explain why populations and species can become extinct as a result of: climate change, competition, hunting by humans, degradation and loss of habitats

Cambridge IGCSE / A‑Level Biology (9700) – Lecture Notes

Topic 1 – Cell Structure

• Prokaryotic vs eukaryotic cells: size, organelles, DNA organisation.
• Microscopy: light (bright‑field, phase‑contrast, fluorescence) and electron (TEM, SEM) techniques.
• Cell wall, plasma membrane & transport: phospholipid bilayer, fluid‑mosaic model, passive (diffusion, osmosis) and active transport (pumps, endocytosis).
• Key diagram: labelled animal & plant cell.

Topic 2 – Biomolecules

• Carbohydrates: monosaccharides, disaccharides, polysaccharides; functions (energy, storage, structural).
• Proteins: amino‑acid structure, peptide bonds, levels of organisation, enzyme specificity (lock‑and‑key, induced fit).
• Lipids: fatty acids, triglycerides, phospholipids, sterols; roles in membranes & energy storage.
• Nucleic acids: DNA & RNA structure, base‑pairing, replication, transcription, translation (overview).

Topic 3 – Enzymes

• Activation energy, catalyst action, enzyme‑substrate complex.
• Factors affecting activity: temperature, pH, substrate concentration, inhibitors (competitive, non‑competitive).
• Practical: effect of temperature on rate of reaction (e.g., amylase).

Topic 4 – Cell Division (Mitosis & Meiosis)

• Purpose of mitosis: growth, repair, asexual reproduction – produces 2 identical diploid cells.
• Phases of mitosis with key events (prophase → telophase) and diagram.
• Meiosis I & II: reduction division, genetic variation (crossing‑over, independent assortment). Produces 4 haploid gametes.

Topic 5 – Nucleic Acids & Protein Synthesis

• DNA replication: semi‑conservative model, enzymes (helicase, DNA polymerase, ligase).
• Transcription: DNA → mRNA (RNA polymerase, promoter, terminator).
• Translation: ribosome, tRNA, codons, peptide‑bond formation; start/stop codons.
• Genetic code is universal, non‑overlapping, degenerate.

Topic 6 – Transport in Plants

• Water transport: cohesion‑tension theory, xylem structure, transpiration stream.
• Mineral transport: root uptake (active & passive), xylem loading, phloem loading/unloading (pressure‑flow hypothesis).
• Practical: dye uptake in celery or rose stems.

Topic 7 – Transport in Animals

• Open vs closed circulatory systems; structure of the human heart (four chambers, double circulation).
• Blood vessels: arteries, veins, capillaries; blood pressure regulation (Baroreceptor reflex).
• Respiratory transport: oxygen binding to haemoglobin (co‑operative binding, Bohr effect).

Topic 8 – Gas Exchange

• Diffusion of gases across respiratory surfaces; surface‑area to volume ratio.
• Human lungs: alveolar structure, ventilation cycle, partial pressures (P_O2, P_CO2).
• Plant gas exchange: stomatal regulation, transpiration, photosynthetic gas exchange.

Topic 9 – Infectious Disease

• Pathogen types (bacteria, viruses, fungi, protozoa); routes of transmission.
• Host defence: innate (skin, phagocytes, inflammation) and adaptive (antibodies, cell‑mediated immunity).
• Vaccination principles (antigenic stimulation, memory cells).

Topic 10 – Immunity

• Humoral immunity: B‑cell activation, antibody classes, neutralisation, opsonisation.
• Cell‑mediated immunity: T‑cell types (helper, cytotoxic), MHC presentation.
• Allergy & auto‑immunity: mechanisms and examples.

Topic 11 – Homeostasis (A‑Level)

• Negative feedback loops – components (receptor, control centre, effector).
• Regulation of blood glucose (insulin, glucagon), body temperature (vasodilation, shivering), water balance (ADH, kidneys).
• Hormonal control: hypothalamus‑pituitary axes.

