What this resource is
This framework re-presents the Diploma Programme Biology Standard 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 thirty-three 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 33-topic flow, at a glance
- 03Where the Strands Come Together — how the applied topics build on the foundations
- 04The Sequence at a Glance — every topic with syllabus code, role and hours
- 05The 33 Topics in Depth — depth, interconnection and investigation angle for each
- 06Time Allocation & Pacing — reconciled to 150 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 — applied topics that draw several earlier strands together, taught once those strands are secure. Inheritance, the integration of body systems, homeostasis, immunity, natural selection, speciation, ecosystem stability, conservation and climate change all sit here.
Standard Level has no late-stage capstone of the kind found at Higher Level — no gene expression, no cladistics, no Hardy–Weinberg. Instead the synthesis happens in these applied topics, where several foundations are brought to bear on a real biological problem at once. 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. The dependency-ordered path in the sections below makes sure those foundations are always in place first.
IB DEMYSTIFIEDExaminers · Moderators · Mentors
The Teaching Spine
Prerequisite-ordered flow · 33 topics · Biology SLTeach 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 A36 teaching hours
1Water
2Carbs & Lipids
3Proteins
4Nucleic Acids
5Enzymes
6Cell Structure
7Membranes
8Water Potential
9Organelles & Cell Spec.
10Cell & Nuclear Div.
▼
PHASE B15 teaching hours
11DNA Replication
12Protein Synthesis
13Mutations
14Inheritance
▼
PHASE C35 teaching hours
15Cell Respiration
16Photosynthesis
17Gas Exchange
18Transport
19Neural Signalling
20Body Systems
21Homeostasis
22Defence
23Reproduction
▼
PHASE D24 teaching hours
24Natural Selection
25Diversity
26Evolution
27Adaptation
28Niches
29Populations
30Energy & Matter
31Stability
32Biodiversity
33Climate Change
Role:
Foundational
Developmental
Synthesis — taught late
Total taught content: 110 h · + 40 h practical scheme & investigation = 150 h
The full 33-topic sequence. Teach top to bottom; each phase is a prerequisite for the next.
IB DEMYSTIFIEDExaminers · Moderators · Mentors
Where the Strands Come Together
How the applied topics build on the foundationsEach synthesis topic (right) can only be taught at depth once its feeders (left) are in place. A gold arrow means ‘is a prerequisite for’.
14. Inheritance
10 · Cell & nuclear division — meiosis4 · Nucleic acids — genes & alleles12 · Protein synthesis — allele to phenotype13 · Mutations — the source of variation
➤
14 · Inheritance
Meiosis is its mechanism; mutation supplies its alleles.
20. Body Systems & Control
19 · Neural signalling18 · Transport systems17 · Gas exchange9 · Cell specialisation
➤
20 · Body Systems & Control
Integration needs the systems it integrates.
22. Defence Against Disease
3 · Proteins — antibody structure18 · Transport — blood & clotting9 · Specialised cells — lymphocytes13 · Mutations — resistance
➤
22 · Defence Against Disease
Immunity is protein chemistry doing a job.
26. Evolution & Speciation
24 · Natural selection14 · Inheritance13 · Mutations — variation25 · Diversity — species & genomes
➤
26 · Evolution & Speciation
Genetics has to be finished first.
33. Climate Change
16 · Photosynthesis15 · Cell respiration30 · Energy & matter — carbon cycling31 · Stability — feedback & tipping points
➤
33 · Climate Change
The last topic because everything feeds it.
