Board exam focus — Biomolecules (CBSE & HBSE)

Carbohydrates (classification, glucose structure, reducing/non-reducing sugars), proteins (amino acids, peptide bond, protein structure levels), nucleic acids (DNA/RNA), vitamins and hormones.

Carbohydrates

Biomolecules

Carbohydrates

Polyhydroxy aldehydes/ketones or substances that hydrolyze to them. General formula: Cx(H2O)y.

Classification

Monosaccharides (cannot be hydrolyzed):

• Trioses (C3): glyceraldehyde
• Pentoses (C5): ribose (in RNA), deoxyribose (in DNA)
• Hexoses (C6): glucose, fructose, galactose

Disaccharides (2 monosaccharides + glycosidic bond):

Polysaccharides (many monosaccharides):

• Starch: alpha-D-glucose; amylose (helical, alpha-1,4) + amylopectin (branched, alpha-1,4 and 1,6)
• Glycogen: animal starch; more branches than amylopectin; liver and muscle storage
• Cellulose: beta-1,4-glucose (straight chains, H-bonded); structural material in plants; not digestible by humans

Structure of Glucose

Molecular formula: C6H12O6 (aldohexose) Open chain: CHO-CHOH-CHOH-CHOH-CHOH-CH2OH

Evidence for structure:

1. Reacts with HCN → cyanohydrin (confirms CHO)
2. Reacts with NH2OH → oxime (confirms C=O)
3. Oxidation with Br2 water → gluconic acid (only CHO oxidized)
4. Reduction → sorbitol (hexahydric alcohol, confirms 6 OH groups)
5. Acetylation with acetic anhydride → pentaacetate (confirms 5 OH groups)

Haworth projection (cyclic form): Glucose forms pyranose ring (6-membered) by reaction of C1-CHO with C5-OH. alpha-D-Glucose: OH at C1 on same side as CH2OH (equatorial/axial) beta-D-Glucose: OH at C1 on opposite side to CH2OH

Mutarotation

Interconversion of alpha and beta anomers in solution through open chain form. alpha-D-Glucose: [alpha]D = +112.2 degrees beta-D-Glucose: [alpha]D = +18.7 degrees Equilibrium mixture: [alpha]D = +52.5 degrees

Reducing and Non-reducing Sugars

Reducing sugars: Have free aldehyde or ketone group (free anomeric OH). Give positive Fehling/Tollens test. Examples: Glucose, fructose, maltose, lactose

Non-reducing sugars: No free anomeric OH (glycosidic bond at anomeric C). Do NOT reduce Fehling. Example: Sucrose (1,2-glycosidic bond between both anomeric carbons)

Proteins and Enzymes

Proteins

Amino Acids

Building blocks of proteins. Structure: H2N-CHR-COOH (alpha-amino acid) R = side chain (distinguishes different amino acids)

Essential amino acids (not synthesized by body): Val, Leu, Ile, Met, Phe, Trp, Thr, Lys

Classification of amino acids:

• Neutral (equal COOH and NH2): glycine, alanine, valine
• Acidic (more COOH): aspartic acid, glutamic acid
• Basic (more NH2): lysine, arginine, histidine

Zwitterion form: H3N+-CHR-COO- (in neutral solution; both +ve and -ve charges) Isoelectric point (pI): pH at which amino acid has zero net charge (zwitterionic form)

Peptide Bond

Amino acids linked by peptide bonds (-CO-NH-) formed by condensation reaction: -COOH + H2N- → -CO-NH- + H2O

Dipeptide: two amino acids; tripeptide: three; polypeptide: many; protein: large polypeptide

Protein Structure

Primary structure: Sequence of amino acids in polypeptide chain (covalent peptide bonds)

Secondary structure: Local conformation stabilized by H-bonds between peptide bonds.

• Alpha-helix: right-handed helix; H-bonds between C=O and N-H 4 residues apart; keratin, myosin
• Beta-sheet (pleated sheet): extended chains side-by-side; H-bonds between adjacent chains; silk fibroin

Tertiary structure: 3D folding of polypeptide by:

• H-bonds
• Disulfide bonds (Cys-S-S-Cys)
• Ionic interactions (salt bridges)
• Hydrophobic interactions (nonpolar residues inside)
• Van der Waals forces

Quaternary structure: Association of two or more polypeptide chains. Example: Hemoglobin: 2 alpha + 2 beta chains.

Denaturation

Loss of 3D structure (secondary, tertiary, quaternary) without breaking peptide bonds. Causes: Heat, strong acid/base, organic solvents, heavy metals (Hg, Pb). Example: Coagulation of egg white (albumin) on boiling. Denatured protein loses biological activity.

Enzymes

Biological catalysts (proteins). Extremely specific. Active site: 3D pocket where substrate binds. Lock-and-Key model: substrate fits active site exactly. Induced fit model: active site changes shape to bind substrate.

Important enzymes:

Nucleic Acids and Vitamins

Nucleic Acids

Components of Nucleic Acids

Nucleotide = Nitrogenous base + pentose sugar + phosphate group

Nitrogenous bases: Purines: Adenine (A), Guanine (G) [two-ring structure] Pyrimidines: Cytosine (C), Thymine (T), Uracil (U) [one-ring structure]

Sugars: DNA: 2-deoxyribose RNA: ribose

DNA vs RNA

Watson-Crick Model of DNA (Double Helix)

1. Two polynucleotide chains wound in right-handed double helix
2. Chains antiparallel (5'→3' and 3'→5')
3. Backbone: sugar-phosphate on outside
4. Bases on inside; base pairs in middle of helix
5. Base pairing: A pairs with T (2 H-bonds); G pairs with C (3 H-bonds)

(Chargaff's rules: [A]=[T]; [G]=[C])

6. 3.4 Angstrom rise per base pair; 34 Angstrom per turn (10 bp/turn)
7. Stabilized by H-bonds (base pairing) and hydrophobic stacking interactions

Types of RNA

Vitamins

Organic compounds required in small amounts for normal metabolism. Cannot be synthesized by body (must be obtained from diet).

Fat-soluble vitamins (stored in body fat): A, D, E, K

Water-soluble vitamins (not stored): B, C

Frequently asked questions

Are these Biomolecules notes free?

Yes — the Biomolecules notes for chemistry (Class 12) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Biomolecules notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Biomolecules chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Biomolecules.