What this resource is
This framework re-presents the Diploma Programme Biology Higher Level course as a single, prerequisite-ordered teaching sequence — not four syllabus themes taught in alphabetical order. It is built on three convictions: that topics should be taught in the order their dependencies require, that each should be taught to its full depth rather than its minimum, and that the connections between topics should be made explicit so that students see biology as one connected subject rather than forty separate ones.
The intended outcome is a student taught deeply and connectedly enough to design and run their own scientific investigation — one who chooses a research question because they understand the biology behind it, rather than one who, never having been shown the depth, outsources it.
Inside this document
- 01The design principle — how topics are classified and ordered
- 02The Teaching Spine — the full 40-topic flow, at a glance
- 03Why the Syntheses Come Last — the prerequisite feeder map
- 04The Sequence at a Glance — every topic with syllabus code, role and hours
- 05The 40 Topics in Depth — depth, interconnection and investigation angle for each
- 06Time Allocation & Pacing — reconciled to 240 hours, finishing by end of January
The design principle
Every topic is classified by the role it plays in the sequence:
- Foundational — self-contained, and a prerequisite for later topics.
- Developmental — extends one or more foundations.
- Synthesis — teachable at depth only once several strands are mature. Inheritance, gene expression, the integration of body systems, homeostasis, immunity, natural selection, speciation, classification, ecosystem stability, conservation and climate change all sit here, and all are placed late by design.
The synthesis topics are scattered across all four themes in the syllabus, yet their feeders cut straight across theme boundaries: inheritance needs meiosis from Theme D and nucleic acids from Theme A; the integration of body systems needs transport from Theme B and signalling from Theme C; climate change needs photosynthesis, respiration and carbon cycling before it means anything at all. That is why a vertical, theme-by-theme march fails, and why the horizontal, dependency-ordered path in the sections below succeeds.
IB DEMYSTIFIEDExaminers · Moderators · Mentors
The Teaching Spine
Prerequisite-ordered flow · 40 topics · Biology HLTeach top to bottom — each phase is a prerequisite for the next. Colour shows each topic’s role; the pill shows the phase’s teaching hours.
PHASE A21 teaching hours
1Water
2Carbs & Lipids
3Proteins
4Nucleic Acids
5Enzymes
▼
PHASE B31 teaching hours
6Cell Structure
7Origins of Cells
8Viruses
9Membranes
10Water Potential
11Organelles
12Cell Specialisation
13Cell & Nuclear Div.
▼
PHASE C28 teaching hours
14Cell Respiration
15Photosynthesis
16Gas Exchange
17Transport
18Muscle & Movement
▼
PHASE D36 teaching hours
19Chemical Signalling
20Neural Signalling
21Body Systems
22Homeostasis
23Defence
24Reproduction
▼
PHASE E30 teaching hours
25DNA Replication
26Protein Synthesis
27Mutations
28Gene Expression
29Inheritance
▼
PHASE F34 teaching hours
30Natural Selection
31Diversity
32Evolution
33Classification
34Adaptation
35Niches
36Populations
37Energy & Matter
38Stability
39Biodiversity
40Climate Change
Role:
Foundational
Developmental
Synthesis — taught late
Total taught content: 180 h · + 60 h practical scheme & investigation = 240 h
The full 40-topic sequence. Teach top to bottom; each phase is a prerequisite for the next.
IB DEMYSTIFIEDExaminers · Moderators · Mentors
Why the Syntheses Come Last
The prerequisite feeder mapEach synthesis topic (right) can only be taught at depth once its feeders (left) are in place. A gold arrow means ‘is a prerequisite for’.
29. Inheritance
13 · Cell & nuclear division — meiosis4 · Nucleic acids — genes & loci26 · Protein synthesis — allele to phenotype27 · Mutations — the source of alleles
➤
29 · Inheritance
Meiosis is its mechanism; mutation supplies its alleles.
32. Evolution & Speciation
30 · Natural selection29 · Inheritance27 · Mutations — variation31 · Diversity — species & genomes
➤
32 · Evolution & Speciation
The whole genetics phase has to be finished first.
21. Body Systems & Control
20 · Neural signalling19 · Chemical signalling17 · Transport systems16 · Gas exchange
➤
21 · Body Systems & Control
Integration needs the systems it integrates.
28. Gene Expression
26 · Protein synthesis — transcription4 · Nucleic acids — nucleosomes12 · Cell specialisation19 · Chemical signalling — cues
➤
28 · Gene Expression
Explains the specialisation taught much earlier.
40. Climate Change
15 · Photosynthesis14 · Cell respiration37 · Energy & matter — carbon cycling38 · Stability — feedback & tipping points
➤
40 · Climate Change
The last topic because everything feeds it.
How to read this
- Right-hand boxes are the five synthesis topics that draw the most strands together.
- Left-hand chips are the topics they depend on.
- A gold arrow means ‘is a prerequisite for’.
- Every feeder sits earlier in the spine, so by the time the topic is taught its feeders are done.
- The whole argument for the order in one image: sequence by dependency, not by theme letter.
Each synthesis topic can be taught at depth only once its feeders are in place.
