Membranes, proteins, DNA and gene expression
Monohybrid and dihybrid inheritance
Monohybrid: one gene. Two heterozygotes (Aa × Aa) give 3 : 1 dominant : recessive. Codominance: both alleles expressed (e.g. Iᴬ Iᴮ blood group AB). Multiple alleles: more than two alleles in the population (Iᴬ, Iᴮ, Iᴼ). Sex linkage: gene on the X chromosome, so males (XY) show recessive traits more often (e.g. haemophilia). Dihybrid: two genes on different chromosomes; AaBb × AaBb gives 9 : 3 : 3 : 1; a test cross with aabb gives 1 : 1 : 1 : 1. Autosomal linkage: genes on the same chromosome are inherited together, giving more parental types and fewer recombinants than expected. Epistasis: one gene affects the expression of another, modifying ratios (e.g. 9 : 3 : 4). The chi-squared test checks whether differences from expected ratios are due to chance.
≈ 55 minCell membrane structure
The fluid mosaic model: a phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) about 7 nm thick, in which proteins float (fluid) in a scattered pattern (mosaic). Channel and carrier proteins transport polar molecules and ions. Cholesterol sits between phospholipids, regulating fluidity and reducing permeability to water and ions. Glycoproteins and glycolipids act as receptors and antigens for cell recognition. In cell signalling, a hormone binds a complementary receptor on the membrane, triggering a cascade inside (often via a second messenger such as cyclic AMP). Higher temperatures increase permeability (phospholipids move more; proteins denature), shown by pigment leaking from beetroot discs.
≈ 40 minDNA structure and replication
A nucleotide = phosphate + pentose sugar (deoxyribose in DNA, ribose in RNA) + nitrogenous base. Purines (A, G) have two rings; pyrimidines (C, T, U) one. Nucleotides join by phosphodiester bonds (condensation) into a sugar-phosphate backbone. DNA is a double helix of two antiparallel strands joined by hydrogen bonds: A–T (2), C–G (3). Semi-conservative replication: helicase breaks hydrogen bonds; free nucleotides pair with each exposed strand; DNA polymerase joins them 5′→3′ (continuous leading strand; fragments on the lagging strand joined by ligase). Each new molecule has one old and one new strand, shown by the Meselson–Stahl ¹⁵N/¹⁴N experiment. RNA is single-stranded with ribose and uracil.
≈ 45 minGas exchange surfaces
Efficient exchange surfaces follow Fick's law: rate ∝ (surface area × concentration difference) ÷ thickness. Alveoli: huge surface area, one-cell-thick squamous epithelium next to capillary endothelium, steep gradient kept by ventilation and blood flow, elastic fibres for recoil. Airways: cartilage (trachea, bronchi) keeps them open; goblet cells make mucus; cilia sweep it away; smooth muscle controls diameter. Ventilation: inspiration (diaphragm and external intercostals contract, volume ↑, pressure ↓); expiration largely passive. Pulmonary ventilation = tidal volume × breathing rate. Fish gills use countercurrent flow to keep a gradient along the whole lamella; insects use tracheae and spiracles; leaves use stomata and air spaces.
≈ 45 minEnzyme action and kinetics
Enzymes are globular proteins that lower activation energy by forming enzyme–substrate complexes at a specific active site. Induced fit: the active site changes shape slightly as the substrate binds. Rate rises with temperature (more collisions) up to an optimum, then falls as bonds break and the enzyme denatures. Away from the optimum pH, ionic and hydrogen bonds change, altering the active site. Rate increases with substrate concentration until all active sites are occupied (Vmax). Km is the substrate concentration at ½Vmax: a low Km means high affinity. Competitive inhibitors bind the active site (overcome by more substrate; Vmax unchanged, Km increases). Non-competitive inhibitors bind elsewhere, changing the active site (Vmax decreases). Immobilised enzymes can be reused and give product free of enzyme.
≈ 50 minProteins and water
Amino acids (H₂N–CHR–COOH) join by condensation into peptide bonds. Primary structure: amino acid sequence. Secondary: α-helix or β-pleated sheet held by hydrogen bonds between C=O and N–H. Tertiary: further folding held by hydrogen, ionic and disulfide bonds and hydrophobic interactions between R groups. Quaternary: more than one polypeptide (and maybe a prosthetic group). Globular proteins (e.g. haemoglobin: 4 chains, haem with Fe²⁺, hydrophilic outside) are soluble and metabolic; fibrous proteins (e.g. collagen: three chains in a triple helix, glycine every third residue, cross-links) are insoluble and structural. Biuret test: purple. Water is polar; hydrogen bonding gives it high specific heat capacity, high latent heat of vaporisation, cohesion and solvent properties.
≈ 50 minTransport across membranes
Diffusion: net movement down a concentration gradient (passive). Facilitated diffusion: polar molecules and ions through channel or carrier proteins (passive). Osmosis: net movement of water from higher to lower water potential (ψ) across a partially permeable membrane; pure water ψ = 0, solutions negative. Plant cells: ; in concentrated solutions they become plasmolysed; in water, turgid. Animal cells burst (lysis) or shrink (crenation). Active transport moves substances against a gradient using ATP and carrier proteins; co-transport (e.g. Na⁺ and glucose in the ileum) uses a gradient set up by active transport. Endocytosis/exocytosis move bulk material in vesicles. Rate of diffusion increases with surface area, concentration gradient and temperature, and decreases with distance.
≈ 50 minTranscription and translation
A gene is a sequence of DNA bases coding for a polypeptide. The code is triplet (3 bases = 1 amino acid), degenerate (most amino acids have several codons), non-overlapping and universal. Transcription (nucleus): RNA polymerase reads the template strand, building pre-mRNA; introns are spliced out, leaving exons. Translation (ribosome): mRNA codons pair with anticodons on tRNA carrying specific amino acids; peptide bonds form; the ribosome moves along until a stop codon. Gene mutations: substitution (may be silent, missense or nonsense), insertion or deletion (frameshift, usually serious). Sickle cell: one base substitution changes glutamic acid to valine in the β chain.
≈ 50 min