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The complete 2025 IB Biology syllabus at a glance. Four themes (A–D), Paper 3 removed, external exams now split across Paper 1 and Paper 2, IA worth 20%. Every subtopic in this guide is tagged SL or AHL so you can see exactly what HL students cover beyond SL.
Shared molecular basis of life across taxa: water, nucleic acids, classification, and biodiversity.
| Code | Subtopic | Level | Summary |
|---|---|---|---|
| A1.1 | Water | SL + HL | Polar molecular structure, hydrogen bonding, cohesion, adhesion, specific heat — explaining how water behaves as a solvent and a temperature buffer. |
| A1.2 | Nucleic acids | SL + HL | DNA and RNA structure, base pairing, antiparallel strands, and the conservation of nucleic acids across all life. |
| A2.1 | Origins of cells (HL only) | HL only (AHL) | Abiotic synthesis, RNA world hypothesis, endosymbiotic theory — the molecular bridge from chemistry to cellular life. |
| A2.2 | Cell structure | SL + HL | Prokaryote vs eukaryote architecture, organelle function, and the compartmentalisation that enables cellular specialisation. |
| A2.3 | Viruses (HL only) | HL only (AHL) | Viral structure, replication cycles, and their role in evolution via horizontal gene transfer. |
| A3.1 | Diversity of organisms | SL + HL | Species concepts, genetic diversity within species, and the three-domain classification. |
| A3.2 | Classification and cladistics | SL + HL | Hierarchical taxonomy, cladograms, molecular clock data, and evidence-based reconstruction of evolutionary relationships. |
| A4.1 | Evolution and speciation | SL + HL | Variation, natural selection, allopatric and sympatric speciation — with case studies from Galápagos finches and modern antibiotic resistance. |
| A4.2 | Conservation of biodiversity | SL + HL | Ecosystem and species-level extinction pressures, Simpson and Shannon indices, ex-situ vs in-situ conservation. |
Structure predicts function at every scale — from enzyme active sites to body-plan physiology.
| Code | Subtopic | Level | Summary |
|---|---|---|---|
| B1.1 | Carbohydrates and lipids | SL + HL | Monomer-polymer logic, condensation and hydrolysis reactions, triglycerides and phospholipids, energy storage vs structural roles. |
| B1.2 | Proteins | SL + HL | Primary, secondary, tertiary, quaternary structure; how sequence determines shape and shape determines function. |
| B2.1 | Membranes and membrane transport | SL + HL | Fluid mosaic model, simple and facilitated diffusion, osmosis, active transport, endocytosis and exocytosis. |
| B2.2 | Organelles and compartmentalisation | SL + HL | The endomembrane system, protein targeting, and how compartmentalisation enables incompatible biochemistry. |
| B2.3 | Cell specialisation (HL only) | HL only (AHL) | Stem cells, differentiation, and the regulatory networks that maintain tissue identity. |
| B3.1 | Gas exchange | SL + HL | Surface-area-to-volume constraint, alveolar architecture, ventilation and perfusion matching, plant stomatal gas exchange. |
| B3.2 | Transport | SL + HL | Xylem and phloem transport, mass flow in animals (circulation), oxygen dissociation curves, and lymphatic return. |
| B3.3 | Muscle and motility (HL only) | HL only (AHL) | Sliding filament theory, neuromuscular junction signalling, motor unit recruitment, and skeletal biomechanics. |
| B4.1 | Adaptation to environment | SL + HL | Ecological niches, coevolution, convergent evolution, and how form tracks environmental pressures. |
| B4.2 | Ecological niches | SL + HL | Fundamental vs realised niche, resource partitioning, and competitive exclusion as a predictor of community structure. |
Living systems are networks — molecular signalling, homeostasis, ecosystems, and global biogeochemical cycles.
| Code | Subtopic | Level | Summary |
|---|---|---|---|
| C1.1 | Enzymes and metabolism | SL + HL | Enzyme kinetics, activation energy, Michaelis-Menten behaviour, and metabolic pathways as regulated networks. |
| C1.2 | Cell respiration | SL + HL | Glycolysis, Krebs cycle, electron transport chain, ATP yield comparisons across aerobic and anaerobic pathways. |
| C1.3 | Photosynthesis | SL + HL | Light-dependent and light-independent reactions, chlorophyll absorption spectrum, limiting factors — irradiance, CO₂, temperature. |
| C1.4 | Chemiosmosis (HL only) | HL only (AHL) | Proton gradients, ATP synthase mechanism, and the shared chemiosmotic logic of respiration and photosynthesis. |
| C2.1 | Chemical signalling (HL only) | HL only (AHL) | Receptor types, second-messenger pathways, signal amplification, and disruption in pharmacology. |
| C2.2 | Neural signalling | SL + HL | Resting and action potentials, saltatory conduction, synaptic transmission, and neurotransmitter classes. |
| C3.1 | Integration of body systems | SL + HL | Homeostatic loops, nervous vs endocrine timescales, negative and positive feedback examples. |
| C3.2 | Defence against disease (HL only) | HL only (AHL) | Innate and adaptive immunity, antibody structure, vaccination, and mechanisms of allergy and autoimmunity. |
| C4.1 | Populations and communities | SL + HL | Predation, competition, mutualism, population dynamics (exponential vs logistic growth), and carrying capacity. |
| C4.2 | Transfer of energy and matter | SL + HL | Food webs, trophic efficiency, biogeochemical cycles (C, N, P), and the ecological costs of energy loss. |
Information is inherited, expressed, and changes — genetics, development, evolution, and the climate-era population.
