Syllabus map · 40 topics · 6 HL only
IB Biology syllabus map
IB Biology is built as a grid: four themes (A to D) crossed with four levels of organisation, from molecules to ecosystems, giving 40 topics with codes such as A1.1 Water or D4.3 Climate change. Open any topic below to see what it covers, what you need to be able to do, the terms to use precisely and the mistake that costs the most marks. Six topics are HL only, and most SL topics carry extra HL content, so HL students should expect more depth on each.
How it's assessed
| Component | SL | HL |
|---|---|---|
| Paper 1 (1A + 1B, one sitting) | 1 h 30 min, 55 marks, 36% | 2 h, 75 marks, 36% |
| Paper 1A: multiple choice | 30 questions, 30 marks | 40 questions, 40 marks |
| Paper 1B: data-based questions | 4 questions, 25 marks | 4 questions, 35 marks |
| Paper 2 | 1 h 30 min, 50 marks, 44% | 2 h 30 min, 80 marks, 44% |
| Paper 2 Section A: data-based and short-answer | 34 marks | 48 marks |
| Paper 2 Section B: extended response | 16 marks, 1 of 2 questions | 32 marks, 2 of 3 questions |
| Internal assessment (scientific investigation) | 10 h, 24 marks, 20% | 10 h, 24 marks, 20% |
| Total exam time | 3 h | 4 h 30 min |
This is the course first examined in May 2025, which covers the 2026 and 2027 sessions: calculators are allowed on both papers and there is no Biology data booklet. From the May 2028 exams a data booklet is supplied, with the same papers, marks and weightings.
Tap a topic to see what it asks, what you need to be able to do, the key terms and the trap. Topic titles follow the course structure; all explanations are ours.
Theme A — Unity and diversity
How the shared chemistry and structure of cells gives rise to the variety of living things, and how that variety is named, evolves and is conserved.
A1.1WaterSL
Why water's polarity and hydrogen bonding make it the medium life runs in. Questions ask you to explain a property such as cohesion, adhesion, solvent behaviour or heat capacity and link it to a specific biological use, or to compare water with air as a habitat.
You need to be able to
- Explain how polar O–H bonds and the bent shape give water δ+ and δ− regions that hydrogen-bond to neighbouring molecules.
- Distinguish cohesion (water to water) from adhesion (water to another polar surface) and use both to explain water rising in xylem.
- Explain why ions and polar molecules dissolve in water but lipids do not, and what that means for transport in blood.
- Compare water and air for buoyancy, viscosity, thermal conductivity and specific heat capacity, naming an organism that depends on each property.
Key terms
- hydrogen bond
- polarity
- cohesion
- adhesion
- hydrophilic
- hydrophobic
- solvent
- specific heat capacity
Common trap Writing that hydrogen bonds hold the atoms inside a water molecule together. The O–H bonds within a molecule are covalent; hydrogen bonds are the weaker attractions between neighbouring molecules.
Practise A1.1 in Heptavo →A1.2Nucleic acidsSL
The structure of DNA and RNA, from a single nucleotide to the double helix. Expect to draw or label a nucleotide, name the bonds involved and state the differences between DNA and RNA.
You need to be able to
- Draw a nucleotide with the phosphate on carbon 5′ and the base on carbon 1′ of the pentose, and show how nucleotides link into a strand.
- State the structural differences between DNA and RNA: number of strands, sugar, and thymine versus uracil.
- Explain complementary base pairing (A–T and C–G; A–U in RNA) and why it allows a sequence to be copied.
- Explain why the near-unlimited variety of base sequences lets DNA store a huge amount of information.
- HL: explain antiparallel strands, purine–pyrimidine pairing and nucleosomes, and how the Hershey–Chase experiment and Chargaff's data supported DNA as the genetic material.
Key terms
- nucleotide
- deoxyribose
- ribose
- phosphodiester bond
- complementary base pairing
- double helix
- antiparallel
- nucleosome
Common trap Saying the two strands are joined by covalent bonds. Covalent phosphodiester bonds run along each strand's backbone; the strands are held to each other by hydrogen bonds between bases.
Practise A1.2 in Heptavo →A2.1Origins of cellsHL
How the first cells might have arisen from non-living chemistry. Questions ask you to evaluate evidence such as the Miller–Urey experiment, explain why RNA may have come before DNA, and interpret dates for the earliest life.
You need to be able to
- Outline the conditions on early Earth (no free oxygen, energy from lightning, UV or vents) that could let simple organic molecules form.
- Evaluate what the Miller–Urey experiment showed and what it did not: monomers can form without life, not that cells can.
- Explain why RNA, which can both store information and catalyse reactions, avoids the problem that DNA needs enzymes and enzymes need DNA.
- Explain how amphipathic molecules form vesicles on their own and why a compartment was a necessary step towards cells.
- Describe LUCA, the evidence for it such as the universal genetic code, and the case for alkaline hydrothermal vents as where it lived.
Key terms
- abiotic synthesis
- protocell
- ribozyme
- RNA world
- LUCA
- amphipathic
- hydrothermal vent
Common trap Treating LUCA as the first living cell. It is the most recent ancestor shared by all life today; other early lineages existed and died out.
Practise A2.1 in Heptavo →A2.2Cell structureSL
What all cells share, how prokaryotes and eukaryotes differ, and how microscopes reveal it. Common questions ask you to identify structures in a micrograph, compare cell types and calculate magnification or actual size.
You need to be able to
- Calculate magnification, image size or actual size, converting to one unit first (1 mm = 1000 µm).
- Compare prokaryotic and eukaryotic cells: nucleoid versus nucleus, 70S versus 80S ribosomes, no membrane-bound organelles versus many.
