Notes Class 11 Chapter 7 Biology Maharashtra Board

Cell Division

Introduction

• Living Organisms: Display vast diversity, classified as unicellular (e.g., bacteria, yeast) or multicellular (e.g., plants, animals).
• Cell: The basic structural and functional unit of life, containing protoplasm with numerous chemical molecules called biomolecules.
• Biochemistry: The study of the chemistry of living organisms, providing insights into biological processes, cell communication, inheritance, and diseases.
• Chemical Composition: All living organisms contain common elements such as carbon, hydrogen, oxygen, nitrogen, sulphur, calcium, phosphorus, magnesium, and trace elements like boron, zinc, and iron.
• Biomolecules: Categorized into:
  • Organic: Include macromolecules (polysaccharides, polypeptides, polynucleotides) and micromolecules (simple sugars, amino acids, nucleotides, lipids).
  • Inorganic: Include macro elements (e.g., potassium, calcium) and trace elements (e.g., boron, zinc).
• Macromolecules: Polymers of simple subunits (monomers):
  • Polysaccharides: Polymers of monosaccharides.
  • Polypeptides: Polymers of amino acids.
  • Polynucleotides: Polymers of nucleotides.
• Lipids: Water-insoluble, small molecular weight compounds, not polymers but derived from fatty acids and glycerol.

6.1 Biomolecules in the Cell

A. Carbohydrates

• Definition: Biomolecules composed of carbon, hydrogen, and oxygen with the general formula Cx(H2O)y, where hydrogen and oxygen are in a 2:1 ratio (hydrates of carbon), also called saccharides.
• Function: Primarily serve as an energy source by being oxidized to release energy.
• Classification: Based on the number of sugar units:
  1. Monosaccharides:
     • Simplest sugars, crystalline, sweet, water-soluble, cannot be hydrolyzed further.
     • General formula: (CH2O)n, where n = 3, 4, 5, 6, or 7.
     • Types (based on carbon atoms):
       • Triose (3C): e.g., glyceraldehyde.
       • Tetrose (4C): e.g., erythrose.
       • Pentose (5C): e.g., ribose (in RNA), deoxyribose (in DNA).
       • Hexose (6C): e.g., glucose (blood sugar), fructose (fruit sugar), galactose.
       • Heptose (7C): e.g., sedoheptulose.
     • Types (based on functional group):
       • Aldoses: Contain an aldehyde group (-CHO), e.g., glucose, xylose.
       • Ketoses: Contain a ketone group (-C=O), e.g., fructose, ribulose.
     • Properties: All monosaccharides are reducing sugars due to free aldehyde or ketone groups, capable of reducing Benedict’s reagent (Cu²⁺ to Cu⁺).
     • Key Monosaccharides:
       • Glucose: Primary fuel in cells, ~90 mg/100 ml in human blood, small size allows passage through cell membranes, metabolized via cellular respiration.
       • Galactose: Structurally similar to glucose, forms lactose with glucose, not interchangeable with glucose in respiration.
       • Fructose: Ketohexose with a five-atom ring, combines with glucose to form sucrose.
  2. Disaccharides:
     • Formed by condensation reaction of two monosaccharides, releasing a water molecule, linked by a glycosidic bond.
     • Soluble in water but too large to pass through cell membranes by diffusion; broken down in the small intestine during digestion.
     • Examples:
       • Sucrose (cane sugar): Glucose + Fructose, non-reducing (no free aldehyde/ketone group).
       • Lactose (milk sugar): Glucose + Galactose, reducing, exists in beta form.
       • Maltose (malt sugar): Two glucose units, reducing.
     • Hydrolysis Reaction: C12H22O11 + H2O → C6H12O6 + C6H12O6 (Disaccharide + Water → Monosaccharide + Monosaccharide).
  3. Polysaccharides:
     • Polymers formed by repeated condensation of monosaccharides (polymerization), broken down by hydrolysis.
     • Properties depend on chain length, branching, folding, and coiling.
     • Types:
       • Homopolysaccharides: Composed of one type of monosaccharide.
         • Starch: Plant storage molecule, exists as:
           • Amylose: Unbranched α-glucose polymer, helical, forms colloidal suspension in hot water.
           • Amylopectin: Branched α-glucose polymer, insoluble in water.
         • Cellulose: Structural component in plant cell walls, β-glucose polymer, straight chains with hydrogen bonding for toughness.
         • Glycogen: Animal storage molecule, branched α-glucose polymer, stored in liver and muscles, hydrolyzed to glucose as needed.
       • Heteropolysaccharides: Composed of different monosaccharides, e.g., hyaluronic acid, heparin, chondroitin sulphate, blood group substances.
• Biological Significance:
  • Energy Supply: Glucose is the main substrate for ATP synthesis via metabolism.
  • Structural Role: Cellulose forms plant cell walls, contributing to structural integrity.
  • Storage: Starch (plants) and glycogen (animals) store energy.
  • Infant Nutrition: Lactose in milk provides energy for lactating babies.