Topic 12 – Energy & Respiration (A‑Level)

• Catabolism of carbohydrates, fats, proteins; ATP yield (aerobic vs anaerobic).
• Glycolysis, pyruvate oxidation, Krebs cycle, oxidative phosphorylation – key enzymes and products.
• Respiratory Quotient (RQ) calculations and interpretation.

Topic 13 – Photosynthesis (A‑Level)

• Overall equation; light‑dependent reactions (photophosphorylation, water splitting, NADPH formation).
• Light‑independent (Calvin) cycle: CO₂ fixation, regeneration of RuBP.
• Factors limiting rate: light intensity, CO₂ concentration, temperature – experimental design.

Topic 14 – Control & Coordination (A‑Level)

• Nervous system: neuron structure, action potential, synaptic transmission, reflex arc.
• Hormonal signalling: endocrine glands, target‑cell receptors, second‑messenger cascades.
• Plant hormones (auxin, cytokinin, gibberellin, abscisic acid, ethylene) and their roles.

Topic 15 – Inheritance (A‑Level)

• Mendelian genetics: monohybrid & dihybrid crosses, law of segregation & independent assortment.
• Linkage & recombination frequency; sex‑linked inheritance (e.g., hemophilia, colour blindness).
• Pedigree analysis: autosomal dominant, recessive, X‑linked patterns.

Topic 16 – Selection & Evolution (A‑Level)

• Natural selection: variation, differential survival/reproduction, inheritance.
• Speciation mechanisms: allopatric, sympatric, peripatric, parapatric.
• Evidence for evolution: fossil record, comparative anatomy, molecular phylogenetics, biogeography.

Topic 17 – Classification, Biodiversity & Conservation (A‑Level)

17.1 Taxonomic Hierarchy & Nomenclature

• Binomial nomenclature: genus (capitalised) + specific epithet (lower‑case), italicised.
• Authority citation (e.g., Homo sapiens Linnaeus, 1758).

17.2 Phylogenetics & Cladistics

• Cladograms illustrate evolutionary relationships based on shared derived characters (synapomorphies).
• Monophyletic groups = common ancestor + all descendants (important for conservation priorities).
• Molecular clocks: using DNA sequence divergence to estimate divergence times.

17.3 Levels of Biodiversity

• Species diversity – number of species in a given area.
• Genetic diversity – allelic variation within and between populations.
• Ecosystem diversity – variety of habitats, ecological processes and interactions.

17.4 Ecosystem Services (Cambridge Syllabus Language)

17.5 Species‑Area Relationship & Extinction Risk

• Log‑log relationship: S = cA^z (S = species number, A = area). Smaller habitats support fewer species → higher extinction probability.
• Minimum Viable Population (MVP) – the smallest population size that can persist long‑term (generally > 50–500 individuals depending on species).

17.6 Drivers of Extinction

Climate Change

• Rising temperatures increase metabolic demand (Q₁₀ effect) and can shift optimal climatic zones pole‑ward or upward.
• Phenological mismatches (e.g., earlier insect emergence vs later plant leaf‑out) reduce food availability.
• Increased frequency of extreme events (droughts, floods, storms) causes direct mortality and habitat degradation.
• Species with limited dispersal ability or specialised niches are most vulnerable.

Competition

• Interspecific competition for a limiting resource can lead to competitive exclusion (the superior competitor drives the other to local extinction).
• Resource partitioning (different feeding times, microhabitats) allows coexistence; loss of partitioning (e.g., by habitat simplification) intensifies competition.
• Intraspecific competition at high densities reduces growth and reproductive output, increasing risk of population collapse.

Hunting & Over‑exploitation by Humans

• Direct removal lowers population size and can skew sex ratios (e.g., trophy hunting of large males).
• Removal of keystone or socially important individuals (dominant breeders, matriarchs) reduces genetic diversity and social stability.
• Allee effect: populations below a critical density experience reduced mate‑finding or cooperative behaviours, leading to rapid decline.
• Historical extinctions: passenger pigeon, great auk, thylacine.

