Class 11 Biology Notes Chapter 9 (Chapter 9) – Examplar Problems (English) Book
Detailed Notes with MCQs of Chapter 9, Biomolecules, from the NCERT Exemplar. This is a crucial chapter for understanding the very basis of life and frequently features in various government examinations. We'll break down the key concepts systematically.
Chapter 9: Biomolecules - Detailed Notes for Government Exam Preparation
1. Introduction: What are Biomolecules?
- All carbon compounds obtained from living tissues are called biomolecules.
- Living organisms are composed of a vast array of organic molecules (containing carbon) and inorganic elements and compounds.
2. How to Analyze Chemical Composition?
- Organic Analysis:
- Take a living tissue (e.g., liver piece, vegetable).
- Grind it in trichloroacetic acid (Cl₃CCOOH).
- Filter the slurry through cheesecloth or cotton.
- Filtrate (Acid-soluble pool): Contains biomicromolecules (molecules with molecular weight < 1000 Daltons). Examples: Amino acids, simple sugars (monosaccharides), nucleotides, fatty acids, glycerol.
- Retentate (Acid-insoluble pool): Contains biomacromolecules (molecules with molecular weight > 1000 Daltons). Examples: Proteins, polysaccharides, nucleic acids. Lipids, although having lower molecular weight (<800 Da), are found in this fraction because they form vesicles/membrane fragments which are insoluble in acid.
- Inorganic Analysis (Ash Analysis):
- Weigh a small amount of living tissue (wet weight).
- Dry it completely (all water evaporates) to get dry weight.
- Fully burn the dried tissue (oxidation of all carbon compounds).
- The remaining material is called 'ash'.
- Ash contains inorganic elements (like Calcium, Magnesium, Sodium, Potassium, etc.) and inorganic compounds (like sulphates, phosphates, etc.).
3. Primary and Secondary Metabolites
- Primary Metabolites:
- Have identifiable functions in normal physiological processes (growth, development, reproduction).
- Universally found in living organisms.
- Examples: Amino acids, sugars, lipids, nucleic acids, vitamins.
- Secondary Metabolites:
- Do not appear to have direct functions in growth or development.
- Found in specific plant, fungal, or microbial cells.
- Often have ecological importance (defense, attraction, etc.) or human uses.
- Examples:
- Alkaloids (Morphine, Codeine)
- Flavonoids (Anthocyanins)
- Rubber
- Essential oils (Lemon grass oil)
- Antibiotics (Penicillin)
- Colored Pigments (Carotenoids)
- Scents
- Gums
- Spices
- Toxins (Abrin, Ricin)
- Lectins (Concanavalin A)
- Drugs (Vinblastin, Curcumin)
4. Biomacromolecules
-
a) Proteins:
- Monomers: Amino acids.
- Amino Acid Structure: An alpha-carbon (α-carbon) bonded to:
- An amino group (-NH₂)
- A carboxyl group (-COOH)
- A hydrogen atom (-H)
- A variable side chain (-R group) - determines the specific amino acid.
- There are 20 standard amino acids used in protein synthesis. Classified as acidic (e.g., Glutamic acid), basic (e.g., Lysine), neutral (e.g., Valine), and aromatic (e.g., Tyrosine, Phenylalanine, Tryptophan).
- Essential vs. Non-essential Amino Acids: Essential amino acids cannot be synthesized by the body and must be obtained through diet.
- Zwitterion: At physiological pH, the amino group is protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO⁻), resulting in a dipolar ion called a zwitterion.
- Peptide Bond: Formed between the carboxyl group of one amino acid and the amino group of the next amino acid via a dehydration reaction (loss of H₂O). Proteins are polypeptides (long chains of amino acids linked by peptide bonds).
- Structure of Proteins:
- Primary (1°): Linear sequence of amino acids. Determines the protein's final shape and function.
- Secondary (2°): Local folding of the polypeptide chain due to hydrogen bonding between backbone atoms. Common structures: α-helix (right-handed coil) and β-pleated sheet (parallel or anti-parallel sheets).
- Tertiary (3°): Overall three-dimensional shape of a single polypeptide chain. Stabilized by various interactions between R-groups: hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulfide bridges (between cysteine residues). Crucial for protein function (e.g., enzyme active site).
