Biomolecules

1Carbohydrates: roles and classification

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that hydrolyse to them. They are the chief source of energy in living organisms (glucose oxidation), the storage form of that energy (starch in plants, glycogen in animals), and the structural scaffolding of plant cell walls (cellulose).

Hydrolysis depth splits them into monosaccharides (cannot be hydrolysed further), oligosaccharides (hydrolyse to two–ten monosaccharide units; disaccharides are the usual case), and polysaccharides (hydrolyse to many units). An independent cut names a monosaccharide by carbonyl type and carbon count: an aldose carries an aldehyde, a ketose a ketone; triose, tetrose, pentose, hexose finish the name. Glucose is an aldohexose (\mathrm{C_6H_{12}O_6}); fructose is a ketohexose with the same molecular formula.

Figure. Hydrolysis classification of carbohydrates: monosaccharides give no further sugar on hydrolysis; oligosaccharides give two–ten monosaccharide units (disaccharides are the usual exam cases); polysaccharides give many. Aldose/ketose × carbon count is a second cut on monosaccharides — carried by this concept's class table, not by this tree.

How it works

1. Ask what it doesEnergy now (glucose), energy stored (starch, glycogen), or structure (cellulose) — three roles, not three formulas.
2. Ask if it hydrolysesNo further hydrolysis → mono. Two–ten units on hydrolysis → oligo (disaccharides dominate). Many units → poly.
3. Name a monosaccharideCarbonyl first (aldose or ketose), then carbon count. Aldo + hexose → aldohexose. Same \mathrm{C_6H_{12}O_6} does not decide glucose versus fructose.

2Glucose preparation and cyclic forms

Glucose is obtained by acid hydrolysis of sucrose: \mathrm{C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+} C_6H_{12}O_6 + C_6H_{12}O_6}, giving one molecule each of glucose and fructose. It is also obtained from starch or cellulose: (\mathrm{C_6H_{10}O_5})_n + n\,\mathrm{H_2O} \xrightarrow{\text{dil.\,H}_2\mathrm{SO}_4,\,393\,\mathrm{K},\,\text{pressure}} n\,\mathrm{C_6H_{12}O_6}.

In water the open-chain aldehyde of glucose closes to a six-membered hemiacetal ring (pyranose). The new chiral centre at C-1 is the anomeric carbon: α and β anomers differ only in the configuration there. Fructose closes to a five-membered hemiketal (furanose) or a six-membered pyranose; sucrose uses the β-D-fructofuranose form.

Figure. Acid hydrolysis of sucrose gives glucose + fructose. Each open chain closes to a hemiacetal/hemiketal ring — pyranose (six) or furanose (five) — with α/β anomers at the anomeric carbon. Ring names are the exam ask without Haworth art.

How it works

1. From sucroseDilute acid hydrolyses the glycosidic bond → equimolar glucose + fructose. Invert sugar is that mixture.
2. From starch or cellulose(\mathrm{C_6H_{10}O_5})_n + n\,\mathrm{H_2O} \rightarrow n\,\mathrm{C_6H_{12}O_6} under dilute \mathrm{H_2SO_4} at 393 K and pressure.
3. Close the chainGlucose: OH of C-5 attacks C-1 → pyranose; α/β at C-1. Fructose: OH of C-5 (or C-6) attacks C-2 → furanose or pyranose.

3Reducing and non-reducing sugars

A reducing sugar has a free anomeric carbon — one that can open to expose an aldehyde (or an α-hydroxy ketone that tautomerises to one). That free carbonyl reduces Tollens' reagent (silver mirror) and Fehling's solution (Cu₂O precipitate). Glucose, fructose, maltose and lactose are reducing.

Sucrose is the standard non-reducing exception: its glycosidic bond joins the anomeric carbon of glucose (C-1) to the anomeric carbon of fructose (C-2), so neither end can open. If a disaccharide's linkage leaves any anomeric carbon free, the sugar is reducing.

Figure. Decision tree for reducing status: free anomeric carbon → reducing (glucose, maltose, lactose, fructose); both anomerics locked → non-reducing (sucrose). The glycosidic bond itself is not drawn — only the yes/no criterion.

