What is the difference between addition and condensation polymerisation?

Addition: monomers join via opening of pi-bonds (alkenes), with no loss of atoms (PE, PVC, polystyrene). Condensation: monomers with two functional groups link with loss of small molecules like water or HCl (nylon, polyester, Bakelite).

Why is cellulose indigestible by humans but starch is not?

Cellulose has beta-1,4 glycosidic bonds; humans lack the enzyme cellulase to hydrolyse them. Starch has alpha-1,4 (and alpha-1,6) bonds, hydrolysed by amylase in saliva and pancreas. Cattle host gut microbes that produce cellulase.

How does the ozone layer protect life?

It absorbs harmful UV-B radiation (280-315 nm) via the Chapman cycle, preventing it from reaching the surface where it would cause skin cancer, cataracts, and damage to phytoplankton and crops.

What is the greenhouse effect?

Greenhouse gases (CO2, CH4, H2O vapour, N2O, CFCs) absorb infrared radiation re-emitted by Earth's surface and re-emit it in all directions, including back down. This warms the troposphere and is essential for life — but enhanced by anthropogenic emissions, it drives climate change.

CO binds to haemoglobin's iron about 200 times more strongly than O2, forming carboxyhaemoglobin and blocking oxygen transport. As little as 0.1% CO in air can be fatal in hours.

Which of the following is a non-reducing sugar?

Sucrose. In sucrose both anomeric carbons are tied up in the glycosidic bond (alpha-1, beta-2), so there is no free hemiacetal -OH; it does not reduce Tollens or Fehling.

Nylon-6,6 is formed by condensation of:

Adipic acid and hexamethylenediamine. The two monomers each have 6 carbons (hence 6,6) and form amide linkages with loss of water.

Vinyl chloride. Vinyl chloride CH2=CHCl polymerises by addition to give -(CH2-CHCl)n-, polyvinyl chloride.

Which gas is primarily responsible for stratospheric ozone depletion?

CFCs (e.g. CCl2F2). UV light cleaves CFCs to release Cl atoms, which catalytically destroy O3. CO2 and CH4 contribute to the greenhouse effect, not ozone depletion.

Vulcanisation of rubber involves:

Cross-linking with sulphur. Heating natural cis-polyisoprene with sulphur creates -S-S- cross-links between chains, converting soft sticky rubber into hard elastic vulcanised rubber.

Macromolecules and Environmental Chemistry MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Macromolecules and Environmental Chemistry

Macromolecules and Environmental Chemistry averages 3 MCQs per MDCAT paper, covering polymers (PE, PVC, nylon, terylene), carbohydrates, lipids, and atmospheric pollutants.

Macromolecules and Environmental Chemistry is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 3 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

Carbohydrates: from monosaccharides to polysaccharides

Carbohydrates have the empirical formula C_n(H₂O)_n. Monosaccharides are aldoses (glucose, galactose) or ketoses (fructose); they cyclise to pyranose (6-ring) or furanose (5-ring) hemiacetal/hemiketal forms. The anomeric carbon (former carbonyl) is α (OH down in Haworth projection of D-glucose) or β (OH up) — they interconvert through the open-chain form (mutarotation). Disaccharides: maltose (2 α-glucose, α-1,4), lactose (galactose-β-1,4-glucose), sucrose (α-glucose-β-fructose, non-reducing). Polysaccharides: starch (amylose α-1,4 + amylopectin α-1,4 + α-1,6 branches), glycogen (more branched than amylopectin), cellulose (β-1,4 glucose, indigestible by humans). Reducing sugars (those with a free anomeric OH) give positive Tollens, Fehling, Benedict; sucrose is non-reducing. The FSc Punjab Textbook Chemistry XII Chapter 16 covers this in MDCAT depth.

Lipids and nucleic acids

Triglycerides are esters of glycerol with three fatty acids; saturated fatty acids (palmitic, stearic) give solid fats, unsaturated (oleic, linoleic) give liquid oils. Saponification with NaOH gives soap (sodium carboxylate) + glycerol. Phospholipids form bilayers in membranes. Nucleic acids (DNA, RNA) are polymers of nucleotides — each nucleotide is a base (A, G, C, T/U) + sugar (deoxyribose in DNA, ribose in RNA) + phosphate. Watson-Crick base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). DNA is a right-handed double helix, ~10 base pairs per turn.

Synthetic polymers

Addition polymers form by free-radical or ionic chain growth from alkene monomers, with no loss of small molecules: polyethylene from ethene (LDPE, HDPE — the latter via Ziegler-Natta), polypropylene from propene, polystyrene, PVC from vinyl chloride, PTFE (Teflon) from tetrafluoroethene, PMMA (Plexiglas). Condensation polymers form by loss of water or another small molecule between two functional groups: nylon-6,6 from hexamethylenediamine + adipic acid (amide linkages), nylon-6 from caprolactam (ring-opening), terylene/PET from ethylene glycol + terephthalic acid (ester linkages), Bakelite from phenol + formaldehyde, melamine-formaldehyde, urea-formaldehyde. Natural rubber is cis-polyisoprene; vulcanisation with sulphur cross-links it to a hard elastic solid (Goodyear, 1839).

Atmospheric pollution and ozone

Tropospheric pollutants: SO₂ and NO_x form acid rain (H₂SO₄, HNO₃) — eroding marble (CaCO₃), killing fish in lakes. Photochemical smog: NO₂ + sunlight → NO + O•; O• + O₂ → O₃; O₃ + hydrocarbons → peroxyacetyl nitrate (PAN) and aldehydes — eye irritation, respiratory damage. CO from incomplete combustion binds haemoglobin 200× more strongly than O₂. Greenhouse gases: CO₂, CH₄, N₂O, CFCs, water vapour — they absorb infrared re-emitted by Earth, raising surface T (the greenhouse effect, foundational to climate change).

Stratospheric ozone and the CFC story

The ozone layer (15-35 km altitude) absorbs UV-B (280-315 nm) via the Chapman cycle: O₂ + hν → 2O•; O• + O₂ → O₃; O₃ + hν → O₂ + O•; O• + O₃ → 2 O₂. Chlorofluorocarbons (CFCs, e.g. CCl₂F₂, Freon-12) are inert at ground level but in the stratosphere UV cleaves the C-Cl bond: CCl₂F₂ + hν → •CClF₂ + Cl•. The chlorine atom catalyses ozone destruction: Cl• + O₃ → ClO• + O₂; ClO• + O• → Cl• + O₂. One Cl atom can destroy ~100,000 O₃ molecules before being removed. The Montreal Protocol (1987) banned CFCs and the ozone hole over Antarctica is now slowly recovering. Replacements are HFCs and HCFCs, which have shorter atmospheric lifetimes. Atkins Physical Chemistry and the FSc Chapter 17 give the kinetics.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Macromolecules and Environmental Chemistry Is Tested

MDCAT questions on Macromolecules and Environmental Chemistry are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.