- Quaternary (4°): Arrangement of multiple polypeptide subunits (if the protein consists of more than one chain). Example: Hemoglobin (4 subunits - 2 alpha, 2 beta).
- Functions: Enzymes (catalysis), hormones (regulation), transport (e.g., Hemoglobin), structural components (e.g., Collagen, Keratin), antibodies (defense), receptors, muscle contraction (Actin, Myosin).
- Most abundant protein in the animal world: Collagen.
- Most abundant protein in the whole biosphere: RuBisCO (Ribulose bisphosphate carboxylase-oxygenase).
-
b) Polysaccharides (Complex Carbohydrates):
- Monomers: Monosaccharides (simple sugars, e.g., Glucose, Fructose, Galactose, Ribose).
- Glycosidic Bond: Formed between two adjacent monosaccharides via a dehydration reaction.
- Types:
- Homopolysaccharides: Composed of only one type of monosaccharide unit.
- Starch: Storage polysaccharide in plants. Made of glucose units. Two components: Amylose (linear, α-1,4 linkages) and Amylopectin (branched, α-1,4 and α-1,6 linkages). Gives blue colour with Iodine.
- Glycogen: Storage polysaccharide in animals ("animal starch"). Highly branched structure of glucose units (α-1,4 and α-1,6 linkages). Gives red colour with Iodine. Stored mainly in liver and muscles.
- Cellulose: Structural polysaccharide in plant cell walls. Linear chain of glucose units linked by β-1,4 glycosidic bonds. Cannot be digested by humans. Most abundant organic molecule on Earth. Does not give colour with Iodine.
- Chitin: Structural polysaccharide found in the exoskeleton of arthropods and cell walls of fungi. A homopolymer of N-acetylglucosamine (a modified sugar) linked by β-1,4 glycosidic bonds.
- Heteropolysaccharides: Composed of more than one type of monosaccharide unit or their derivatives (e.g., Hyaluronic acid, Heparin).
- Homopolysaccharides: Composed of only one type of monosaccharide unit.
- Reducing vs. Non-reducing Sugars: Sugars with a free aldehyde or ketone group can reduce Cu²⁺ (Benedict's test). In polysaccharides, the right end is typically the reducing end, and the left end is the non-reducing end.
-
c) Nucleic Acids:
- Monomers: Nucleotides.
- Nucleotide Structure:
- A Pentose Sugar (Ribose in RNA, Deoxyribose in DNA)
- A Nitrogenous Base (attached to C1' of sugar)
- A Phosphate Group (attached to C5' of sugar)
- Nitrogenous Bases:
- Purines: Adenine (A), Guanine (G) (Double-ring structure)
- Pyrimidines: Cytosine (C), Thymine (T - only in DNA), Uracil (U - only in RNA) (Single-ring structure)
- Nucleoside: Sugar + Nitrogenous Base (e.g., Adenosine, Guanosine, Cytidine, Thymidine, Uridine).
- Phosphodiester Bond: Links the phosphate group of one nucleotide (at 5' position) to the hydroxyl group of the sugar of the next nucleotide (at 3' position). Forms the sugar-phosphate backbone.
- DNA (Deoxyribonucleic Acid):
- Genetic material in most organisms.
- Double helix structure (Watson & Crick model).
- Two antiparallel strands (5'→3' and 3'→5').
- Sugar: Deoxyribose.
- Bases: A, G, C, T.
- Base Pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds) - Chargaff's rules.
- RNA (Ribonucleic Acid):
- Involved in protein synthesis and gene regulation; genetic material in some viruses.
- Usually single-stranded.
- Sugar: Ribose.
- Bases: A, G, C, U (Uracil replaces Thymine).
- Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).
-
d) Lipids:
- Generally water-insoluble.
- Not strictly polymers (monomers not linked in repeating chains).
- Found in the acid-insoluble fraction due to membrane association/vesicle formation.
- Fatty Acids: Hydrocarbon chain with a carboxyl group (-COOH).
- Saturated: No double bonds between carbons in the chain (e.g., Palmitic acid, Stearic acid). Usually solid at room temp.
- Unsaturated: One or more double bonds in the chain (e.g., Oleic acid, Linoleic acid). Usually liquid at room temp (oils).
- Glycerol: A simple alcohol (trihydroxy propane).
- Ester Bond: Formed between the carboxyl group of a fatty acid and the hydroxyl group of glycerol (dehydration).
