Class 12 Chemistry Notes Chapter 14 (Biomolecules) – Examplar Problems Book

Alright class, let's begin our focused revision of Chapter 14, Biomolecules, keeping in mind the requirements for your government exam preparation. This chapter deals with the molecules that build up living organisms and are essential for their functions.

Biomolecules: Detailed Notes

1. Introduction
Biomolecules are complex, lifeless organic molecules which combine in a specific manner to produce life or control biological reactions. Major classes include carbohydrates, proteins, nucleic acids, lipids, enzymes, vitamins, and hormones.

2. Carbohydrates ('Hydrates of Carbon' - Cₓ(H₂O)y)

• Definition: Optically active polyhydroxy aldehydes or ketones or compounds which produce such units on hydrolysis. Also called 'saccharides'.
• Classification:
  • Monosaccharides: Simplest carbohydrates, cannot be hydrolysed further. Examples: Glucose, Fructose, Galactose, Ribose. General formula: (CH₂O)n where n=3-7.
    • Glucose (C₆H₁₂O₆): An aldohexose ('grape sugar', 'dextrose'). The primary source of energy.
      • Structure: Open chain structure (Fischer projection) shows an aldehyde group (-CHO) and five hydroxyl (-OH) groups. Cyclic structures (pyranose ring, Haworth projection) are more stable (α-D-glucose and β-D-glucose - anomers differing at C1).
      • Reducing sugar (due to free aldehydic group in open chain form, reacts with Tollen's and Fehling's reagents).
    • Fructose (C₆H₁₂O₆): A ketohexose ('fruit sugar', 'levulose').
      • Structure: Open chain structure shows a ketone group (>C=O) at C2. Cyclic structures (furanose ring, Haworth projection) exist (α-D-fructose and β-D-fructose).
      • Reducing sugar (isomerises to glucose in alkaline medium).
  • Oligosaccharides: Yield 2 to 10 monosaccharide units on hydrolysis.
    • Disaccharides (C₁₂H₂₂O₁₁): Yield 2 monosaccharide units. Formed by glycosidic linkage between two monosaccharides (with loss of H₂O).
      • Sucrose (Cane Sugar): Hydrolysis gives Glucose + Fructose. Linkage: α-C1 of glucose and β-C2 of fructose (α,β-1,2-glycosidic linkage). Non-reducing sugar (reducing groups of both monosaccharides are involved in linkage). Invert sugar: Hydrolysis product of sucrose is laevorotatory.
      • Maltose (Malt Sugar): Hydrolysis gives 2 units of Glucose. Linkage: α-C1 of one glucose and C4 of another glucose (α-1,4-glycosidic linkage). Reducing sugar.
      • Lactose (Milk Sugar): Hydrolysis gives Galactose + Glucose. Linkage: β-C1 of galactose and C4 of glucose (β-1,4-glycosidic linkage). Reducing sugar.
  • Polysaccharides: Yield a large number of monosaccharide units on hydrolysis. Not sweet, also called 'non-sugars'. Mainly act as food storage or structural materials.
    • Starch: Main storage polysaccharide in plants. Polymer of α-D-glucose. Components:
      • Amylose: Water-soluble, linear chain (α-1,4-glycosidic linkage), 15-20%. Gives blue colour with iodine.
      • Amylopectin: Water-insoluble, branched chain (α-1,4 linkage in chain, α-1,6 linkage at branch points), 80-85%. Gives reddish-brown colour with iodine.
    • Cellulose: Main structural component of plant cell walls. Polymer of β-D-glucose. Straight chain (β-1,4-glycosidic linkage). Cannot be digested by humans.
    • Glycogen: Storage polysaccharide in animals ('animal starch'). Structure similar to amylopectin but more highly branched. Stored in liver, muscles.