The sequence at a glance
Every topic in teaching order, with its syllabus code, its role and its teaching hours. The suggested Year 1 / Year 2 boundary falls at the end of Phase D and is adjustable.
| # | Topic | Syllabus | Role | Hours |
|---|---|---|---|---|
| Phase A — Molecular Foundations (21 h) | ||||
| 1 | Water | A1.1 | Foundational | 3 h |
| 2 | Carbohydrates & Lipids | B1.1 | Foundational | 4 h |
| 3 | Proteins | B1.2 | Foundational | 4 h |
| 4 | Nucleic Acids | A1.2 | Foundational | 5 h |
| 5 | Enzymes & Metabolism | C1.1 | Developmental | 5 h |
| Phase B — The Cell (31 h) | ||||
| 6 | Cell Structure | A2.2 | Foundational | 6 h |
| 7 | Origins of Cells | A2.1 | Developmental | 3 h |
| 8 | Viruses | A2.3 | Developmental | 2 h |
| 9 | Membranes & Transport | B2.1 | Developmental | 5 h |
| 10 | Water Potential & Osmosis | D2.3 | Developmental | 3 h |
| 11 | Organelles & Compartmentalisation | B2.2 | Developmental | 3 h |
| 12 | Cell Specialisation | B2.3 | Developmental | 3 h |
| 13 | Cell & Nuclear Division | D2.1 | Foundational | 6 h |
| Phase C — Bioenergetics & Exchange (28 h) | ||||
| 14 | Cell Respiration | C1.2 | Developmental | 7 h |
| 15 | Photosynthesis | C1.3 | Developmental | 7 h |
| 16 | Gas Exchange | B3.1 | Developmental | 5 h |
| 17 | Transport Systems | B3.2 | Developmental | 6 h |
| 18 | Muscle & Movement | B3.3 | Developmental | 3 h |
| Phase D — Signalling, Control & Defence (36 h) | ||||
| 19 | Chemical Signalling | C2.1 | Developmental | 4 h |
| 20 | Neural Signalling | C2.2 | Developmental | 6 h |
| 21 | Body Systems & Control | C3.1 | Synthesis | 7 h |
| 22 | Homeostasis | D3.3 | Synthesis | 5 h |
| 23 | Defence Against Disease | C3.2 | Synthesis | 6 h |
| 24 | Reproduction | D3.1 | Developmental | 8 h |
| Phase E — Genetics & Continuity (30 h) | ||||
| 25 | DNA Replication & Biotechnology | D1.1 | Developmental | 5 h |
| 26 | Protein Synthesis | D1.2 | Developmental | 7 h |
| 27 | Mutations & Gene Editing | D1.3 | Developmental | 4 h |
| 28 | Gene Expression | D2.2 | Synthesis | 6 h |
| 29 | Inheritance | D3.2 | Synthesis | 8 h |
| Phase F — Evolution, Diversity & Ecology (34 h) | ||||
| 30 | Natural Selection | D4.1 | Synthesis | 4 h |
| 31 | Diversity of Organisms | A3.1 | Developmental | 5 h |
| 32 | Evolution & Speciation | A4.1 | Synthesis | 4 h |
| 33 | Classification & Cladistics | A3.2 | Synthesis | 3 h |
| 34 | Adaptation & Environment | B4.1 | Foundational | 3 h |
| 35 | Ecological Niches | B4.2 | Developmental | 3 h |
| 36 | Populations & Communities | C4.1 | Developmental | 3 h |
| 37 | Energy & Matter in Ecosystems | C4.2 | Developmental | 3 h |
| 38 | Stability & Environmental Change | D4.2 | Synthesis | 2 h |
| 39 | Biodiversity & Conservation | A4.2 | Synthesis | 2 h |
| 40 | Climate Change | D4.3 | Synthesis | 2 h |
| Total taught content | 180 h | |||
Scroll the table sideways on narrow screens.
The 40 topics in depth
Each topic carries its teaching depth, its interconnections with other topics, and the investigative angle through which it prepares a student for the scientific investigation.
Filter by role
Showing all 40 topics in teaching order.
Phase A — Molecular Foundations
Nothing here depends on later material, so it is taught first. Water, the four classes of carbon compound and the enzymes that act on them are the vocabulary in which every later cell, organ and ecosystem is described — and hydrogen bonding, condensation and hydrolysis are introduced once, here, then reused for the rest of the course.
1. Water
- Teach to this depth
- Polarity first, then hydrogen bonding as the single idea that explains everything else: cohesion, adhesion and the transpiration stream, surface tension as a habitat, the high specific heat capacity and latent heat of vaporisation, and water as a solvent. Use methane as the deliberate contrast so that polarity is isolated as the variable. Distinguish hydrophilic from hydrophobic solutes and follow the consequence into how glucose, amino acids, oxygen, fats and cholesterol are actually carried in blood. Treat buoyancy, viscosity and thermal conductivity as real habitat variables, and close with the evidence for the origin of water on Earth.
- Connects to
- Hydrogen bonding returns in DNA base pairing, in protein secondary structure and in the cohesion–tension mechanism of xylem; solvent behaviour underpins membranes, osmosis, blood transport and every enzyme reaction that follows.
- Investigation & inquiry angle
- Thermal-property and surface-tension investigations, solvent effects on solute movement, and pilot osmosis work that later becomes a full water-potential study.
2. Carbohydrates & Lipids
- Teach to this depth
- Begin with the tetravalency of carbon and the reason it builds diverse stable molecules, then teach condensation and hydrolysis once, as a single reaction pattern reused for all three macromolecule families. Take monosaccharides through to polysaccharides with the structure–function argument made explicitly — why cellulose is straight and strong, why starch and glycogen are branched stores. Cover glycoproteins, then triglycerides, phospholipids and steroids, distinguishing saturated, unsaturated, cis and trans fatty acids and evaluating real health data rather than asserting conclusions. Compare energy density and explain it.
- Connects to
- The amphipathic phospholipid is the bilayer of the next phase; the storage polysaccharides feed respiration and photosynthesis; glycoproteins reappear in membrane recognition and immunity; the condensation pattern recurs in proteins and nucleic acids.
- Investigation & inquiry angle
- Calorimetry of food energy content, amylase acting on starch, lipid extraction, and the critical evaluation of published diet-and-health correlation data.