- Compare plant, animal and fungal cells, including cell wall material (cellulose or chitin), plastids, vacuoles and centrioles.
- Distinguish magnification from resolution and explain why electron microscopes resolve much smaller structures than light microscopes.
- Explain how atypical cells such as red blood cells, striated muscle fibres, aseptate fungal hyphae and phloem sieve tubes test cell theory.
Key terms
- cell theory
- magnification
- resolution
- nucleoid
- 70S ribosome
- organelle
- cell wall
- eukaryote
Common trap Dividing image size by magnification without converting units. Put the measurement and the answer unit on the same scale first; a mitochondrion that comes out at 2 mm or 2 nm means a conversion was skipped.
Practise A2.2 in Heptavo →A2.3VirusesHL
Why viruses are not cells, how they replicate inside hosts and why they evolve so fast. Questions ask you to compare the lytic and lysogenic cycles, explain how varied viruses are, and apply evolution to drug resistance or new variants.
You need to be able to
- List the features every virus has (genetic material in a protein capsid, no ribosomes or metabolism) and those only some have (a lipid envelope, an RNA genome).
- Sequence the lytic cycle of a bacteriophage, saying what the virus supplies and what the host cell supplies.
- Compare the lytic and lysogenic cycles, including integration as a prophage and later switching to the lytic cycle.
- Explain how a retrovirus such as HIV uses reverse transcriptase to make DNA that integrates into the host chromosome.
- Explain why fast mutation and short generation times let viruses such as influenza and HIV evolve quickly, and what that means for vaccines and antiviral drugs.
Key terms
- capsid
- envelope
- bacteriophage
- lytic cycle
- lysogenic cycle
- prophage
- reverse transcriptase
- retrovirus
Common trap Saying a drug or vaccine makes a virus mutate. Variants arise from random copying errors; the treatment only selects the ones that already resist it.
Practise A2.3 in Heptavo →A3.1Diversity of organismsSL
How the variation between living things is recognised and sorted into species. Expect questions on species concepts and where they break down, binomial names, karyograms and genome size.
You need to be able to
- Write binomial names correctly: genus capitalised, species name lower case, both in italics (underlined when handwritten).
- Apply the biological species concept and explain where it fails: asexual organisms, fossils, fertile hybrids and ring species.
- Explain how a karyogram is prepared and use one to identify sex and an abnormal chromosome number such as trisomy 21.
- Compare chromosome numbers and genome sizes between species and explain why genome size does not track complexity.
- Use or construct a dichotomous key to identify organisms.
Key terms
- species
- binomial nomenclature
- biological species concept
- morphological species concept
- karyogram
- homologous chromosomes
- genome
- hybrid
Common trap Calling a mule evidence that horses and donkeys are one species. The hybrid is sterile, so no genes pass between the two gene pools and they remain separate species.
Practise A3.1 in Heptavo →A3.2Classification and cladisticsHL
How organisms are grouped by evolutionary relationship rather than appearance alone. Questions ask you to read a cladogram, pick out clades, use sequence differences as a molecular clock and explain why groups get reclassified.
You need to be able to
- List the taxonomic ranks in order from domain to species and explain why the traditional ranks do not always match evolutionary history.
- Read a cladogram: treat each node as a common ancestor, identify a clade and say which two groups are most closely related.
- Use base or amino acid sequence differences to judge relatedness and estimate when lineages split, stating the assumptions involved.
- Distinguish homologous from analogous traits and explain why only homologous traits are evidence for a clade.
- Explain the three-domain system and why ribosomal RNA evidence split prokaryotes into Bacteria and Archaea.
Key terms
- clade
- cladogram
- node
- common ancestor
- molecular clock
- homologous trait
- analogous trait
- domain
Common trap Judging relatedness by how close two names sit on the page. Relatedness comes from the most recent shared node, and branches can be rotated around any node without changing the tree.
Practise A3.2 in Heptavo →A4.1Evolution and speciationSL
The evidence that species change over time and how one species becomes two. Questions ask you to interpret evidence from sequences, selective breeding and homologous structures, and explain how isolation leads to speciation.
You need to be able to
- Define evolution as a change in the heritable characteristics of a population over generations, not a change within one individual.
- Use base and amino acid sequence data, selective breeding and homologous structures as evidence for evolution.
- Distinguish homologous structures (divergent evolution) from analogous structures (convergent evolution), with an example of each.
- Explain allopatric speciation: a barrier stops gene flow, then different selection pressures and chance push the gene pools apart.
- Distinguish prezygotic from postzygotic isolation, and explain speciation without a barrier, such as by polyploidy.
Key terms
- evolution
- homologous structure
- analogous structure
- convergent evolution
- speciation
- reproductive isolation
- gene flow
- allopatric speciation
Common trap Writing that the barrier itself creates the new species. A barrier only stops gene flow; speciation needs the separated populations to diverge until they can no longer interbreed.
Practise A4.1 in Heptavo →A4.2Conservation of biodiversitySL
What biodiversity is, why it is being lost and how it can be protected. Questions ask you to explain causes of decline with a named example, evaluate in situ against ex situ conservation, and interpret biodiversity data.
You need to be able to
- Describe biodiversity at three levels: genetic diversity, species diversity and ecosystem diversity.
- Explain human causes of biodiversity loss (land-use change, overexploitation, invasive species, pollution, climate change) using a named case.
- Explain why small, fragmented populations lose genetic diversity and how wildlife corridors help.
- Compare in situ and ex situ conservation, with a strength and a limitation of each.
- Explain how the EDGE approach prioritises species that are both evolutionarily distinct and globally endangered.