How to read this
- Right-hand boxes are the five applied 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 in the middle of Phase C and is adjustable.
| # | Topic | Syllabus | Role | Hours |
|---|---|---|---|---|
| Phase A — Molecular & Cellular Foundations (36 h) | ||||
| 1 | Water | A1.1 | Foundational | 2 h |
| 2 | Carbohydrates & Lipids | B1.1 | Foundational | 4 h |
| 3 | Proteins | B1.2 | Foundational | 3 h |
| 4 | Nucleic Acids | A1.2 | Foundational | 4 h |
| 5 | Enzymes & Metabolism | C1.1 | Developmental | 4 h |
| 6 | Cell Structure | A2.2 | Foundational | 5 h |
| 7 | Membranes & Transport | B2.1 | Developmental | 4 h |
| 8 | Water Potential & Osmosis | D2.3 | Developmental | 2 h |
| 9 | Organelles & Cell Specialisation | B2.2 | Developmental | 4 h |
| 10 | Cell & Nuclear Division | D2.1 | Foundational | 4 h |
| Phase B — Genetics & Continuity (15 h) | ||||
| 11 | DNA Replication & Biotechnology | D1.1 | Developmental | 3 h |
| 12 | Protein Synthesis | D1.2 | Developmental | 4 h |
| 13 | Mutations & Variation | D1.3 | Developmental | 3 h |
| 14 | Inheritance | D3.2 | Synthesis | 5 h |
| Phase C — Organisms: Energy, Exchange & Control (35 h) | ||||
| 15 | Cell Respiration | C1.2 | Developmental | 3 h |
| 16 | Photosynthesis | C1.3 | Developmental | 4 h |
| 17 | Gas Exchange | B3.1 | Developmental | 3 h |
| 18 | Transport Systems | B3.2 | Developmental | 4 h |
| 19 | Neural Signalling | C2.2 | Developmental | 3 h |
| 20 | Body Systems & Control | C3.1 | Synthesis | 6 h |
| 21 | Homeostasis | D3.3 | Synthesis | 3 h |
| 22 | Defence Against Disease | C3.2 | Synthesis | 5 h |
| 23 | Reproduction | D3.1 | Developmental | 4 h |
| Phase D — Evolution, Diversity & Ecology (24 h) | ||||
| 24 | Natural Selection | D4.1 | Synthesis | 2 h |
| 25 | Diversity of Organisms | A3.1 | Developmental | 3 h |
| 26 | Evolution & Speciation | A4.1 | Synthesis | 3 h |
| 27 | Adaptation & Environment | B4.1 | Foundational | 2 h |
| 28 | Ecological Niches | B4.2 | Developmental | 2 h |
| 29 | Populations & Communities | C4.1 | Developmental | 3 h |
| 30 | Energy & Matter in Ecosystems | C4.2 | Developmental | 3 h |
| 31 | Stability & Environmental Change | D4.2 | Synthesis | 2 h |
| 32 | Biodiversity & Conservation | A4.2 | Synthesis | 2 h |
| 33 | Climate Change | D4.3 | Synthesis | 2 h |
| Total taught content | 110 h | |||
Scroll the table sideways on narrow screens.
The 33 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 33 topics in teaching order.
Phase A — Molecular & Cellular Foundations
Nothing here depends on later material, so it is taught first. Water, the carbon compounds and the enzymes that act on them are the vocabulary in which every later cell, organ and ecosystem is described; the cell is then assembled from those molecules rather than presented as a labelled diagram. Meiosis closes the phase so that inheritance and reproduction have their mechanism ready when they arrive.
1. Water
- Teach to this depth
- Polarity first, then hydrogen bonding as the single idea that explains everything else: cohesion, adhesion, 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 and fats are actually carried in blood. Treat buoyancy, viscosity and thermal conductivity as real habitat variables.
- Connects to
- Hydrogen bonding returns in DNA base pairing, in protein 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, 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, 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.
- Connects to
- The amphipathic phospholipid becomes the bilayer two topics later; storage polysaccharides feed respiration and photosynthesis; the condensation pattern recurs in proteins and nucleic acids.
- Investigation & inquiry angle
- Calorimetry of food energy content, amylase acting on starch, 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, and contrast globular with fibrous proteins through function. Treat denaturation by pH and temperature as bond disruption, not as a rule to memorise.