3. Proteins
- Teach to this depth
- Amino acid structure and the diversity that R-groups create; peptide bond formation as another condensation. Build primary, secondary, tertiary and quaternary structure as a chain of consequences rather than four labels, name the bonds holding each level, and contrast globular with fibrous proteins through function. Treat denaturation by pH and temperature as bond disruption, not as a rule to memorise, and introduce the proteome as larger than the genome.
- Connects to
- Proteins are the direct product of protein synthesis and the substrate of the enzyme work that follows; haemoglobin, channel and pump proteins, antibodies, actin and myosin are all applications of this one topic; denaturation is the explanation behind every enzyme temperature curve.
- Investigation & inquiry angle
- Denaturation of egg albumin or milk protein under controlled conditions, enzyme activity against pH, and gel electrophoresis or chromatography of protein mixtures.
4. Nucleic Acids
- Teach to this depth
- Nucleotide structure, the sugar–phosphate backbone, complementary base pairing and the antiparallel double helix, with DNA and RNA compared feature by feature. Establish the genetic code as a language of triplets before any mechanism is taught. At HL, add nucleosomes and the packaging problem they solve. Teach the evidence as evidence: Hershey–Chase for DNA as the genetic material and Chargaff’s ratios for base pairing, and use molecular visualisation software so students handle the structure rather than only draw it.
- Connects to
- This is the foundation for replication, transcription and translation, for mutation and gene expression, and for the genome comparison, barcoding and environmental DNA that underpin classification and conservation.
- Investigation & inquiry angle
- DNA extraction with yield optimised against a controlled variable, molecular visualisation, and database work with published sequences.
5. Enzymes & Metabolism
- Teach to this depth
- Enzymes as globular protein catalysts: active site, specificity, induced fit. Explain the temperature, pH and substrate-concentration curves through collision theory and denaturation so the shapes are derived rather than recalled, and treat activation energy quantitatively. Distinguish intracellular from extracellular enzymes and anabolic from catabolic pathways. At HL, add competitive and non-competitive inhibition with graph interpretation, allosteric sites, end-product inhibition as a metabolic control loop, and immobilised enzymes in industry.
- Connects to
- Rests entirely on protein structure; it is the mechanism behind respiration, photosynthesis, digestion, replication and transcription; inhibition explains the action of many drugs and metabolic poisons.
- Investigation & inquiry angle
- The single richest source of investigations in the course — catalase, amylase, pectinase or lactase, with rate measured against temperature, pH, concentration or an inhibitor, and the initial-rate method used properly.
Phase B — The Cell
With the molecules in place, the cell can be assembled from them rather than described as a diagram. Surface-area-to-volume ratio, introduced here, becomes the argument that later explains gas exchange, transport and body size; meiosis is taught in this phase so that inheritance, reproduction and variation all have their mechanism ready.
6. Cell Structure
- Teach to this depth
- Cell theory together with the cases that strain it — aseptate fungal hyphae, striated muscle, giant algae. Microscopy with real magnification and scale calculations, biological drawing, and the interpretation of electron micrographs. Prokaryotic and eukaryotic ultrastructure compared; the functions of life demonstrated in Paramecium and Chlamydomonas. Establish surface-area-to-volume ratio as the limiting argument for cell size, then take it into differentiation, the emergence of multicellularity and the emergent properties that follow.
- Connects to
- SA:V drives gas exchange, transport systems and body size later in the course; ultrastructure sets up organelles, membranes and cell specialisation; the origin of eukaryotes anticipates evolution and classification.
- Investigation & inquiry angle
- Agar-cube diffusion models of SA:V, cell size measurement with a calibrated eyepiece graticule, and quantitative counts from prepared slides.
7. Origins of Cells
- Teach to this depth
- The conditions under which carbon compounds form spontaneously, and Miller–Urey as a designed test of a hypothesis. Vesicle formation as the origin of compartments, the RNA-world argument for a self-replicating polymer, LUCA and the evidence placing it at deep-sea vents, and the endosymbiotic origin of mitochondria and chloroplasts with the specific evidence — double membranes, circular DNA, 70S ribosomes, binary fission. Discuss how the first cells are dated.
- Connects to
- Builds directly on nucleic acids, membranes and cell structure; feeds evolution, classification and the three-domain system.
- Investigation & inquiry angle
- Literature- and database-based investigation, or modelling of spontaneous vesicle formation from amphipathic molecules.
8. Viruses
- Teach to this depth
- Capsid, genetic material and envelope, and the striking diversity of viral genomes. The lytic and lysogenic cycles with a named example, and HIV as a retrovirus with reverse transcription made explicit. Rapid evolution and antigenic change, and why viruses sit awkwardly against both the definition of life and any classification scheme; the competing hypotheses for their origin.
- Connects to
- Needs cell structure and nucleic acids; feeds immunity, vaccination and epidemiology; the mutation rate of viruses is natural selection observed on a human timescale.
- Investigation & inquiry angle
- Modelling of epidemic spread, or analysis of published outbreak and viral sequence data.
9. Membranes & Transport
- Teach to this depth
- Derive the bilayer from phospholipid amphipathy rather than presenting it. Integral and peripheral proteins and their functions; the Davson–Danielli to Singer–Nicolson shift taught as a nature-of-science case in which evidence overturned a model. Simple and facilitated diffusion, channels against carriers, osmosis, and active transport with the sodium–potassium pump worked through stoichiometrically. Membrane fluidity and the role of cholesterol; glycoproteins and glycolipids in recognition and adhesion; endocytosis and exocytosis; gated channels.
- Connects to
- Uses lipids and proteins directly; it is the basis of water potential, of the resting and action potential, of mineral uptake in roots, and of chemiosmosis in both respiration and photosynthesis.