3. Proteins

• Definition: Polymers of α-amino acids linked by peptide bonds (-CO-NH-). Most abundant biomolecules in living systems. Essential for growth, maintenance, and structure.
• α-Amino Acids: Contain amino (-NH₂) and carboxyl (-COOH) groups attached to the same carbon atom (α-carbon). General structure: R-CH(NH₂)-COOH. The 'R' group varies.
  • Classification: Based on the nature of 'R' group: Neutral (e.g., Glycine, Alanine, Valine), Acidic (e.g., Aspartic acid, Glutamic acid), Basic (e.g., Lysine, Arginine).
  • Essential vs. Non-essential: Essential amino acids cannot be synthesized by the body and must be obtained through diet (e.g., Valine, Leucine, Lysine). Non-essential amino acids can be synthesized by the body.
  • Zwitterion: Amino acids exist as dipolar ions (containing both positive and negative charges) in aqueous solution. They are amphoteric.
  • Isoelectric Point (pI): The pH at which the amino acid exists predominantly as a zwitterion and has no net charge (does not migrate in an electric field).
• Peptide Bond: Formed between the -COOH group of one amino acid and the -NH₂ group of another, with the elimination of a water molecule. (-CO-NH- linkage). Dipeptide, tripeptide, polypeptide.
• Structure of Proteins:
  • Primary (1°): The specific sequence of amino acids linked by peptide bonds. Determines the protein's function.
  • Secondary (2°): The shape in which a long polypeptide chain can exist. Arises due to hydrogen bonding between >C=O and -NH- groups of the peptide backbone. Common structures:
    • α-Helix: Right-handed helix, H-bonds within the same chain. Example: Keratin (hair, wool).
    • β-Pleated Sheet: Polypeptide chains arranged side-by-side, H-bonds between adjacent chains. Example: Silk fibroin.
  • Tertiary (3°): Overall folding of the polypeptide chain, giving a specific 3D shape. Stabilized by hydrogen bonds, disulfide linkages (-S-S-), van der Waals forces, ionic interactions. Essential for biological activity. Two shapes: Fibrous (insoluble, e.g., collagen) and Globular (soluble, e.g., enzymes, albumin).
  • Quaternary (4°): Arrangement of multiple polypeptide chains (subunits) in proteins composed of more than one chain. Example: Haemoglobin (4 subunits).
• Denaturation: Loss of biological activity of a protein due to disruption of its 2° and 3° structures (4° if present) by changes in temperature, pH, or chemical agents. Primary structure remains intact. Example: Coagulation of egg white on boiling, curdling of milk.

4. Enzymes

• Definition: Biological catalysts, mostly globular proteins. Highly specific and efficient.
• Mechanism: Lower the activation energy of biochemical reactions. Form an enzyme-substrate complex. Active site is where the substrate binds.
• Factors Affecting Activity: Temperature (optimum temp), pH (optimum pH), substrate concentration, presence of inhibitors.

5. Vitamins

• Definition: Organic compounds required in small amounts in the diet to perform specific biological functions for normal health, growth, and maintenance. Cannot be synthesized by the body (except Vit D partially).
• Classification:
  • Fat-Soluble: Soluble in fats and oils, stored in liver and adipose tissue. (Vitamins A, D, E, K). Excess intake can be harmful.
  • Water-Soluble: Soluble in water, must be supplied regularly in diet as they are readily excreted in urine (except Vit B₁₂). (Vitamins B complex, C).
• Important Vitamins, Sources, and Deficiency Diseases:
  • Vit A (Retinol): Vision, growth. Sources: Carrots, fish liver oil, milk. Deficiency: Xerophthalmia (hardening of cornea), Night blindness.
  • Vit B₁ (Thiamine): Carbohydrate metabolism. Sources: Yeast, milk, cereals. Deficiency: Beri-beri (loss of appetite, retarded growth).
  • Vit B₂ (Riboflavin): FAD/FMN coenzymes. Sources: Milk, egg white, liver. Deficiency: Cheilosis (fissuring at corners of mouth), digestive disorders.
  • Vit B₆ (Pyridoxine): Amino acid metabolism. Sources: Yeast, milk, egg yolk. Deficiency: Convulsions.
  • Vit B₁₂ (Cyanocobalamin): RBC formation (contains Cobalt). Sources: Meat, fish, egg, curd. Deficiency: Pernicious anaemia.
  • Vit C (Ascorbic Acid): Antioxidant, collagen formation. Sources: Citrus fruits, amla, green leafy vegetables. Deficiency: Scurvy (bleeding gums). Heat sensitive.
  • Vit D (Calciferol): Calcium absorption. Sources: Sunlight exposure, fish, egg yolk. Deficiency: Rickets (bone deformities in children), Osteomalacia (soft bones in adults).
  • Vit E (Tocopherol): Antioxidant, fertility. Sources: Vegetable oils, sunflower oil. Deficiency: Increased fragility of RBCs, muscular weakness, sterility (less common).
  • Vit K (Phylloquinone): Blood clotting. Sources: Green leafy vegetables. Deficiency: Increased blood clotting time.