- Connects to
- Proteins are the direct product of protein synthesis and the substrate of the enzyme work that follows; haemoglobin, channel proteins and antibodies are all applications; denaturation explains every enzyme temperature curve.
- Investigation & inquiry angle
- Denaturation of egg albumin or milk protein under controlled conditions, and enzyme activity against pH.
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, and make the point that the code is essentially universal — a fact that becomes an evolutionary argument later in the course.
- Connects to
- The foundation for replication, transcription and translation, for mutation and variation, and for the genome comparison that underpins classification and conservation.
- Investigation & inquiry angle
- DNA extraction with yield optimised against a controlled variable, 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 properly. Distinguish intracellular from extracellular enzymes and anabolic from catabolic pathways, and use the initial-rate method rather than a single end-point reading.
- Connects to
- Rests entirely on protein structure; it is the mechanism behind respiration, photosynthesis, digestion and replication.
- Investigation & inquiry angle
- The single richest source of investigations in the course — catalase, amylase, pectinase or lactase, with rate measured against temperature, pH or concentration.
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 micrographs. Prokaryotic and eukaryotic ultrastructure compared; the functions of life demonstrated in a named unicellular organism. Establish surface-area-to-volume ratio as the limiting argument for cell size and carry it forward deliberately.
- Connects to
- SA:V drives gas exchange, transport systems and body size later in the course; ultrastructure sets up membranes, organelles and cell specialisation.
- Investigation & inquiry angle
- Agar-cube diffusion models of SA:V, and cell size measurement with a calibrated eyepiece graticule.
7. Membranes & Transport
- Teach to this depth
- Derive the bilayer from phospholipid amphipathy rather than presenting it. Integral and peripheral proteins and their functions; the fluid mosaic model taught alongside the evidence that replaced the earlier model, as a nature-of-science case. Simple and facilitated diffusion, osmosis, and active transport with the sodium–potassium pump; endocytosis and exocytosis; glycoproteins and glycolipids in recognition and adhesion.
- Connects to
- Uses lipids and proteins directly; it is the basis of water potential, of nerve impulses, and of mineral uptake in roots and absorption in the gut.
- Investigation & inquiry angle
- Beetroot pigment leakage against temperature or solvent concentration, measured colorimetrically.
8. Water Potential & Osmosis
- Teach to this depth
- 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, plasmolysis, turgor — and the estimation of tissue water potential from mass-change data by locating the zero-change point.
- Connects to
- Applies membrane transport; explains guard cell action, root uptake and the transpiration stream.
- 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.
9. Organelles & Cell Specialisation
- Teach to this depth
- Organelle structure and function, then the argument for compartmentalisation itself: local conditions maintained, incompatible reactions separated, substrates concentrated. Then differentiation as differential gene expression, stated that way from the start; stem cells and potency; named specialised cells with the structure–function argument made in each case; therapeutic applications and the ethical debate handled with real cases.
- Connects to
- Builds on cell structure and membranes; underpins the tissue-level physiology of gas exchange, transport and immunity later in the course.
- Investigation & inquiry angle
- Quantitative micrograph work, stomatal density comparisons, and cell-type counts against a controlled variable.
10. 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 and tumour formation; the mitotic index calculated and used as evidence. Then meiosis as reduction division, with crossing over and independent assortment drawn out as sources of genetic variation.
- Connects to
- Meiosis is the mechanism behind inheritance and behind the variation natural selection acts on — which is why it is taught here rather than alongside inheritance.
- Investigation & inquiry angle
- Mitotic index in root tips under a controlled treatment such as temperature or a named chemical.
Phase B — Genetics & Continuity
The molecular genetics strand is taught in one block, in mechanistic order: copy the information, express it, corrupt it, then inherit it. Inheritance closes the phase because it needs meiosis, mutation and the gene–allele distinction all secure — and the phase as a whole sits before evolution, because natural selection cannot be taught honestly without it.