- Investigation & inquiry angle
- Beetroot pigment leakage against temperature or solvent concentration, measured colorimetrically, with a properly justified range.
10. Water Potential & Osmosis
- Teach to this depth
- Solvation, then solute potential and pressure potential and water potential as their sum, with movement always down the gradient. Hypertonic, hypotonic and isotonic effects in animal and plant cells — lysis, crenation, plasmolysis, turgor — and the estimation of tissue water potential from mass-change data by locating the zero-change point. Explain why isotonic solutions matter in medicine and tissue culture.
- Connects to
- Applies membrane transport; explains guard cell action, root uptake and the transpiration stream; the arithmetic of the estimate is a model of quantitative practical reasoning.
- Investigation & inquiry angle
- The classic potato or plant-tissue sucrose series, done properly: replicates, controlled surface area, a calculated isotonic point and honest treatment of uncertainty.
11. Organelles & Compartmentalisation
- Teach to this depth
- Organelle structure and function, then the argument for compartmentalisation itself: local conditions maintained, incompatible reactions separated, substrates concentrated, membrane surface increased. The specific adaptations of mitochondria and chloroplasts, and the protein-secretion pathway followed through nucleus, rough ER, Golgi and vesicle. Identify organelles from electron micrographs.
- Connects to
- Builds on cell structure and membranes; it is the physical setting for chemiosmosis, for secretion, and for the specialisation that follows.
- Investigation & inquiry angle
- Quantitative micrograph analysis — organelle density compared across cell types with a stated sampling method.
12. Cell Specialisation
- Teach to this depth
- Differentiation as differential gene expression, stated that way from the start. The stem cell niche; totipotent, pluripotent and multipotent cells; named specialised cells with the structure–function argument made in each case; the constraints on cell size and shape. Therapeutic applications and the ethical debate handled with real cases.
- Connects to
- Needs cell structure; anticipates gene expression, which supplies its mechanism; underpins the tissue-level physiology of gas exchange, transport and immunity.
- Investigation & inquiry angle
- Quantitative micrograph work, stomatal density comparisons, and cell-type counts against a controlled variable.
13. Cell & Nuclear Division
- Teach to this depth
- The cell cycle with interphase properly subdivided; mitosis stage by stage with chromosome behaviour, and cytokinesis contrasted in plant and animal cells. Cyclins and checkpoint control; mutagens, oncogenes, tumour formation and metastasis; the mitotic index calculated and used as evidence. Then meiosis as reduction division — homologous pairing, crossing over, independent assortment, non-disjunction — with the three sources of genetic variation drawn out explicitly.
- Connects to
- Meiosis is the mechanism behind inheritance and behind the variation that natural selection acts on; the cell cycle links to gene expression and cancer; the mitotic index is one of the few genuinely quantitative cell practicals.
- Investigation & inquiry angle
- Mitotic index in root tips under a controlled treatment such as temperature or a named chemical; modelling of crossing over; analysis of published tumour data.
Phase C — Bioenergetics & Exchange
Respiration and photosynthesis are placed immediately after the cell because they are cell processes: they need enzymes, membranes and organelles and nothing else. Gas exchange and transport then follow as the organism-level answer to the problem those two processes create — getting substrates in and products out at a size where diffusion alone will not do it.
14. Cell Respiration
- Teach to this depth
- ATP as the energy currency, with the reason it suits the role. Oxidation and reduction defined in terms of hydrogen and electron transfer, and NAD and FAD as carriers. Glycolysis in outline with the net yield accounted for; anaerobic pathways in humans and in yeast, with their products and yields. At HL, the link reaction, the Krebs cycle, the electron transport chain, chemiosmosis and ATP synthase, and oxygen’s role as terminal electron acceptor. Compare yields, cover respiratory substrates and the respiratory quotient, and relate mitochondrial structure to each stage.
- Connects to
- Uses enzymes, membranes and organelles; it is the exact counterpart of photosynthesis, with chemiosmosis common to both; it explains muscle fatigue, the need for gas exchange and transport, and energy loss between trophic levels.
- Investigation & inquiry angle
- Respirometry with germinating seeds or invertebrates, yeast fermentation rate against temperature or substrate, and the determination of a respiratory quotient.
15. Photosynthesis
- Teach to this depth
- The overall equation read as an ecological statement, not a formula. Pigment separation by chromatography with Rf values; absorption and action spectra taught together so students can state the argument connecting them. Limiting factors with real graph interpretation and carbon dioxide enrichment data. At HL, photosystems I and II, photolysis, non-cyclic and cyclic photophosphorylation, chemiosmosis in the thylakoid, and the Calvin cycle with carbon fixation — closing on the interdependence of the light-dependent and light-independent reactions.
- Connects to
- Mirrors respiration in its use of chemiosmosis; supplies the carbon compounds that every ecosystem runs on; it is the basis of productivity, the carbon cycle and the climate change topic that ends the course.
- Investigation & inquiry angle
- Leaf-disc flotation or aquatic plant oxygen evolution against light intensity, wavelength or carbon dioxide concentration; quantitative chromatography.
16. Gas Exchange
- Teach to this depth
- The shared properties of exchange surfaces, derived from the SA:V argument rather than listed. Ventilation mechanics, alveolar structure and the roles of type I and type II pneumocytes; lung volumes read from spirometer traces. Leaf structure, stomata and guard cell action, with stomatal density measured. At HL, haemoglobin and the oxygen dissociation curve, cooperative binding, the Bohr shift, and the fetal and myoglobin curves interpreted against it.
- Connects to
- Uses SA:V, membranes and protein structure; feeds transport, exercise physiology and homeostasis; the dissociation curve is quaternary protein structure doing visible work.