- Triglycerides (Fats and Oils): Glycerol esterified with three fatty acids. Major storage form of energy.
- Phospholipids: Glycerol + 2 Fatty Acids + Phosphate group + (often) a nitrogenous base. Amphipathic (hydrophilic head, hydrophobic tail). Main component of cell membranes (e.g., Lecithin).
- Steroids: Complex ring structure (e.g., Cholesterol - precursor for steroid hormones, bile acids, vitamin D; component of animal cell membranes).
- Waxes: Esters of long-chain fatty acids with long-chain alcohols.
5. Dynamic State of Body Constituents - Concept of Metabolism
- Biomolecules in a living system are constantly being synthesized, changed, and broken down. This turnover is called the dynamic state.
- Metabolism: The sum total of all chemical reactions occurring in a living organism.
- Metabolic Pathways: Sequences of linked chemical reactions. Can be linear or circular (e.g., Glycolysis, Krebs cycle). Each step is usually catalyzed by a specific enzyme.
- Anabolism (Biosynthesis): Building complex molecules from simpler ones. Requires energy input (endergonic). Example: Protein synthesis from amino acids.
- Catabolism (Degradation): Breaking down complex molecules into simpler ones. Releases energy (exergonic). Example: Glucose breakdown during respiration.
- Energy released during catabolism is often trapped in the form of ATP (Adenosine Triphosphate) - the energy currency of the cell.
6. The Living State
- Living organisms exist in a non-equilibrium steady-state.
- Systems at equilibrium cannot perform work. Living processes require continuous work.
- This steady-state is maintained by constant energy input and metabolic flux (flow of metabolites through pathways).
- Life is characterized by the ability to maintain this non-equilibrium state, resisting the tendency towards equilibrium (death).
7. Enzymes: The Biocatalysts
- Almost all enzymes are proteins (exception: Ribozymes - RNA enzymes).
- Function: Increase the rate of biochemical reactions without being consumed in the process. They do not change the equilibrium of a reaction, only speed up reaching it.
- Mechanism: Lower the activation energy required for a reaction to proceed.
- Active Site: A specific region (pocket or crevice) on the enzyme where the substrate binds and catalysis occurs. Formed by specific amino acid residues in the tertiary structure.
- Enzyme-Substrate Complex (ES Complex): Transient complex formed when the substrate binds to the active site.
- Models of Binding:
- Lock and Key (Emil Fischer): Rigid active site fits only specific substrate.
- Induced Fit (Koshland): Substrate binding induces a conformational change in the active site for a better fit. (More accepted model).
- Factors Affecting Enzyme Activity:
- Temperature: Activity increases with temperature up to an optimum temperature. Beyond this, the enzyme denatures (loses tertiary structure and activity) due to heat. Enzymes from thermophilic organisms have higher optimum temperatures.
- pH: Each enzyme has an optimum pH at which it functions best. Extreme pH values can alter the ionization state of amino acid residues in the active site and denature the enzyme. (e.g., Pepsin ~pH 2, Trypsin ~pH 8).
- Substrate Concentration: Initially, increasing substrate concentration increases reaction velocity. Eventually, the enzyme becomes saturated with substrate (all active sites occupied), and the velocity reaches a maximum (Vmax).
- Michaelis Constant (Km): The substrate concentration at which the reaction velocity is half of Vmax. It indicates the affinity of the enzyme for its substrate (Lower Km = Higher affinity).
- Inhibitors: Substances that decrease enzyme activity.
- Competitive Inhibition: Inhibitor resembles the substrate and competes for the active site. Can be overcome by increasing substrate concentration. Increases apparent Km, Vmax remains unchanged. (e.g., Malonate inhibiting succinate dehydrogenase).
- Non-competitive Inhibition: Inhibitor binds to a site other than the active site (allosteric site), changing the enzyme's conformation and reducing its efficiency. Cannot be overcome by increasing substrate concentration. Decreases Vmax, Km usually remains unchanged.
- Classification and Nomenclature (IUBMB System): Enzymes are classified into 6 major classes based on the type of reaction they catalyze:
- Oxidoreductases: Catalyze oxidation-reduction reactions (transfer of H or electrons). (e.g., Dehydrogenases).
- Transferases: Catalyze the transfer of a functional group (other than H) from one substrate to another. (e.g., Kinases - transfer phosphate group).
