IB Biology HL cheat sheet

Three-domain classification (Bacteria, Archaea, Eukarya) rests on rRNA sequence comparisons; Archaea and Eukarya share a more recent common ancestor than either does with Bacteria.

Binomial nomenclature: genus capitalized, species lowercase, both italicized or underlined; e.g. Homo sapiens.

Cladistics groups taxa by shared derived characters (synapomorphies); analogous structures from convergent evolution are excluded because they are homoplasies.

Molecular clocks use the roughly constant accumulation of neutral mutations to date divergence events; the universal genetic code and shared metabolic pathways (glycolysis, ATP) are evidence for a common ancestor.

Biological species concept: members interbreed to produce fertile offspring and are reproductively isolated from other groups; it fails for asexual organisms and fossils.

Reproductive isolation modes: prezygotic (temporal, behavioural, habitat, mechanical, gametic) and postzygotic (hybrid inviability, hybrid infertility, hybrid breakdown).

Binomial classification hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Eukaryotic kingdoms typically recognized: Protista (paraphyletic), Fungi, Plantae, Animalia; viruses are non-cellular and not classified in this scheme.

Evidence for evolution: fossil record, homologous structures, embryology, biogeography, and molecular (DNA/protein) comparisons.

Surface area to volume ratio scales as 1/length; smaller or flattened structures have higher ratios, favoring diffusion; large organisms need exchange and transport systems.

Mammalian lung: ventilation by negative pressure (diaphragm and external intercostals contract, volume increases, pressure falls); alveoli provide a large thin moist surface with a rich capillary network.

Enzyme kinetics: rate rises with substrate concentration to Vmax; Michaelis constant Km equals substrate concentration at half Vmax and is inversely related to enzyme affinity.

Competitive inhibitors raise apparent Km without changing Vmax; non-competitive inhibitors lower Vmax without changing Km.

Hemoglobin: cooperative oxygen binding gives a sigmoid dissociation curve; a right shift (higher pCO2, lower pH, higher temperature, higher 2,3-BPG) decreases affinity and favors unloading in tissues.

Water potential Ψ = Ψs + Ψp; water moves from higher to lower (more negative) water potential; pure water at atmospheric pressure has Ψ = 0.

Neuron action potential: resting potential about −70 mV (Na+/K+ pump, K+ leak); depolarization by Na+ influx, repolarization by K+ efflux, refractory period enforces one-way conduction.

Muscle contraction: sliding filament model; Ca2+ binds troponin, moving tropomyosin to expose actin-binding sites; ATP hydrolysis drives myosin head cycling.

Translocation in plants: phloem loading by active transport lowers Ψ in sieve tubes, water enters by osmosis, and pressure flow moves sap from source to sink.

Population growth: exponential dN/dt = rN; logistic dN/dt = rN(K − N)/K, where K is carrying capacity; growth is fastest at K/2 in the logistic model.

Energy transfer between trophic levels is typically about 10% (range roughly 5–20%); pyramids of energy are always upright, pyramids of numbers may invert.

Species interactions: mutualism (+,+), commensalism (+,0), parasitism (+,−), predation (+,−), herbivory (+,−), competition (−,−).

Carbon cycle: photosynthesis fixes CO2; respiration, combustion, and decomposition release it; fossil fuel combustion and deforestation are the main anthropogenic drivers of rising CO2.

Nitrogen cycle: nitrogen fixation (Rhizobium, cyanobacteria), ammonification, nitrification (Nitrosomonas, Nitrobacter), denitrification; plants absorb nitrate and ammonium.

Greenhouse effect: gases absorb outgoing long-wave infrared and re-emit it; key gases are CO2, CH4, N2O, and water vapor; methane has a higher warming potential per molecule than CO2.

Conservation biology: edge effects, habitat fragmentation, and small population size (inbreeding, genetic drift) raise extinction risk; keystone species have disproportionate community effects.

Trophic cascade: removing a top predator releases herbivores, which overgraze producers; example: sea otter–urchin–kelp.

DNA replication is semi-conservative; DNA polymerase adds nucleotides 5'→3' only, so the lagging strand is synthesized discontinuously as Okazaki fragments.

Central dogma: DNA → RNA (transcription) → protein (translation); mRNA codons are read 5'→3' in triplets against tRNA anticodons.

Meiosis: homologous chromosomes pair and cross over in prophase I; independent assortment in metaphase I; four haploid cells result; crossing over and independent assortment generate genetic variation.

Mendelian ratios: monohybrid F2 3:1; dihybrid F2 9:3:3:1 for unlinked genes; test cross distinguishes homozygous dominant from heterozygous.

Gene mutations: substitution (missense, nonsense, silent), insertion/deletion causing frameshift; sickle cell anemia is a single base substitution (GAG→GTG, glutamic acid→valine).

Human genetics: autosomal recessive (cystic fibrosis), autosomal dominant (Huntington's), X-linked recessive (hemophilia, red-green color blindness); carriers are heterozygous and unaffected.

Hardy–Weinberg: p + q = 1; p² + 2pq + q² = 1; allele frequencies stay constant if no mutation, migration, selection, drift, or non-random mating.

Natural selection requires heritable variation, differential reproductive success, and selection pressure; antibiotic resistance in bacteria is a standard example of directional selection.

PCR: denaturation (~95 °C), annealing of primers (~55 °C), extension by Taq polymerase (~72 °C); gel electrophoresis separates DNA by size (smaller fragments migrate farther).

Independent variable is manipulated; dependent variable is measured; controlled variables are kept constant; a control group receives no treatment or a placebo.

Validity = the investigation measures what it claims; reliability = consistent repeatable results (repeat trials, control conditions); accuracy = closeness to true value.

Uncertainty: for a single measurement use ± half the smallest scale division; for repeated measurements use the range or standard deviation; percentage uncertainty = (uncertainty/value) × 100.

Correlation is not causation; a scatter graph with a line of best fit and R² or correlation coefficient assesses association, not mechanism.

Statistical test choice: t-test compares two means; chi-squared tests observed vs expected frequencies; both require a null hypothesis and a stated significance level (usually p < 0.05).

Chi-squared: χ² = Σ (O − E)²/E; degrees of freedom = (rows − 1)(columns − 1); reject the null hypothesis if χ² exceeds the critical value.

Rate calculations use the gradient of a tangent or Δy/Δt; dilution series use C1V1 = C2V2; always report units, significant figures, and uncertainties.

Internal Assessment (scientific investigation) criteria: research question and background, methodology, data collection and processing, analysis and evaluation, and communication; personal engagement is embedded.

Ethical and safety considerations: animal welfare approval, informed consent for human subjects, aseptic technique, and appropriate risk assessment.