| Code | Subtopic | Level | Summary |
|---|---|---|---|
| D1.1 | DNA replication | SL + HL | Semiconservative replication, leading and lagging strands, proofreading, and the origins of replication fidelity. |
| D1.2 | Protein synthesis | SL + HL | Transcription, RNA processing, translation, the genetic code, and its near-universal conservation. |
| D1.3 | Mutation and gene editing (HL only) | HL only (AHL) | Point mutations, frameshift mutations, CRISPR-Cas9, and the ethical frontier of germline editing. |
| D2.1 | Cell and nuclear division | SL + HL | Mitosis and meiosis, crossing over and independent assortment as sources of genetic variation, and the cell cycle checkpoints. |
| D2.2 | Gene expression (HL only) | HL only (AHL) | Transcription factors, enhancers, epigenetic regulation, and how cell identity is maintained through division. |
| D2.3 | Water potential | SL + HL | Water potential components, plasmolysis and turgor, with quantitative links to the osmotic IA you probably just wrote. |
| D3.1 | Reproduction | SL + HL | Sexual vs asexual strategies, pollination in angiosperms, menstrual cycle control, and IVF as applied reproductive biology. |
| D3.2 | Inheritance | SL + HL | Mendelian genetics, sex linkage, codominance, pedigree analysis, and Hardy-Weinberg equilibrium. |
| D3.3 | Homeostasis | SL + HL | Blood glucose, osmoregulation, thermoregulation — systems-level view of how multicellular organisms maintain internal state. |
| D4.1 | Natural selection | SL + HL | Variation, heritability, differential fitness, and the feedback loop that drives adaptation. |
| D4.2 | Stability and change | SL + HL | Anthropogenic climate change, tipping points in biological systems, and conservation biology in a changing world. |
| D4.3 | Climate change | SL + HL | Carbon cycle disruption, ocean acidification, shifting phenology, and evidence-based mitigation strategies. |
Three structural changes. First, content is reorganised into four themes (A: Unity and Diversity; B: Form and Function; C: Interaction and Interdependence; D: Continuity and Change) replacing the legacy 1-11 topic numbering. Second, Paper 3 (options) is removed — assessment is now split across two papers only. Third, the Internal Assessment rubric was reduced to four criteria (Research Design, Data Analysis, Conclusion, Evaluation) each worth 6 marks, with greater emphasis on Conclusion and Evaluation.
The first assessment was the May 2025 exam session. November 2025 was the second. All students first taught from August/September 2023 sit the new syllabus.
Standard Level (SL) is 150 teaching hours. Higher Level (HL) is 240 teaching hours, of which 90 are AHL (Additional Higher Level) content. Both levels share the four-theme structure — HL students cover additional subtopics marked as AHL within each theme.
Theme A: Unity and Diversity (water, nucleic acids, cells, classification, evolution, biodiversity). Theme B: Form and Function (biomolecules, membranes, gas exchange, transport, adaptation). Theme C: Interaction and Interdependence (enzymes, respiration, photosynthesis, signalling, populations, ecosystems). Theme D: Continuity and Change (DNA replication, protein synthesis, cell division, reproduction, inheritance, natural selection, climate change).
No. The four legacy options (Neurobiology and Behaviour, Biotechnology and Bioinformatics, Ecology and Conservation, Human Physiology) have been removed as a separate assessed unit. Some option content has been folded into the four themes as AHL content — for example human physiology material appears in Theme C.
Externally assessed papers are now Paper 1 and Paper 2. Paper 1 combines multiple choice questions (Paper 1A) with data-based short-response questions (Paper 1B). Paper 2 is extended response and data analysis. Together the external papers contribute 80% of the final grade; the Internal Assessment contributes the remaining 20%.
Cerebrum's 2-year programme is already mapped to the 4-theme 2025 syllabus with examiner-led live classes and a 2-point score guarantee.
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