Key terms
- biodiversity
- extinction
- habitat fragmentation
- invasive species
- in situ conservation
- ex situ conservation
- endemic species
- EDGE
Common trap Assuming biodiversity falls only when a species goes extinct. Losing genetic diversity within a species, or losing a type of ecosystem, also counts as biodiversity loss.
Practise A4.2 in Heptavo →Theme B — Form and function
How the structure of molecules, cells, organs and whole organisms fits what they do, from sugars and membranes to lungs, hearts, muscles and niches.
B1.1Carbohydrates and lipidsSL
How sugars and lipids are built and why their structures suit storage, structural and membrane roles. Questions ask you to show condensation and hydrolysis, compare polysaccharides and explain the properties of fats and phospholipids.
You need to be able to
- Show how condensation forms a glycosidic or ester bond and releases water, and how hydrolysis reverses it.
- Distinguish α-glucose from β-glucose and explain how this gives starch, glycogen and cellulose different properties.
- Show a triglyceride forming from glycerol and three fatty acids, and explain why it suits long-term energy storage and insulation.
- Explain why saturated fats are solid and unsaturated oils liquid at room temperature, including the effect of cis double bonds.
- Explain how amphipathic phospholipids arrange themselves into a bilayer in water.
Key terms
- monosaccharide
- polysaccharide
- glycosidic bond
- condensation
- hydrolysis
- triglyceride
- ester bond
- amphipathic
Common trap Calling cellulose an energy store or starch a structural molecule. Starch and glycogen are stores; cellulose's straight β-glucose chains bundle into the fibres of plant cell walls.
Practise B1.1 in Heptavo →B1.2ProteinsSL
How amino acids join into polypeptides and why the folded shape decides what a protein does. Expect to draw a peptide bond and explain denaturation; HL adds the levels of protein structure.
You need to be able to
- Draw a generalised amino acid and show a condensation reaction forming a peptide bond.
- Explain why the number of possible polypeptides is effectively unlimited (20 options at every position along the chain).
- Explain how heat and pH changes denature a protein by breaking the bonds that hold its shape, while the peptide bonds stay intact.
- Explain why some amino acids are essential in the diet.
- HL: describe primary to quaternary structure and the bonds holding each, and compare globular with fibrous proteins.
Key terms
- amino acid
- R group
- peptide bond
- polypeptide
- denaturation
- essential amino acid
- conformation
- globular protein
Common trap Saying denaturation breaks peptide bonds. It disrupts the weaker bonds that hold the fold; the amino acid sequence is unchanged.
Practise B1.2 in Heptavo →B2.1Membranes and membrane transportSL
The structure of cell membranes and the routes substances take across them. Questions ask you to label the fluid mosaic model, choose the transport mechanism for a named substance, and explain osmosis or active transport.
You need to be able to
- Explain why the hydrophobic core of the bilayer is a barrier to ions and polar molecules but not to small non-polar ones.
- Distinguish simple diffusion, facilitated diffusion through channels and carriers, osmosis through aquaporins, and active transport by pumps.
- Distinguish integral from peripheral proteins and describe the roles of glycoproteins and glycolipids.
- Explain endocytosis and exocytosis in terms of membrane fluidity.
- HL: explain how fatty-acid saturation and cholesterol affect fluidity, and describe gated channels, the sodium–potassium pump and sodium-dependent glucose cotransport.
Key terms
- phospholipid bilayer
- fluid mosaic model
- integral protein
- channel protein
- facilitated diffusion
- active transport
- aquaporin
- endocytosis
Common trap Calling facilitated diffusion active because a protein is involved. Only movement against the concentration gradient, using energy usually from ATP, is active transport.
Practise B2.1 in Heptavo →B2.2Organelles and compartmentalizationSL
Why eukaryotic cells divide their interior into membrane-bound compartments. Questions ask you to explain the advantage of a named compartment and, at HL, relate the structure of mitochondria, chloroplasts and the nucleus to their functions.
You need to be able to
- Explain how separating the nucleus from the cytoplasm lets mRNA be processed before translation, which prokaryotes cannot do.
- Explain the advantages of compartments: enzymes and substrates concentrated together, incompatible reactions kept apart, lysosomal enzymes kept away from the cytoplasm.
- Decide whether a structure counts as an organelle, and explain why the cell wall and cytoskeleton are usually not treated as organelles.
- HL: relate cristae, the intermembrane space and the matrix to ATP production, and thylakoids and stroma to photosynthesis.
- HL: trace a secreted protein from ribosomes on the rough ER through the Golgi apparatus to exocytosis, and compare free and bound ribosomes.
Key terms
- organelle
- compartmentalization
- nuclear envelope
- nuclear pore
- lysosome
- cristae
- thylakoid
- Golgi apparatus
Common trap Naming a compartment without saying what it gains. Give the specific benefit, for example that a lysosome's membrane keeps its digestive enzymes and low pH away from the rest of the cell.
Practise B2.2 in Heptavo →B2.3Cell specializationSL
How cells become specialised and why cell size is limited. Questions ask you to classify stem cells by potency, calculate surface area to volume ratios, and explain how a named cell's structure fits its job.
You need to be able to
- Explain that differentiation comes from differences in which genes are expressed, not from differences in the genes present.
- Distinguish totipotent, pluripotent and multipotent stem cells with an example of each, and discuss uses of stem cells.
- Calculate surface area to volume ratios for cubes of different sizes and explain why the ratio limits cell size.
- Describe adaptations that raise the surface area to volume ratio, such as microvilli and flattened cells.
- HL: relate the structure of type I and type II pneumocytes, cardiac and striated muscle cells, and sperm and egg cells to their functions.
Key terms
- differentiation
- stem cell
- totipotent
- pluripotent
- multipotent
- stem cell niche
- surface area to volume ratio
Common trap Saying a larger cell has a larger surface area to volume ratio. Volume grows faster than surface area, so the ratio falls as a cell gets bigger.
