class 11 notes

Class 11 Biology Notes Chapter 9 (Biomolecules) – Biology Book

Philoid

Philoid

— 8 min read

Biology
Detailed Notes with MCQs of Chapter 9, 'Biomolecules'. This is a fundamental chapter for understanding life processes at the molecular level and is crucial for various government exams. Pay close attention to the definitions, structures, and functions.

Chapter 9: Biomolecules - Detailed Notes

1. Introduction: What are Biomolecules?

  • Living organisms are composed of inanimate matter, but exhibit life phenomena due to the presence and organization of specific complex organic molecules called biomolecules.
  • These include carbohydrates, proteins, nucleic acids, lipids, vitamins, minerals, etc.
  • Found in a wide range of sizes and structures, performing diverse functions.

2. How to Analyze Chemical Composition?

  • Ash Analysis: Weighing a small amount of living tissue (wet weight), drying it (dry weight), and then fully burning it yields 'ash' containing inorganic elements (like Calcium, Magnesium).
  • Organic Compound Analysis:
    • Grind living tissue (e.g., liver piece, vegetable) in trichloroacetic acid (Cl₃CCOOH) using a mortar and pestle.
    • Filter the resulting slurry through cheesecloth or cotton.
    • Two fractions are obtained:
      • Filtrate (Acid-soluble pool): Contains biomicromolecules (molecules with molecular weight generally < 1000 Daltons). Examples: Amino acids, simple sugars, nucleotides, ions.
      • Retentate (Acid-insoluble fraction): Contains biomacromolecules (molecules with molecular weight > 1000 Daltons, except lipids). Examples: Proteins, Polysaccharides, Nucleic acids. Lipids are also found here despite lower molecular weight because they form vesicles/membrane fragments that are insoluble in acid.

3. Primary and Secondary Metabolites

  • Primary Metabolites:
    • Have identifiable functions in normal physiological processes.
    • Directly involved in growth, development, and reproduction.
    • Found in almost all cells.
    • Examples: Amino acids, sugars, fats, nucleic acids, metabolic intermediates (like pyruvic acid).
  • Secondary Metabolites:
    • Not directly involved in normal growth, development, or reproduction.
    • Often have ecological importance (defense, attraction, etc.) or are useful to humans (drugs, dyes, etc.).
    • Found in specific plant, fungal, or microbial cells.
    • Examples:
      • Alkaloids: Morphine, Codeine (Painkillers)
      • Flavonoids: Anthocyanins (Pigments)
      • Rubber, Gums: Industrial uses
      • Essential oils: Lemon grass oil (Fragrance)
      • Toxins: Abrin, Ricin
      • Lectins: Concanavalin A (Agglutinates RBCs)
      • Drugs: Vinblastin, Curcumin (Anti-cancer, Anti-inflammatory)
      • Polymeric substances: Rubber, Gums, Cellulose

4. Biomacromolecules

These are large polymeric molecules formed by the linking of smaller monomeric units.

A. Proteins:

  • Monomers: Amino acids.
  • Structure of Amino Acid: Central alpha-carbon (Cα) bonded to:
    • Amino group (-NH₂) - Basic
    • Carboxyl group (-COOH) - Acidic
    • Hydrogen atom (-H)
    • Variable side chain (-R group) - Determines the specific amino acid.
  • There are 20 standard amino acids used in protein synthesis. Classified based on R-group: Acidic (e.g., Glutamic acid), Basic (e.g., Lysine), Neutral (e.g., Valine), Aromatic (e.g., Tyrosine, Phenylalanine, Tryptophan).
  • Zwitterion: At physiological pH, amino acids exist as dipolar ions (zwitterions) with both positive (-NH₃⁺) and negative (-COO⁻) charges, but overall neutral.
  • Peptide Bond: Formed between the -COOH group of one amino acid and the -NH₂ group of the next amino acid, with the removal of a water molecule (dehydration). A chain of amino acids linked by peptide bonds is a polypeptide.
  • Levels of Protein Structure:
    • Primary (1°): Linear sequence of amino acids in a polypeptide chain. Determines the protein's final structure and function.
    • Secondary (2°): Local folding of the polypeptide chain into regular structures, stabilized by hydrogen bonds between backbone atoms. Common forms:
      • α-Helix: Right-handed coil (e.g., keratin).
      • β-Pleated Sheet: Parallel or anti-parallel sheets (e.g., silk fibroin).
    • Tertiary (3°): Overall three-dimensional shape of a single polypeptide chain, formed by interactions between R-groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges -S-S-). Essential for function (e.g., myoglobin, enzymes).
    • Quaternary (4°): Arrangement of multiple polypeptide subunits (if present) to form a functional protein. Stabilized by the same interactions as 3° structure (e.g., Hemoglobin - 2 alpha + 2 beta chains).
  • Functions: Enzymes (catalysis), Hormones (regulation), Transport (e.g., Hemoglobin), Structural (e.g., Collagen, Keratin), Defense (Antibodies), Receptors, Movement (Actin, Myosin). Collagen is the most abundant animal protein; RuBisCO is the most abundant protein in the whole biosphere.