- Investigation & inquiry angle
- Stomatal density against habitat, light or water availability; ventilation rate and recovery against exercise intensity.
17. Transport Systems
- Teach to this depth
- Arteries, capillaries and veins with the structure–function argument made in each case; pulse and blood pressure measurement; coronary heart disease approached through risk-factor data and its limitations. At HL, the mammalian heart, the cardiac cycle read from pressure and volume traces, valve action, myogenic contraction and the SA node; tissue fluid and lymph formation. On the plant side, xylem and the cohesion–tension mechanism, root uptake, transpiration and its measurement, xerophytic adaptations, and phloem translocation by pressure flow with sources and sinks.
- Connects to
- Applies water potential, membranes and gas exchange; feeds homeostasis, immunity and the integration of body systems; cohesion–tension is the payoff for the hydrogen bonding taught in the opening topic.
- Investigation & inquiry angle
- Potometer work on transpiration against light, wind or humidity; heart-rate recovery profiles; the evaluation of published epidemiological data on risk factors.
18. Muscle & Movement
- Teach to this depth
- The sliding filament model with actin, myosin, tropomyosin and troponin, and the roles of calcium and ATP made mechanistic. Sarcomere banding interpreted from electron micrographs, including what happens to each band on contraction. Antagonistic muscle pairs, joint structure and range of movement, the skeleton as a system of levers, and adaptations for locomotion in different media.
- Connects to
- Applies protein quaternary structure and ATP from respiration; complements neural signalling, which triggers the process; the lever analysis is a genuinely physical piece of biology.
- Investigation & inquiry angle
- Grip strength or fatigue against a controlled variable; sarcomere length measured from micrographs; lever mechanics in a named joint.
Phase D — Signalling, Control & Defence
Control is taught only once there is something to control. With exchange and transport in place, the signalling topics can be taught mechanistically rather than descriptively, and the three genuine syntheses of organism biology — integration of body systems, homeostasis and immunity — each arrive with every feeder already secure.
19. Chemical Signalling
- Teach to this depth
- Teach the logic first — ligand, receptor, transduction, response — then populate it. Hormones, cytokines, neurotransmitters and pheromones compared by range and speed. Receptor location follows from solubility: hydrophilic ligands bind at the surface, hydrophobic ones cross and bind inside. Work the epinephrine G-protein and cAMP cascade through in full, then intracellular receptors acting as transcription factors. Close on specificity, amplification, feedback, and the regulation of signalling by up- and down-regulation, tolerance and receptor-blocking drugs.
- Connects to
- Builds on membranes and protein structure; it is the mechanism behind homeostasis, gene expression, tropisms and the integration of body systems; receptor regulation explains drug tolerance and addiction.
- Investigation & inquiry angle
- Plant hormone effects on growth or germination; otherwise database and model-based work on dose–response data.
20. Neural Signalling
- Teach to this depth
- Neuron structure and myelination; the resting potential derived from the sodium–potassium pump and membrane permeability rather than stated. Depolarisation, repolarisation and the action potential as a sequence of gated channel events; the all-or-nothing law; the refractory period and what it makes possible. Saltatory conduction and the factors that set conduction velocity; oscilloscope traces read and explained. The synapse in full — calcium influx, vesicle fusion, receptor binding, removal of transmitter — with excitatory and inhibitory summation, and named agents such as neonicotinoids acting on it. Brain regions in outline.
- Connects to
- Applies membranes, channel proteins and active transport; feeds reflex arcs, homeostasis and muscle contraction; sets up the comparison with hormonal control.
- Investigation & inquiry angle
- Reaction-time investigations with one genuinely controlled variable and adequate replication; analysis of published electrophysiological traces.
21. Body Systems & Control
- Teach to this depth
- The organism treated as an integrated system rather than a list of organs. Nervous and endocrine control compared on speed, duration and specificity, with the reason for each difference. The reflex arc traced as a complete pathway, including a pain-withdrawal example; the division of labour between brain and spinal cord; the hypothalamus–pituitary link as the junction of the two control systems. Movement control from motor cortex to muscle fibre. On the plant side, tropisms, auxin and the mechanism of phototropism, apical dominance, growth regulators, and ethylene in fruit ripening.
- Connects to
- This is where neural signalling, chemical signalling, transport, gas exchange and muscle are brought together — the integration argument that gives the theme its name, and the reason this topic cannot be taught early.
- Investigation & inquiry angle
- Reaction time under a controlled variable; phototropic curvature against light direction or intensity; ethylene and ripening rate measured objectively.
22. Homeostasis
- Teach to this depth
- The set point, receptor, control centre and effector loop, then negative feedback worked through in three systems. Blood glucose regulation by insulin and glucagon, with type I and type II diabetes distinguished mechanistically and glucose-tolerance data interpreted. Thermoregulation through vasodilation and vasoconstriction, shivering, sweating and behaviour. Osmoregulation with ADH; kidney structure, ultrafiltration, selective reabsorption and the countercurrent action of the loop of Henle; urine composition read as diagnostic evidence.
- Connects to
- Requires transport, membranes, water potential and both signalling topics; it is the physiological case study of negative feedback and the model for the ecological feedbacks met later.
- Investigation & inquiry angle
- Cooling-curve models of thermoregulation with surface area or insulation as the variable; interpretation of published glucose tolerance or renal function data.
23. Defence Against Disease
- Teach to this depth
- Pathogens and what makes a disease infectious; skin and mucous membranes as primary defence; the clotting cascade in outline. Phagocytosis and the innate response, then lymphocytes, antigen recognition, antibody structure and the specificity argument. HIV transmission and the mechanism by which it causes AIDS. Antibiotics, their mode of action, why they do not work on viruses, and the evolution of resistance with MRSA as the worked case. Zoonoses; vaccination, memory cells and herd immunity; and throughout, the evaluation of epidemiological data.
