Class 12 Chemistry Chapter 14 Notes: Biomolecules | Important Questions & PYQs

 Class 12 Chemistry Chapter 14 Notes: Biomolecules Notes

🔹 14.1 Biomolecules – Introduction

• Biomolecules are organic compounds present in living organisms

• Main biomolecules:

  • Carbohydrates

  • Proteins

  • Enzymes

  • Vitamins

  • Nucleic acids

  • Hormones

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🔹 14.1 Carbohydrates

✔ Definition

• Carbohydrates are polyhydroxy aldehydes or ketones or substances which give these on hydrolysis

• General formula: (CH₂O)ₙ

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✔ Classification of Carbohydrates

(A) Monosaccharides

• Cannot be hydrolysed further

• Types:

  • Aldoses → contain –CHO

  • Ketoses → contain >C=O

• Examples: Glucose, fructose, ribose

(B) Disaccharides

• Formed by two monosaccharides

• Examples:

  • Sucrose → glucose + fructose

  • Lactose → glucose + galactose

  • Maltose → glucose + glucose

(C) Polysaccharides

• Large number of monosaccharides

• Examples: Starch, cellulose, glycogen

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✔ Reducing and Non-Reducing Sugars

• Reducing sugars: Have free –CHO or >C=O group
  → Give Tollens’ & Fehling test
  Example: Glucose, fructose, lactose

• Non-reducing sugars: No free aldehyde or ketone group
  Example: Sucrose

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✔ Glycosidic Linkage

• Ether linkage (–O–) joining two monosaccharides

• Formed by loss of one molecule of water

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✔ Starch vs Cellulose (VERY IMPORTANT)

Property	Starch	Cellulose
Type of glucose	α-D-glucose	β-D-glucose
Linkage	α-1,4	β-1,4
Digestible	Yes	No
Function	Energy storage	Structural

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✔ Glycogen

• Animal starch

• Highly branched polymer of α-D-glucose

• Stored in liver and muscles

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🔹 14.2 Proteins

✔ Definition

• Proteins are polymers of α-amino acids

• Linked by peptide bonds

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✔ Amino Acids

• Contain:

  • –NH₂ (basic)

  • –COOH (acidic)

• Exist as zwitter ion

⁺NH₃–CHR–COO⁻

→ Shows amphoteric behaviour

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✔ Essential & Non-Essential Amino Acids

• Essential: Not synthesised by body
  Example: Valine, leucine

• Non-essential: Synthesised by body
  Example: Glycine, alanine

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✔ Peptide Bond

• Amide linkage between –COOH and –NH₂

• Loss of water molecule

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✔ Structure of Proteins

(1) Primary Structure

• Sequence of amino acids

• Most important

(2) Secondary Structure

• α-Helix

• β-Pleated sheet

• Stabilised by hydrogen bonding

(3) Tertiary Structure

• Folding of secondary structure

• Responsible for biological activity

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✔ α-Helix Structure

• Right-handed helix

• Stabilised by intramolecular H-bonding

• Found in wool, hair, muscles

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✔ β-Pleated Sheet

• Zig-zag arrangement

• Intermolecular H-bonding

• Found in silk (fibroin)

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✔ Globular vs Fibrous Proteins

Property	Globular	Fibrous
Shape	Spherical	Thread-like
Solubility	Soluble	Insoluble
Function	Biological	Structural
Example	Enzymes	Keratin, collagen

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✔ Denaturation of Proteins

• Loss of biological activity

• Caused by:

  • Heat

  • Change in pH

• Secondary & tertiary structures destroyed

• Primary structure remains intact

• Example: Boiling of egg

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🔹 14.3 Enzymes

• Enzymes are biological catalysts

• Highly specific

• Work best at:

  • Optimum temperature

  • Optimum pH

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🔹 14.4 Vitamins

✔ Classification

(A) Water-Soluble

• Vitamin B-complex

• Vitamin C

• Cannot be stored in body

(B) Fat-Soluble

• Vitamins A, D, E, K

• Stored in liver & fat tissues

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✔ Important Vitamins

Vitamin	Deficiency Disease
A	Night blindness
C	Scurvy
D	Rickets
K	Blood clotting

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🔹 14.5 Nucleic Acids

✔ Definition

• Biomolecules present in nucleus

• Responsible for inheritance

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✔ Types

• DNA – Deoxyribonucleic acid

• RNA – Ribonucleic acid

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✔ Nucleoside vs Nucleotide

Nucleoside	Nucleotide
Sugar + base	Sugar + base + phosphate
No phosphoric acid	Has phosphoric acid