11. DNA Replication & Biotechnology
- Teach to this depth
- Semi-conservative replication with complementary base pairing established as the copying principle; helicase, DNA polymerase and ligase in outline. PCR with the function of primers and of 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 genome comparison, conservation genetics and the evidence used in classification.
- Investigation & inquiry angle
- Gel electrophoresis of dyes or prepared samples; database analysis of published profiles.
12. Protein Synthesis
- Teach to this depth
- Transcription with RNA polymerase and the template strand. The genetic code — codons, degeneracy, universality — and the evolutionary inference that universality supports. Translation: ribosome structure, tRNA and the anticodon, and the sequence from initiation to termination. Finish by tracing a single base change through to the polypeptide, which makes the next topic follow naturally.
- Connects to
- The mechanistic bridge from nucleic acids to proteins, and therefore the topic that makes mutation and variation explicable rather than assertable.
- Investigation & inquiry angle
- Bioinformatics: comparing published sequences across species and predicting the resulting polypeptides.
13. Mutations & Variation
- 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; mutagens; mutation established as the ultimate source of all variation. Gene editing in outline, with the ethical debate given real cases.
- Connects to
- Rests on protein synthesis; supplies the raw material that natural selection and speciation depend on, and the variation that inheritance distributes.
- Investigation & inquiry angle
- Sequence comparison across species using published databases; analysis of published mutation-rate data.
14. 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. Continuous variation with the polygenic explanation, and the difference between discrete and continuous data handled properly.
- Connects to
- Meiosis is its mechanism and mutation supplies its alleles; the shift from individual crosses to population allele frequencies is exactly the step into natural selection.
- Investigation & inquiry angle
- A properly sampled continuous-variation study with appropriate statistics, or the analysis of published pedigree or cross data.
Phase C — Organisms: Energy, Exchange & Control
Respiration and photosynthesis come first as cell processes needing only enzymes and membranes; gas exchange and transport then follow as the organism-level answer to the problem those processes create. Control is taught only once there is something to control, so the three applied topics — integration of body systems, homeostasis and immunity — arrive with every feeder already in place.
15. Cell Respiration
- Teach to this depth
- ATP as the energy currency, with the reason it suits the role. Aerobic and anaerobic respiration compared on yield, products and location; respiration in humans and in yeast; the measurement of respiration rate and what it does and does not tell you.
- Connects to
- Uses enzymes and membranes; it is the counterpart of photosynthesis and the basis of energy flow through ecosystems; it explains why gas exchange and transport systems exist at all.
- Investigation & inquiry angle
- Respirometry with germinating seeds or invertebrates, and yeast fermentation rate against temperature or substrate concentration.
16. 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 evaluated rather than accepted.
- Connects to
- Mirrors respiration; 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.
17. Gas Exchange
- Teach to this depth
- The shared properties of exchange surfaces, derived from the surface-area-to-volume argument rather than listed. Ventilation mechanics and alveolar structure; lung volumes read from spirometer traces. Leaf structure, stomata and guard cell action, with stomatal density measured from real material.
- Connects to
- Uses SA:V from cell structure and the membranes topic; feeds transport, exercise physiology and homeostasis.
- Investigation & inquiry angle
- Stomatal density against habitat, light or water availability; ventilation rate and recovery against exercise intensity.
18. 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. On the plant side, xylem and the cohesion–tension mechanism, root and stem structure, transpiration and its measurement, and xerophytic adaptations.
- Connects to
- Applies water potential, membranes and gas exchange; feeds homeostasis and immunity; 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; evaluation of published epidemiological data.
19. 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 and the action potential as a sequence of gated channel events; the all-or-nothing law; saltatory conduction. The synapse — calcium influx, vesicle fusion, receptor binding and the removal of transmitter — with the reason transmission is one-way.
- Connects to
- Applies membranes, channel proteins and active transport; feeds reflexes and the coordination topic that follows.
