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Class 12 Chemistry Notes Chapter 5 (Biomolecules) – Chemistry-II Book

Philoid

Philoid

— 8 min read

Chemistry-II
Alright students, let's begin our focused revision of Chapter 5: Biomolecules from your NCERT Class 12 Chemistry-II textbook. This chapter is crucial not just for your board exams but also forms a significant part of the syllabus for various government competitive exams where General Science or Chemistry is included. Pay close attention to the definitions, classifications, structures, and functions.

Chapter 5: Biomolecules - Detailed Notes for Government Exam Preparation

1. Introduction

  • Biomolecules: Complex, lifeless organic molecules which combine in a specific manner to produce life or control biological reactions. Examples: Carbohydrates, Proteins, Nucleic Acids, Lipids, Vitamins, Hormones.
  • They are essential for the growth, maintenance, and functioning of living organisms.

2. Carbohydrates ('Hydrates of Carbon')

  • Definition: Optically active polyhydroxy aldehydes or polyhydroxy ketones, or compounds which produce such units on hydrolysis.

  • General Formula: Often Cₓ(H₂O)y (not always true, e.g., Rhamnose C₆H₁₂O₅).

  • Classification:

    • Monosaccharides: Simplest carbohydrates, cannot be hydrolysed further.
      • Based on functional group: Aldoses (aldehyde group, e.g., Glucose) and Ketoses (ketone group, e.g., Fructose).
      • Based on number of carbon atoms: Trioses (3C), Tetroses (4C), Pentoses (5C, e.g., Ribose), Hexoses (6C, e.g., Glucose, Fructose, Galactose).
      • Glucose (C₆H₁₂O₆): An aldohexose, also known as dextrose. Primary source of energy. Found in grapes, honey.
        • Structure: Open chain structure and cyclic structure (pyranose ring - six-membered). Exists as α-D-glucose and β-D-glucose (anomers - differ in configuration at C1).
        • Preparation: From sucrose hydrolysis, from starch hydrolysis.
        • Reactions: Shows reactions of -OH and -CHO groups (e.g., oxidation, reduction, esterification, reaction with HCN, NH₂OH).
      • Fructose (C₆H₁₂O₆): A ketohexose. Sweetest natural sugar. Found in fruits, honey.
        • Structure: Open chain structure and cyclic structure (furanose ring - five-membered). Exists as α-D-fructose and β-D-fructose (anomers - differ in configuration at C2).
      • D/L Configuration: Based on the configuration of the lowest asymmetric carbon atom relative to glyceraldehyde. Most natural sugars are D-sugars.
      • Anomers: Isomers differing in configuration only at the anomeric/glycosidic carbon (C1 for aldoses, C2 for ketoses). α and β forms.
      • Mutarotation: Change in specific rotation of an optically active compound in solution with time, to an equilibrium value (e.g., α-D-glucose and β-D-glucose in water).
    • Oligosaccharides: Yield 2 to 10 monosaccharide units on hydrolysis.
      • Disaccharides (C₁₂H₂₂O₁₁): Yield two monosaccharide units on hydrolysis. Linked by Glycosidic Linkage.
        • Sucrose (Cane Sugar): Hydrolysis yields α-D-Glucose + β-D-Fructose. Non-reducing sugar (anomeric carbons of both units involved in linkage). Invert sugar (hydrolysis product is laevorotatory).
        • Maltose (Malt Sugar): Hydrolysis yields 2 units of α-D-Glucose. Reducing sugar (one anomeric carbon is free). Linkage: C1 of one α-glucose to C4 of another α-glucose (α-1,4 glycosidic linkage).
        • Lactose (Milk Sugar): Hydrolysis yields β-D-Galactose + β-D-Glucose. Reducing sugar. Linkage: C1 of β-galactose to C4 of β-glucose (β-1,4 glycosidic linkage).
    • Polysaccharides: Yield a large number of monosaccharide units on hydrolysis. Not sweet, known as non-sugars. Act as food storage or structural materials.
      • Starch: Main storage polysaccharide in plants. Polymer of α-D-Glucose. Consists of two components:
        • Amylose: Water-soluble, linear chain of α-D-glucose units linked by C1-C4 glycosidic linkages. Forms helical structure, gives blue colour with iodine. (15-20%)
        • Amylopectin: Water-insoluble, branched chain. Linear chain C1-C4 linkage, branching occurs by C1-C6 glycosidic linkage. Gives reddish-brown colour with iodine. (80-85%)
      • Cellulose: Main structural component of plant cell walls. Straight chain polymer of β-D-Glucose units linked by C1-C4 glycosidic linkages (β-1,4). Cannot be digested by humans.
      • Glycogen: Storage polysaccharide in animals ('animal starch'). Structure similar to amylopectin but more highly branched. Stored in liver, muscles. Gives reddish-brown colour with iodine.