- Connects to
- Pulls together proteins, cell specialisation, transport and viruses; antibiotic resistance is natural selection observed in real time, which is why it repays being taught before the evolution phase rather than after.
- Investigation & inquiry angle
- Zone-of-inhibition assays with antiseptics, plant extracts or antibiotics; analysis of published vaccination coverage or resistance-surveillance data.
24. Reproduction
- Teach to this depth
- Asexual and sexual reproduction compared on their consequences for variation. Gametogenesis in both sexes as an application of meiosis; fertilisation with the acrosome and cortical reactions and the blocks to polyspermy; implantation, placental structure and exchange, and the hormones of pregnancy and birth. Puberty and the sex hormones; the menstrual cycle taught as an interacting hormonal system with graph interpretation rather than four curves to memorise; IVF and hormone replacement therapy with their data. Plant reproduction, pollination, seed dispersal and the conditions for germination.
- Connects to
- Requires meiosis, hormonal signalling and transport; it feeds inheritance directly, and through variation it feeds natural selection.
- Investigation & inquiry angle
- Germination rate against a controlled factor; pollen tube growth in different sucrose concentrations; analysis of published fertility or cycle data.
Phase E — Genetics & Continuity
The molecular genetics strand is taught in one block, in mechanistic order: copy the information, express it, corrupt it, regulate it, then inherit it. Inheritance sits at the end of the phase because it needs meiosis, mutation and the gene–allele distinction all secure — and the whole phase sits before evolution because natural selection is meaningless without it.
25. DNA Replication & Biotechnology
- Teach to this depth
- Semi-conservative replication with the Meselson–Stahl experiment taught as the evidence that settled it. Helicase, DNA polymerase, primase and ligase, and at HL the leading and lagging strands with Okazaki fragments — with the antiparallel structure of the helix used to explain why replication has to work this way. PCR with the function of primers and each temperature step; gel electrophoresis and the interpretation of banding patterns; DNA profiling and its forensic, medical and evolutionary applications.
- Connects to
- Applies nucleic acid structure and enzyme action; it is the technique base for gene editing, genome sequencing, cladistics, environmental DNA and conservation genetics.
- Investigation & inquiry angle
- Gel electrophoresis of dyes or restriction digests; database analysis of published profiles or amplicon data.
26. Protein Synthesis
- Teach to this depth
- Transcription with RNA polymerase, the template strand, promoter and terminator. The genetic code — codons, degeneracy, universality — and the evolutionary inference that universality supports. Translation in full: ribosome structure, tRNA and the anticodon, initiation, elongation and termination. Free against bound ribosomes and what that means for a protein’s destination. At HL, post-transcriptional modification, introns and exons, alternative splicing, and the reason the proteome exceeds the genome. Finish by tracing a single base change through to the polypeptide.
- Connects to
- The mechanistic bridge from nucleic acids to proteins, and therefore the topic that makes mutation, gene expression and biotechnology explicable rather than assertable.
- Investigation & inquiry angle
- Bioinformatics: codon usage comparison, sequence alignment, splice-variant analysis from published databases.
27. Mutations & Gene Editing
- Teach to this depth
- Base substitution, insertion and deletion, and the frameshift argument reasoned through rather than asserted. Silent, missense and nonsense outcomes with sickle-cell anaemia as the worked case from base to phenotype. Germline against somatic mutation and the different consequences of each; mutagens and mutation rate; mutation established as the ultimate source of all variation. Conserved sequences and what conservation implies about function. CRISPR-Cas9 mechanism and applications, with the ethical debate given real cases.
- Connects to
- Rests on protein synthesis; supplies the raw material for natural selection and speciation; conserved sequences feed straight into cladistics.
- Investigation & inquiry angle
- Sequence comparison and conservation analysis across species using published databases.
28. Gene Expression
- Teach to this depth
- Start from the fact that needs explaining: every cell has the same genome and no two cell types look alike. Transcriptional regulation by promoters, enhancers, repressors and transcription factors; epigenetic regulation by DNA methylation and histone modification, and the evidence that some of it is heritable; translational and post-translational control. Then environmental effects on expression — coat colour in Himalayan rabbits and Siamese cats, and monozygotic twin studies — and the general point that phenotype is genome plus environment.
- Connects to
- Needs protein synthesis, nucleosome structure and chemical signalling; it supplies the mechanism for cell specialisation taught earlier, and it explains the gap between genotype and phenotype that continuous variation depends on.
- Investigation & inquiry angle
- Temperature or light effects on pigment expression in a suitable organism; analysis of published twin or methylation datasets.
29. Inheritance
- Teach to this depth
- Insist on precise vocabulary — gene, allele, locus, genotype, phenotype, dominance, codominance, incomplete dominance. Monohybrid crosses and Punnett squares; multiple alleles through ABO blood groups; sex determination and sex linkage with haemophilia and colour blindness; pedigree analysis where students deduce the pattern rather than confirm it. At HL, dihybrid crosses with the 9:3:3:1 ratio explained by independent assortment, autosomal linkage and recombinants as the deviation from it, continuous variation and polygenic inheritance, and the chi-squared test applied to real cross data.
- Connects to
- Meiosis is its mechanism, mutation supplies its alleles, and the shift from genotype counts to allele frequencies is exactly the step into natural selection and Hardy–Weinberg.
- Investigation & inquiry angle
- A real cross analysed with chi-squared — Drosophila, maize cobs or seedling phenotypes — or a properly sampled continuous-variation study.
