Chemistry: Class 12 — Chapter 5: Biomolecules

• Monosaccharides: Carbohydrates that cannot be further hydrolyzed into simpler units of polyhydroxy aldehyde or ketone (e.g., glucose, fructose, ribose).
• Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units on hydrolysis. They are further classified as disaccharides, trisaccharides, etc. (e.g., sucrose, maltose).
• Polysaccharides: High molecular mass carbohydrates that yield a large number of monosaccharide units on hydrolysis (e.g., starch, cellulose, glycogen). These are generally non-sugars and are not sweet.
• Reducing Sugars: Carbohydrates containing free aldehydic or ketonic groups that can reduce Fehling's solution and Tollens' reagent. All monosaccharides are reducing sugars.
• Non-reducing Sugars: Carbohydrates in which the reducing groups are bonded and cannot reduce these reagents (e.g., sucrose).

Glucose ($C_6H_{12}O_6$)

Also known as dextrose, glucose is an aldohexose and is highly abundant on Earth.

• Preparation:
  1. From Sucrose: Boiling sucrose with dilute $HCl$ or $H_2SO_4$ in alcoholic solution yields equal amounts of glucose and fructose: $$\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_{12}\text{O}_6 \text{ (Glucose)} + \text{C}_6\text{H}_{12}\text{O}_6 \text{ (Fructose)}$$
  2. From Starch: Commercial production by hydrolysis of starch with dilute $H_2SO_4$ at $393\text{ K}$ under pressure: $$(\text{C}_6\text{H}_{10}\text{O}_5)_n + n\text{H}_2\text{O} \xrightarrow[393\text{ K, 2-3 atm}]{\text{H}^+} n\text{C}_6\text{H}_{12}\text{O}_6$$
• Structure Elucidation:
  • Prolonged heating with $HI$ yields n-hexane, proving a straight chain of six carbon atoms.
  • Reaction with hydroxylamine ($NH_2OH$) forms an oxime, and with hydrogen cyanide ($HCN$) forms a cyanohydrin, confirming the presence of a carbonyl group ($>C=O$).
  • Mild oxidation with bromine water yields gluconic acid, indicating that the carbonyl group is an aldehyde ($-CHO$).
  • Acetylation with acetic anhydride yields glucose pentaacetate, confirming the presence of five hydroxyl ($-OH$) groups on different carbon atoms.
  • Oxidation with nitric acid ($HNO_3$) yields saccharic acid (a dicarboxylic acid), indicating the presence of a primary alcoholic group.
• Cyclic Structure: Glucose exists in two crystalline cyclic hemiacetal forms, $\alpha$ and $\beta$, which differ only in the configuration of the hydroxyl group at C1 (the anomeric carbon). These isomers are called anomers. The six-membered cyclic structure is referred to as a pyranose structure.

Fructose ($C_6H_{12}O_6$)

Fructose is a ketohexose found in fruits, honey, and vegetables. It belongs to the D-series and is laevorotatory, represented as D-(-)-fructose. It exists in cyclic forms containing a five-membered ring called a furanose structure.

Disaccharides consist of two monosaccharide units joined by an oxide linkage formed by the loss of a water molecule, known as a glycosidic linkage.

• Sucrose: Composed of $\alpha$-D-glucose and $\beta$-D-fructose linked via C1 of glucose and C2 of fructose. It is a non-reducing sugar. Hydrolysis of sucrose reverses its optical rotation from dextrorotatory to laevorotatory, producing invert sugar.
• Maltose: Composed of two $\alpha$-D-glucose units linked via a C1–C4 glycosidic bond. It is a reducing sugar.
• Lactose: Known as milk sugar, composed of $\beta$-D-galactose and $\beta$-D-glucose linked via a C1–C4 glycosidic bond. It is a reducing sugar.

• Starch: The main storage polysaccharide in plants. It consists of two components:
  • Amylose: Water-soluble, linear polymer of $\alpha$-D-glucose units linked by C1–C4 glycosidic bonds (15-20% of starch).
  • Amylopectin: Water-insoluble, branched polymer of $\alpha$-D-glucose units with C1–C4 linkages in the main chain and C1–C6 linkages at branching points (80-85% of starch).
• Cellulose: A straight-chain structural polysaccharide found in plant cell walls, composed of $\beta$-D-glucose units linked by C1–C4 glycosidic bonds.
• Glycogen: Known as animal starch, stored in the liver, muscles, and brain. Its structure is highly branched and similar to amylopectin.

