Chapter: Biomolecules – Detailed Notes for NEET/JEE Mains

1. Introduction to Biomolecules

• Biomolecules: Complex organic molecules that are found in living organisms and are essential for their biological processes. These include carbohydrates, proteins, nucleic acids, lipids, vitamins, etc.

2. Carbohydrates

• Definition: Polyhydroxy aldehydes or ketones, or substances which yield these upon hydrolysis. They are the most abundant organic molecules on Earth.
• General Formula: Cx​(H2​O)y​ (where x≥3).
• Classification:
  1. Monosaccharides: Simplest carbohydrates that cannot be hydrolyzed further.
     • Examples: Glucose (C6​H12​O6​), Fructose (C6​H12​O6​), Ribose (C5​H10​O5​), Deoxyribose.
     • Aldoses: Monosaccharides containing an aldehyde group (e.g., Glucose, Ribose).
     • Ketoses: Monosaccharides containing a ketone group (e.g., Fructose).
     • Reducing Sugars: All monosaccharides are reducing sugars as they have free aldehyde or ketone groups that can reduce Tollens’ or Fehling’s reagents.
  2. Oligosaccharides: Yield 2-10 monosaccharide units on hydrolysis.
     • Disaccharides: Yield two monosaccharide units on hydrolysis.
       • Sucrose: Glucose + Fructose (non-reducing sugar, as the glycosidic linkage is formed between the aldehyde group of glucose and ketone group of fructose, making both reducing groups unavailable).
       • Maltose: Glucose + Glucose (reducing sugar, linkage between C1 of one glucose and C4 of another; one aldehyde group remains free).
       • Lactose: Glucose + Galactose (reducing sugar, linkage between C1 of galactose and C4 of glucose; one aldehyde group remains free).
     • Trisaccharides: Yield three monosaccharide units on hydrolysis (e.g., Raffinose: Glucose + Fructose + Galactose).
  3. Polysaccharides: Yield a large number of monosaccharide units on hydrolysis (hundreds to thousands).
     • Examples: Starch, Cellulose, Glycogen.
     • Starch: Main storage polysaccharide for plants. A polymer of α-D-glucose.
       • Composed of two components:
         • Amylose: Linear polymer of α-D-glucose, linked by α-1,4-glycosidic bonds. Water soluble.
         • Amylopectin: Branched polymer of α-D-glucose, linked by α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at branching points. Water insoluble.
     • Cellulose: Structural polysaccharide in plants. A linear polymer of β-D-glucose, linked by β-1,4-glycosidic bonds.
     • Glycogen: Animal starch. Main storage polysaccharide in animals. Highly branched polymer of α-D-glucose, similar to amylopectin but more highly branched.
• Glucose (C6​H12​O6​):
  • Preparation:
    • From Sucrose (hydrolysis): C12​H22​O11​+H2​OH+​C6​H12​O6​ (Glucose)+C6​H12​O6​ (Fructose)
    • From Starch (hydrolysis): (\text{C}_6\text{H}_{10}\text{O}_5)_n + n\text{H}_2\text{O} \xrightarrow{\text{H}^+, 393K, 2-3 atm}} n\text{C}_6\text{H}_{12}\text{O}_6
  • Structure: Open-chain structure with one aldehyde group and five hydroxyl groups (an aldohexose). Also exists in cyclic hemiacetal forms (α-D-glucose and β-D-glucose) due to reaction between C1 (aldehyde) and C5 (-OH). These two cyclic forms are called anomers.
  • Mutarotation: The change in optical rotation of an aqueous solution of glucose (or other monosaccharides) over time until an equilibrium mixture of α- and β-anomers is reached.

3. Proteins

• Definition: Polymers of α-amino acids linked by peptide bonds. They are fundamental building blocks of life.
• Amino Acids: Organic compounds containing at least one amino group (−NH2​) and one carboxyl group (−COOH).
  • α-amino acids are those where both the amino group and carboxyl group are attached to the same carbon atom (the α-carbon).
  • Zwitter ion: In aqueous solution, amino acids exist as dipolar ions called zwitter ions, where the amino group is protonated (−NH3+​) and the carboxyl group is deprotonated (−COO−).
  • Isoelectric point: The pH at which an amino acid exists predominantly as a zwitter ion and has no net charge, so it does not migrate in an electric field.
• Peptide Bond: An amide linkage (-CO-NH-) formed by the condensation reaction between the carboxyl group of one amino acid and the amino group of another, with the elimination of a water molecule.
• Polypeptide: A chain of many amino acids linked by peptide bonds. Proteins are long polypeptides.
• Classification of Proteins:
  1. Based on molecular shape:
     • Fibrous Proteins: Long, thread-like molecules. Insoluble in water. Provide structural support.
       • Examples: Keratin (hair, nails, wool), Myosin (muscles), Collagen (tendons, bones).
     • Globular Proteins: Spherical shape due to extensive coiling. Soluble in water. Perform biological functions.
       • Examples: Enzymes, Hormones (insulin), Haemoglobin, Antibodies.
  2. Based on hydrolysis products:
     • Simple Proteins: Yield only amino acids on hydrolysis.
     • Conjugated Proteins: Yield amino acids and a non-protein part (prosthetic group) on hydrolysis.
• Structure of Proteins:
  • Primary Structure: The specific sequence of amino acids in a polypeptide chain. Determines all higher-level structures.
  • Secondary Structure: Regular folding patterns of the polypeptide chain due to hydrogen bonding between the backbone atoms.
    • α-helix: Right-handed helical structure.
    • β-pleated sheet: Folded, zig-zag structure.
  • Tertiary Structure: The overall three-dimensional folding of the polypeptide chain, resulting from interactions (H-bonds, disulfide bridges, ionic bonds, hydrophobic interactions) between the side chains of amino acids. Gives globular proteins their compact structure.
  • Quaternary Structure: The arrangement of two or more polypeptide subunits (each with its own tertiary structure) to form a functional protein (e.g., haemoglobin).
• Denaturation of Proteins: The process where proteins lose their biological activity and often their secondary, tertiary, and quaternary structures (but not primary structure) due to physical or chemical changes (e.g., heat, changes in pH, strong acids/bases, salts of heavy metals, alcohol). It typically involves the disruption of hydrogen bonds and other weak interactions.
  • Example: Coagulation of egg white on boiling.