B. Lipids

• Definition: Greasy substances with long hydrocarbon chains, containing carbon, hydrogen, and oxygen, with a hydrogen-to-oxygen ratio greater than 2:1 (unlike carbohydrates’ 2:1), soluble in non-polar solvents.
• Fatty Acids: Organic acids with a hydrocarbon chain ending in a carboxyl group (-COOH).
  • Saturated Fatty Acids: No double bonds, e.g., palmitic acid, stearic acid (found in animal and plant fats).
  • Unsaturated Fatty Acids: One or more double bonds, e.g., oleic acid (in fats), linoleic acid (in seed oils).
• Classification:
  1. Simple Lipids:
     • Esters of fatty acids with alcohols.
     • Fats: Esters of glycerol with three fatty acids (triglycerides).
       • Unsaturated fats are liquid at room temperature (oils), e.g., vegetable oil; saturated fats are solid, e.g., butter.
       • Hydrogenation converts unsaturated fats to solids, e.g., vanaspati ghee.
       • Functions: High calorific value, stored in plant seeds (for embryo nourishment) and animal adipose tissue, act as insulators (subcutaneous fat), and cushion internal organs.
     • Waxes: Esters of long-chain fatty acids with long-chain alcohols, solid, found in beehives, skin, plant surfaces (stems, leaves, fruits).
       • Functions: Water-insoluble coatings, protect against water loss and damage.
  2. Compound Lipids:
     • Contain additional groups like phosphate or sugar.
     • Phospholipids: Glycerol, two fatty acids, phosphate group (often with a nitrogenous compound, e.g., lecithin).
       • Structure: Hydrophilic polar head (phosphate/nitrogenous group), hydrophobic non-polar tails (fatty acids).
       • Function: Form cell membrane bilayers, regulating substance passage.
     • Glycolipids: Glycerol, fatty acids, simple sugars (e.g., galactose), also called cerebrosides.
       • Function: Found in brain white matter, myelin sheath, contribute to membrane structure.
  3. Derived Lipids:
     • Sterols: Composed of fused hydrocarbon rings and a hydrocarbon side chain, e.g., cholesterol (in animal cells, especially nervous tissue).
       • Functions: Precursor for adrenocorticoids, sex hormones (progesterone, testosterone), and vitamin D.
       • Plant Sterols: Phytosterols, e.g., diosgenin (from yam, used in birth control pills).
• Biological Significance:
  • Energy storage (fats in adipocytes, seeds).
  • Insulation and shock absorption (subcutaneous fat, fat around organs).
  • Structural role in cell membranes (phospholipids, glycolipids).
  • Hormone and vitamin synthesis (sterols).

C. Proteins

• Definition: Complex organic nitrogenous compounds, named by Berzelius (1830), essential in all animal and plant cells.
• Characteristics:
  • Large molecules with 100-3000 amino acid units, high molecular weight.
  • Amino acids linked by peptide bonds (carboxyl group of one amino acid to amino group of another).
  • Protein Structures:
    • Primary: Linear sequence of amino acids.
    • Secondary: Folding into alpha-helix (e.g., keratin in hair) or beta-pleated sheets (e.g., silk fibroin), stabilized by hydrogen bonds.
    • Tertiary: Further folding due to disulfide bonds, hydrophobic interactions, e.g., myoglobin, enzymes.
    • Quaternary: Multiple polypeptide chains, e.g., haemoglobin.
  • Amphoteric: Act as both acids and bases, influenced by pH.
  • Basic Proteins: Rich in lysine, arginine, exist as cations at pH 7.4, e.g., histones.
  • Acidic Proteins: Rich in acidic amino acids, exist as anions, e.g., most blood proteins.
• Classification:
  1. Simple Proteins:
     • Yield only amino acids on hydrolysis, soluble in specific solvents.
     • Examples:
       • Histones: Soluble in water, not coagulated by heat, part of nucleoproteins.
       • Albumins: Soluble in water, coagulate on heating, e.g., egg albumin, serum albumin, legumelin (pulses).
  2. Conjugated Proteins:
     • Simple protein + non-protein prosthetic group.
     • Examples:
       • Haemoglobin: Globin (protein) + haem (prosthetic group).
       • Nucleoproteins: Histone + nucleic acids.
       • Mucoproteins: Carbohydrate-protein complexes, e.g., mucin (saliva), heparin (blood).
       • Lipoproteins: Lipid-protein complexes, e.g., in brain, plasma membrane, milk.
  3. Derived Proteins:
     • Not naturally occurring, formed by hydrolysis of native proteins, e.g., metaproteins, peptones.
• Biological Significance:
  • Structural support (keratin, collagen).
  • Catalysis (enzymes).
  • Transport (haemoglobin for oxygen).
  • Immunity (antibodies).
  • Cell signaling and regulation.