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✔ DNA vs RNA

DNA	RNA
Double stranded	Single stranded
Deoxyribose sugar	Ribose sugar
Thymine present	Uracil present
Genetic material	Protein synthesis

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✔ Base Pairing in DNA

• A pairs with T → 2 H-bonds

• G pairs with C → 3 H-bonds

• Hence strands are complementary

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✔ Types of RNA

1. m-RNA – Message carrier

2. t-RNA – Transfers amino acids

3. r-RNA – Ribosome structure

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🔹 14.6 Hormones

• Chemical messengers

• Secreted by endocrine glands

• Regulate growth & metabolism

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🔥 EXAM HOTSPOTS (VERY IMPORTANT)

✔ Reducing vs non-reducing sugars
✔ Starch vs cellulose
✔ Zwitter ion
✔ α-helix & β-sheet
✔ Denaturation
✔ DNA vs RNA
✔ Vitamins classification

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are biomolecules?
   Biomolecules are organic compounds present in living organisms essential for life.

2. Define carbohydrates.
   Carbohydrates are polyhydroxy aldehydes or ketones or substances that give these on hydrolysis.

3. What are reducing sugars?
   Sugars having free aldehyde or ketone group and giving Tollens’/Fehling test.

4. Why is sucrose a non-reducing sugar?
   Because it has no free aldehyde or ketone group.

5. What is a glycosidic linkage?
   An ether linkage (–O–) joining two monosaccharide units.

6. What is a zwitter ion?
   A dipolar ion having both positive and negative charges in the same molecule.

7. What is a peptide bond?
   Amide linkage formed between –COOH and –NH₂ groups of amino acids.

8. Which protein structure is most important?
   Primary structure.

9. What is denaturation of protein?
   Loss of biological activity due to destruction of secondary and tertiary structures.

10. Which base pairs with guanine in DNA?
    Cytosine.

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📝 Long Answer Questions

1. Classify carbohydrates and explain reducing and non-reducing sugars.

Classification of carbohydrates:

(a) Monosaccharides

• Cannot be hydrolysed further

• Examples: glucose, fructose, ribose

(b) Disaccharides

• Formed by two monosaccharides

• Examples: sucrose, lactose, maltose

(c) Polysaccharides

• Large number of monosaccharides

• Examples: starch, cellulose, glycogen

Reducing sugars

• Have free –CHO or >C=O group

• Give Tollens’ and Fehling tests

• Examples: glucose, fructose, lactose

Non-reducing sugars

• No free carbonyl group

• Example: sucrose

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2. Explain structure of proteins and denaturation.

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

Levels of protein structure:

• Primary structure: Sequence of amino acids

• Secondary structure: α-helix and β-pleated sheet (H-bonding)

• Tertiary structure: Folding of secondary structure, gives biological activity

Denaturation of proteins

• Loss of biological activity

• Caused by heat, pH change, chemicals

• Secondary and tertiary structures destroyed

• Primary structure remains unchanged

• Example: boiling of egg

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3. Differentiate between starch and cellulose.

Property	Starch	Cellulose
Glucose type	α-D-glucose	β-D-glucose
Linkage	α-1,4	β-1,4
Digestibility	Digestible	Non-digestible
Function	Energy storage	Structural
Example	Rice, potato	Cotton, wood

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❓ FAQs (Concept Clarity)

1. Why is glucose called a reducing sugar?
   Because it has a free aldehyde group.

2. Why are proteins amphoteric?
   Due to presence of both acidic (–COOH) and basic (–NH₂) groups.

3. Why are enzymes highly specific?
   Because they have specific active sites for substrates.

4. Why is cellulose not digested by humans?
   Because humans lack enzyme to break β-1,4 glycosidic bonds.

5. Why is DNA double stranded?
   Due to complementary base pairing and hydrogen bonding.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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