6. Nucleic Acids

• Definition: Polymers of nucleotides, responsible for heredity and protein synthesis. Also called polynucleotides. Two types: DNA and RNA.
• Components of Nucleotides:
  • Pentose Sugar: Deoxyribose (in DNA) or Ribose (in RNA).
  • Nitrogenous Base:
    • 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: PO₄³⁻ group, links nucleotides together via phosphodiester bonds.
• Nucleoside vs. Nucleotide:
  • Nucleoside = Pentose Sugar + Nitrogenous Base
  • Nucleotide = Pentose Sugar + Nitrogenous Base + Phosphate Group (Nucleoside + Phosphate Group)
• DNA (Deoxyribonucleic Acid):
  • Structure: Double helix structure (Watson & Crick model). Two polynucleotide strands coiled around each other. Sugar-phosphate backbone outside, bases project inwards. Strands are complementary and held together by hydrogen bonds between specific base pairs: A pairs with T (A=T, 2 H-bonds), G pairs with C (G≡C, 3 H-bonds). Strands run antiparallel.
  • Function: Stores genetic information, responsible for heredity. Replication ensures transmission of genetic info.
• RNA (Ribonucleic Acid):
  • Structure: Usually single-stranded helix. Contains Ribose sugar and Uracil instead of Thymine.
  • Types & Functions:
    • Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes.
    • Ribosomal RNA (rRNA): Structural component of ribosomes.
    • Transfer RNA (tRNA): Transfers specific amino acids to the ribosome during protein synthesis.
• Key Differences between DNA and RNA:

  Feature	DNA	RNA
  Sugar	Deoxyribose	Ribose
  Bases	A, G, C, T	A, G, C, U
  Structure	Double Helix	Mostly Single Strand
  Location	Mainly Nucleus	Nucleus & Cytoplasm
  Primary Function	Heredity, Genetic Info	Protein Synthesis

• DNA Fingerprinting: Technique to identify individuals based on unique sequences in their DNA.

7. Hormones

• Definition: Molecules produced by endocrine (ductless) glands, released directly into the bloodstream, and transported to target organs to regulate physiological processes. Act as intercellular messengers.
• Classification (based on structure):
  • Steroid Hormones: Derived from cholesterol (e.g., Estrogens, Androgens like Testosterone, Cortisol).
  • Peptide/Protein Hormones: Polymers of amino acids (e.g., Insulin, Glucagon, Pituitary hormones).
  • Amine Hormones: Derived from amino acids (e.g., Epinephrine/Adrenaline, Thyroxine).
• Examples & Functions:
  • Insulin: Peptide hormone (produced by pancreas). Regulates blood glucose levels (lowers it). Deficiency leads to Diabetes Mellitus.
  • Adrenaline (Epinephrine): Amine hormone (produced by adrenal medulla). Prepares body for emergency situations ('fight or flight').
  • Testosterone: Steroid hormone (androgen). Main male sex hormone, responsible for development of male secondary sexual characteristics.
  • Estrogen: Steroid hormone. Main female sex hormone, responsible for development of female secondary sexual characteristics and regulation of menstrual cycle.

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 amino acids
   (b) The formation of α-helix
   (c) The overall folding of the polypeptide chain
   (d) The assembly of multiple subunits

3. Deficiency of which vitamin 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. Amylopectin is a polymer of α-D-glucose units linked by:
   (a) α-1,4 glycosidic linkage only
   (b) β-1,4 glycosidic linkage only
   (c) α-1,4 and α-1,6 glycosidic linkages
   (d) β-1,4 and β-1,6 glycosidic linkages

6. Amino acids generally exist in the form of Zwitterions. This means they contain:
   (a) Only basic groups
   (b) Only acidic groups
   (c) Both acidic and basic groups but are overall neutral
   (d) Both acidic and basic groups in the same molecule

7. Which of the following is a fat-soluble vitamin?
   (a) Vitamin C
   (b) Vitamin B₆
   (c) Vitamin K
   (d) Vitamin B₁

8. The two strands in a DNA double helix are held together by:
   (a) Peptide bonds
   (b) Glycosidic bonds
   (c) Hydrogen bonds
   (d) Phosphodiester bonds

9. Denaturation of proteins involves the loss of which structure(s)?
   (a) Primary structure only
   (b) Secondary and Tertiary structures
   (c) Primary and Secondary structures
   (d) Primary, Secondary, Tertiary, and Quaternary structures

10. Insulin, a hormone responsible for regulating blood sugar, is chemically a:
    (a) Steroid
    (b) Protein
    (c) Amine derivative
    (d) Carbohydrate

Answers to MCQs:

1. (b) Sucrose
2. (a) The sequence of amino acids
3. (c) Vitamin C
4. (d) Uracil
5. (c) α-1,4 and α-1,6 glycosidic linkages
6. (d) Both acidic and basic groups in the same molecule (leading to the dipolar ion form)
7. (c) Vitamin K
8. (c) Hydrogen bonds
9. (b) Secondary and Tertiary structures (and Quaternary, if present)
10. (b) Protein (specifically, a peptide hormone)

Make sure you understand the structures (especially cyclic forms of glucose/fructose, basic amino acid structure, nucleotide components), classifications, linkages (glycosidic, peptide, phosphodiester), and the functions/deficiency diseases associated with these biomolecules. This forms the core for most competitive exams. Revise these notes thoroughly.