- Investigation & inquiry angle
- Reaction-time investigations with one genuinely controlled variable and adequate replication.
20. 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; the division of labour between brain and spinal cord; hormonal control including the regulation of blood glucose; movement control from brain to muscle; and feedback mechanisms established as a general principle before homeostasis applies them.
- Connects to
- This is where neural signalling, transport and gas exchange 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; measured responses to a controlled stimulus with proper ethics clearance.
21. Homeostasis
- Teach to this depth
- The set point, receptor, control centre and effector loop, then negative feedback worked through in two 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 behavioural responses.
- Connects to
- Requires transport, membranes and neural and hormonal signalling; it is the physiological case study of negative feedback and the model for the ecological feedbacks met at the end of the course.
- Investigation & inquiry angle
- Cooling-curve models of thermoregulation with surface area or insulation as the variable; interpretation of published glucose tolerance data.
22. 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, antibodies 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 as a worked case. Zoonoses; vaccination, memory cells and herd immunity; and throughout, the evaluation of epidemiological data.
- Connects to
- Pulls together proteins, cell specialisation and transport; antibiotic resistance is natural selection observed in real time, which makes it worth teaching before the evolution phase rather than after.
- Investigation & inquiry angle
- Zone-of-inhibition assays with antiseptics or plant extracts; analysis of published vaccination coverage or resistance-surveillance data.
23. Reproduction
- Teach to this depth
- Asexual and sexual reproduction compared on their consequences for variation. Gametes as the product of meiosis, and fertilisation; the menstrual cycle taught as an interacting hormonal system with graph interpretation rather than four curves to memorise. Plant reproduction, pollination, seed dispersal and the conditions required for germination.
- Connects to
- Requires meiosis and hormonal control; it feeds inheritance directly and, through variation, natural selection.
- Investigation & inquiry angle
- Germination rate against a controlled factor; pollen tube growth in different sucrose concentrations; flower morphometrics.
Phase D — Evolution, Diversity & Ecology
The applied conclusion of the course, placed last by design. Natural selection needs inheritance and mutation; speciation needs natural selection; and the ecology topics that close the sequence draw on photosynthesis, respiration, transport and populations at once. Climate change is taught last of all because almost every earlier topic bears on it.
24. 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 unprompted. Selection pressures with named, data-backed examples: peppered moths, antibiotic resistance, beak morphology. Then the population-level restatement in terms of changing allele frequency, and the interpretation of real selection data.
- Connects to
- Needs inheritance, mutation and variation to be secure; it is the mechanism behind speciation, adaptation, antibiotic resistance and conservation.
- Investigation & inquiry angle
- Allele-frequency simulations; analysis of published resistance or morphometric datasets.
25. 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, and the uses to which sequence data are put.
- Connects to
- Applies nucleic acid and genome ideas from Phase B; feeds speciation and conservation.
- Investigation & inquiry angle
- Morphometric variation between two populations; comparison of published genome or barcode data.
26. Evolution & Speciation
- Teach to this depth
- Treat the evidence as several independent lines that converge — fossils, selective breeding, homologous structures, biogeography and molecular sequence data — and make clear why convergent evolution produces analogous rather than homologous features. Then reproductive isolation and the formation of new species, with a named case worked through.
- Connects to
- The point where genetics, natural selection and diversity converge. The argument only lands if inheritance and variation are already secure, which is why it is placed here and not in the opening weeks.
- Investigation & inquiry angle
- Analysis of published molecular or morphological datasets; a simple phylogeny compared against the fossil record.
27. 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.
- Connects to
- Applies the exchange and transport physiology of Phase C; 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.
28. Ecological Niches
- Teach to this depth
- The niche as the full set of conditions and resources a species uses, not merely where it lives. Modes of nutrition — autotroph, heterotroph, saprotroph, mixotroph — 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 of Phase C; it is the conceptual base for populations and energy flow.