4. Nucleic Acids

• Definition: Biopolymers that carry genetic information. Two main types: DNA and RNA.
• Monomers: Nucleotides.
• Nucleotide: Composed of three components:
  1. A Pentose Sugar: Ribose (in RNA) or 2-deoxyribose (in DNA).
  2. A Nitrogenous Base:
     • Purines: Adenine (A) and Guanine (G).
     • Pyrimidines: Cytosine (C), Thymine (T, in DNA only), and Uracil (U, in RNA only).
  3. A Phosphate Group (H3​PO4​).
• Nucleoside: Sugar + Nitrogenous base (without phosphate group).
• Polynucleotide: Many nucleotides linked together by phosphodiester bonds (between 5′-phosphate of one nucleotide and 3′-OH of the next).
• DNA (Deoxyribonucleic Acid):
  • Structure: Double helix model (Watson and Crick). Two polynucleotide strands coiled around each other, held together by hydrogen bonds between complementary base pairs.
  • Base Pairing: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds (A=T). Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds (G≡C).
  • Genetic Information: Carries the genetic blueprint of an organism.
  • Replication: DNA can self-replicate.
• RNA (Ribonucleic Acid):
  • Structure: Single-stranded polynucleotide.
  • Sugar: Ribose.
  • Bases: A, G, C, U (Uracil replaces Thymine).
  • Functions: Involved in protein synthesis. Three main types:
    • mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.
    • rRNA (ribosomal RNA): A component of ribosomes.
    • tRNA (transfer RNA): Carries specific amino acids to the ribosomes during protein synthesis.

5. Vitamins

• Definition: Organic compounds required in small amounts in the diet to perform specific biological functions for the maintenance of optimal growth and health. They cannot be synthesized by the body (except Vitamin D).
• Classification:
  1. Fat-soluble Vitamins: A, D, E, K. Stored in adipose tissue and liver.
     • Vitamin A (Retinol): Vision, normal growth. Deficiency causes night blindness.
     • Vitamin D (Calciferol): Absorption of Ca and P. Deficiency causes rickets (children), osteomalacia (adults).
     • Vitamin E (Tocopherol): Antioxidant, prevents oxidation of polyunsaturated fatty acids. Deficiency causes muscular weakness, increased fragility of RBCs.
     • Vitamin K (Phylloquinone): Blood clotting. Deficiency causes increased blood clotting time/hemorrhage.
  2. Water-soluble Vitamins: B-complex vitamins and Vitamin C. Must be supplied regularly as they are excreted in urine and not stored (except Vitamin B12).
     • Vitamin B1 (Thiamine): Carbohydrate metabolism. Deficiency causes Beri-beri.
     • Vitamin B2 (Riboflavin): Metabolism. Deficiency causes cheilosis (cracks at corners of mouth), digestive disorders.
     • Vitamin B6 (Pyridoxine): Amino acid metabolism. Deficiency causes convulsions.
     • Vitamin B12 (Cyanocobalamin): RBC formation, nerve function. Deficiency causes pernicious anemia.
     • Vitamin C (Ascorbic Acid): Collagen formation, antioxidant. Deficiency causes Scurvy (bleeding gums, joint pain).

6. Hormones

• Definition: Chemical messengers secreted by endocrine glands directly into the bloodstream. They act on target organs to regulate various physiological processes.
• Classification:
  • Steroid Hormones: (e.g., Estrogen, Testosterone, Progesterone)
  • Peptide/Protein Hormones: (e.g., Insulin, Glucagon)
  • Amino acid derivatives: (e.g., Adrenaline, Thyroxine)
• Functions: Regulate growth, metabolism, reproduction, mood, etc.

7. Enzymes

• Definition: Biocatalysts. Proteins that catalyze biochemical reactions in living organisms.
• Mechanism: They lower the activation energy of a reaction. Highly specific in their action.
• Active Site: A specific region on the enzyme molecule where the substrate binds.
• Factors Affecting Enzyme Activity: Temperature (optimal temp for maximum activity), pH (optimal pH), substrate concentration.
• Lock and Key Model: Substrate fits precisely into the active site of the enzyme.
• Induced Fit Model: Active site changes shape slightly to fit the substrate more perfectly upon binding.