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LearnEdexcelYBI112018 specificationWBI12 · Unit 2: Cells, Development, Biodiversity and ConservationPlant structure and function, biodiversity and conservation

WBI12 · Module

Plant structure and function, biodiversity and conservation

4 subtopics≈ 185 min in total

Start the module: Classification and biodiversity Open graphing calculator

Classification and biodiversity
Organisms are classified into three domains (Bacteria, Archaea, Eukarya) and a hierarchy: domain, kingdom, phylum, class, order, family, genus, species; named binomially (Homo sapiens). Eukarya kingdoms: protoctists, fungi, plants, animals. Biodiversity includes ecosystem, species (richness and evenness) and genetic diversity. Measure with random sampling (quadrats at random coordinates) or systematic sampling (transects across a gradient). Mobile animals: mark–release–recapture, $N=\frac{n_1\times n_2}{m_2}$ . Simpson's index $D=1-\sum\left(\frac nN\right)^2$ (0 to 1; higher = more diverse). Spearman's rank tests correlation between two variables: $r_s=1-\frac{6\sum d^2}{n(n^2-1)}$ .
≈ 50 min
Plant fibres, starch and drug testing
Plant cells have a cellulose cell wall, middle lamella (calcium pectate), plasmodesmata, amyloplasts (starch storage), chloroplasts and a large vacuole with tonoplast. Sclerenchyma fibres are dead, elongated cells with thick lignified walls: high tensile strength, used for rope, textiles and composites (more sustainable than oil-based plastics). Measure tensile strength by hanging masses until a fibre breaks. Mineral ions: nitrates (proteins, DNA), calcium (middle lamella), magnesium (chlorophyll). Plants make antimicrobial substances (e.g. garlic, tea tree); test with discs on bacterial lawns. Drug testing: William Withering's foxglove trials (digitalis, 1785) were early dose–response studies. Modern trials: preclinical (cells, animals) → phase 1 (small healthy group, safety) → phase 2 (patients, effectiveness and dose) → phase 3 (large, randomised, double-blind, placebo-controlled).
≈ 45 min
Xylem, phloem and transpiration
Xylem vessels are dead, hollow tubes with no end walls and lignified walls (strength, waterproof) with pits. Water evaporates from mesophyll cells and diffuses out through stomata (transpiration), creating tension; cohesion between water molecules (hydrogen bonds) and adhesion to vessel walls pull a continuous column up: cohesion–tension. Water crosses the root via the apoplast (cell walls) and symplast (cytoplasm), but the Casparian strip forces it into the symplast at the endodermis. Transpiration increases with light, temperature and wind and decreases with humidity; measured with a potometer (water uptake). Xerophytes reduce water loss (sunken stomata, hairs, rolled leaves, thick cuticle). Phloem sieve tubes with companion cells move sucrose by mass flow from source to sink: sucrose loaded by co-transport lowers ψ, water enters, hydrostatic pressure pushes sap.
≈ 50 min
Conservation and human impact
Biodiversity is worth conserving for ethical, economic (medicines, food, tourism), ecological (stable ecosystems) and aesthetic reasons. Threats: habitat loss, climate change, pollution, invasive species, overexploitation. In situ conservation protects species in their habitat (national parks, marine reserves, controlling invasive species, managing succession by grazing or coppicing). Ex situ: zoos (captive breeding, genetic diversity using studbooks), botanic gardens, seed banks (dried, frozen seeds), frozen zoos (sperm, eggs, embryos), assisted reproduction (IVF, embryo transfer). CITES controls international trade in endangered species; the Convention on Biological Diversity promotes sustainable use. Reintroduction needs suitable habitat and genetic variety.
≈ 40 min