20.1 Macromolecules: Polymers, Carbohydrates, Proteins, Lipids, Enzymes & Nucleic Acids

Macromolecules: Polymers, Carbohydrates, Proteins, Lipids, Enzymes & Nucleic Acids

A macromolecule is a very large molecule, often with a molar mass of tens of thousands to millions of grams per mole. Most are polymers: long chains built from many small repeating units called monomers. Synthetic polymers give us plastics and fibres; natural polymers (carbohydrates, proteins and nucleic acids) make up living things.

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1. Polymerization

Important Addition Polymers

The polymer formula is written as the repeat unit in brackets, for example $-[\text{CH}_2-\text{CHCl}]_n-$ for PVC. The degree of polymerization $n$ is the number of repeat units; for a polyethene sample of molar mass $28{,}000\text{ g/mol}$, $n = \frac{28{,}000}{28} = 1000$.

Important Condensation Polymers

• Nylon-6,6: hexamethylenediamine $\text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2$ and adipic acid $\text{HOOC}(\text{CH}_2)_4\text{COOH}$ join by amide links with loss of water. "6,6" refers to the six carbons in each monomer. Used for fibres, ropes and carpets. Nylon is a polyamide, like proteins.
• Polyester (Terylene or Dacron, PET): ethylene glycol and terephthalic acid join by ester links. Used for clothing fibres and drink bottles.
• Bakelite: phenol and formaldehyde form a cross-linked network. Used for electrical switches and handles.
• Epoxy resins: strong adhesives and coatings that set by cross-linking.

Thermoplastics and Thermosets

• Thermoplastics have long chains with weak forces between them. They soften on heating and can be remoulded many times (polyethene, PVC, polystyrene).
• Thermosetting plastics form a rigid three-dimensional network of covalent cross-links when first heated. They do not soften again and char on strong heating (Bakelite, epoxy).

Most synthetic polymers are non-biodegradable, which causes litter and waste-disposal problems; burning PVC releases toxic HCl and other harmful gases.

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2. Carbohydrates

Carbohydrates have the general formula $\text{C}_x(\text{H}_2\text{O})_y$ and are polyhydroxy aldehydes or ketones, or compounds that hydrolyse to them.

• Starch is the energy store of plants; it gives a blue-black colour with iodine.
• Glycogen is the energy store of animals, kept in the liver and muscles.
• Cellulose forms plant cell walls; humans cannot digest it because they lack the enzyme to break its linkages, but it is valuable dietary fibre.
• Glucose and other reducing sugars reduce Fehling's and Benedict's solutions; sucrose is a non-reducing sugar.

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3. Proteins

Proteins are polypeptides made from amino acids joined by peptide bonds (Section 19.6).

Denaturation is the loss of the secondary, tertiary and quaternary structure by heat, strong acids or bases, organic solvents or heavy metal ions. The primary structure remains, but the protein loses its function, as when egg white sets on boiling.

Proteins are classified as fibrous (insoluble, structural: keratin in hair, collagen in tendons) or globular (soluble, functional: enzymes, haemoglobin, many hormones).

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4. Lipids

Lipids are naturally occurring esters that are insoluble in water but soluble in organic solvents. Fats and oils are triglycerides: esters of glycerol with three long-chain fatty acids.

• Fats (mostly from animals) are solid at room temperature and contain mainly saturated fatty acids.
• Oils (mostly from plants) are liquid and contain more unsaturated fatty acids with $\text{C}=\text{C}$ bonds.
• Hydrogenation of vegetable oils with hydrogen and a nickel catalyst produces semi-solid fats such as vegetable ghee and margarine.
• Saponification: boiling a fat or oil with NaOH hydrolyses the ester to glycerol and sodium salts of fatty acids, which are soaps.
• Rancidity is the development of an unpleasant smell when fats are oxidized or hydrolysed on storage.
• The iodine number measures unsaturation: the more C=C bonds, the more iodine the fat absorbs.

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5. Enzymes

Enzymes are biological catalysts, almost all proteins. They speed reactions by providing a pathway with a lower activation energy, without being used up.

• Specificity: each enzyme acts on one substrate or type of reaction. The lock-and-key model pictures the substrate fitting the enzyme's active site.
• Temperature: activity rises to an optimum (about $37^\circ\text{C}$ for human enzymes) and then falls sharply as the enzyme is denatured.
• pH: each enzyme has an optimum pH: pepsin in the stomach works best near pH 2, while many other enzymes work best near pH 7.
• Inhibitors reduce activity; coenzymes and activators (such as some metal ions) are needed by some enzymes.

Examples: amylase (starch to maltose), invertase (sucrose to glucose and fructose), zymase (glucose to ethanol), urease (urea to ammonia and $\text{CO}_2$).

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6. Nucleic Acids

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers of nucleotides. Each nucleotide contains a phosphate group, a five-carbon sugar and a nitrogenous base.

In DNA the strands are held together by hydrogen bonds between complementary bases: A pairs with T and G pairs with C (A with U in RNA). The sequence of bases carries the genetic code that directs the order of amino acids in proteins.