  • Reducing vs. Non-reducing Sugars:

    • Reducing Sugars: Contain a free aldehyde or ketone group (in open chain form) or a hemiacetal/hemiketal group (in cyclic form). Reduce Tollens' reagent and Fehling's solution. Examples: All monosaccharides, Maltose, Lactose.
    • Non-reducing Sugars: Do not have a free aldehyde/ketone group as the anomeric carbons are involved in the glycosidic bond. Do not reduce Tollens' or Fehling's. Example: Sucrose.

  • Functions of Carbohydrates: Energy source, structural components (cellulose, chitin), components of nucleic acids (ribose, deoxyribose).

3. Proteins

  • Definition: High molecular mass complex biopolymers of α-amino acids. Most abundant biomolecules in living systems.
  • Amino Acids: Organic compounds containing both an amino (-NH₂) group and a carboxyl (-COOH) group.
    • α-Amino Acids: Amino group is attached to the α-carbon (carbon next to -COOH group). General formula: R-CH(NH₂)-COOH.
    • Nature: Amphoteric (react with both acids and bases). Exist as Zwitterions (dipolar ion) in aqueous solution at a specific pH called the Isoelectric Point (at which there is no net charge and no migration in an electric field).
    • Classification:
      • Based on R group: Acidic (e.g., Aspartic acid, Glutamic acid), Basic (e.g., Lysine, Arginine), Neutral (e.g., Glycine, Alanine, Valine).
      • Essential Amino Acids: Cannot be synthesized by the body, must be obtained through diet (e.g., Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Threonine, Methionine, Lysine - approx. 9-10).
      • Non-essential Amino Acids: Can be synthesized by the body (e.g., Glycine, Alanine, Serine, Aspartic acid).
  • Peptide Bond: The amide linkage (-CO-NH-) formed between the -COOH group of one α-amino acid and the -NH₂ group of another α-amino acid, with the elimination of a water molecule.
    • Dipeptide (2 amino acids), Tripeptide (3), Polypeptide (many). Proteins are polypeptides with >100 amino acid residues and high molecular mass (>10,000 u).
  • Structure of Proteins:
    • Primary Structure: The specific sequence in which α-amino acids are linked in a polypeptide chain. Determines the protein's function.
    • Secondary Structure: The shape in which a long polypeptide chain can exist due to hydrogen bonding between >C=O and -NH- groups of the peptide backbone.
      • α-Helix: Polypeptide chain coils into a right-handed helix. H-bonds form between C=O of one residue and N-H of the fourth residue along the chain.
      • β-Pleated Sheet: Polypeptide chains lie side-by-side, held together by inter-chain H-bonds. Can be parallel or anti-parallel.
    • Tertiary Structure: The overall three-dimensional folding of the polypeptide chain, representing the folding of secondary structures. Stabilized by various interactions: Hydrogen bonds, Disulphide bridges (-S-S-), van der Waals forces, Electrostatic forces (salt bridges), Hydrophobic interactions. Gives rise to Fibrous and Globular shapes.
    • Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein complex. Example: Haemoglobin (4 subunits).
  • Classification based on Molecular Shape:
    • Fibrous Proteins: Polypeptide chains run parallel, held by H-bonds and disulphide bonds. Fibre-like structure. Insoluble in water. Structural role. Examples: Keratin (hair, wool, silk), Myosin (muscles), Collagen (tendons).
    • Globular Proteins: Polypeptide chains coil around to give a spherical shape. Soluble in water. Functional roles (enzymes, hormones, transport). Examples: Insulin, Albumin, Haemoglobin, Enzymes.
  • Denaturation of Proteins: Loss of biological activity of a protein due to disruption of its secondary, tertiary, and quaternary structures (primary structure remains intact) upon exposure to physical changes (heat, UV) or chemical changes (acids, bases, salts). Example: Coagulation of egg white on boiling, curdling of milk.
  • Functions of Proteins: Structure (collagen), Enzymes (catalysis), Transport (haemoglobin), Hormones (insulin), Antibodies (defense), Movement (actin, myosin).