Practise B2.3 in Heptavo →B3.1Gas exchangeSL
How lungs and leaves exchange gases with their surroundings. Questions ask you to relate the features of exchange surfaces to diffusion, explain ventilation, read lung-volume data and explain how leaves control gas exchange and water loss.
You need to be able to
- List the properties of an efficient gas exchange surface and explain how a steep concentration gradient is maintained.
- Explain inhalation and exhalation in terms of muscle action, thoracic volume and pressure.
- Read tidal volume, vital capacity and the inspiratory and expiratory reserve volumes from a spirometer trace.
- Relate leaf tissue layout to gas exchange, and explain how guard cells open and close stomata and affect transpiration.
- HL: interpret oxygen dissociation curves, including the Bohr shift and why fetal haemoglobin binds oxygen more readily than adult haemoglobin.
Key terms
- ventilation
- alveolus
- concentration gradient
- tidal volume
- vital capacity
- stoma
- guard cell
- transpiration
Common trap Saying air is sucked in because the lungs expand. Muscle contraction enlarges the thorax, pressure inside falls below atmospheric, and air is pushed in from outside.
Practise B3.1 in Heptavo →B3.2TransportSL
How blood vessels in animals and xylem in plants move materials around the body. Questions ask you to compare arteries, veins and capillaries, explain transpiration pull and, at HL, describe the heart, tissue fluid and phloem transport.
You need to be able to
- Compare the walls and lumens of arteries, veins and capillaries and relate each to the pressure it handles and its function.
- Explain what causes a pulse and how to measure pulse rate.
- Explain how a coronary artery becomes blocked and the main risk factors for coronary heart disease.
- Explain transpiration pull using cohesion, adhesion and tension, and relate xylem vessel structure to it.
- HL: describe the heart and the cardiac cycle, how tissue fluid forms and returns, and how phloem moves sugars from source to sink.
Key terms
- artery
- vein
- capillary
- lumen
- xylem
- transpiration pull
- cohesion
- tension
Common trap Defining arteries as vessels carrying oxygenated blood. Arteries carry blood away from the heart and veins carry it back; the pulmonary artery carries deoxygenated blood.
Practise B3.2 in Heptavo →B3.3Muscle and motilityHL
How muscles contract and how skeletons and joints turn contraction into movement. Questions ask you to explain the sliding filament model, interpret sarcomere diagrams, and analyse antagonistic muscles and joints.
You need to be able to
- Label a sarcomere and predict which bands narrow during contraction.
- Sequence one cross-bridge cycle, giving the roles of ATP, calcium ions, troponin and tropomyosin.
- Explain the role of titin and why muscles work in antagonistic pairs, such as the internal and external intercostals.
- Describe the parts of a synovial joint and how each aids movement, and how range of motion is measured.
- Explain why animals move and how marine mammals are adapted for swimming.
Key terms
- sarcomere
- actin
- myosin
- cross-bridge
- tropomyosin
- troponin
- antagonistic muscles
- synovial joint
Common trap Saying the filaments shorten. Actin and myosin stay the same length and slide past each other, so the A band stays constant while the I band and H zone narrow.
Practise B3.3 in Heptavo →B4.1Adaptation to environmentSL
How abiotic conditions decide where species can live and how organisms are adapted to them. Questions ask you to explain the adaptations of named organisms, interpret transect or tolerance data and link climate to biomes.
You need to be able to
- Explain the adaptations of xerophytes to dry conditions and halophytes to salty ones.
- Interpret a range-of-tolerance curve and use transect data to relate a species' distribution to an abiotic variable.
- State the conditions coral reefs need (light, warmth, clear water, normal salinity) and explain why.
- Explain how temperature and rainfall determine which biome develops, using a climate graph or biome diagram.
- Describe adaptations of plants and animals to hot deserts and tropical rainforests.
Key terms
- habitat
- abiotic factor
- adaptation
- xerophyte
- halophyte
- range of tolerance
- transect
- biome
Common trap Writing that an organism adapted to its environment by changing during its life. Adaptations are inherited features shaped by natural selection over many generations.
Practise B4.1 in Heptavo →B4.2Ecological nichesSL
How organisms obtain energy and nutrients and the role each plays in its community. Questions ask you to classify modes of nutrition, compare fundamental and realised niches, and explain adaptations for feeding or defence.
You need to be able to
- Distinguish autotrophs, heterotrophs and mixotrophs, and holozoic from saprotrophic nutrition.
- Classify organisms as obligate aerobes, facultative anaerobes or obligate anaerobes from where they grow.
- Explain adaptations of herbivores and the plants they eat, and of predators and their prey.
- Relate tooth shape to diet, including what fossil teeth suggest about hominid diets.
- Distinguish the fundamental from the realised niche and explain competitive exclusion.
Key terms
- niche
- autotroph
- heterotroph
- mixotroph
- saprotroph
- holozoic nutrition
- fundamental niche
- realised niche
Common trap Using habitat and niche as the same thing. The habitat is where a species lives; the niche is its whole role, including what it eats, what eats it and the conditions it tolerates.
Practise B4.2 in Heptavo →Theme C — Interaction and interdependence
How molecules, cells, body systems and species act on one another: metabolism, signalling, defence, and the flow of energy and matter through ecosystems.
C1.1Enzymes and metabolismSL
How enzymes speed up reactions and what changes their rate. Questions ask you to explain rate graphs for temperature, pH and substrate concentration, design an enzyme experiment and, at HL, distinguish types of inhibition.
You need to be able to
- Explain enzyme action using the active site, induced fit and a lower activation energy.
- Explain the shape of rate curves for temperature, pH and substrate concentration in terms of collisions and denaturation.