D. Nucleic Acids

• Discovery: Friederich Miescher (1869) isolated nucleic acids from pus cells.
• Types: Deoxyribose Nucleic Acid (DNA) and Ribose Nucleic Acid (RNA).
• Components: Nucleotides, each containing:
  • 5-carbon sugar (deoxyribose in DNA, ribose in RNA).
  • Phosphoric acid (phosphate group).
  • Nitrogenous base (purine or pyrimidines).
• Nitrogenous Bases:
  • Pyrimidines: Single-ring, e.g., cytosine, thymine (DNA), uracil (RNA).
  • Purines: Double-ring, e.g., adenine, guanine.
• Structure of DNA:
  • Watson and Crick Model: Double helix with two antiparallel polynucleotide strands (3′ end of one aligns with 5′ end of the other).
  • Backbone: Alternating sugar (deoxyribose) and phosphate groups, linked by phosphodiester bonds.
  • Base Pairing: Adenine pairs with thymine (2 hydrogen bonds), guanine with cytosine (3 hydrogen bonds).
  • Dimensions: One turn = 34 Å, nucleotide spacing = 3.4 Å, diameter = 20 Å.
  • Exceptions: Some organisms (e.g., bacteriophage φx174) have single-stranded DNA.
• Structure of RNA:
  • Single-stranded, with ribose sugar and uracil instead of thymine.
  • Folded in regions for stability, may be double-stranded in some viruses (e.g., reovirus, wound tumour virus).
  • Types of RNA:
    • Messenger RNA (mRNA): 3% of cellular RNA, carries genetic code for protein synthesis, linear, synthesized from 5′ to 3′ end of DNA.
    • Ribosomal RNA (rRNA): 80-90% of cellular RNA, structural component of ribosomes, coiled due to complementary base pairing.
    • Transfer RNA (tRNA): 70-80 nucleotides, clover-leaf shape, transfers amino acids during translation, has anticodon loop and amino acid binding site.
• Chargaff’s Rule: In DNA, adenine = thymine, guanine = cytosine; A+T/G+C ratio is constant for a species but varies across species.
• Differences between DNA and RNA:
  • Sugar: DNA (deoxyribose), RNA (ribose).
  • Strands: DNA (double helix), RNA (single, sometimes folded or double-stranded).
  • Bases: DNA (thymine), RNA (uracil).
  • Function: DNA (hereditary material), RNA (protein synthesis).