B. Polysaccharides (Complex Carbohydrates):

  • Monomers: Monosaccharides (simple sugars, e.g., Glucose, Fructose, Galactose, Ribose).
  • Glycosidic Bond: Formed between two adjacent monosaccharides via dehydration.
  • Types:
    • Homopolysaccharides: Composed of only one type of monosaccharide unit.
      • Starch: Energy storage in plants. Polymer of α-glucose. 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: Energy storage in animals ("animal starch"). Highly branched polymer of α-glucose (more branched than amylopectin). Stored mainly in liver and muscles. Gives red colour with Iodine.
      • Cellulose: Structural component of plant cell walls. Linear polymer of β-glucose (β-1,4 linkages). Cannot be digested by humans. Does not give colour with Iodine. Most abundant organic molecule on Earth.
      • Chitin: Structural polysaccharide in fungal cell walls and arthropod exoskeletons. Polymer of N-acetylglucosamine (a modified sugar).
      • Inulin: Polymer of fructose. Used to measure glomerular filtration rate.
    • Heteropolysaccharides: Composed of more than one type of monosaccharide or their derivatives (e.g., Hyaluronic acid, Heparin).

C. Nucleic Acids:

  • Monomers: Nucleotides.
  • Structure of Nucleotide:
    • Pentose Sugar: Ribose (in RNA) or Deoxyribose (in DNA).
    • Nitrogenous Base: Attached to C1' of sugar.
      • Purines: Adenine (A), Guanine (G) (Double ring structure).
      • Pyrimidines: Cytosine (C), Thymine (T) (in DNA), Uracil (U) (in RNA) (Single ring structure).
    • Phosphate Group: Attached to C5' of sugar.
  • Nucleoside: Sugar + Nitrogenous Base.
  • Phosphodiester Bond: Links the 3'-hydroxyl of one sugar to the 5'-phosphate of the next nucleotide, forming 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-phosphate backbone on the outside, bases projecting inwards.
    • Complementary Base Pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds). (Chargaff's rules: A+G = T+C).
    • The sequence of bases encodes genetic information.
  • RNA (Ribonucleic Acid):
    • Generally single-stranded.
    • Sugar is ribose.
    • Uracil (U) replaces Thymine (T).
    • Types: mRNA (messenger RNA), tRNA (transfer RNA), rRNA (ribosomal RNA). Involved in protein synthesis. Some viruses have RNA as genetic material. Ribozymes are catalytic RNA molecules.

D. Lipids:

  • Generally water-insoluble. Not strictly polymers/macromolecules based on MW, but found in the acid-insoluble fraction.
  • Fatty Acids: Hydrocarbon chain (-R) with a carboxyl group (-COOH).
    • Saturated: No double bonds in the R chain (e.g., Palmitic acid - 16C, Stearic acid - 18C). Usually solid at room temp.
    • Unsaturated: One or more double bonds in the R chain (e.g., Oleic acid - 18C, 1 double bond; Linoleic acid - 18C, 2 double bonds). Usually liquid at room temp (oils). Have lower melting points.
  • Glycerol: Simple alcohol (trihydroxypropane).
  • Fats and Oils (Triglycerides/Triacylglycerols): Esters of fatty acids with glycerol. Three fatty acids esterified to one glycerol molecule. Function primarily as stored energy. Oils generally have unsaturated fatty acids.
  • Phospholipids: Lipids containing phosphorus. Typically have glycerol, two fatty acids, and a phosphate group (often linked to another group like choline - e.g., Lecithin). Amphipathic (hydrophilic head, hydrophobic tail). Major component of cell membranes.
  • Steroids: Lipids with a characteristic four-fused ring structure. Example: Cholesterol (component of animal cell membranes, precursor for steroid hormones like testosterone, estrogen, cortisol, and bile acids).

5. Concept of Metabolism

  • The sum total of all chemical reactions occurring in a living organism.
  • Metabolic Pathways: Sequences of reactions where metabolites are converted into other metabolites. 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 (endergonic). E.g., Protein synthesis from amino acids, photosynthesis.
  • Catabolism (Degradation): Breaking down complex molecules into simpler ones. Releases energy (exergonic). E.g., Respiration (glucose breakdown). Energy released is often trapped as ATP.
  • ATP (Adenosine Triphosphate): The energy currency of the cell.

6. Metabolic Basis for Living: Dynamic State of Body Constituents

  • Biomolecules within a cell are constantly being synthesized and broken down – they exist in a dynamic steady-state.
  • Concentrations are relatively stable, but there is continuous flow (flux) of metabolites through pathways.
  • This constant turnover and flow of energy is essential for life, preventing equilibrium (which means death).