Phase F — Evolution, Diversity & Ecology
The genuine capstones, placed last by design. Natural selection needs inheritance and mutation; speciation needs natural selection; cladistics needs genome comparison; and the ecology topics that close the course draw on photosynthesis, respiration, transport and populations at once. Climate change is taught last of all because almost every earlier topic bears on it.
30. Natural Selection
- Teach to this depth
- Build the argument in four steps — variation, overproduction, differential survival, heritable advantage — and insist that students can state it. Selection pressures with named, data-backed examples: peppered moths, antibiotic resistance, beak morphology in Darwin’s finches. Then the population-level restatement: gene pools and allele frequencies. Stabilising, directional and disruptive selection with graph interpretation. At HL, Hardy–Weinberg equilibrium, its five assumptions, and the calculation used to test whether a population is evolving. Artificial selection compared with the natural case.
- Connects to
- Needs inheritance, mutation and variation to be secure; it is the mechanism behind speciation, adaptation, antibiotic resistance and conservation genetics.
- Investigation & inquiry angle
- Allele-frequency simulations with a stated selection coefficient; analysis of published resistance or morphometric datasets; continuous-variation fieldwork.
31. Diversity of Organisms
- Teach to this depth
- Species concepts and the honest difficulty of the biological species definition — ring species, fertile hybrids, asexual organisms. Chromosome number, karyotypes and karyograms, and what they can and cannot show. Genome size and its poor correlation with organismal complexity; genome sequencing and cross-species comparison; cross-breeding and hybrid fertility. Dichotomous keys constructed as well as used; environmental DNA and barcoding for biodiversity survey.
- Connects to
- Applies nucleic acid and genome ideas from the genetics phase; feeds classification, speciation and conservation; eDNA links molecular technique to ecological fieldwork.
- Investigation & inquiry angle
- Key construction for a real group of organisms; morphometric variation between two populations; comparison of published genome or barcode data.
32. Evolution & Speciation
- Teach to this depth
- Treat the evidence as several independent lines that converge — fossils, selective breeding, homologous structures, vestigial organs, biogeography and molecular sequence data — and make clear why convergent evolution produces analogous rather than homologous features. Then reproductive isolation, and allopatric and sympatric speciation with named cases. Gradualism against punctuated equilibrium; adaptive radiation; and polyploidy as instantaneous speciation in plants.
- Connects to
- The point where genetics, natural selection and diversity converge. The argument only lands if inheritance, mutation and population thinking are already secure — which is precisely why it is placed here and not in the opening weeks.
- Investigation & inquiry angle
- Analysis of published molecular or morphological datasets; construction of a phylogeny and comparison against the fossil record.
33. Classification & Cladistics
- Teach to this depth
- The hierarchy of taxa and binomial nomenclature; natural against artificial classification and why the difference matters. Clades and cladograms, with branch points read correctly; molecular clocks and base-sequence difference as quantitative evidence; parsimony as the selection rule. Work through a case where molecular evidence forced reclassification, such as the figwort family, and finish with the three-domain system and the evidence behind it.
- Connects to
- Rests on genome comparison, conserved sequences and evolution; it is the direct application of the molecular evidence built up across the genetics phase.
- Investigation & inquiry angle
- Constructing a cladogram from published sequence data and comparing it with a morphology-based classification.
34. Adaptation & Environment
- Teach to this depth
- Habitat distinguished from niche; abiotic factors as limiting factors, with the range of tolerance and its zones. Adapted traits in named marine, freshwater and terrestrial organisms, argued rather than listed. The biome concept and the temperature and rainfall variables that define biome distribution. Sampling species distribution against a measured abiotic gradient, using transects and correlation properly.
- Connects to
- Applies the exchange and transport physiology taught earlier; it is the foundation for niches, populations and the conservation argument.
- Investigation & inquiry angle
- Transect fieldwork correlating the abundance of one species with a measured abiotic factor, with a stated sampling strategy and a correlation coefficient.
35. Ecological Niches
- Teach to this depth
- The niche as the full set of conditions and resources a species uses, not merely where it lives. Obligate anaerobes, facultative anaerobes and obligate aerobes; autotrophs, heterotrophs, mixotrophs and saprotrophs, with named examples. Adaptations for feeding compared across related species. The fundamental against the realised niche, competitive exclusion, and the effect of competition on observed distribution.
- Connects to
- Builds on adaptation and on the metabolic topics from Phase C; it is the conceptual base for populations, communities and energy flow.
- Investigation & inquiry angle
- Competition experiments with duckweed or yeast under controlled conditions; field distribution data for two competing species.
36. Populations & Communities
- Teach to this depth
- Population size estimated by quadrat sampling and by capture–mark–recapture with the Lincoln index, including the assumptions each method makes. Carrying capacity and limiting factors; the sigmoid growth curve explained phase by phase; exponential growth in the laboratory. Intraspecific and interspecific relationships — competition, predation, herbivory, mutualism, parasitism, disease — and predator–prey cycles read from real data. The chi-squared test used for association between two species.
- Connects to
- Uses the niche concept; feeds energy flow, ecosystem stability and conservation; the chi-squared test returns here from the genetics phase, which is worth pointing out explicitly.
- Investigation & inquiry angle
- A quadrat-based association study analysed with chi-squared; yeast or duckweed growth curves fitted to the sigmoid model.
37. Energy & Matter in Ecosystems
- Teach to this depth
- The ecosystem as a system through which energy flows and matter cycles — the distinction stated sharply. Trophic levels and food webs; the reasons energy transfer between levels is inefficient and the consequence for the number of levels a system can support. Pyramids of energy; gross and net primary productivity and secondary productivity, calculated from data. The carbon cycle with photosynthesis, respiration, decomposition and sequestration in peat and fossil fuels; carbon pools and fluxes; mesocosms as a study method.