E. Enzymes

• Definition: Protein catalysts that accelerate biochemical reactions without being consumed, discovered by Edward Buchner (termed “enzyme” from Greek: en = in, zyma = yeast).
• Nature:
  • Proteinaceous: Most are proteins, except ribozymes (RNA).
  • Conjugated Enzymes: Comprise apoenzyme (protein part) + prosthetic group (non-protein, e.g., haem), forming a holoenzyme.
  • Co-enzymes: Organic compounds, often vitamin-derived, tightly bound, e.g., NAD, flavin mononucleotide (FMN).
  • Co-factors: Inorganic ions, loosely bound, e.g., magnesium, iron (Fe²⁺ for catalase), manganese (for peptidases).
• Properties:
  • Proteinaceous Nature: Composed of proteins with specific 3D structures.
  • Three-Dimensional Conformation: Have active sites for substrate binding.
  • Catalytic Property: Speed up reactions, remain unchanged post-reaction.
  • Specificity: Act on specific substrates or substrate groups (e.g., sucrase on sucrose).
  • High Efficiency: Small quantities catalyze large substrate amounts (e.g., sucrase hydrolyzes 100,000 times its weight in sucrose).
  • Sensitivity to Temperature: Optimal at 20-35°C, denatured above 60-70°C, inactive but not destroyed below 4°C.
  • Sensitivity to pH: Optimal pH varies (e.g., pepsin at pH 2, trypsin at pH 9.5).
• Classification:
  1. Oxidoreductases: Catalyze oxidation-reduction reactions, e.g., alcohol dehydrogenase (alcohol + NAD⁺ → aldehyde + NADH₂).
  2. Transferases: Transfer groups between molecules, e.g., glucokinase (glucose + ATP → glucose-6-phosphate + ADP).
  3. Hydrolases: Catalyze hydrolysis, e.g., sucrase (sucrose + H₂O → glucose + fructose).
  4. Lyases: Remove groups to form double bonds, e.g., histidine decarboxylase (histidine → histamine + CO₂).
  5. Isomerases: Rearrange molecular structures, e.g., xylose isomerase (glucose-6-phosphate → fructose-6-phosphate).
  6. Ligases/Synthetases: Form covalent bonds using ATP, e.g., pyruvate carboxylase (pyruvate + CO₂ + ATP → oxaloacetate + ADP + Pi).
• Nomenclature:
  • Suffix “-ase” added to substrate (e.g., protease, sucrase).
  • Based on function (e.g., dehydrogenase, carboxylase).
  • Based on source (e.g., papain from papaya).
  • International code: Double name (substrate + reaction), e.g., pyruvic decarboxylase.
• Mechanism of Action:
  • Lock and Key Model (Emil Fischer, 1894): Substrate fits precisely into the enzyme’s rigid active site, like a key in a lock.
  • Induced Fit Model (Koshland, 1959): Active site reshapes to fit the substrate upon binding, more flexible and widely accepted.
  • Process: Substrate binds to active site, forms enzyme-substrate complex, undergoes reaction, releases products, and enzyme is regenerated.
• Factors Affecting Enzyme Activity:
  1. Substrate Concentration: Increases reaction velocity until saturation, follows Michaelis-Menten kinetics (Km = substrate concentration at half Vmax, low Km indicates high affinity).
  2. Enzyme Concentration: Reaction rate is proportional to enzyme concentration.
  3. Temperature: Optimal at 37°C (normal body temperature), denatured above 40°C, inactive below 4°C.
  4. pH: Each enzyme has an optimal pH (e.g., pepsin at pH 2, trypsin at pH 9.5), activity declines outside this range.
  5. Other Substances: Co-enzymes and activators (e.g., metal ions) enhance activity; inhibitors retard it.

F. Concept of Metabolism

• Definition: The sum of all chemical reactions in a cell, providing energy for vital processes and synthesizing new organic material.
• Types of Pathways:
  • Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (e.g., glycolysis: starch → glucose, producing ATP).
  • Anabolic Pathways: Synthesize complex molecules from simpler ones, consuming energy (e.g., glycogen synthesis from glucose, protein synthesis from amino acids).
• Metabolic Pool: A reservoir of biomolecules in the cell, acted upon by enzymes to produce useful products as needed (e.g., carbohydrates converted to fats or vice versa).
  • Significance: Maintains homeostasis by balancing catabolism and anabolism, providing metabolites for cellular needs.
  • Example: Glycolysis and Krebs cycle produce ATP and metabolites for synthesizing cellular components.

G. Secondary Metabolites (SMs)

• Definition: Small organic molecules produced by organisms (bacteria, fungi, plants) that are not essential for growth, development, or reproduction.
• Classification:
  1. Terpenes: Made from mevalonic acid, primarily carbon and hydrogen, e.g., essential oils.
  2. Phenolics: Made from simple sugars, contain benzene rings, hydrogen, and oxygen, e.g., tannins.
  3. Nitrogen-Containing Compounds: Diverse, may include sulphur, e.g., alkaloids (nicotine, morphine).
• Economic Importance:
  • Medicine: Drugs from SMs treat infections, cancer, hypertension, inflammation (e.g., morphine from Papaver somniferum for pain relief).
  • Food Industry: Flavors (e.g., glucosinolates in cabbage), preservatives (due to antibiotic properties).
  • Recreation/Stimulation: Alkaloids (nicotine, cocaine), terpenes (cannabinol).
  • Agriculture: Pest protection (e.g., glucosinolates, tannins), improve astringency in wines and chocolate.