4. Enzymes

  • Definition: Biological catalysts, mostly globular proteins, that increase the rate of biochemical reactions without being consumed.
  • Characteristics: Highly specific, highly efficient, work under mild conditions (temperature, pH).
  • Mechanism: Lower the activation energy of reactions. Often described by Lock and Key or Induced Fit models.

5. Vitamins

  • Definition: Organic compounds required in small amounts in the diet for specific biological functions, normal growth, and health. Cannot be synthesized by the body (except Vit D partially).
  • Classification:
    • Fat-Soluble Vitamins: Soluble in fats and oils, insoluble in water. Stored in liver and adipose tissue. Examples: A, D, E, K.
    • Water-Soluble Vitamins: Soluble in water. Must be supplied regularly as they are readily excreted (except Vit B₁₂). Examples: B complex (B₁, B₂, B₆, B₁₂, etc.), C.
  • Important Vitamins, Sources, and Deficiency Diseases (High Yield for Exams):
    • Vitamin A (Retinol): Fish liver oil, carrots, butter, milk. Deficiency: Xerophthalmia (hardening of cornea), Night blindness.
    • Vitamin B₁ (Thiamine): Yeast, milk, green vegetables, cereals. Deficiency: Beri-beri (loss of appetite, retarded growth).
    • Vitamin B₂ (Riboflavin): Milk, egg white, liver, kidney. Deficiency: Cheilosis (fissuring at corners of mouth and lips), digestive disorders, burning sensation of skin.
    • Vitamin B₆ (Pyridoxine): Yeast, milk, egg yolk, cereals, grams. Deficiency: Convulsions.
    • Vitamin B₁₂ (Cyanocobalamin): Meat, fish, egg, curd. Contains Cobalt. Deficiency: Pernicious anaemia (RBC deficient in haemoglobin).
    • Vitamin C (Ascorbic Acid): Citrus fruits, amla, green leafy vegetables. Deficiency: Scurvy (bleeding gums). Heat sensitive.
    • Vitamin D (Calciferol): Exposure to sunlight, fish, egg yolk. Deficiency: Rickets (bone deformities in children), Osteomalacia (soft bones, joint pain in adults).
    • Vitamin E (Tocopherol): Vegetable oils, sunflower oil, wheat germ oil. Deficiency: Increased fragility of RBCs, muscular weakness. Antioxidant.
    • Vitamin K (Phylloquinone): Green leafy vegetables. Deficiency: Increased blood clotting time.