- Connects to
- Draws directly on photosynthesis and respiration; it is the bridge from organism physiology to global change, and the quantitative basis of the climate topic.
- Investigation & inquiry angle
- Mesocosm productivity measurement; leaf-litter decomposition rate against a controlled variable; analysis of published flux data.
38. Stability & Environmental Change
- Teach to this depth
- What a stable ecosystem actually requires. Keystone species and the measured consequences of their removal; tipping points; the sustainability of harvesting and agriculture. Eutrophication worked through as a complete causal chain from nutrient input to biochemical oxygen demand and death; biomagnification of persistent pollutants; plastics and microplastics. Ecological succession, primary and secondary, with succession arrested by human activity; ecosystem restoration and rewilding.
- Connects to
- Requires populations, energy flow and nutrient cycling; it is the applied face of the ecology strand and the immediate precursor to conservation and climate change.
- Investigation & inquiry angle
- Succession along a real gradient; nutrient enrichment and algal growth; BOD measurement in contrasting water samples.
39. Biodiversity & Conservation
- Teach to this depth
- Biodiversity at three levels — genetic, species and ecosystem — and the measurement of species diversity with an index that accounts for evenness as well as richness. The evidence on current extinction rates and the case for a sixth mass extinction; named causes with case studies; habitat loss and fragmentation and why fragmentation is not simply area loss. In situ and ex situ approaches, rewilding, EDGE species and the role of the IUCN Red List, with the ecological, economic and ethical arguments distinguished.
- Connects to
- Draws on diversity, evolution, populations and stability; it is the applied conclusion of the whole course and the natural home for the ethical dimension of the subject.
- Investigation & inquiry angle
- Diversity index comparison between two sites under different management; analysis of published Red List or monitoring data.
40. Climate Change
- Teach to this depth
- The greenhouse mechanism explained physically, and the enhanced effect distinguished from the natural one. The absorption properties of carbon dioxide and methane and why they differ; anthropogenic sources; the evidence linking emissions to temperature, taught through ice cores and the Keeling curve rather than assertion. Ocean acidification and its carbonate chemistry; effects on distribution, phenology and coral reefs; positive feedback loops such as permafrost methane release and albedo loss; carbon sequestration and mitigation; and the evolutionary responses now being recorded.
- Connects to
- The final synthesis: photosynthesis, respiration, carbon cycling, populations, distribution, phenology and natural selection all bear on it at once. Placed last, it can be taught as biology rather than as current affairs.
- Investigation & inquiry angle
- Analysis of long-run published climate and phenology datasets; ocean-acidification models using shells or calcifying organisms.
Time allocation & two-year pacing
The IB recommends 240 teaching hours for Higher Level Biology — 180 hours of taught content plus 60 hours for the practical scheme of work, which itself contains the 10-hour collaborative sciences project and the 10-hour scientific investigation. The allocations in the sequence table above distribute the 180 content hours across all 40 topics; below they are reconciled to the official per-theme totals and paced so that teaching is complete by the end of January in Year 2.
Reconciliation to the official IB allocation
| IB syllabus theme | Official IB hours | Allocated here |
|---|---|---|
| A — Unity and diversity | 33 h | 33 h |
| B — Form and function | 39 h | 39 h |
| C — Interaction and interdependence | 48 h | 48 h |
| D — Continuity and change | 60 h | 60 h |
| Taught content subtotal | 180 h | 180 h |
| Practical scheme of work (incl. 10 h collaborative sciences project + 10 h scientific investigation) | 60 h | 60 h |
| HL course total | 240 h | 240 h |
Because this framework re-sequences topics across theme boundaries, a phase’s hours will not match a theme’s. The reconciliation above proves that nothing has been added or lost: every topic still carries the hours its theme is entitled to.
The two-year pacing plan
Built on roughly four to four-and-a-half teaching hours per week across the two years, with the practical scheme woven through the terms rather than bolted on at the end. The cumulative column tracks progress toward the 240-hour total; the gold rows fall outside the teaching budget.
| Period | Focus | Hours | Cumul. |
|---|---|---|---|
| YEAR 1 | |||
| Autumn term | Phase A — Molecular Foundations · begin Phase B (cell structure → membranes) · tools, safety and practical-skills induction | 47 | 47 |
| Spring term | Finish Phase B (water potential → cell and nuclear division) · Phase C — Bioenergetics & Exchange · practical work | 53 | 100 |
| Summer term | Phase D — Signalling, Control & Defence · Collaborative Sciences Project · practical work | 52 | 152 |
| YEAR 2 (to end January) | |||
| Autumn term | Phase E — Genetics & Continuity · Scientific Investigation written and submitted · begin Phase F (natural selection → classification) | 60 | 212 |
| To end of January | Phase F completed — adaptation, niches, populations, energy and matter, stability, conservation and climate change · remaining practical work | 28 | 240 |
| FEBRUARY – APRIL | Dedicated revision: past papers, Paper 1A / 1B and Paper 2 drills, and timed mocks (additional to the 240 teaching hours) | — | — |
| MAY | IB examinations | — | — |
A note on the practical scheme. The 60 hours are not a separate course. Roughly 40 hours are practical work distributed across the topics — the enzyme, osmosis, respirometry, potometer, transpiration, mitotic index and fieldwork investigations named in the topic entries — with 10 hours for the collaborative sciences project at the end of Year 1 and 10 hours for the scientific investigation in the first term of Year 2. Placing the investigation there is deliberate: by that point students have met the enzyme, osmosis, respiration, photosynthesis, transpiration and ecology techniques, so they choose a question from a position of knowledge, and there is still time to absorb an unexpected setback.
Revision time (February–April) is deliberately additional to the 240 teaching hours, in line with the subject guide’s reminder that adequate time must be set aside for examination revision.