- Hydrolases: Catalyze hydrolysis (breakdown using water). (e.g., Proteases, Lipases, Amylases, Nucleases).
- Lyases: Catalyze the removal of groups from substrates by mechanisms other than hydrolysis, leaving double bonds, or adding groups to double bonds. (e.g., Decarboxylases, Aldolases).
- Isomerases: Catalyze the interconversion of isomers (geometric, positional, or optical). (e.g., Mutases, Epimerases).
- Ligases (Synthetases): Catalyze the joining of two molecules, coupled with the hydrolysis of ATP or similar triphosphate. (e.g., DNA Ligase, Synthetases).
- Cofactors: Non-protein components required by some enzymes for activity.
- Apoenzyme: The protein part of the enzyme.
- Holoenzyme: Apoenzyme + Cofactor (catalytically active).
- Types of Cofactors:
- Prosthetic Groups: Organic molecules tightly bound (often covalently) to the apoenzyme (e.g., Heme in peroxidase and catalase).
- Co-enzymes: Organic molecules loosely bound to the apoenzyme, often derived from vitamins (e.g., NAD derived from Niacin, FAD derived from Riboflavin, Coenzyme A). Act as transient carriers of specific functional groups.
- Metal Ions: Required for enzyme activity, form coordination bonds with side chains at the active site and simultaneously with the substrate (e.g., Zn²⁺ for carboxypeptidase, Mg²⁺ for hexokinase).
Multiple Choice Questions (MCQs)
-
Which fraction obtained during the chemical analysis of living tissue represents the biomacromolecules?
a) Acid-soluble pool
b) Filtrate
c) Ash
d) Acid-insoluble pool -
Which of the following is a secondary metabolite used as a drug?
a) Rubber
b) Morphine
c) Ricin
d) Vinblastin -
The bond formed between two amino acids in a polypeptide chain is called a:
a) Glycosidic bond
b) Phosphodiester bond
c) Peptide bond
d) Ester bond -
Which level of protein structure is determined solely by the sequence of amino acids?
a) Primary
b) Secondary
c) Tertiary
d) Quaternary -
Cellulose, a structural polysaccharide in plants, is a polymer of:
a) α-glucose linked by α-1,4 bonds
b) β-glucose linked by β-1,4 bonds
c) α-glucose linked by α-1,6 bonds
d) N-acetylglucosamine linked by β-1,4 bonds -
Which of the following statements correctly distinguishes DNA from RNA?
a) DNA contains ribose sugar, while RNA contains deoxyribose sugar.
b) DNA contains Uracil, while RNA contains Thymine.
c) DNA is usually double-stranded, while RNA is usually single-stranded.
d) DNA forms peptide bonds, while RNA forms glycosidic bonds. -
Enzymes increase the rate of a reaction primarily by:
a) Increasing the activation energy
b) Lowering the activation energy
c) Changing the equilibrium constant of the reaction
d) Increasing the temperature of the system -
The Michaelis constant (Km) of an enzyme represents:
a) The maximum velocity of the reaction
b) The substrate concentration at which the reaction velocity is half maximal
c) The concentration of the enzyme
d) The turnover number of the enzyme -
An enzyme that catalyzes the transfer of a functional group (e.g., phosphate) from one molecule to another belongs to which class?
a) Oxidoreductases
b) Hydrolases
c) Transferases
d) Ligases -
NAD and FAD, which act as co-enzymes, are derived from:
a) Proteins
b) Lipids
c) Minerals
d) Vitamins
Answer Key:
- d) Acid-insoluble pool
- d) Vinblastin (Morphine is an alkaloid, Ricin is a toxin, Rubber is polymeric)
- c) Peptide bond
- a) Primary
- b) β-glucose linked by β-1,4 bonds
- c) DNA is usually double-stranded, while RNA is usually single-stranded.
- b) Lowering the activation energy
- b) The substrate concentration at which the reaction velocity is half maximal
- c) Transferases
- d) Vitamins (NAD from Niacin/B3, FAD from Riboflavin/B2)
Study these notes thoroughly. Pay attention to definitions, examples, structural details (especially bonds and monomer units), and the functions of different biomolecules and enzymes. Understanding these fundamentals is key for tackling questions in competitive exams. Let me know if any part needs further clarification.