How it works

1. Look at the anomeric carbonFree hemiacetal/hemiketal → can open → reducing. Both anomerics tied in a full acetal → non-reducing.
2. MonosaccharidesGlucose and fructose are reducing even though fructose is a ketose — it tautomerises under the test conditions.
3. DisaccharidesMaltose and lactose each leave one anomeric free. Sucrose locks both — the exam's favourite trap.

4Di- and polysaccharides

Three disaccharides carry most of the marks. Maltose is two α-D-glucose units joined by an α-1,4-glycosidic linkage and is reducing (one anomeric free). Lactose is β-D-galactose joined to β-D-glucose by a β-1,4 linkage and is likewise reducing. Sucrose joins C-1 of α-D-glucose to C-2 of β-D-fructose, locks both anomerics, and is non-reducing.

Starch is the plant storage polysaccharide: amylose is an unbranched α-1,4 chain of D-glucose; amylopectin is branched, with α-1,4 links in the chains and α-1,6 links at the branch points. Glycogen is the animal storage form — like amylopectin but more highly branched. Cellulose is a structural β-1,4 polymer of D-glucose; humans lack the enzyme that cleaves that β linkage.

Figure. Disaccharide identity is the linkage: maltose α-1,4 and lactose β-1,4 stay reducing; sucrose ties both anomerics (α-1,β-2) and is non-reducing. Polysaccharides add branching (α-1,6) on top of α-1,4 chains.

How it works

1. Name the units and the linkMaltose: Glc–α-1,4–Glc. Lactose: Gal–β-1,4–Glc. Sucrose: Glc–α-1,β-2–Fru.
2. Ask which anomeric is freeOne free → reducing (maltose, lactose). Both tied → non-reducing (sucrose).
3. Storage versus structureStarch (amylose + amylopectin) and glycogen store glucose with α links. Cellulose builds walls with β-1,4 links.

5Amino acids and zwitterions

α-Amino acids carry an amino group and a carboxyl group on the same (α) carbon. They are amphoteric: the –COOH can donate a proton and the –NH₂ can accept one. In the solid and in aqueous solution near neutrality they exist as zwitterions, ^{+}\mathrm{H_3N{-}CHR{-}COO^-}, not as the uncharged \mathrm{H_2N{-}CHR{-}COOH} form.

The isoelectric point (pI) is the pH at which the net charge is zero — the zwitterion dominates and the amino acid does not migrate in an electric field. Side chains sort the twenty proteinogenic acids into acidic (Asp, Glu — an extra carboxyl), basic (Lys, Arg, His — an extra amino-like nitrogen), and neutral (equal acidic and basic groups on the α-framework).

Essential amino acids cannot be synthesised by humans at a rate that meets need and must come from the diet. NCERT lists ten essentials: Val, Leu, Ile, Phe, Thr, Trp, Met, Lys, Arg and His. The rest are non-essential — the body can make them — even though Arg and His are sometimes called semi-essential outside the NCERT list.

Figure. The zwitterion ⁺H₃N–CHR–COO⁻ dominates near the isoelectric point pI, where net charge is zero. Lower pH protonates the carboxylate (+1); higher pH deprotonates the ammonium (−1).

How it works

1. Two ionisable groups–COOH is acidic; –NH₃⁺ is acidic too once protonated. An acidic or basic side chain may add a third ionisable group and shift the pI.
2. Zwitterion at pIAt the isoelectric pH the average charge is zero: ^{+}\mathrm{H_3N{-}CHR{-}COO^-}. Below pI the cationic form dominates; above pI the anionic form does.
3. Side-chain classAcidic = Asp, Glu. Basic = Lys, Arg, His. Neutral = the others (Gly, Ala, Ser, …). Class follows extra –COOH versus extra –NH₂ on the side chain.
4. Essential vs non-essentialEssential means diet-required. NCERT's ten: Val, Leu, Ile, Phe, Thr, Trp, Met, Lys, Arg, His. Non-essential means the body can synthesise them.

