Macromolecules: Class 12 Chemistry

Key points

Macromolecules are very large molecules with molar masses from several thousand up to millions. Most are polymers, built by joining many small units called monomers. The process is polymerisation. The number of monomer units in the chain is the degree of polymerisation (n). Polymers can be natural (starch, proteins, cellulose, rubber, DNA) or synthetic (polythene, PVC, nylon).

Types of polymerisation:

• Addition polymerisation: monomers with a C=C bond join one after another without losing any atoms. The polymer has the same empirical formula as the monomer. Examples: ethene to polythene, chloroethene to PVC, tetrafluoroethene to PTFE (Teflon) and propene to polypropene.
• Condensation polymerisation: monomers with two reactive groups join with loss of a small molecule, usually water. Examples: nylon-6,6 from hexane-1,6-diamine and hexanedioic acid, and polyester (terylene) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Addition polymers form by a free radical chain mechanism. An initiator such as a peroxide makes radicals (initiation). The radical attacks a monomer and the new radical attacks the next (propagation), and the chain stops when two radicals meet (termination). Low density polythene is made at high pressure and temperature by this route, while high density polythene is made with Ziegler-Natta catalyst at low pressure and has a more regular, tightly packed structure.

Polymers can be linear, branched or cross-linked. Linear and branched polymers soften on heating and are called thermoplastics (polythene, PVC). Cross-linked polymers cannot be remoulded after setting and are called thermosetting plastics (Bakelite, melamine). Natural rubber is a polymer of isoprene (2-methylbuta-1,3-diene) that becomes harder and more elastic when heated with sulfur, called vulcanisation.

Carbohydrates are polyhydroxy aldehydes or ketones and their polymers, with a general formula near Cₓ(H₂O)ᵧ:

• Monosaccharides cannot be hydrolysed further: glucose (an aldose), fructose (a ketose) and galactose, all C₆H₁₂O₆.
• Disaccharides give two monosaccharides: sucrose (glucose and fructose), maltose (two glucose) and lactose (glucose and galactose). The units join through a glycosidic bond.
• Polysaccharides give many monosaccharides: starch (storage in plants, a mixture of amylose and amylopectin), glycogen (storage in animals) and cellulose (structural, in plant cell walls, made of β-glucose).

Glucose reduces Tollens' and Fehling's reagents, so it is a reducing sugar. Sucrose is not reducing because the glycosidic bond uses the carbon atoms that would give the aldehyde and ketone groups. Starch turns blue-black with iodine solution. Hydrolysis with dilute acid or enzymes breaks the glycosidic bonds, as in (C₆H₁₀O₅)ₙ + nH₂O → nC₆H₁₂O₆.

Proteins are polymers of alpha amino acids, H₂N-CHR-COOH. Amino acids are amphoteric, since they have both a basic NH₂ group and an acidic COOH group, and in water they exist as zwitterions. Two amino acids join by a peptide bond (-CO-NH-) with loss of water. Chains of many amino acids form polypeptides. Twenty common amino acids occur in proteins, and eight to nine of them are essential in the human diet.

Levels of protein structure:

• Primary: the sequence of amino acids in the chain.
• Secondary: regular folding held by hydrogen bonds, as the alpha helix and beta pleated sheet.
• Tertiary: the overall three-dimensional shape from interactions between side chains (hydrogen bonds, disulfide bridges, ionic and hydrophobic interactions).
• Quaternary: the arrangement of two or more polypeptide chains, as in haemoglobin.

Denaturation is the loss of the natural shape of a protein caused by heat, strong acid or alkali, or heavy metal salts. Biological activity is lost. Enzymes are protein catalysts that are very specific and work best at certain temperature and pH values. Proteins give a purple colour in the biuret test and a yellow colour with concentrated nitric acid (xanthoproteic test).

Lipids are fats, oils and waxes. Fats and oils are triesters of glycerol with three fatty acids (triglycerides). Fats are solids with mostly saturated acids, and oils are liquids with unsaturated acids. Alkaline hydrolysis gives glycerol and soap. Lipids store energy (about 38 kJ/g, more than carbohydrates), insulate the body and form cell membranes.

Nucleic acids (DNA and RNA) are polymers of nucleotides. A nucleotide has a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base. DNA has A, T, G and C, and RNA has U in place of T. DNA is a double helix with bases paired A with T and G with C by hydrogen bonds. It stores genetic information, and RNA helps to make proteins.

Synthetic polymers are useful but many do not break down in nature. Plastic waste blocks drains and harms animals. Burning PVC releases toxic gases. Remedies are reducing use, reusing, recycling and using biodegradable polymers.

Study tip: Prepare a table of the main polymers with monomer, type of polymerisation and one use. For biomolecules, learn one clear diagram in words for each: peptide bond, glycosidic bond, and base pairing. Numericals are simple ratio problems, so write the balanced equation or the unit mass first.

Definitions and flashcards

Say the meaning first, then open the card to check.

Formula sheet

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Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Numericals with worked steps

Common mistakes

• Saying addition polymers lose small molecules. That is condensation polymerisation.
• Forgetting to subtract the water when finding the mass of a peptide, or subtracting one water too many (it is x − 1 for x residues).
• Writing starch as a polymer of fructose or cellulose as a polymer of α-glucose. Cellulose has β-glucose.
• Mixing up thermoplastic and thermosetting plastics.
• Writing U in DNA or T in RNA. DNA has thymine, and RNA has uracil.

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Practice questions and timed test Printable worksheet