis. They are classified as monosaccharides, oligosaccharides and polysaccharides.

2. Glucose and Fructose

Glucose is an aldohexose and fructose is a ketohexose. In aqueous solution, glucose exists predominantly in cyclic forms, while a small amount is in open-chain equilibrium.

3. Disaccharides

Sucrose is formed from glucose and fructose and is a non-reducing sugar because the relevant anomeric centres are involved in the glycosidic linkage. Maltose and lactose are reducing sugars.

4. Polysaccharides

• Starch: plant storage carbohydrate.
• Glycogen: major storage carbohydrate in animals.
• Cellulose: structural polysaccharide in plant cell walls.

5. Proteins

Proteins are polymers of amino acids linked by peptide bonds. Their structure is described at primary, secondary, tertiary and quaternary levels.

6. Enzymes

Enzymes are biological catalysts. Their activity depends on factors such as temperature, pH and substrate concentration, with an optimum range for many enzymes.

7. Vitamins and Nucleic Acids

Vitamins are required in small amounts and are commonly classified as water-soluble or fat-soluble. Nucleic acids such as DNA and RNA are polymers of nucleotides and are central to storage, transmission and expression of genetic information.

🧠 Biomolecules Map

📝 Competency Questions

1. Why is sucrose classified as a non-reducing sugar?
2. Distinguish starch, glycogen and cellulose by biological role.
3. Why does enzyme activity change with temperature and pH?
4. How does a peptide bond connect amino acids?

🎯 Board Practice

1. Classify carbohydrates with examples.
2. Differentiate reducing and non-reducing sugars.
3. Explain levels of protein structure.
4. Describe enzyme action and factors affecting it.
5. Compare DNA and RNA at the prescribed level.

MCQs

1. Glucose is an aldohexose. True.
2. Fructose is a ketohexose. True.
3. Sucrose is a reducing sugar. False.
4. Peptide bonds join amino acid residues in proteins. True.

Biomolecules: Structure to Function

Carbohydrates

Monosaccharides are the simplest carbohydrate units. They can join through glycosidic bonds to form larger carbohydrates. Distinguish reducing and non-reducing sugars using structural reasoning and the prescribed examples.

Proteins

Amino acids join through peptide bonds. Protein structure is described at primary, secondary, tertiary and quaternary levels. Hydrogen bonding and other interactions help stabilise higher structures.

Enzymes

Enzymes are biological catalysts. Their specificity is related to the structure of the active site and its interaction with the substrate.

Vitamins & Nucleic Acids

Know the broad classification and functions of vitamins prescribed in the syllabus. DNA and RNA are polynucleotides built from nucleotides; understand their basic structural differences and biological roles.

Structural Concept Map

Common Mistakes

• Confusing peptide bonds with glycosidic bonds.
• Calling every carbohydrate a reducing sugar.
• Confusing nucleotide and nucleoside.
• Mixing DNA and RNA structural features.

Practice

1. Identify carbohydrate linkages.
2. Distinguish reducing and non-reducing sugars.
3. Identify levels of protein structure.
4. Compare DNA and RNA.
5. Classify vitamins and state their broad functions.

Biomolecules — Structure & Function Map

Carbohydrates

Understand monosaccharides, oligosaccharides and polysaccharides, together with the structural relationship between glucose, fructose, sucrose, starch and cellulose.

Proteins

Proteins are polymers of amino acids linked by peptide bonds. Primary, secondary, tertiary and quaternary levels describe progressively higher organisation of protein structure.

Enzymes

Enzymes are biological catalysts. Their activity depends on factors such as substrate concentration, temperature and pH, and their specificity is related to the active site.

Nucleic Acids

DNA and RNA are polynucleotides containing sugar, phosphate and nitrogenous bases. Their structures support their roles in storing and transmitting biological information.

Worked Concept Question

To distinguish starch from a reducing sugar, select an appropriate qualitative test and interpret the observation rather than relying only on molecular formula.

CBSE Practice

1. Classify carbohydrates with examples.
2. Explain peptide-bond formation.
3. Differentiate primary and secondary protein structure.
4. Compare DNA and RNA.
5. Explain enzyme specificity.
6. Match vitamins with important deficiency conditions.