6Peptide bonds and protein structure levels

Proteins are polymers of α-amino acids joined by peptide (amide) bonds, written -\mathrm{CO{-}NH}-. The carboxyl carbon of one residue links to the amino nitrogen of the next, releasing water. The sequence of residues is the primary structure — and it alone determines everything that folds later.

Secondary structure is local backbone folding: the α-helix and the β-pleated sheet, both held by hydrogen bonds between backbone \mathrm{C{=}O} and \mathrm{N{-}H}. Tertiary structure is the full three-dimensional fold of one chain; quaternary structure is how multiple folded chains pack together (haemoglobin's four subunits).

Shape and solubility sort proteins into fibrous and globular. Fibrous proteins are fibre-like and generally water-insoluble — keratin (hair, nails), collagen (tendons) and myosin (muscles). Globular proteins are roughly spherical and usually water-soluble — enzymes, haemoglobin and albumin. The helix and sheet are three-dimensional objects this vocabulary cannot draw honestly.

Figure. The four structure levels as a hierarchy: sequence → local fold → one-chain 3D → multi-chain assembly. Bar and ribbon geometry of the α-helix and β-sheet are not drawn — only the naming ladder the exam uses. Fibrous versus globular is a separate shape/solubility sort, carried by the topic-level table.

How it works

1. Peptide bond–COOH of residue i + –NH₂ of residue i+1 → –CO–NH– + H₂O. Planar amide; restricted rotation about the C–N bond.
2. Primary → secondarySequence first (covalent peptide bonds). Then local H-bonded patterns: α-helix or β-sheet.
3. Tertiary → quaternaryOne chain's full fold is tertiary (side-chain interactions). Multiple chains assembling is quaternary — absent in a single-chain protein.
4. Fibrous vs globularFibrous: elongated, insoluble — keratin, collagen, myosin. Globular: compact, usually soluble — enzymes, haemoglobin, albumin.

7Protein denaturation

Denaturation is loss of the native fold: secondary, tertiary and — when present — quaternary structure are disrupted, so the protein loses its biological activity. The primary structure, the covalent sequence of peptide bonds, remains intact.

Heat, a sharp change of pH, urea, heavy-metal salts and organic solvents are the usual agents — they break the non-covalent interactions that hold the fold. Boiling egg albumin is the textbook coagulation example: the white goes from clear and soluble to opaque and solid as the chains unfold and tangle. Curdling of milk (lactic acid from bacteria) is the same idea at lower temperature.

Because activity needs the native shape, a denatured enzyme or haemoglobin cannot do its job even though every peptide bond is still there. Renaturation is sometimes possible if conditions return gently; irreversible coagulation is common once chains aggregate.

Figure. Native fold → denatured chain under heat, pH or urea. The figure names which levels vanish and which survive; it does not invent a 3D ribbon of the fold or a numerical activity–temperature curve.

How it works

1. What is lostH-bonds, hydrophobic packing, ionic links and (where present) subunit contacts break. α-helix and β-sheet unroll; the chain opens.
2. What survivesPrimary structure — the –CO–NH– sequence — is covalent and stays. Denaturation is not hydrolysis of the backbone.
3. AgentsHeat; extreme pH; urea; heavy-metal salts; organic solvents. Each loosens the non-covalent fold.
4. ConsequenceBiological activity collapses. Egg albumin coagulates on boiling; milk curdles when acid unfolds casein.

8DNA versus RNA

Nucleic acids are polymers of nucleotides. A nucleoside is a nitrogenous base linked to a pentose sugar; a nucleotide is that nucleoside plus a phosphate group. The bases split into purines — adenine (A) and guanine (G) — and pyrimidines — cytosine (C), thymine (T) in DNA, and uracil (U) in RNA.

In the chain, the phosphate joins the 3′-OH of one sugar to the 5′-OH of the next: a phosphodiester backbone with a free 5′ end and a free 3′ end. DNA uses 2-deoxyribose and thymine; RNA uses ribose and uracil. DNA has thymine and deoxyribose; RNA has uracil and ribose — swapping either marker is the classic trap.

