The Chemistry of Life
Biomolecules are the organic compounds that make life possible — carbohydrates for energy, proteins for structure and function, nucleic acids for genetic information, and vitamins for metabolism. This chapter bridges chemistry and biology, and the questions are largely classification-based.
CBSE Class 12 gives 3-5 marks. NEET tests 1-2 questions. The content is classification-heavy — learn the categories, key examples, and distinguishing tests, and you score full marks.
Carbohydrates
General formula: — literally “hydrates of carbon” (though this formula is not universal — deoxyribose and rhamnose do not fit it).
Classification by sugar units
| Type | Number of units | Hydrolysis | Examples |
|---|---|---|---|
| Monosaccharides | 1 | Cannot be hydrolysed further | Glucose, fructose, galactose, ribose |
| Disaccharides | 2 | Give 2 monosaccharides | Sucrose, maltose, lactose, cellobiose |
| Oligosaccharides | 2-10 | Give 2-10 monosaccharides | Raffinose (3), stachyose (4) |
| Polysaccharides | Many | Give many monosaccharides | Starch, cellulose, glycogen, chitin |
Glucose ()
Structure: An aldohexose (6 carbons, aldehyde group). In solution, exists mainly as a cyclic form (pyranose ring — 6-membered ring). Two anomers: -glucose (OH at C1 is axial/down) and -glucose (OH at C1 is equatorial/up).
Key reactions:
- With HI: gives n-hexane (proves 6-carbon straight chain)
- With HNO: oxidises to saccharic acid (proves primary OH at C6)
- With Br water: oxidises aldehyde to gluconic acid
- Tollens’ test (silver mirror) and Fehling’s test (brick-red precipitate): positive — glucose is a reducing sugar
Fructose
A ketohexose (ketone group at C2). Forms a furanose ring (5-membered) in solution. Also a reducing sugar despite having a ketone, because it isomerises to glucose in alkaline conditions.
Reducing vs non-reducing sugars
Reducing sugars have a free anomeric carbon (free aldehyde or ketone in open-chain form) that can reduce Tollens’ or Fehling’s reagent. All monosaccharides are reducing. Among disaccharides: maltose, lactose are reducing. Sucrose is non-reducing because both anomeric carbons (C1 of glucose, C2 of fructose) are involved in the glycosidic bond — neither is free.
“Why is sucrose non-reducing?” is one of the most frequently asked questions in CBSE and NEET. The answer: both anomeric carbons are locked in the glycosidic linkage, so no free aldehyde or ketone is available to reduce the test reagent.
Disaccharides
| Disaccharide | Monomers | Linkage | Reducing? |
|---|---|---|---|
| Sucrose (cane sugar) | Glucose + Fructose | -1,2 | No |
| Maltose (malt sugar) | Glucose + Glucose | -1,4 | Yes |
| Lactose (milk sugar) | Galactose + Glucose | -1,4 | Yes |
Hydrolysis of sucrose is called inversion because the rotation of polarised light changes sign: sucrose is dextrorotatory (+66.5°), but the mixture of glucose (+52.5°) and fructose (-92°) is levorotatory. Hence “invert sugar.”
Polysaccharides
Starch: storage carbohydrate in plants. Made of -glucose. Two components: amylose (linear, -1,4 linkages, ~20%) and amylopectin (branched, -1,4 + -1,6 branches, ~80%). Gives blue colour with iodine.
Cellulose: structural carbohydrate in plant cell walls. Made of -glucose with -1,4 linkages. Humans cannot digest it (we lack the enzyme cellulase). Most abundant organic compound on Earth.
Glycogen: storage carbohydrate in animals. Similar to amylopectin but more highly branched. Stored in liver and muscle.
Both are polysaccharides of glucose, but starch uses -glycosidic linkages (digestible by humans) while cellulose uses -glycosidic linkages (not digestible). This single bond orientation difference is why we can eat rice but not wood.
Proteins
Polymers of amino acids joined by peptide bonds (). There are 20 standard amino acids, each with an amino group (), a carboxyl group (), an H atom, and a unique side chain (R group) attached to the alpha carbon.
Amino acid properties
- Zwitterion: At physiological pH, amino acids exist as dipolar ions:
- Isoelectric point (pI): The pH at which the amino acid has no net charge and does not move in an electric field
- Essential amino acids: Cannot be synthesised by the body — must be obtained from diet (valine, leucine, isoleucine, etc.)
