Biology
IB BiologyLife from molecule to ecosystem — four themes (unity & diversity, form & function, interaction & interdependence, continuity & change) and the experimental craft of biology. SL + HL paths share lessons; HL-extension topics are flagged with a violet badge.
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Water's polarity and hydrogen bonding explain cohesion, adhesion, solvent properties and the physical conditions of aquatic habitats; AHL extends to its possible extraterrestrial origin.
DNA and RNA store and transfer genetic information; their structure (nucleotides, antiparallel double helix, complementary base pairing) underpins faithful replication and expression.
Nucleotides and the polynucleotide backbone
Components of a nucleotide · Phosphodiester bonds and 5′→3′ directionality
The double helix and base pairing
Antiparallel strands & base pairing · DNA vs RNA & the universal code
Evidence for DNA as genetic material (AHL)HL
Nucleosome packaging · Hershey-Chase experiment · Chargaff's rules & falsifiability
Plausible hypotheses for the origin of life and the emergence of the last universal common ancestor (LUCA) from non-living chemistry.
Features common to all cells and the differences between prokaryotic and eukaryotic organisation, studied with light and electron microscopy.
What every cell has in common
Cell theory tenets · The universal kit · Eight functions of life
Prokaryotic vs eukaryotic cells
Prokaryote structure · Eukaryotic organelles & kingdoms · Endosymbiosis (HL) · Differentiation (HL)
Microscopy and cell size
Mag vs resolution · Scale bar calculation · Modern microscopy techniques
Viral structural diversity, the lytic and lysogenic cycles, and how rapid viral evolution drives emerging disease.
Defining species, the genetic and morphological variation within and between species, and binomial nomenclature.
Traditional taxonomy versus cladistics — building cladograms from shared derived characters and molecular sequence data.
Multiple lines of evidence for evolution; homologous vs analogous structures; and the mechanisms by which populations diverge into new species.
Evidence for evolution
What evolution is, and the evidence we can measure · Homologous vs analogous — what bones reveal
Allopatric and sympatric speciation
Speciation — splitting one gene pool into two · Allopatric vs sympatric — and the hybrid problem (HL)
Adaptive radiation + polyploidy (AHL)
Adaptive radiation — biodiversity bursts (HL) · Polyploidy — instant speciation in plants (HL)
The current biodiversity crisis (the sixth mass extinction), its anthropogenic drivers, and conservation strategies from species level to ecosystem level.
Monosaccharides, disaccharides and polysaccharides; triglycerides and phospholipids; and why each class is suited to particular biological roles.
Carbohydrates — sugars and polymers
How carbon builds the molecules of life · Monosaccharides → polysaccharides → glycoproteins
Lipids — triglycerides and phospholipids
Lipids — triglycerides + saturated vs unsaturated · Phospholipid bilayer + steroid permeability
Why energy stores are lipid-rich
Why triglycerides — not carbs — are used for long-term storage · Insulation + carbohydrate-vs-lipid comparison
Proteins from amino acids to four levels of structure, with function determined by 3D shape; AHL extends to R-group diversity and denaturation.
Amino acids and the primary structure
The amino acid and the peptide bond · 20 amino acids — essential vs non-essential
Secondary, tertiary and quaternary structure (HL)HL
Secondary structure — α-helix and β-sheet (HL) · Tertiary + quaternary structure (HL)
Denaturation and R-group diversity
pH and temperature on protein structure — denaturation · R-group diversity → conformation (HL)
The fluid mosaic model of membrane structure and the passive and active transport mechanisms it supports — diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
The fluid mosaic model
The phospholipid bilayer · The fluid mosaic model
Passive transport — diffusion and osmosis
Simple diffusion + osmosis · Channel proteins — facilitated diffusion
Active transport and bulk transport
Active transport — pumps against the gradient · Bulk transport, Na/K pump, cotransport (HL)
Membrane fluidity, gated channels and cell adhesion (HL)HL
Membrane fluidity (HL) · Gated channels and cell adhesion (HL)
Eukaryotic organelles (nucleus, mitochondria, chloroplasts, ribosomes, ER, Golgi, vesicles) and why cells partition their interior — concentration of enzymes + separation of incompatible processes.
