Notes Class 11 Chapter 6 Biology Maharashtra Board

Biomolecules

Introduction

• Living organisms are diverse, classified as unicellular (e.g., bacteria, yeast) or multicellular (e.g., plants, animals).
• The cell is the basic structural and functional unit, containing protoplasm with biomolecules.
• Biochemistry studies the chemistry of living organisms, understanding biological processes, cell communication, inheritance, and diseases.
• Chemical analysis reveals common elements: carbon, hydrogen, oxygen, nitrogen, sulphur, calcium, phosphorus, magnesium, etc.
• Biomolecules are categorized into:
  • Organic: Macromolecules (polysaccharides, polypeptides, polynucleotides) and micromolecules (sugars, amino acids, nucleotides, lipids).
  • Inorganic: Macro elements (e.g., potassium, calcium) and trace elements (e.g., boron, zinc).

6.1 Biomolecules in the Cell

A. Carbohydrates

• Definition: Biomolecules made of carbon, hydrogen, and oxygen (general formula: Cx(H2O)y), with a hydrogen-to-oxygen ratio of 2:1.
• Function: Broken down to release energy for metabolism.
• Classification (Based on sugar units):
  1. Monosaccharides:
     • Simplest sugars, crystalline, sweet, water-soluble, non-hydrolysable.
     • General formula: (CH2O)n, where n = 3-7.
     • Types based on carbon atoms:
       • Triose (3C): e.g., glyceraldehyde.
       • Tetrose (4C): e.g., erythrose.
       • Pentose (5C): e.g., ribose (RNA), deoxyribose (DNA).
       • Hexose (6C): e.g., glucose, fructose, galactose.
       • Heptose (7C): e.g., sedoheptulose.
     • Types based on functional group:
       • Aldoses: Contain aldehyde group (-CHO), e.g., glucose.
       • Ketoses: Contain ketone group (-C=O), e.g., fructose.
     • Properties: Reducing sugars due to free aldehyde/ketone groups, reducing Benedict’s reagent (Cu²⁺ to Cu⁺).
     • Examples:
       • Glucose: Primary fuel, 90 mg/100 ml in human blood, metabolized via cellular respiration.
       • Galactose: Combines with glucose to form lactose, not interchangeable with glucose in respiration.
       • Fructose: Fruit sugar, ketohexose, forms sucrose with glucose.
  2. Disaccharides:
     • Formed by condensation of two monosaccharides, releasing water, linked by a glycosidic bond.
     • Soluble in water, too large for diffusion through cell membranes, broken down in the small intestine.
     • Examples:
       • Sucrose: Glucose + Fructose (non-reducing, no free aldehyde/ketone).
       • Lactose: Glucose + Galactose (reducing, exists in beta form).
       • Maltose: Two glucose units (reducing).
     • Hydrolysis: C12H22O11 + H2O → C6H12O6 + C6H12O6.
  3. Polysaccharides:
     • Polymers formed by condensation of multiple monosaccharides (polymerization).
     • Broken down by hydrolysis into monosaccharides.
     • Properties depend on length, branching, folding, and coiling.
     • Types:
       • Homopolysaccharides: Single type of monosaccharide.
         • Starch: Plant storage, two forms:
           • Amylose: Unbranched, helical, forms colloidal suspension.
           • Amylopectin: Branched, insoluble.
         • Cellulose: Structural, β-glucose, forms tough plant cell walls via hydrogen bonding.
         • Glycogen: Animal storage, branched, stored in liver/muscles, hydrolyzed to glucose.
       • Heteropolysaccharides: Different monosaccharides, e.g., hyaluronic acid, heparin, chondroitin sulphate.
     • Biological Significance:
       • Energy source (glucose for ATP).
       • Lactose provides energy for infants.
       • Structural roles (cellulose in cell walls).
       • Storage (starch, glycogen).