- Design an enzyme experiment with clear independent, dependent and controlled variables, and calculate a rate from data.
- Distinguish anabolic from catabolic reactions in metabolism.
- HL: compare competitive and non-competitive inhibition, and explain end-product inhibition at an allosteric site and mechanism-based inhibition such as penicillin.
Key terms
- active site
- substrate
- induced fit
- activation energy
- denaturation
- metabolism
- competitive inhibition
- allosteric site
Common trap Writing that enzymes are killed at high temperature. Enzymes are proteins, not living things; heat denatures them by changing the shape of the active site.
Practise C1.1 in Heptavo →C1.2Cell respirationSL
How cells release energy from organic molecules as ATP. SL questions focus on ATP, aerobic versus anaerobic respiration and respirometer experiments; HL adds the stages inside the mitochondrion and chemiosmosis.
You need to be able to
- Explain ATP's role as an energy carrier that is constantly hydrolysed and regenerated, not stored.
- Distinguish cell respiration from gas exchange.
- Compare aerobic and anaerobic respiration in humans and yeast: products, ATP yield and where each happens.
- Design and evaluate a respirometer experiment, including how carbon dioxide is absorbed and temperature controlled.
- HL: trace glycolysis, the link reaction, the Krebs cycle and the electron transport chain, and explain chemiosmosis with oxygen as the final electron acceptor.
Key terms
- ATP
- aerobic respiration
- anaerobic respiration
- glycolysis
- pyruvate
- lactate
- respirometer
- chemiosmosis
Common trap Confusing respiration with breathing. Respiration is the release of energy by chemical reactions inside cells; ventilation and gas exchange only supply the oxygen and remove the carbon dioxide.
Practise C1.2 in Heptavo →C1.3PhotosynthesisSL
How light energy is captured and used to build carbon compounds. SL questions centre on pigments, spectra and limiting factors; HL adds the light-dependent reactions and the Calvin cycle.
You need to be able to
- Separate photosynthetic pigments by chromatography and calculate Rf values.
- Distinguish an absorption spectrum from an action spectrum and explain why their peaks line up.
- Interpret limiting-factor graphs for light intensity, carbon dioxide concentration and temperature.
- Explain how carbon dioxide enrichment experiments test predictions about future rates of photosynthesis.
- HL: explain photosystems, photolysis of water, chemiosmosis in the thylakoid and the Calvin cycle (rubisco, RuBP, GP, triose phosphate).
Key terms
- chlorophyll
- absorption spectrum
- action spectrum
- chromatography
- limiting factor
- photolysis
- Calvin cycle
- rubisco
Common trap Saying plants photosynthesise by day and respire at night. Plants respire all the time; in good light the rate of photosynthesis is usually higher than the rate of respiration.
Practise C1.3 in Heptavo →C2.1Chemical signallingHL
How cells communicate using signalling chemicals and receptors. Questions ask you to classify signals and receptors, explain how a named hormone acts on its target cell, and compare fast pathways with slow ones.
You need to be able to
- Distinguish hormones, neurotransmitters, cytokines and calcium ions as signals, and local from distant signalling.
- Explain quorum sensing in bacteria as signalling that depends on population density.
- Compare receptors in the membrane with receptors inside the cell, and explain why steroid hormones can use internal receptors.
- Explain how adrenaline (epinephrine) acts through a G protein-coupled receptor and a second messenger, and insulin through a tyrosine kinase receptor.
- Explain how acetylcholine opens a ligand-gated ion channel.
Key terms
- ligand
- receptor
- signal transduction
- second messenger
- G protein-coupled receptor
- tyrosine kinase
- ligand-gated channel
- quorum sensing
Common trap Saying a hormone travels only to its target cells. The blood carries it to almost every cell; only cells with a matching receptor respond.
Practise C2.1 in Heptavo →C2.2Neural signallingSL
How neurons generate nerve impulses and pass them on. Questions ask you to explain the resting and action potentials, why some axons conduct faster than others, and what happens at a synapse.
You need to be able to
- Explain how the sodium–potassium pump and potassium leak channels keep the resting potential at about −70 mV.
- Describe depolarisation and repolarisation during an action potential, and explain the threshold and the all-or-nothing response.
- Explain why myelinated and wider axons conduct impulses faster, with the impulse jumping between nodes of Ranvier.
- Sequence events at a synapse, from calcium ions entering the presynaptic terminal to the neurotransmitter being broken down.
- HL: interpret oscilloscope traces, explain summation of excitatory and inhibitory inputs, and the effects of chemicals such as cocaine or neonicotinoids.
Key terms
- resting potential
- action potential
- depolarisation
- repolarisation
- threshold
- myelin sheath
- synapse
- neurotransmitter
Common trap Saying a stronger stimulus produces a bigger action potential. Action potentials are all-or-nothing; a stronger stimulus raises how often impulses fire, not their size.
Practise C2.2 in Heptavo →C3.1Integration of body systemsSL
How the nervous and endocrine systems coordinate the body, and at HL how plants coordinate growth. Questions ask you to trace a reflex arc, give the roles of brain regions and hormones, and explain how heart and breathing rates are adjusted.
You need to be able to
- Trace a pain reflex arc from receptor to effector and explain why it is fast and involuntary.
- State the roles of the cerebellum, medulla, hypothalamus and pituitary gland.
- Explain how the hypothalamus and pituitary link nervous and hormonal control, and the roles of melatonin and adrenaline.
- Explain how chemoreceptors and the medulla adjust heart rate and ventilation rate during exercise.
- HL: explain phototropism through redistribution of auxin, and positive feedback in fruit ripening driven by ethylene.