Amino acids contain both amino ($-NH_2$) and carboxyl ($-COOH$) functional groups.

• Classification:
  • Acidic, Basic, or Neutral: Based on the relative number of amino and carboxyl groups.
  • Essential Amino Acids: Cannot be synthesized by the body and must be obtained through diet (e.g., valine, leucine).
  • Non-essential Amino Acids: Can be synthesized by the body (e.g., glycine, alanine).
• Zwitterion: In aqueous solution, the carboxyl group loses a proton and the amino group accepts a proton, forming a dipolar, neutral ion with both positive and negative charges. Zwitterions exhibit amphoteric behavior.
• Optical Activity: Except for glycine, all naturally occurring $\alpha$-amino acids are optically active and generally possess the L-configuration.

Amino acids are linked together by peptide bonds (amide linkages, $-CO-NH-$) formed between the carboxyl group of one amino acid and the amino group of another.

• Classification by Shape:
  • Fibrous Proteins: Parallel polypeptide chains held together by hydrogen and disulfide bonds; insoluble in water (e.g., keratin, myosin).
  • Globular Proteins: Polypeptide chains folded into spherical shapes; soluble in water (e.g., insulin, albumin).
• Levels of Protein Structure:
  1. Primary Structure: The specific linear sequence of amino acids in the polypeptide chain.
  2. Secondary Structure: The spatial conformation of the polypeptide chain, stabilized by hydrogen bonding (e.g., $\alpha$-helix and $\beta$-pleated sheet).
  3. Tertiary Structure: The overall folding of the secondary structure into fibrous or globular shapes, stabilized by hydrogen bonds, disulfide linkages, van der Waals forces, and electrostatic attractions.
  4. Quaternary Structure: The spatial arrangement of multiple polypeptide subunits relative to one another.

When a native protein is subjected to physical changes (like temperature) or chemical changes (like pH), its hydrogen bonds are disrupted. The globules unfold and helices uncoil, causing the protein to lose its biological activity. During denaturation, secondary and tertiary structures are destroyed, but the primary structure remains intact (e.g., coagulation of egg white, curdling of milk).

• Fat-Soluble Vitamins: Soluble in fats and oils but insoluble in water. Stored in the liver and adipose tissues (Vitamins A, D, E, and K).
• Water-Soluble Vitamins: Soluble in water. Must be supplied regularly in the diet as they are readily excreted in urine and cannot be stored, except for Vitamin $B_{12}$ (B-group vitamins and Vitamin C).

• Vitamin A: Xerophthalmia, Night blindness.
• Vitamin $B_1$ (Thiamine): Beri-beri.
• Vitamin $B_2$ (Riboflavin): Cheilosis.
• Vitamin C (Ascorbic acid): Scurvy.
• Vitamin D: Rickets, Osteomalacia.
• Vitamin K: Increased blood clotting time.

Complete hydrolysis of nucleic acids yields:

• A pentose sugar: $\beta$-D-2-deoxyribose in DNA, and $\beta$-D-ribose in RNA.
• Phosphoric acid.
• Nitrogenous bases:
  • Purines: Adenine (A) and Guanine (G).
  • Pyrimidines: Cytosine (C), Thymine (T) [only in DNA], and Uracil (U) [only in RNA].

• Nucleoside: A unit formed by attaching a nitrogenous base to the 1' position of a pentose sugar.
• Nucleotide: Formed when a nucleoside is linked to phosphoric acid at the 5' position of the sugar.
• Polynucleotide Chain: Nucleotides are joined together by phosphodiester linkages between the 5' and 3' carbon atoms of adjacent pentose sugars.
• DNA Double Helix: Proposed by Watson and Crick. Two complementary polynucleotide strands are wound around each other and held together by hydrogen bonds between specific base pairs (A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds).
• RNA Structure: Usually single-stranded. Three main types exist: messenger RNA (m-RNA), ribosomal RNA (r-RNA), and transfer RNA (t-RNA).

A unique sequence of bases on DNA exists for every individual, which cannot be altered by surgery. This is utilized in forensic laboratories, paternity testing, and evolutionary studies.