B. Lipids

• Definition: Greasy substances with long hydrocarbon chains, higher hydrogen-to-oxygen ratio than carbohydrates (>2:1), soluble in non-polar solvents.
• Fatty Acids: Organic acids with a hydrocarbon chain and carboxyl group (-COOH).
  • Saturated: No double bonds, e.g., palmitic, stearic acids.
  • Unsaturated: One or more double bonds, e.g., oleic, linoleic acids.
• Classification:
  1. Simple Lipids:
     • Esters of fatty acids with alcohols.
     • Fats: Esters with glycerol (triglycerides: one glycerol + three fatty acids).
       • Unsaturated fats are liquid (oils), hydrogenated to form solids (e.g., vanaspati ghee).
       • Functions: High-energy source, stored in plant seeds and animal adipose tissue, insulation, shock absorption.
     • Waxes: Esters of fatty acids with long-chain alcohols, solid, water-insoluble, found in beehives, skin, plant surfaces.
  2. Compound Lipids:
     • Contain additional groups (e.g., phosphate, sugar).
     • Phospholipids: Glycerol, two fatty acids, phosphate (often with nitrogenous compound, e.g., lecithin).
       • Hydrophilic head, hydrophobic tails, form cell membrane bilayers.
     • Glycolipids: Glycerol, fatty acids, sugars (e.g., cerebrosides in brain, myelin sheath).
  3. Derived Lipids:
     • Sterols: Fused hydrocarbon rings, e.g., cholesterol (in animals, not plants).
       • Functions: Precursor for adrenocorticoids, sex hormones, vitamin D.
       • Plant sterols: Phytosterols, e.g., diosgenin (used in birth control pills).
• Biological Significance:
  • Energy storage, insulation, membrane structure, hormone synthesis, protection of organs.

C. Proteins

• Definition: Complex nitrogenous compounds, termed by Berzelius (1830), essential in all cells.
• Characteristics:
  • Large molecules (100-3000 amino acids), high molecular weight.
  • Amino acids linked by peptide bonds (carboxyl to amino group).
  • Structures:
    • Primary: Linear amino acid sequence.
    • Secondary: Alpha-helix or beta-pleated sheets, stabilized by hydrogen bonds (e.g., keratin, silk fibroin).
    • Tertiary: 3D folding due to disulfide bonds (e.g., myoglobin).
    • Quaternary: Multiple polypeptide chains (e.g., haemoglobin).
  • Amphoteric (act as acids and bases), influenced by pH.
  • Basic proteins: Rich in lysine, arginine (e.g., histones).
  • Acidic proteins: Rich in acidic amino acids (e.g., blood proteins).
• Classification:
  1. Simple Proteins:
     • Yield only amino acids on hydrolysis.
     • Examples: Albumins (e.g., egg albumin, soluble, coagulate on heating), Histones (soluble, DNA packaging).
  2. Conjugated Proteins:
     • Simple protein + non-protein prosthetic group.
     • Examples: Haemoglobin (globin + haem), Nucleoproteins (histone + nucleic acid), Mucoproteins (e.g., mucin), Lipoproteins (lipid + protein).
  3. Derived Proteins:
     • Formed by hydrolysis of native proteins, e.g., metaproteins, peptones.
• Importance:
  • Structural (keratin, collagen), enzymatic (catalyze reactions), transport (haemoglobin), immunity (antibodies).

D. Nucleic Acids

• Discovery: Friederich Miescher (1869) isolated nucleic acids from pus cells.
• Types: DNA (deoxyribose nucleic acid) and RNA (ribose nucleic acid).
• Components: Nucleotides (5-carbon sugar, phosphoric acid, nitrogenous base).
• Nitrogenous Bases:
  • Pyrimidines: Single-ring, e.g., cytosine, thymine (DNA), uracil (RNA).
  • Purines: Double-ring, e.g., adenine, guanine.
• Structure of DNA:
  • Double helix (Watson and Crick model), antiparallel strands (3′ to 5′).
  • Sugar-phosphate backbone, bases paired via hydrogen bonds (A-T: 2 bonds, G-C: 3 bonds).
  • One turn = 34 Å, nucleotide distance = 3.4 Å, diameter = 20 Å.
  • Some organisms (e.g., bacteriophage φx174) have single-stranded DNA.
• Structure of RNA:
  • Single-stranded, ribose sugar, uracil instead of thymine.
  • Folded for stability, may be double-stranded in some viruses (e.g., reovirus).
• Types of RNA:
  1. mRNA (3% of cellular RNA): Carries genetic code for protein synthesis.
  2. rRNA (80-90%): Structural component of ribosomes.
  3. tRNA (70-80 nucleotides): Transfers amino acids, clover-leaf shape with anticodon loop.
• Differences between DNA and RNA:
  • Sugar: DNA (deoxyribose), RNA (ribose).
  • Strands: DNA (double), RNA (single).
  • Bases: DNA (thymine), RNA (uracil).
  • Function: DNA (hereditary), RNA (protein synthesis).
• Chargaff’s Rule: In DNA, A = T, G = C; A+T/G+C ratio is species-specific.