Figure. A nucleotide is three parts: nitrogenous base, pentose sugar and phosphate. Drop the phosphate and what remains is a nucleoside. Purine versus pyrimidine, 2-deoxyribose versus ribose, and thymine versus uracil then decide DNA versus RNA — that split sits in this concept's comparison table. The helix itself is not drawn.

How it works

1. Name the unitNucleoside = base + sugar. Nucleotide = base + sugar + phosphate.
2. Sort the basesPurines: A, G (two rings). Pyrimidines: C, T, U (one ring). DNA uses T; RNA uses U.
3. Link the chain3′–5′ phosphodiester bonds join successive sugars; the strand direction is written 5′ → 3′.
4. Mark DNA vs RNADNA: 2-deoxyribose + T, usually double helix. RNA: ribose + U, usually single-stranded.

9Base pairing and hydrogen bonds

In DNA, adenine pairs with thymine and guanine pairs with cytosine. Chargaff's rules follow at once: mole percent A equals T, and G equals C. The A–T pair is held by two hydrogen bonds; the G–C pair by three. A G–C-rich duplex therefore has more H-bonds per base pair and a higher melting temperature.

That is why a stem that gives one base percent is enough to recover all four, and why stability arguments point at the triple-bonded G–C pair rather than at the length of the chain alone.

Figure. Base composition for the worked example (30% A). Chargaff forces T = 30% and G = C = 20%. Annotations recall the H-bond counts: two in each A–T pair, three in each G–C pair — the reason G–C-rich DNA melts higher. Bar heights are the percents by construction.

How it works

1. Pair the basesA = T (two H-bonds) and G ≡ C (three). One percent fixes its partner; the other two share what remains, equally.
2. Count H-bondsEach A–T pair contributes 2; each G–C pair contributes 3. Sum over all pairs for the duplex.
3. Read stabilityHigher G–C content → more triple H-bonds → higher melting temperature.

10Fat-soluble and water-soluble vitamins

Vitamins are organic micronutrients needed in small amounts. They split by solubility: A, D, E and K are fat-soluble and can be stored in adipose tissue; the B-complex vitamins and vitamin C are water-soluble and are not stored to the same degree, so a regular dietary supply matters more.

NCERT pairs each vitamin with sources and a deficiency disease — night blindness or xerophthalmia for A, rickets for D, scurvy for C, beri-beri for B₁, pernicious anaemia for B₁₂, and longer clotting time for K. Solubility class and deficiency name are asked as a pair more often than either alone.

Figure. Two solubility classes side by side. Fat-soluble vitamins A, D, E, K can accumulate in adipose tissue; water-soluble B-complex and C do not. Sources and deficiency names sit in this concept's vitamin table — the figure only carries the split.

How it works

1. Split by solubilityFat-soluble: A, D, E, K. Water-soluble: B-complex and C.
2. Storage consequenceFat-soluble vitamins accumulate in adipose tissue; water-soluble ones clear more readily.
3. Link source and diseaseRead the vitamin table in this concept — each row ties solubility, NCERT sources and the deficiency name.

11Enzymes as specific catalysts

Enzymes are biological catalysts, almost always proteins, that act on a narrow set of substrates at an active site. Catalysis lowers the activation energy of the reaction; the enzyme is regenerated and not consumed, so one molecule turns over many substrate molecules.

Specificity is the exam idea: the active site recognises shape and chemistry, so one enzyme does not stand in for another. Harsh heat or extreme pH can denature the fold that builds that site and kill activity — the same fragility that sets temperature and pH optima — without changing the definition of catalysis itself.

How it works

1. Active siteThe folded protein creates a pocket that binds one substrate (or a tight family).
2. Lower EaBinding and catalytic groups cut the activation barrier; rate rises, ΔG of the reaction is unchanged.
3. RegeneratedProduct leaves; free enzyme turns over again. It is a catalyst, not a reactant.
4. Fragile foldDenaturation destroys the active-site geometry, so activity falls — a brief link to protein structure, not a second concept.