6. Nucleic Acids

  • Definition: Biopolymers responsible for storing and transmitting genetic information and protein synthesis. Polymers of nucleotides. Also known as polynucleotides.
  • Types: Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).
  • Chemical Composition:
    • Pentose Sugar: β-D-ribose (in RNA) or β-D-2-deoxyribose (in DNA).
    • Nitrogenous Bases:
      • Purines: Adenine (A), Guanine (G) - Present in both DNA and RNA.
      • Pyrimidines: Cytosine (C) - Present in both DNA and RNA. Thymine (T) - Present only in DNA. Uracil (U) - Present only in RNA.
    • Phosphate Group: Phosphoric acid (H₃PO₄). Forms phosphoester bonds with sugar.
  • Nucleoside: Base + Sugar (linked via N-glycosidic linkage).
  • Nucleotide: Base + Sugar + Phosphate group (phosphate linked to 5'-OH of sugar).
  • Polynucleotide Chain: Nucleotides are linked together by Phosphodiester Linkage between the 5'-hydroxyl of one sugar and the 3'-hydroxyl of the next sugar. Forms the sugar-phosphate backbone.
  • DNA (Deoxyribonucleic Acid):
    • Structure: Double helix (Watson-Crick model). Two polynucleotide strands coiled around a common axis. Strands run anti-parallel.
    • Base Pairing: Specific hydrogen bonding between bases on opposite strands: Adenine pairs with Thymine (A=T, two H-bonds), Guanine pairs with Cytosine (G≡C, three H-bonds). This is complementary base pairing.
    • Function: Repository of genetic information. Responsible for heredity. Replication (making identical copies).
  • RNA (Ribonucleic Acid):
    • Structure: Usually single-stranded, can fold back on itself. Contains Ribose sugar and Uracil instead of Thymine.
    • Types & Functions:
      • Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes for protein synthesis.
      • Ribosomal RNA (rRNA): Structural component of ribosomes.
      • Transfer RNA (tRNA): Reads the genetic code on mRNA and transports the specific amino acid to the ribosome.
  • Biological Functions: Heredity (DNA replication), Protein Synthesis (Transcription - DNA to RNA; Translation - RNA to Protein). DNA Fingerprinting.

7. Hormones (Brief Overview)

  • Definition: Molecules produced by endocrine (ductless) glands, transported by the bloodstream to target organs to regulate physiological processes. Intercellular messengers.
  • Examples:
    • Steroid hormones (e.g., Estrogens, Androgens, Cortisol)
    • Polypeptide hormones (e.g., Insulin, Glucagon)
    • Amine hormones (e.g., Epinephrine/Adrenaline, Thyroxine)
  • Function: Regulate metabolism, growth, mood, reproduction, etc. Required in small amounts.

Multiple Choice Questions (MCQs)

  1. Which of the following is a non-reducing sugar?
    (a) Glucose
    (b) Sucrose
    (c) Maltose
    (d) Lactose

  2. The primary structure of a protein represents:
    (a) The sequence of α-amino acids
    (b) The formation of α-helix structure
    (c) The overall three-dimensional shape
    (d) The arrangement of multiple subunits

  3. Which vitamin deficiency causes Scurvy?
    (a) Vitamin A
    (b) Vitamin B₁₂
    (c) Vitamin C
    (d) Vitamin D

  4. Which base is present in RNA but not in DNA?
    (a) Adenine
    (b) Guanine
    (c) Cytosine
    (d) Uracil

  5. Cellulose is a polymer of:
    (a) α-D-Glucose
    (b) β-D-Glucose
    (c) α-D-Fructose
    (d) β-D-Galactose

  6. The linkage connecting monosaccharide units in polysaccharides is called:
    (a) Peptide linkage
    (b) Glycosidic linkage
    (c) Phosphodiester linkage
    (d) Hydrogen bond

  7. Which of the following is an essential amino acid?
    (a) Glycine
    (b) Alanine
    (c) Valine
    (d) Aspartic acid

  8. Denaturation of proteins leads to the loss of structure, but the structure remains intact.
    (a) Primary
    (b) Secondary
    (c) Tertiary
    (d) Quaternary

  9. Which vitamin contains the metal Cobalt?
    (a) Vitamin B₁
    (b) Vitamin B₂
    (c) Vitamin B₆
    (d) Vitamin B₁₂

  10. In the double helix structure of DNA, Adenine pairs with:
    (a) Guanine via two H-bonds
    (b) Cytosine via three H-bonds
    (c) Thymine via two H-bonds
    (d) Uracil via two H-bonds

Answer Key for MCQs:

  1. (b)
  2. (a)
  3. (c)
  4. (d)
  5. (b)
  6. (b)
  7. (c)
  8. (a)
  9. (d)
  10. (c)

Make sure you understand the structures, especially the cyclic forms of glucose and fructose, the linkages (glycosidic, peptide, phosphodiester), and the classification tables (carbohydrates, vitamins, amino acids). Relate the structure to properties like reducing nature or protein shape. Good luck with your preparation!

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