1. Amino acids
α-Amino acids H₂N–CH(R)–COOH; all chiral (L) except glycine. Essential (from diet: valine, leucine, isoleucine, lysine, threonine, methionine, phenylalanine, tryptophan, arginine, histidine) vs non-essential (made in the body: glycine, alanine, glutamic acid). Neutral (one –NH₂, one –COOH), acidic (extra –COOH: aspartic, glutamic acid), basic (extra –NH₂: lysine, arginine).
Zwitter ion H₃N⁺–CH(R)–COO⁻: salt-like — high m.p., soluble in water not benzene, amphoteric. Isoelectric point (pI): pH of zero net charge, no migration in an electric field; below pI → cation (to cathode), above pI → anion (to anode). For neutral amino acids pI ≈ (pKa₁ + pKa₂)/2.
| pH vs pI | Form | Moves to |
|---|---|---|
| pH < pI | Cation | Cathode |
| pH = pI | Zwitter ion | No movement |
| pH > pI | Anion | Anode |
Formulas
- Zwitter ion as a base
- Zwitter ion as an acid
Key terms
- Zwitter ion
A dipolar ion carrying both a positive (–NH₃⁺) and a negative (–COO⁻) charge, neutral overall.
- Isoelectric point
The pH at which an amino acid has no net charge and does not move in an electric field.
2. Peptides and proteins
Peptide bond –CO–NH– from –COOH of one amino acid + –NH₂ of the next, losing H₂O. n amino acids → n − 1 peptide bonds and n − 1 H₂O; M(peptide) = ΣM(amino acids) − 18(n − 1). N-terminal (free –NH₂) written on the left.
Fibrous (keratin, collagen, myosin): parallel chains, H- and disulfide bonds, insoluble, structural. Globular (insulin, albumin, enzymes): compact folded chains, soluble, functional.
Levels (definitions only): primary — amino-acid sequence; secondary — α-helix or β-pleated sheet (H-bonds); tertiary — overall 3D folding of a chain; quaternary — arrangement of subunits (haemoglobin: four).
| Type | Shape | Example |
|---|---|---|
| Fibrous | Long, parallel chains | Keratin |
| Globular | Compact, folded | Insulin |
Formulas
- Glycylalanine
Key terms
- Peptide bond
The amide link –CO–NH– joining two amino-acid residues.
- Primary structure
The sequence of amino acids in a polypeptide chain.
3. Denaturation, enzymes and hormones
Denaturation: heat, pH change, alcohol or heavy-metal ions break H-bonds and other interactions; 2°, 3° (and 4°) structure lost, 1° intact; protein coagulates and loses activity (boiled egg, curdled milk, 70% alcohol on bacteria).
Enzymes: biological catalysts, mostly globular proteins; highly specific, efficient, work best near body temperature and an optimum pH; lower activation energy. Invertase (sucrose), maltase (maltose), pepsin (proteins → peptides), urease: .
Hormones: chemical messengers from endocrine glands carried in blood — insulin (lowers blood glucose), glucagon (raises it), thyroxine (metabolic rate; contains iodine), adrenaline (emergency), steroid sex hormones.
| Hormone | Role |
|---|---|
| Insulin | Lowers blood glucose |
| Glucagon | Raises blood glucose |
| Thyroxine | Metabolic rate |
Formulas
- Urease
Key terms
- Denaturation
Loss of the secondary and tertiary structure, and of biological activity, of a protein without breaking peptide bonds.
- Enzyme
A biological catalyst, usually a globular protein, specific for its substrate.
4. Vitamins
Fat-soluble: A, D, E, K (stored in liver and fat; excess can be harmful). Water-soluble: B group, C (excreted; must be supplied regularly; lost in cooking water). B₁₂ is stored in the liver.
| Vitamin | Deficiency |
|---|---|
| A | Night blindness, xerophthalmia |
| B₁ | Beriberi |
| B₂ | Cheilosis, digestive disorders |
| B₆ | Convulsions |
| B₁₂ | Pernicious anaemia |
| C | Scurvy (bleeding gums) |
| D | Rickets (children), osteomalacia (adults) |
| E | Fragile red blood cells, muscular weakness |
| K | Increased blood-clotting time |
| Group | Members |
|---|---|
| Fat-soluble | A, D, E, K |
| Water-soluble | B group, C |
Key terms
- Vitamin
An organic compound needed in small amounts in the diet for normal growth and health.
- Fat-soluble vitamin
One of vitamins A, D, E and K, which dissolve in fat and are stored in the body.
5. Nucleic acids
Nucleotide = base + pentose + phosphate (nucleoside = base + sugar); joined by phosphodiester links. Purines: adenine, guanine. Pyrimidines: cytosine, thymine (DNA), uracil (RNA).
DNA: 2-deoxyribose; double helix (Watson and Crick, 1953); antiparallel, complementary strands; A–T (2 H-bonds), G–C (3 H-bonds); A = T, G = C. Stores hereditary information; replicates.
RNA: ribose; A, G, C, U; usually single-stranded; mRNA (message), tRNA (carries amino acids), rRNA (ribosome); protein synthesis.
Base counting: total bases = 2 × base pairs.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Pyrimidines | C, T | C, U |
| Strands | Two | One |
Key terms
- Nucleotide
The monomer of nucleic acids: a nitrogenous base, a pentose sugar and a phosphate group.
- Complementary strands
Two DNA strands whose bases pair A with T and G with C.
Exam tips
Common mistakes
- Giving the zwitter ion a net charge, or confusing which way an amino acid moves below its pI.
- Counting n peptide bonds for n amino acids (it is n − 1).
- Saying denaturation breaks peptide bonds.
- Listing vitamin C as fat-soluble, or putting thymine in RNA.
How marks are usually awarded
- Zwitter ion (1 mark): correct charges; amphoteric equations (1 mark each).
- Vitamins (½–1 mark each): vitamin, group and deficiency disease.
- DNA vs RNA (1 mark per feature): sugar, bases, strands, function.