Levels of protein structure
| Level | Description | Bonds involved |
|---|---|---|
| Primary | Amino acid sequence (linear chain) | Peptide bonds (covalent) |
| Secondary | Local folding — -helix or -pleated sheet | Hydrogen bonds between backbone C=O and N-H |
| Tertiary | Overall 3D shape of a single polypeptide | H-bonds, disulphide bridges, hydrophobic interactions, ionic bonds |
| Quaternary | Multiple polypeptide subunits assembled together | Same as tertiary, between subunits |
Denaturation: Loss of secondary, tertiary and quaternary structure by heat, pH change, heavy metals, or organic solvents. The primary structure (amino acid sequence) remains intact, but the protein loses its biological activity. Example: boiling egg white — albumin unfolds and coagulates irreversibly.
Enzymes
Biological catalysts — proteins (mostly) with highly specific active sites.
Lock-and-key model: The substrate fits exactly into the active site, like a key in a lock. Induced fit model: The active site changes shape slightly when the substrate binds, achieving optimal fit.
Properties: extreme specificity, work at mild conditions (body temperature, neutral pH), very high efficiency (turnover numbers of 10 to 10 per second).
Nucleic Acids
DNA and RNA store and transmit genetic information.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Structure | Double helix | Usually single-stranded |
| Function | Genetic blueprint (long-term storage) | Protein synthesis (mRNA, tRNA, rRNA) |
| Location | Nucleus (mainly) | Nucleus + cytoplasm |
| Stability | Very stable (double helix + deoxyribose) | Less stable |
Nucleotide structure
A nucleotide = nitrogenous base + sugar + phosphate group. Nucleotides are the monomers of nucleic acids.
- Purines (double ring): Adenine (A), Guanine (G)
- Pyrimidines (single ring): Cytosine (C), Thymine (T in DNA), Uracil (U in RNA)
Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). The extra H-bond in G-C makes GC-rich DNA more thermally stable (higher melting temperature).
Vitamins
Organic compounds needed in small amounts for metabolism. Not synthesised in sufficient quantities by the body — must be obtained from diet.
Complete vitamin table
| Vitamin | Chemical name | Solubility | Deficiency disease | Source |
|---|---|---|---|---|
| A | Retinol | Fat | Night blindness, xerophthalmia | Carrots, liver, fish oil |
| B | Thiamine | Water | Beriberi | Whole grains, pork |
| B | Riboflavin | Water | Cheilosis (cracked lips) | Milk, eggs, liver |
| B | Pyridoxine | Water | Convulsions | Meat, fish, potatoes |
| B | Cyanocobalamin | Water | Pernicious anaemia | Meat, dairy (absent in plants) |
| C | Ascorbic acid | Water | Scurvy | Citrus fruits, amla |
| D | Calciferol | Fat | Rickets (children), osteomalacia (adults) | Sunlight, fish oil |
| E | Tocopherol | Fat | Reproductive issues, sterility | Vegetable oils, nuts |
| K | Phylloquinone | Fat | Slow blood clotting, haemorrhage | Green vegetables, liver |
| H (Biotin) | Biotin | Water | Dermatitis | Egg yolk, liver |
Fat-soluble vitamins: A, D, E, K — stored in the body (can accumulate to toxic levels). Water-soluble vitamins: B-complex and C — excreted if in excess (need daily intake). Mnemonic: “All Dogs Eat Kibble” for fat-soluble vitamins.
Hormones vs Vitamins vs Enzymes
| Feature | Hormones | Vitamins | Enzymes |
|---|---|---|---|
| Nature | Can be protein, steroid, amine | Organic compounds | Mostly proteins |
| Synthesis | By endocrine glands | Not synthesised (or insufficient) — dietary | By cells |
| Function | Chemical messengers | Coenzymes, metabolic cofactors | Biological catalysts |
| Consumed in reaction? | Yes (bind receptors) | No (recycled as coenzymes) | No (regenerated) |
Solved Examples
In sucrose, the glycosidic bond involves C1 of glucose and C2 of fructose — both anomeric carbons. Neither can open to expose a free aldehyde/ketone. In maltose, the bond is between C1 of one glucose and C4 of another — the second glucose still has a free C1 that can open to the aldehyde form and reduce Tollens’/Fehling’s reagent.
Both are glucose polymers. Add iodine solution: starch gives a blue-black colour (iodine fits inside the amylose helix). Cellulose gives no colour (no helical structure for iodine to fit). Alternatively, test with human saliva (salivary amylase): starch is digested to maltose; cellulose is not (we lack cellulase).
A protein has 150 amino acids. Number of peptide bonds = number of amino acids - 1 = 149. Each peptide bond is formed by a condensation reaction between the of one amino acid and the of the next, with loss of one water molecule.