E. Enzymes

• Definition: Protein catalysts speeding up biochemical reactions without being consumed.
• Discovery: Edward Buchner coined “enzyme” (in yeast) after discovering yeast extract catalyzed fermentation.
• Nature:
  • Proteinaceous: Most are proteins, except ribozymes (RNA).
  • Conjugated: Apoenzyme (protein) + prosthetic group = holoenzyme.
  • Co-enzymes: Organic (e.g., NAD, FMN), tightly bound.
  • Co-factors: Inorganic ions (e.g., Fe²⁺ for catalase, Mn²⁺ for peptidases).
• Properties:
  • Proteinaceous, 3D conformation with active sites.
  • Catalytic, unchanged post-reaction.
  • Highly specific to substrates.
  • Sensitive to temperature (optimal: 20-35°C, denature above 40°C) and pH (e.g., pepsin: pH 2, trypsin: pH 9.5).
• Classification:
  1. Oxidoreductases: Catalyze oxidation-reduction, e.g., alcohol dehydrogenase.
  2. Transferases: Transfer groups, e.g., glucokinase.
  3. Hydrolases: Hydrolytic reactions, e.g., sucrase.
  4. Lyases: Eliminate groups, e.g., histidine decarboxylase.
  5. Isomerases: Rearrange structures, e.g., xylose isomerase.
  6. Ligases: Form covalent bonds, e.g., pyruvate carboxylase.
• Nomenclature:
  • Suffix “-ase” (e.g., sucrase, protease).
  • Based on function (e.g., dehydrogenase, carboxylase).
  • Source-based (e.g., papain from papaya).
• Mechanism:
  • Lock and Key Model (Emil Fischer, 1894): Substrate fits enzyme’s active site like a key in a lock.
  • Induced Fit Model (Koshland, 1959): Active site reshapes to fit substrate, more accepted.
• Factors Affecting Enzyme Activity:
  1. Substrate Concentration: Increases rate until saturation, follows Michaelis-Menten kinetics (Km = substrate concentration at half Vmax).
  2. Enzyme Concentration: Rate proportional to enzyme concentration.
  3. Temperature: Optimal at 37°C, denatures above 40°C.
  4. pH: Specific optimum pH for each enzyme.
  5. Other Substances: Co-enzymes/activators enhance, inhibitors retard.

F. Concept of Metabolism

• Definition: Sum of all chemical reactions in a cell, providing energy and synthesizing materials.
• Pathways:
  • Catabolic: Breakdown of complex molecules (e.g., starch to glucose), releases ATP.
  • Anabolic: Synthesis of complex molecules (e.g., glycogen from glucose), consumes energy.
• Metabolic Pool: Reservoir of biomolecules, allowing interconversion (e.g., carbohydrates to fats), maintaining homeostasis.

G. Secondary Metabolites (SMs)

• Definition: Small organic molecules not essential for growth, produced by bacteria, fungi, plants.
• Classification:
  1. Terpenes: Carbon and hydrogen, e.g., essential oils.
  2. Phenolics: Benzene rings, e.g., tannins.
  3. Nitrogen-containing: May include sulphur, e.g., alkaloids.
• Economic Importance:
  • Medicine: Morphine (pain relief), antibiotics.
  • Food: Flavors (glucosinolates in cabbage), preservatives.
  • Recreation: Nicotine, cocaine.
  • Agriculture: Pest protection (tannins).