Key terms
- central nervous system
- reflex arc
- sensory neuron
- motor neuron
- effector
- hypothalamus
- pituitary gland
- auxin
Common trap Saying a spinal reflex is controlled by the brain. Relay neurons in the spinal cord connect sensory to motor neurons; the brain only registers the event, slightly later.
Practise C3.1 in Heptavo →C3.2Defence against diseaseSL
How the body keeps pathogens out and destroys those that get in. Questions ask you to compare innate and adaptive immunity, explain antibody production and memory, and evaluate antibiotics, vaccines and herd immunity.
You need to be able to
- Describe skin and mucous membranes as barriers, and blood clotting from platelets to a fibrin mesh.
- Compare innate and adaptive immunity, and explain how phagocytes engulf and digest pathogens.
- Explain clonal selection: antigen presentation, helper T cells activating B cells, plasma cells and memory cells.
- Explain why antibiotics act on bacteria but not viruses, and how antibiotic resistance spreads.
- Explain how vaccines and herd immunity protect a population, and how HIV damages the immune system.
Key terms
- pathogen
- antigen
- antibody
- phagocyte
- lymphocyte
- plasma cell
- memory cell
- herd immunity
Common trap Saying bacteria become resistant because they get used to an antibiotic. Resistance genes are already present in a few bacteria by chance; the antibiotic kills the others and those few multiply.
Practise C3.2 in Heptavo →C4.1Populations and communitiesSL
How population size is estimated and controlled, and how species in a community interact. Questions ask you to estimate population size from sample data, explain growth curves, classify interactions and test for association between species.
You need to be able to
- Estimate population size with random quadrat sampling and capture–mark–release–recapture (the Lincoln index), stating the assumptions.
- Explain each phase of a sigmoid population growth curve and the role of carrying capacity.
- Classify interactions between species: mutualism, parasitism, pathogenicity, predation, herbivory and competition.
- Use a chi-squared test to decide whether two species are associated in quadrat data.
- Explain predator–prey cycles and the difference between top-down and bottom-up control.
Key terms
- population
- community
- carrying capacity
- density-dependent factor
- Lincoln index
- mutualism
- invasive species
- chi-squared test
Common trap Treating a non-significant chi-squared result as proof of no association. It means the data give no evidence of association at the 5% level; compare χ² with the critical value for the correct degrees of freedom.
Practise C4.1 in Heptavo →C4.2Transfers of energy and matterSL
How energy flows through ecosystems while matter is recycled. Questions ask you to calculate energy transfer efficiencies, build pyramids of energy, explain why food chains are short and interpret the carbon cycle or the Keeling curve.
You need to be able to
- Explain why ecosystems need a constant input of energy from sunlight while chemical elements are recycled.
- Construct food chains, food webs and pyramids of energy, and calculate the percentage of energy passed between trophic levels.
- Explain where energy is lost between levels (heat from respiration, parts not eaten or not digested) and why chains rarely exceed four or five levels.
- Distinguish gross from net primary production.
- Draw the carbon cycle and interpret the Keeling curve, including its rise and its yearly oscillation.
Key terms
- trophic level
- producer
- decomposer
- food web
- pyramid of energy
- primary production
- carbon sink
- Keeling curve
Common trap Writing that energy is recycled in an ecosystem. Matter cycles; energy flows through once and leaves as heat.
Practise C4.2 in Heptavo →Theme D — Continuity and change
How genetic information is copied, expressed, changed and inherited, and how bodies, populations and ecosystems stay stable or shift over time.
D1.1DNA replicationSL
How DNA is copied accurately and how copying is used in the lab. Questions ask you to explain semi-conservative replication and the roles of key enzymes, and to apply PCR and gel electrophoresis to testing.
You need to be able to
- Explain semi-conservative replication and how complementary base pairing keeps copying accurate.
- State the roles of helicase and DNA polymerase.
- Outline the three temperature steps of PCR and calculate the number of copies after n cycles (2ⁿ per starting molecule).
- Explain how gel electrophoresis separates DNA fragments by size, and apply PCR and electrophoresis to cases such as COVID-19 testing and DNA profiling.
- HL: explain leading and lagging strands, Okazaki fragments, primase, DNA ligase and proofreading.
Key terms
- semi-conservative replication
- helicase
- DNA polymerase
- complementary base pairing
- PCR
- primer
- gel electrophoresis
- Okazaki fragment
Common trap Saying DNA polymerase unwinds the helix. Helicase separates the strands by breaking hydrogen bonds; DNA polymerase then adds nucleotides to build each new strand.
Practise D1.1 in Heptavo →D1.2Protein synthesisSL
How a gene's base sequence becomes a polypeptide through transcription and translation. Questions ask you to transcribe or translate a sequence, explain the roles of mRNA, tRNA and ribosomes, and predict the effect of a base change.
You need to be able to
- Transcribe a DNA template strand into mRNA, using U in place of T.
- Translate mRNA into an amino acid sequence with a codon table, including start and stop codons.
- Explain the roles of mRNA, tRNA and ribosomes, and codon–anticodon pairing, in translation.
- Explain what it means for the genetic code to be degenerate and universal.
- HL: explain 5′→3′ synthesis, promoters, removal of introns, alternative splicing and modification of the finished polypeptide.
Key terms
- transcription
- translation
- RNA polymerase
- codon
- anticodon
- tRNA
- ribosome
- degenerate code
Common trap Reading the DNA template strand straight into the codon table. Codon tables list mRNA codons, so transcribe first, or read the coding strand and swap T for U.
Practise D1.2 in Heptavo →D1.3Mutation and gene editingSL
How changes in base sequence arise and what they do to proteins. Questions ask you to classify mutations, predict their effect on the polypeptide (sickle cell is the standard example) and distinguish somatic from germ-line mutations.