Habitat Degradation & Loss

• Habitat loss – conversion to agriculture, urban areas, infrastructure reduces total area available.
• Fragmentation – remaining patches become isolated, limiting gene flow and increasing edge effects (altered micro‑climate, higher predation).
• Degradation – pollution, soil erosion, salinisation, eutrophication lower habitat quality.
• When habitat falls below the MVP, stochastic events (disease, fire) can cause extinction.

Invasive Species (Additional Driver)

• Non‑native organisms out‑compete, predate or transmit novel diseases (e.g., cane toad in Australia, brown tree snake on Guam).
• Hybridisation can dilute genetic integrity of native species.

Pollution (Additional Driver)

• Chemical contaminants (pesticides, heavy metals) cause mortality, reproductive failure, bioaccumulation.
• Eutrophication → algal blooms → hypoxia → “dead zones”.

Disease (Additional Driver)

• Emerging pathogens (chytrid fungus in amphibians, white‑nose syndrome in bats) can cause rapid population crashes.
• Low genetic diversity often increases susceptibility.

Over‑exploitation of Marine Resources (Additional Driver)

• Unsustainable fishing removes top predators and keystone species, altering trophic structure.
• By‑catch and destructive gear (bottom trawling) damage benthic habitats.

17.7 Genetic Factors Increasing Extinction Risk

• Loss of genetic diversity reduces adaptive potential to environmental change.
• Inbreeding depression – increased homozygosity exposes deleterious recessive alleles, lowering fitness.
• Genetic drift in small, isolated populations can fix harmful alleles.
• Genetic rescue (translocation of individuals) can increase heterozygosity and improve viability.

17.8 Conservation Tools & Strategies

In‑situ Conservation

• Protected areas (national parks, wildlife reserves, Marine Protected Areas).
• Habitat restoration – re‑forestation, wetland creation, removal of invasive species.
• Ecological corridors to reconnect fragmented populations.
• Legislation & international agreements (CITES, CBD, Ramsar).
• Community‑based management & sustainable use programmes.

Ex‑situ Conservation

• Captive breeding programmes, seed banks, gene banks.
• Assisted migration and re‑introduction of threatened species.
• Cryopreservation of gametes, embryos, and somatic cells.
• Emerging technologies: CRISPR‑mediated rescue of endangered alleles.

Climate‑change Mitigation for Conservation

• Reducing greenhouse‑gas emissions (renewable energy, carbon capture).
• Protecting climate‑refugia (areas likely to remain suitable under future climates).
• Enhancing ecosystem resilience (diverse, structurally complex habitats).

17.9 IUCN Red List Categories (Assessment of Extinction Risk)

17.10 Suggested Diagram – “Cascade of Extinction Drivers”

A central species icon with arrows pointing outward to the eight primary drivers (climate change, competition, hunting, habitat loss, invasive species, pollution, disease, over‑exploitation). Each arrow leads to “Reduced population size → Loss of genetic diversity → Extinction”. This visual links the concepts for exam answers.

Topic 18 – Genetic Technology (A‑Level)

• DNA extraction, PCR amplification, gel electrophoresis – principles and interpretation of bands.
• DNA sequencing (Sanger, next‑generation) and applications (species identification, phylogenetics).
• Genetically modified organisms (GMOs): gene cloning, transformation, selectable markers, biosafety.
• Gene therapy basics: viral vectors, CRISPR‑Cas9 genome editing, ethical considerations.
• Biotechnological applications in agriculture, medicine and conservation (e.g., gene drives for pest control).

Key Practical Skills

Summary

This set of notes follows the Cambridge IGCSE/A‑Level Biology syllabus, providing concise yet comprehensive coverage of all 19 topics. The conservation section (Topic 17) links the mechanisms that drive populations and species to extinction – climate change, competition, hunting, habitat degradation, invasive species, pollution, disease and over‑exploitation – with genetic consequences, ecosystem services, and the IUCN Red List framework. Together with the brief but complete outlines of the remaining topics, these notes give students a solid foundation for both coursework and examination preparation.