All subjects · Biology · Unit 2

WBI12 Cells, Development, Biodiversity and Conservation

Edexcel IAL Biology, Unit 2 (AS unit, usually Year 12). 25 lessons so far, in the suggested order.

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Lessons in WBI12

  1. Eukaryotic and prokaryotic cells
    Eukaryotic: nucleus, membrane-bound organelles, 80S ribosomes. Prokaryotic: no nucleus, circular DNA, plasmids, 70S ribosomes.
  2. Cell theory, tissues and bacterial structures
    Cells form tissues, tissues form organs, organs form systems. Bacterial structures: wall (support), capsule (protection), plasmids (extra genes), flagellum (mov
  3. Organelles and the secretory pathway
    Rough ER makes and folds proteins, the Golgi modifies and packages them, and vesicles release them by exocytosis. Know each organelle's function and how it look
  4. Magnification, resolution and staining
    Magnification = image size / actual size (same units). Resolution is the smallest separation at which two points can be seen as separate: about 200 nm for light
  5. Core practical 5: drawing cells and using a graticule
    Calibrate the eyepiece graticule with a stage micrometer for each objective. Drawings: pencil, clear lines, no shading, ruled labels, title and scale.
  6. Meiosis, loci and linkage
    Meiosis gives four genetically different haploid cells. Independent assortment (metaphase I) gives 2n combinations; crossing over (prophase I) swaps sections o
  7. Mammalian gametes and fertilisation
    Acrosome reaction: enzymes digest the zona pellucida. Membranes fuse, the sperm nucleus enters. Cortical reaction: the zona pellucida hardens to block other spe
  8. Fertilisation in flowering plants
    The pollen tube grows down the style using enzymes; the generative nucleus gives two male nuclei. One fuses with the egg (diploid zygote), the other with two po
  9. The cell cycle and mitosis
    Interphase (G1, S: DNA replication, G2), then prophase, metaphase, anaphase, telophase, cytokinesis. Mitosis gives two genetically identical cells for growth, r
  10. Core practical 6: root tip squash and mitotic index
    Acid separates cells and stops division; stain binds chromosomes; squash to one layer. Mitotic index = cells in mitosis / total cells.
  11. Stem cells and their use in medicine
    Totipotent: any cell type, including placenta (zygote, early embryo). Pluripotent: any body cell (inner cell mass of the blastocyst). Stem cells may treat many
  12. Cell specialisation and gene expression
    Specialisation comes from differential gene expression: only some genes are transcribed and translated. Alternative splicing of pre-mRNA lets one gene code for
  13. Epigenetics and continuous variation
    Phenotype = genotype + environment. DNA methylation and histone modification switch genes on or off without changing the base sequence, and are passed on in cel
  14. Plant cell ultrastructure
    Plant cells add a cellulose wall, middle lamella, plasmodesmata, pits, chloroplasts, amyloplasts and a vacuole with its tonoplast. Animal cells have centrioles
  15. Cellulose, starch and plant fibres
    Cellulose: β-glucose, straight chains linked by hydrogen bonds into microfibrils, layered at angles. Lignin adds rigidity in xylem and sclerenchyma. Xylem carri
  16. Core practical 7: plant tissues under the microscope
    Plan diagrams show tissue outlines only, not cells. Stem: ring of bundles (sclerenchyma, phloem, xylem from outside in). Root: central X-shaped xylem with phloe
  17. Plant fibres, sustainability and mineral ions
    Plant fibres and starch are renewable, biodegradable alternatives to oil-based materials. Nitrate: amino acids, DNA, chlorophyll. Calcium: calcium pectate. Magn
  18. Core practical 8: tensile strength of plant fibres
    Hang increasing masses from a clamped fibre until it breaks. Force = mass x 9.81; tensile strength = breaking force / cross-sectional area. Control fibre length
  19. Bacterial growth and core practical 9: antimicrobial plants
    Bacteria need nutrients, water, suitable temperature and pH, and often oxygen. Use aseptic technique, an ethanol-only control disc and incubation at 25 °C; meas
  20. Developing drugs: from digitalis to clinical trials
    Withering found foxglove (digitalis) treated dropsy using trial and error on patients. Today: pre-clinical, then phase 1 (healthy volunteers, safety), phase 2 (
  21. Classification and the three domains
    Species: can interbreed to give fertile offspring. Hierarchy: domain to species. Molecular evidence (DNA, protein, rRNA sequences) shows relationships; Woese's
  22. Biodiversity, endemism and measuring diversity
    Biodiversity covers species and genetic variety; endemic species live in only one area. Species richness = number of species. H = heterozygotes / individuals. D
  23. Niches and adaptation
    A niche is a species' role in its habitat. Adaptations are behavioural (actions), anatomical (structures) or physiological (body processes), and arise through n
  24. Hardy-Weinberg, natural selection and speciation
    p + q = 1 and p2 + 2pq + q2 = 1; start from q2. Mutation and natural selection change allele frequencies. Reproductive isolation stops gene flow, frequencies
  25. Conservation: zoos and seed banks
    Zoos: captive breeding (studbooks to avoid inbreeding), reintroduction (Arabian oryx), research and education. Seed banks: dried seeds stored at -20 °C, tested

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