You need to be able to
- Distinguish substitutions, insertions and deletions and predict the effect of each on the amino acid sequence.
- Explain sickle cell disease from the single base substitution to the altered haemoglobin and red blood cell shape.
- Explain why insertions and deletions usually cause frameshifts while a substitution changes at most one codon.
- State causes of mutation (mutagens such as UV and certain chemicals, copying errors) and explain why mutation is random.
- HL: explain gene knockout and how CRISPR–Cas9 uses a guide RNA to cut a chosen sequence.
Key terms
- gene mutation
- substitution
- insertion
- deletion
- frameshift
- mutagen
- somatic mutation
- germ-line mutation
Common trap Assuming every mutation is harmful. Many substitutions are silent or neutral and a few are beneficial; mutation is the only source of new alleles.
Practise D1.3 in Heptavo →D2.1Cell and nuclear divisionSL
How cells divide by mitosis and meiosis and why the two differ. Questions ask you to identify phases in micrographs, compare mitosis with meiosis, explain how meiosis creates variation and how non-disjunction causes Down syndrome.
You need to be able to
- Identify the phases of mitosis in micrographs and describe what the chromosomes do in each.
- Compare mitosis and meiosis: number of divisions, chromosome number and genetic identity of the daughter cells.
- Explain how crossing over and random orientation of pairs in meiosis I generate variation.
- Explain non-disjunction and how it can lead to trisomy 21.
- HL: describe the cell cycle and the role of cyclins, calculate a mitotic index, and explain how mutagens and oncogenes can lead to tumours.
Key terms
- chromatid
- centromere
- homologous chromosomes
- mitosis
- meiosis
- cytokinesis
- crossing over
- non-disjunction
Common trap Counting chromatids as chromosomes. After replication each chromosome is two sister chromatids joined at a centromere, so a human cell in metaphase of mitosis still has 46 chromosomes.
Practise D2.1 in Heptavo →D2.2Gene expressionHL
How cells control which of their genes are expressed, and how environment and epigenetics shape that. Questions ask you to explain transcriptional control, epigenetic tags and evidence such as studies of identical twins.
You need to be able to
- Explain how transcription factors binding at promoters and enhancers control whether a gene is transcribed.
- Explain how the rate at which mRNA is broken down controls how much protein a gene yields.
- Explain how DNA methylation and histone acetylation switch genes off or on without changing the base sequence.
- Distinguish the genome, transcriptome and proteome of a cell.
- Explain how epigenetic tags pass to daughter cells, and sometimes to offspring, using evidence from monozygotic twins.
Key terms
- gene expression
- transcription factor
- promoter
- epigenetics
- methylation
- histone acetylation
- transcriptome
- proteome
Common trap Saying differentiated cells lose the genes they do not use. They keep the whole genome; transcription factors and epigenetic tags decide which genes are expressed.
Practise D2.2 in Heptavo →D2.3Water potentialSL
How water moves by osmosis between cells and their surroundings. Questions ask you to predict the direction of water movement, explain what happens to plant and animal cells in different solutions and, at HL, calculate water potential.
You need to be able to
- Predict the net direction of osmosis between two solutions of different solute concentration.
- Describe the effects of hypotonic, isotonic and hypertonic solutions on animal and plant cells (bursting, crenation, turgor, plasmolysis).
- Explain why isotonic solutions are used for intravenous fluids and for keeping organs for transplant.
- Estimate the solute concentration of a tissue from a graph of percentage mass change against solution concentration.
- HL: use ψ = ψs + ψp to predict water movement, keeping track of the minus signs.
Key terms
- osmosis
- solute
- hypotonic
- hypertonic
- isotonic
- turgor
- plasmolysis
- water potential
Common trap Writing that water moves "from high to low concentration" without saying of what. Water moves towards the solution with more solute, from higher to lower water potential, so name the quantity you mean.
Practise D2.3 in Heptavo →D3.1ReproductionSL
Sexual reproduction in humans and flowering plants. Questions ask you to label reproductive structures, explain the hormonal control of the menstrual cycle, and outline pollination, fertilisation, seed dispersal and germination.
You need to be able to
- Compare sexual and asexual reproduction and explain why meiosis and fertilisation produce varied offspring.
- Label the male and female reproductive systems and give the function of each part.
- Explain how FSH, LH, oestradiol and progesterone control the menstrual cycle, including the feedback loops between them.
- Outline IVF and the hormone treatments used in it.
- Distinguish pollination from fertilisation in flowering plants, compare insect and wind pollination, and explain self-incompatibility, dispersal and germination.
Key terms
- gamete
- fertilisation
- follicle
- ovulation
- corpus luteum
- pollination
- self-incompatibility
- seed dispersal
Common trap Using pollination and fertilisation as the same thing. Pollination is pollen landing on the stigma; fertilisation is the later fusion of male and female gametes inside the ovule.
Practise D3.1 in Heptavo →D3.2InheritanceSL
How alleles pass from parents to offspring. Questions ask you to complete Punnett squares, interpret pedigree charts, and explain codominance, sex linkage and continuous variation.
You need to be able to
- Use Punnett squares for monohybrid crosses and state the expected phenotype ratios.
- Explain codominance and multiple alleles using the ABO blood groups.
- Explain sex-linked inheritance with haemophilia or red–green colour blindness, and why males are affected more often.
- Deduce the pattern of inheritance from a pedigree chart.
- Distinguish continuous from discrete variation and explain polygenic inheritance; HL adds dihybrid crosses, gene linkage and chi-squared tests.
Key terms
- allele
- genotype
- phenotype
- dominant
- recessive
- codominance
- sex linkage
- pedigree chart
Common trap Writing sex-linked alleles without the X. Put the allele on the X chromosome (for example Xᴴ and Xʰ) and show the Y with no allele, or the cross cannot show why sons and daughters differ.
