Biomolecules studies the chemistry of the molecules of life: carbohydrates, proteins and amino acids, enzymes and vitamins, nucleic acids, hormones and lipids. You learn how glucose and fructose are built, how sugars link through glycosidic bonds, how amino acids fold into proteins, and how DNA and RNA store genetic information. It is a high-yield, fact-dense NEET chapter that rewards memory of structures, reducing or non-reducing sugars, vitamin deficiencies and base pairing.
Class 12 Chemistry · Chapter 14 – swipe through all 10 cards to understand the whole chapter.
Carbohydrates: Classification
Carbohydrates are optically active polyhydroxy aldehydes or ketones (or yield them on hydrolysis).
The Cₓ(H2O)ᵧ rule is loose — sugars like rhamnose break it.
- Mono- (glucose), oligo-/di- (sucrose), poly- (starch) by hydrolysis
- Sugars = sweet, soluble; non-sugars = polysaccharides, insoluble
- Glucose = aldohexose, fructose = ketohexose
Glucose & Fructose Structure
Classic chemical tests fix glucose as a straight-chain D-(+)-aldohexose, C6H12O6.
Pentaacetate ⇒ 5 –OH; HI → n-hexane ⇒ straight 6-C chain; 1 HCN ⇒ one –CHO.
- Fructose = ketohexose at C-2 but still a reducing sugar
- Glucose closes to a 6-ring pyranose; fructose to a 5-ring furanose
- C-1 –OH (anomeric C) α (down) or β (up) → two anomers
Mutarotation
Pure α- and β-glucose slowly interconvert through the open chain to a fixed rotation.
Rotation changes, molecular formula does not; equilibrium ≈ 36% α, 64% β.
- Anomers differ only at the anomeric C-1, not at C-4 (that’s an epimer)
- 0.36(111) + 0.64(19.2) ≈ 52° matches observed +52.7°
- Caused by ring repeatedly opening and re-closing
Di- & Polysaccharides
A glycosidic linkage (C–O–C, loss of water) joins sugars; the α/β angle decides digestibility.
Sucrose hydrolysis → invert sugar; rotation flips +66.5° → −39.7°.
- Maltose (2 α-glucose, C1–C4) & lactose (β-gal + glucose) are reducing
- Starch = amylose (linear α-1,4) + amylopectin (branched α-1,4 & α-1,6)
- α-links = digestible storage; β-1,4 = tough structural cellulose
Amino Acids & the Zwitterion
α-Amino acids carry –NH2 and –COOH on the same carbon and exist as dipolar ions.
Essential ones (valine, leucine, lysine) come from diet; rest are non-essential.
- Zwitterion ⇒ high melting, water-soluble, salt-like
- All except glycine are optically active (chiral α-carbon)
- Peptide bond = amide link –CO–NH– between two amino acids
Four Levels of Protein Structure
A protein’s function comes from how its amino-acid chain folds.
Denaturation breaks secondary/tertiary structure; primary sequence stays intact.
- Secondary held by H-bonds; tertiary adds –S–S– disulphide bridges
- Quaternary = ≥2 subunits, e.g. haemoglobin (4 chains)
- Egg white / curdled milk = denaturation in action
Enzymes & Vitamins
Enzymes are protein biocatalysts; vitamins are trace micronutrients you must eat.
B12 is the water-soluble vitamin that IS stored (in the liver).
- Enzymes = globular proteins, lock-and-key, names end in -ase
- Maltase→maltose, invertase→sucrose, zymase→glucose to ethanol
- C→scurvy, A→night blindness, D→rickets, K→poor clotting
Nucleic Acids (DNA & RNA)
Nucleic acids are polymers of nucleotides that store and transfer genetic information.
Backbone = 3′–5′ phosphodiester bonds; strands held by H-bonds.
- DNA: deoxyribose + thymine; RNA: ribose + uracil
- Purines A, G (2 rings); pyrimidines C, T, U (1 ring)
- Pairing A=T (2 H-bonds), G≡C (3 H-bonds); strands antiparallel
Chargaff’s Rule
In double-stranded DNA each base pairs with its complement, so percentages are linked.
Use only for double-stranded DNA, not single-stranded RNA.
- If %A = 30, then %T = 30, so %G = %C = 20
- G≡C-rich DNA is more stable (3 H-bonds, needs more heat)
- Know one strand ⇒ you can write the complementary strand
Hormones, Lipids & Metabolism
Hormones are chemical messengers; lipids build membranes; metabolism runs on hydrolysis and glycolysis.
Thyroxine = the iodine-containing hormone; Co in B12, Fe in haemoglobin, Mg in chlorophyll.
- Steroid (testosterone), peptide (insulin), amine (adrenaline, thyroxine)
- Phospholipid = glycerol + 2 fatty acids + 1 phosphate → cell membranes
- Glycolysis: glucose → 2 pyruvate (+ATP); digestion = hydrolysis
📝 Practice Biomolecules — 10 NEET PYQs
Real previous-year questions · with answers & solutions
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Frequently Asked Questions
Biomolecules are the organic molecules that make up and run living cells, mainly carbohydrates, proteins, nucleic acids, lipids, enzymes, vitamins and hormones. In Class 12 Chemistry the chapter focuses on their structure, classification and biological roles.
An amino acid carries an acidic –COOH and a basic –NH2 group on the same carbon, so a proton shifts to form a dipolar ion called a zwitterion, written as H3N+ –CHR–COO-. This salt-like ionic nature makes amino acids high melting and water soluble, and at the isoelectric point (pI) the net charge is zero.
A reducing sugar has a free aldehyde or ketone (free anomeric –OH) and can reduce Tollens or Fehling reagent, like glucose, fructose, maltose and lactose. Sucrose is non-reducing because both anomeric carbons are locked in the glycosidic linkage between glucose and fructose.
Mutarotation is the slow change in optical rotation when pure alpha-D-glucose (+111 degrees) or beta-D-glucose (+19.2 degrees) is dissolved in water, until both reach an equilibrium rotation of about +52.7 degrees. It happens because the cyclic ring keeps opening to the open chain and re-closing, interconverting the two anomers.
Yes, Biomolecules is part of the NEET Class 12 Chemistry syllabus and is a scoring chapter because most questions are direct and memory based. Commonly tested points are reducing or non-reducing sugars, vitamin deficiency diseases, DNA versus RNA, base pairing and types of hormones.