From one fertilized egg to ~37 trillion specialised cells — stem cells, potency (toti/pluri/multi), niches, and how specialised cells adapt to their functions (cell size, SA:V ratio, RBC, pneumocytes, muscle, gametes).
Surfaces specialised for gas exchange in animals (alveoli) and plants (stomata and spongy mesophyll), and the diffusion gradients that drive them.
Properties of good exchange surfaces
Four properties of a good exchange surface · Maintaining the concentration gradient
Mammalian gas exchange
Alveoli and ventilation mechanics · Haemoglobin and the Bohr shift (HL)
Gas exchange in plants
Leaf tissues and stomata · Transpiration and stomatal density
Bulk transport systems — animal circulatory systems with vessels and a heart, plant xylem driven by transpiration, and plant phloem driven by pressure flow.
Skeletal muscle contraction by the sliding-filament mechanism, locomotion in different animal groups, and seed dispersal.
Species are adapted to their habitats; how terrestrial biomes are determined by abiotic factors such as temperature, water availability and seasonality.
The ecological niche concept, modes of nutrition (autotroph, heterotroph, mixotroph) and the link between adaptation and niche.
The niche concept
Niche + fundamental vs realised · Competitive exclusion
Modes of nutrition
Autotroph vs heterotroph and O₂ tolerance · Mixotrophs, saprotrophs, and archaea
Niche and adaptation
Dentition and the herbivore-plant arms race · Predator-prey arms race and forest light niches
Enzymes are globular protein catalysts whose active site lowers activation energy via induced-fit binding. Rate is governed by temperature, pH and substrate concentration (collision theory + denaturation). HL: intracellular vs extracellular reactions, cyclic vs linear pathways, and the four inhibition types (competitive · non-competitive · feedback · mechanism-based) with statins, isoleucine and penicillin as named examples.
Enzyme structure and induced-fit specificity
Active site + induced fit · Molecular motion + immobilised enzymes
Factors affecting enzyme activity
Temperature, pH, [S] + collision theory · Activation energy + denaturation
Anabolic vs catabolic pathways (AHL)HL
Pathways — location + topology (HL) · Four inhibition types (HL)
ATP as the cell's energy currency. Aerobic vs anaerobic respiration in humans (substrates, locations, ATP yields, waste products). HL: glycolysis as a 10-step pathway with 4 phases; lactate/ethanol fermentation regenerate NAD⁺. Link reaction + Krebs cycle in the matrix; electron transport chain pumps H⁺ to generate a gradient that drives ATP synthase (chemiosmosis); O₂ is the terminal electron acceptor. Lipids yield ~2× ATP per gram of carbohydrate.
Aerobic vs anaerobic respiration
ATP energy currency · Aerobic vs anaerobic respiration
Glycolysis and fermentation (AHL)HL
Glycolysis pathway (HL) · Fermentation: lactate + ethanol (HL)
Krebs cycle and the electron transport chain (AHL)HL
Link reaction + Krebs cycle (HL) · ETC + chemiosmosis + yields (HL)
Photosynthesis converts light energy into chemical energy in glucose, releasing O₂ from photolysis of water. Pigments separable by chromatography (Rf). Three limiting factors (light, CO₂, temperature) shape rate; FACE experiments predict future plant response. HL: Calvin cycle fixes CO₂ via Rubisco (RuBP + CO₂ → 2 GP → TP using NADPH + ATP) — RuBP regenerated. Thylakoid light-dependent reactions: photosystems with antenna pigments, photolysis at PSII (O₂ origin), chemiosmosis via H⁺ gradient, NADP⁺ reduction at PSI; light + dark reactions are interdependent.
Pigments and the light-dependent reactions
Overview + pigments + chromatography · Spectra + limiting factors + FACE
The Calvin cycle (AHL)HL
Rubisco + carbon fixation (HL) · RuBP regeneration (HL)
Photosynthesis in context (AHL)HL
Photosystems + photolysis + chemiosmosis (HL) · NADP reduction + interdependence (HL)
HL-only. Receptors are proteins with specific binding sites for ligands (hormones, neurotransmitters, cytokines, Ca²⁺). Hydrophilic signals bind transmembrane receptors; lipid-soluble steroids bind intracellular receptors that affect gene expression. Signal transduction cascades: quorum sensing in bacteria, ligand-gated ion channels (ACh), G-protein-coupled receptors with cAMP (adrenaline), and tyrosine kinase receptors (insulin → GLUT4). Endocrine integration via oestradiol + progesterone, with positive and negative feedback loops.
