Schooling · Class 12 · Chemistry · Biomolecules · Chapter on a page
CBSE Class 12 · Chemistry · NCERT Chemistry-II
Biomolecules: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Carbohydrates
- Monosaccharides
- Amino acids
- Protein structure
- Enzymes
- Nucleic acids
- Vitamins
- Formulas and facts
- Glucose molecular formula
- Fructose molecular formula
- Peptide bond formation
- Isoelectric point
- DNA double helix
- Reducing sugars
- Non-reducing sugars
- Worked examples
- Identifying a reducing sugar
- Calculating peptide bonds in a…
- Base pairing in DNA
- Watch out for
- Confusing aldoses and ketoses
- Writing the wrong number of H-bonds…
- Calling sucrose a reducing sugar
- Mixing up α and β anomers
- Forgetting that enzymes are…
What this chapter is about
Biomolecules are the organic molecules that form the structural and functional basis of all living organisms. This chapter introduces three major classes of biomolecules (carbohydrates, proteins and nucleic acids) along with vitamins. A Class 12 student meets this topic because it connects organic chemistry concepts (functional groups, stereochemistry, condensation reactions) to biological systems, showing how chemistry explains life processes at the molecular level.
Key ideas
- Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield them on hydrolysis; they are classified as monosaccharides, disaccharides and polysaccharides based on the number of sugar units.
- Monosaccharides exist in open-chain and cyclic (Haworth) forms; glucose forms a six-membered pyranose ring, while fructose forms a five-membered furanose ring. The α and β anomers differ in the orientation of the –OH group at the anomeric carbon (C-1 in glucose, C-2 in fructose).
- Amino acids contain both an amino group (–NH₂) and a carboxyl group (–COOH); in aqueous solution they exist as zwitterions (H₃N⁺–CHR–COO⁻). They are linked by peptide bonds (–CO–NH–) to form proteins.
- Protein structure has four levels: primary (sequence of amino acids), secondary (α-helix or β-pleated sheet held by hydrogen bonds), tertiary (overall 3-D folding stabilised by disulphide bridges, ionic and hydrophobic interactions), and quaternary (association of multiple polypeptide chains).
- Enzymes are biological catalysts, mostly proteins, that are highly specific and lower the activation energy of biochemical reactions. They have an active site where substrate binding occurs.
- Nucleic acids (DNA and RNA) are polymers of nucleotides; each nucleotide consists of a nitrogenous base, a pentose sugar and a phosphate group. DNA uses deoxyribose and bases A, G, C, T; RNA uses ribose and bases A, G, C, U.
- Vitamins are organic compounds required in small amounts for normal metabolism; they are classified as water-soluble (B-complex, C) and fat-soluble (A, D, E, K). Deficiency diseases result when intake is insufficient.
Formulas and facts to remember
- Glucose molecular formula: C₆H₁₂O₆ — an aldohexose with an aldehyde group at C-1.
- Fructose molecular formula: C₆H₁₂O₆ — a ketohexose with a ketone group at C-2.
- Peptide bond formation: amino acid 1 + amino acid 2 → dipeptide + H₂O (condensation reaction between –COOH of one and –NH₂ of another).
- Isoelectric point: the pH at which an amino acid exists as a zwitterion with no net charge.
- DNA double helix: two antiparallel strands held by hydrogen bonds; A pairs with T (two H-bonds), G pairs with C (three H-bonds).
- Reducing sugars: carbohydrates with a free aldehyde or ketone group that can reduce Fehling's solution or Tollens' reagent (e.g., glucose, maltose).
- Non-reducing sugars: sucrose is non-reducing because both anomeric carbons are involved in the glycosidic bond.
- Vitamin deficiency examples: Vitamin A deficiency causes night blindness; Vitamin C deficiency causes scurvy; Vitamin D deficiency causes rickets.
Common mistakes
- Confusing aldoses and ketoses → Remember: aldoses have –CHO at terminal carbon; ketoses have C=O at C-2.
- Writing the wrong number of H-bonds in base pairs → A–T has 2 hydrogen bonds; G–C has 3 hydrogen bonds.
- Calling sucrose a reducing sugar → Sucrose is non-reducing because both anomeric carbons are engaged in the glycosidic linkage.
- Mixing up α and β anomers → In α-anomer, the –OH at anomeric carbon is on the opposite side of CH₂OH in the Haworth projection; in β-anomer, they are on the same side.
- Forgetting that enzymes are substrate-specific → Each enzyme acts on a particular substrate due to the shape of its active site (lock-and-key model).
Quick revision
- Carbohydrates: polyhydroxy aldehydes (aldoses) or ketones (ketoses); classified by number of sugar units.
- Amino acids join via peptide bonds (–CO–NH–); proteins have primary, secondary, tertiary and quaternary structures.
- DNA: deoxyribose + A, G, C, T; RNA: ribose + A, G, C, U; base pairing A–T (2 H-bonds), G–C (3 H-bonds).
- Vitamins: water-soluble (B, C) and fat-soluble (A, D, E, K); deficiency causes specific diseases.
- Enzymes lower activation energy and are highly specific to substrates.