- Investigation & inquiry angle
- Competition experiments with duckweed or yeast under controlled conditions; field distribution data for two competing species.
29. Populations & Communities
- Teach to this depth
- Population size estimated by quadrat sampling and by capture–mark–recapture, including the assumptions each method makes. Carrying capacity and limiting factors; the sigmoid growth curve explained phase by phase. Intraspecific and interspecific relationships — competition, predation, herbivory, mutualism, parasitism — 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.
- Investigation & inquiry angle
- A quadrat-based association study analysed with chi-squared; yeast or duckweed growth curves fitted to the sigmoid model.
30. 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 calculated from data. The carbon cycle with photosynthesis, respiration, decomposition and sequestration in peat and fossil fuels; nutrient cycling.
- 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.
31. 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. Eutrophication worked through as a complete causal chain from nutrient input to oxygen demand and death; biomagnification of persistent pollutants; plastic pollution; and ecosystem restoration with a real case.
- Connects to
- Requires populations, energy flow and nutrient cycling; it is the applied face of the ecology strand and the precursor to conservation and climate change.
- Investigation & inquiry angle
- Nutrient enrichment and algal growth; comparison of a managed and an unmanaged site.
32. Biodiversity & Conservation
- Teach to this depth
- Biodiversity at three levels — genetic, species and ecosystem — and its measurement with an index that accounts for evenness as well as richness. The evidence on current extinction rates; named causes with case studies; habitat loss and fragmentation, and why fragmentation is not simply area loss. In situ and ex situ approaches 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 course and the natural home for its ethical dimension.
- Investigation & inquiry angle
- Diversity index comparison between two sites under different management; analysis of published Red List or monitoring data.
33. 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. Effects on species distribution, phenology and coral reefs; ocean acidification; carbon sequestration and mitigation.
- Connects to
- The closing synthesis: photosynthesis, respiration, carbon cycling, populations and distribution 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; simple ocean-acidification models using shells or calcifying organisms.
Time allocation & two-year pacing
The IB recommends 150 teaching hours for Standard Level Biology — 110 hours of taught content plus 40 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 110 content hours across all 33 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 | 19 h | 19 h |
| B — Form and function | 26 h | 26 h |
| C — Interaction and interdependence | 31 h | 31 h |
| D — Continuity and change | 34 h | 34 h |
| Taught content subtotal | 110 h | 110 h |
| Practical scheme of work (incl. 10 h collaborative sciences project + 10 h scientific investigation) | 40 h | 40 h |
| SL course total | 150 h | 150 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 two-and-a-half to three 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 150-hour total; the gold rows fall outside the teaching budget.
| Period | Focus | Hours | Cumul. |
|---|---|---|---|
| YEAR 1 | |||
| Autumn term | Phase A — Molecular & Cellular Foundations, water → membranes · tools, safety and practical-skills induction | 34 | 34 |
| Spring term | Finish Phase A (water potential → cell and nuclear division) · Phase B — Genetics & Continuity · practical work | 33 | 67 |
| Summer term | Begin Phase C — respiration, photosynthesis, gas exchange, transport and neural signalling · Collaborative Sciences Project | 31 | 98 |
| YEAR 2 (to end January) | |||
| Autumn term | Finish Phase C (body systems, homeostasis, defence, reproduction) · begin Phase D (natural selection → ecological niches) · Scientific Investigation written and submitted | 40 | 138 |
| To end of January | Phase D completed — populations, energy and matter, stability, conservation and climate change | 12 | 150 |
| FEBRUARY – APRIL | Dedicated revision: past papers, Paper 1A / 1B and Paper 2 drills, and timed mocks (additional to the 150 teaching hours) | — | — |
| MAY | IB examinations | — | — |
A note on the practical scheme. The 40 hours are not a separate course. Roughly 20 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 and transpiration 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 150 teaching hours, in line with the subject guide’s reminder that adequate time must be set aside for examination revision.