Hormones and receptor types (AHL)HL
Receptors + categories (HL) · Chemical diversity + receptor location (HL)
Signal transduction cascades (AHL)HL
Quorum sensing + ACh channel (HL) · GPCR + cAMP · Tyrosine kinase (HL)
Endocrine integration of body systems (AHL)HL
Steroid receptors + transcription (HL) · Feedback + oestradiol/progesterone (HL)
Resting and action potentials in neurons, propagation along myelinated axons, chemical synaptic transmission, and (HL) neural integration at the axon hillock plus pharmacology of drugs and neurotoxins.
Resting and action potentials
Resting potential (−70 mV) and Na⁺/K⁺ pump · Action potential cycle and saltatory conduction
Chemical synapses and neurotransmission
Five-step synaptic transmission · EPSP vs IPSP and NT removal
Neural integration and drug effects (AHL)HL
Spatial and temporal summation at the axon hillock · Agonists, antagonists, AChE inhibitors, reuptake inhibitors
Hierarchical organisation of organisms, coordination by nervous and endocrine systems, CNS/PNS and reflex arcs, and (HL) the hypothalamus-pituitary axis plus fight-or-flight integration via adrenergic receptor diversity.
Hierarchical organisation and two coordination systems
Hierarchy: cells → tissues → organs → systems · Nervous + endocrine coordination
Central nervous system and reflex arcs
CNS / PNS and sensory + motor neurons · Reflex arcs (monosynaptic + polysynaptic)
Hypothalamus-pituitary axis and fight-or-flight (AHL)HL
Hypothalamus + anterior + posterior pituitary · Epinephrine fight-or-flight + adrenergic receptor subtypes
Pathogens and primary defences (skin, mucus, acid, clotting), innate immunity (phagocytosis + inflammation), adaptive immunity (B cells, T cells, antibodies, clonal selection, memory), vaccination + herd immunity, HIV destroying helper T cells, and the evolution of antibiotic resistance.
Pathogens and primary defence
Pathogen types (bacteria, viruses, fungi, protists, prions) and transmission · Primary defences: skin, mucus + cilia, stomach acid + lysozyme, clotting
Innate and adaptive immunity
Phagocytosis and inflammation (innate) · Clonal selection, B + T cells, antibodies, memory (adaptive)
Vaccination, HIV, and antibiotic resistance
Vaccination + memory + herd immunity · HIV destroys helper T cells; antibiotic resistance evolves under selection
Exponential and logistic population growth, carrying capacity, intra- and interspecific competition (Gause), predator-prey oscillation (Lotka-Volterra), and community structure shaped by keystone species and the three symbioses.
Population growth models
Exponential growth (J-curve) · Logistic growth and carrying capacity K
Competition and predation
Intra- vs interspecific competition and Gause's exclusion · Predator-prey oscillation and reproductive lag
Community structure
Keystone species and trophic cascades · Mutualism, commensalism, parasitism
Trophic levels and food webs, energy pyramids and Lindeman's 10% rule, biogeochemical cycles (carbon, nitrogen, water), GPP vs NPP, and global productivity patterns.
Trophic levels and food webs
Producers, consumers, decomposers · Food webs and energy pyramids (10% rule)
Biogeochemical cycles (C, N, water)
Carbon cycle reservoirs and fluxes (incl. anthropogenic) · Nitrogen cycle (4 bacterial steps) and water cycle
Productivity and ecosystem efficiency
GPP vs NPP · 10% rule and global NPP variation
Semi-conservative DNA replication: the three candidate models and the Meselson-Stahl experiment; the four enzymes at the fork (helicase, primase, DNA polymerase, ligase) and leading-vs-lagging strand synthesis; AHL applications via PCR (DNA profiling, RT-PCR for COVID).