7. Enzymes: The Catalysts of Life

  • Nature: Biological catalysts that speed up the rate of metabolic reactions without being consumed. Mostly proteins (except ribozymes). Highly specific for their substrates.
  • Mechanism of Action:
    • Lower the activation energy required for a reaction to proceed.
    • Have an active site: A specific region (pocket or crevice) with a unique 3D structure where the substrate binds.
    • Substrate (S) binds to the active site of the Enzyme (E) forming an Enzyme-Substrate complex (ES).
    • Enzyme facilitates the chemical change, converting substrate to product (P).
    • Enzyme releases the product(s), and the enzyme is free to bind another substrate molecule.
    • E + S ⇌ ES → EP → E + P
  • Factors Affecting Enzyme Activity:
    • Temperature: Activity increases with temperature up to an optimum temperature, beyond which the enzyme denatures (loses structure and activity) due to heat. Low temperatures cause inactivation, not denaturation.
    • pH: Each enzyme has an optimum pH at which it shows maximum activity. Extreme pH values cause denaturation. (e.g., Pepsin ~pH 2, Trypsin ~pH 8, Salivary amylase ~pH 6.8).
    • Substrate Concentration: With fixed enzyme concentration, the reaction rate (velocity, V) increases as substrate concentration [S] increases, until the enzyme becomes saturated (all active sites are occupied). The maximum velocity is called Vmax.
    • Michaelis-Menten Constant (Km): The substrate concentration at which the reaction velocity is half of Vmax (Vmax/2). Km indicates the affinity of the enzyme for its substrate (Low Km = High affinity, High Km = Low 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. Km increases, 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. Vmax decreases, Km usually remains unchanged.
  • Classification and Nomenclature (IUBMB): Enzymes are named based on the reaction they catalyze and classified into 6 major classes:
    1. Oxidoreductases/Dehydrogenases: Catalyze oxidation-reduction reactions.
    2. Transferases: Transfer a functional group (e.g., amino, phosphate) from one molecule to another.
    3. Hydrolases: Catalyze hydrolysis (breakdown using water). (e.g., Amylase, Protease, Lipase).
    4. Lyases: Remove groups from substrates non-hydrolytically, often forming double bonds.
    5. Isomerases: Catalyze rearrangement of atoms within a molecule (isomerization).
    6. Ligases: Join two molecules together, usually coupled with ATP hydrolysis.
  • Co-factors: Non-protein components required by some enzymes for activity.
    • Apoenzyme: The protein part of the enzyme.
    • Holoenzyme: Apoenzyme + Co-factor (catalytically active).
    • Types of Co-factors:
      • Prosthetic Groups: Organic molecules tightly bound 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, NADP derived from Niacin; FAD derived from Riboflavin; Coenzyme A). Often act as carriers of specific groups.
      • Metal Ions: 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)

  1. When analyzing the chemical composition of living tissue by grinding it in trichloroacetic acid, lipids are found in which fraction and why?
    a) Acid-soluble fraction, because they are small molecules.
    b) Acid-insoluble fraction, because they are polymers.
    c) Acid-insoluble fraction, because they form micelles/membrane fragments insoluble in acid.
    d) Acid-soluble fraction, because they are hydrophobic.

  2. Which of the following is a secondary metabolite used as a drug?
    a) Glucose
    b) Alanine
    c) Vinblastin
    d) Lecithin

  3. The quaternary structure of a protein refers to:
    a) The sequence of amino acids in the polypeptide chain.
    b) The formation of α-helices and β-sheets.
    c) The overall 3D shape of a single polypeptide chain.
    d) The assembly and arrangement of multiple polypeptide subunits.

  4. Which polysaccharide is the main structural component of plant cell walls and is a polymer of β-glucose?
    a) Starch
    b) Glycogen
    c) Chitin
    d) Cellulose

  5. Which type of bond links nucleotides together in a nucleic acid strand?
    a) Peptide bond
    b) Glycosidic bond
    c) Phosphodiester bond
    d) Hydrogen bond

  6. Lecithin, a major component of cell membranes, is classified as a:
    a) Triglyceride
    b) Steroid
    c) Phospholipid
    d) Wax

  7. An enzyme that catalyzes the joining of two molecules using ATP belongs to which class?
    a) Hydrolases
    b) Lyases
    c) Ligases
    d) Transferases

  8. How does a competitive inhibitor affect enzyme kinetics?
    a) Increases Vmax, Km remains unchanged.
    b) Decreases Vmax, decreases Km.
    c) Vmax remains unchanged, increases Km.
    d) Decreases Vmax, Km remains unchanged.

  9. NAD and FAD, which act as hydrogen carriers in metabolic reactions, are examples of:
    a) Prosthetic groups
    b) Metal ions
    c) Co-enzymes
    d) Apoenzymes

  10. The constant synthesis and breakdown of biomolecules within a cell, maintaining relatively stable concentrations, is referred to as:
    a) Static equilibrium
    b) Metabolic flux
    c) Dynamic steady-state
    d) Catabolism only

Answer Key for MCQs:

  1. c
  2. c
  3. d
  4. d
  5. c
  6. c
  7. c
  8. c
  9. c
  10. c

Study these notes thoroughly. Understand the structures, the bonds linking monomers, the functions, and especially the enzyme kinetics part. Good luck with your preparation!

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