Vitamin D is synthesised in the skin when exposed to UV-B sunlight. Despite abundant sunlight in India, vitamin D deficiency is widespread because: (1) dark skin pigmentation reduces UV absorption, (2) cultural practices of covering skin, (3) air pollution blocks UV-B in cities, (4) indoor lifestyles. Fortification of milk and supplements are recommended.
Common Mistakes to Avoid
Calling sucrose a reducing sugar. Sucrose is non-reducing because both anomeric carbons are involved in the glycosidic bond. Maltose and lactose are reducing sugars.
Confusing DNA bases with RNA bases. DNA has thymine (T). RNA has uracil (U). Both have A, G, C. The mnemonic: DNA has Thymine because DNA is the Template.
Mixing water-soluble and fat-soluble vitamins. Water-soluble (B, C) are excreted daily — need regular intake. Fat-soluble (A, D, E, K) accumulate in fat tissue — excess can be toxic (hypervitaminosis).
Saying enzymes are consumed in reactions. Enzymes are catalysts — they are regenerated at the end of the reaction. They speed up reactions but are not used up. This distinguishes them from reagents.
Confusing and glucose. In -glucose, the OH on C1 is on the opposite side of the ring from C6 (axial/down in Haworth). In -glucose, it is on the same side (equatorial/up). This determines whether the polymer is starch () or cellulose ().
Exam Weightage and Strategy
Biomolecules carries 3-5 marks in CBSE Class 12 boards and 1-2 NEET questions per year. The questions are mostly table-based and classification-based. Memorise: reducing vs non-reducing sugars, starch vs cellulose, protein structure levels, DNA vs RNA, and the vitamin deficiency table.
Three tables to memorise: (1) disaccharides — monomers, linkage, reducing status, (2) DNA vs RNA — 5 differences, (3) vitamins — name, solubility, deficiency disease. These three tables cover 80% of exam questions on biomolecules.
Practice Questions
Q1. What is the difference between starch and cellulose?
Both are polysaccharides of glucose. Starch uses -glucose with -glycosidic linkages (digestible by humans via amylase). Cellulose uses -glucose with -glycosidic linkages (not digestible by humans — we lack cellulase). Starch is food storage in plants; cellulose is structural (cell walls). Starch gives blue colour with iodine; cellulose does not.
Q2. Name the products of hydrolysis of sucrose.
Glucose + Fructose. This hydrolysis is called “inversion of cane sugar” because the optical rotation of the solution changes sign — from dextrorotatory (sucrose) to levorotatory (mixture, because fructose has a stronger levorotation than glucose’s dextrorotation).
Q3. What is a peptide bond?
A covalent bond between the carboxyl group () of one amino acid and the amino group () of another, with loss of water: . It is formed by a condensation reaction. It is planar (partial double-bond character) and has restricted rotation, which influences protein structure.
Q4. What is denaturation of proteins?
Loss of secondary, tertiary and quaternary structure due to heat, pH change, heavy metals, or organic solvents. The primary structure (amino acid sequence) remains intact, but the protein unfolds and loses its biological activity. Example: boiling an egg denatures albumin — the transparent liquid becomes a white solid. Most denaturation is irreversible.
Q5. A person has bleeding gums and slow wound healing. Which vitamin is likely deficient?
Vitamin C (ascorbic acid). Deficiency causes scurvy — characterised by bleeding gums, loose teeth, slow wound healing, and general weakness. Vitamin C is essential for collagen synthesis — without it, connective tissue breaks down. Found in citrus fruits, amla, and tomatoes.
FAQs
Why are enzymes specific?
Enzymes have an active site with a specific 3D shape complementary to only one substrate (lock-and-key model) or that adjusts to fit the substrate (induced-fit model). This shape specificity is determined by the protein’s tertiary structure. A different substrate simply will not fit.
What is the isoelectric point?
The pH at which an amino acid carries no net charge (zwitterionic form with equal positive and negative charges). At this pH, the amino acid does not move in an electric field (electrophoresis). Each amino acid has a characteristic pI depending on its R group.
Why is DNA double-stranded while RNA is single-stranded?
DNA’s role is long-term information storage — the double helix provides stability (two copies of information, base-pairing for error correction). RNA’s role is temporary information transfer — single-stranded is sufficient and allows the molecule to fold into functional shapes (tRNA, rRNA). The 2’-OH on ribose in RNA also makes it less chemically stable than DNA (prone to hydrolysis).
What is the difference between essential and non-essential amino acids?
Essential amino acids cannot be synthesised by the human body and must be obtained from diet (there are about 9: valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, lysine, histidine). Non-essential amino acids can be synthesised by the body from other molecules. Both are equally important for protein synthesis — the terms refer only to dietary requirement, not biological importance.