The semi-conservative model
Three candidate models (conservative, semi-conservative, dispersive) · Meselson-Stahl experiment (¹⁵N → ¹⁴N + CsCl gradient) · Why the band pattern uniquely supports semi-conservative
Enzymes of replication
Helicase, primase, DNA polymerase, ligase · Leading vs lagging strand and Okazaki fragments
PCR and replication biotechnology (AHL)HL
PCR cycle (denature / anneal / extend) and Taq polymerase · DNA profiling (STRs) and RT-PCR for COVID-19
DNA → mRNA via transcription (RNA polymerase, antisense template); mRNA → polypeptide via translation at the ribosome (A/P/E sites, tRNAs, the triplet code with redundancy + universality + start/stop signals); sickle-cell as a worked example of a single base change causing disease; HL extensions cover pre-mRNA processing (5' cap, poly-A tail, splicing + alternative splicing) and post-translational modification (cleavage of preproinsulin → insulin, ubiquitin/proteasome recycling).
Transcription
RNA polymerase reads the antisense strand · Gene expression starts with transcription
The genetic code and translation
The genetic code is a 3-letter language (redundancy + universality + start/stop) · Translation: ribosome A/P/E sites + tRNAs + sickle-cell worked example
Splicing and post-translational modification (AHL)HL
Pre-mRNA processing (5' cap, poly-A tail, splicing) + alternative splicing · Post-translational modification (preproinsulin → insulin) + ubiquitin/proteasome
Mutations as permanent changes in DNA sequence: substitutions vs indels (frameshift if not 3n); spontaneous, physical, and chemical sources (UV / X-ray / benzopyrene); their consequences — silent, missense, nonsense substitutions, with sickle-cell and β-thalassaemia as worked examples; germline vs somatic and the link to evolution and cancer; HL extension to deliberate gene editing with CRISPR-Cas9 (mechanism, Casgevy for sickle-cell, ethics of germline vs somatic editing).
Types of mutation
Substitution vs in-frame indel vs frameshift indel · Spontaneous, physical, and chemical mutagens
Consequences of mutation
Silent, missense, nonsense (sickle-cell + β-thalassaemia) · Germline vs somatic; mutation as source of variation for evolution
Gene editing with CRISPR (AHL)HL
CRISPR-Cas9 mechanism (gRNA, PAM, double-strand cut, NHEJ vs HDR) · Applications (Casgevy for sickle-cell) and ethics (somatic vs germline)
The cell cycle and its checkpoints; mitosis for growth and repair; meiosis with crossing-over and independent assortment for gamete production.
Regulation of gene expression via transcription factors, epigenetic modifications, and environment-driven plasticity.
Water potential combines solute and pressure components; osmotic movement of water across cell membranes in animals (haemolysis/crenation) and plants (turgor/plasmolysis).
Asexual and sexual reproduction strategies in plants and animals — gametogenesis, fertilisation, pollination and seed dispersal.
Asexual vs sexual reproduction
Clonal reproduction · Genetic variation from sex
Reproduction in flowering plants
Pollination · Seed and fruit formation
Reproduction in mammals
Gametogenesis · Fertilisation and pregnancy
Hormonal control of reproduction (AHL)
Menstrual cycle · IVF and contraception
Haploid and diploid number, Mendelian inheritance, dihybrid crosses, sex linkage, codominance, and pedigree analysis.
Mendelian inheritance and Punnett squares
Alleles, dominance, recessive · Monohybrid cross
Dihybrid crosses and independent assortment
Dihybrid cross and 9:3:3:1 ratio · Test crosses
Sex linkage and pedigree charts
X-linked inheritance · Reading pedigrees
Polygenic inheritance and codominance (AHL)
Codominance and incomplete dominance · Polygenic traits
Maintaining internal conditions via negative feedback — blood glucose, body temperature, water and solute balance.
Variation, differential reproduction and inheritance drive allele-frequency change; Hardy-Weinberg assumptions; comparison with Lamarckism.
Ecosystem stability and the disturbances (succession, fragmentation, introduced species) that drive change; tipping points and resilience.
Anthropogenic CO₂ and CH₄ emissions; greenhouse effect mechanism; evidence from ice cores; and biological impacts (range shifts, phenology, ocean acidification).