Practise D3.2 in Heptavo →D3.3HomeostasisSL
How the body holds internal conditions within narrow limits by negative feedback. Questions ask you to explain the control of blood glucose and body temperature, compare type 1 and type 2 diabetes and, at HL, explain the kidney's role in osmoregulation.
You need to be able to
- Describe a negative feedback loop with a receptor, a control centre and an effector.
- Explain how insulin and glucagon from the pancreas regulate blood glucose.
- Compare the causes and treatment of type 1 and type 2 diabetes.
- Explain thermoregulation, including vasodilation, vasoconstriction, sweating, shivering and brown adipose tissue.
- HL: explain ultrafiltration, selective reabsorption and the role of ADH in the kidney.
Key terms
- homeostasis
- negative feedback
- set point
- insulin
- glucagon
- glycogen
- thermoregulation
- vasodilation
Common trap Writing glucagon where you mean glycogen, or the reverse. Glucagon is the hormone that raises blood glucose; glycogen is the storage polysaccharide it makes the liver break down.
Practise D3.3 in Heptavo →D4.1Natural selectionSL
How natural selection changes populations over generations. Questions ask you to explain the steps of selection with a named example, distinguish it from inheritance of acquired characteristics and, at HL, analyse allele frequencies and patterns of selection.
You need to be able to
- Explain natural selection step by step: heritable variation, overproduction, competition, differential survival and reproduction, a change in allele frequency.
- Explain how mutation and sexual reproduction supply the variation that selection acts on.
- Explain why characteristics acquired during a lifetime are not passed on, unlike heritable variation.
- Explain sexual selection and interpret a modelling experiment such as Endler's work on guppies.
- HL: distinguish stabilising, directional and disruptive selection, and use the Hardy–Weinberg equation to test whether a population is evolving.
Key terms
- natural selection
- variation
- fitness
- adaptation
- allele frequency
- gene pool
- sexual selection
- Hardy–Weinberg
Common trap Saying individuals evolve or adapt because they need to. Individuals either survive and reproduce or do not; the population evolves as the frequency of advantageous alleles rises over generations.
Practise D4.1 in Heptavo →D4.2Stability and changeSL
What keeps ecosystems stable and what pushes them past tipping points. Questions ask you to explain keystone species, eutrophication and biomagnification, and evaluate the sustainability of harvesting and farming.
You need to be able to
- Explain what makes an ecosystem stable and how deforestation can push one past a tipping point.
- Explain the role of a keystone species with a named example.
- Evaluate the sustainability of harvesting wild populations and of farming practices.
- Explain eutrophication from nutrient run-off to fish deaths, including the role of decomposers.
- Explain biomagnification of persistent pollutants along food chains, and the effects of plastic pollution.
Key terms
- ecosystem stability
- tipping point
- keystone species
- sustainability
- eutrophication
- biochemical oxygen demand
- biomagnification
- succession
Common trap Blaming the algae directly for fish deaths in eutrophication. Bacteria decomposing the dead algae use up the dissolved oxygen, and that oxygen demand is what kills the fish.
Practise D4.2 in Heptavo →D4.3Climate changeSL
The causes of recent climate change and its effects on living things. Questions ask you to explain the enhanced greenhouse effect, positive feedback and tipping points, interpret data on range shifts, polar ice or coral reefs, and evaluate ways of storing carbon.
You need to be able to
- Explain the enhanced greenhouse effect and name the main human sources of carbon dioxide and methane.
- Explain positive feedback such as ice loss and thawing permafrost, and a tipping point such as boreal forest switching from carbon sink to source.
- Explain effects on polar habitats, nutrient upwelling, poleward and upslope range shifts, and coral reefs.
- Evaluate afforestation, forest regeneration and peatland restoration as ways of sequestering carbon.
- HL: explain phenology and how warming disrupts the timing of linked events, adds insect generations per year and drives evolution.
Key terms
- greenhouse gas
- enhanced greenhouse effect
- positive feedback
- tipping point
- carbon sequestration
- range shift
- coral bleaching
- phenology
Common trap Mixing up global warming with ozone depletion. A thinner ozone layer lets in more UV; warming is caused by greenhouse gases absorbing infrared radiation emitted by the Earth's surface.
Practise D4.3 in Heptavo →Try 4 Heptavo questions
Real questions from the app, with the explanation after each answer.
A1.2 · Nucleic acids
The two strands of a DNA double helix are held together by
- covalent bonds between complementary bases
- hydrogen bonds between complementary bases
- phosphodiester bonds between the bases
- ionic bonds between the sugars
Show the answer and why
Answer: B
Hydrogen bonds hold the two strands to each other; covalent phosphodiester bonds hold each single strand together along its backbone. Confusing the two is the classic error here.
B2.2 · Organelles and compartmentalization
In a micrograph a mitochondrion measures 20 mm across and the magnification is ×10 000. Its actual width is
- 0.002 µm
- 0.02 µm
- 0.2 µm
- 2 µm
Show the answer and why
Answer: D
Actual size is image size divided by magnification: 20 mm ÷ 10 000 = 0.002 mm, and 0.002 mm is 2 µm. The three smaller figures are the same division with the millimetre-to-micrometre step done wrong by one, two or three factors of ten — and each can be rejected on sight, because a mitochondrion is about the size of a bacterium, one to three micrometres across, not a few nanometres.
C1.2 · Cell respiration
During intense exercise, a runner's blood lactate concentration rises. This is because
- the muscles have stopped respiring aerobically altogether
- lactate is produced whenever the Krebs cycle is working hard
- oxygen supply cannot meet demand, so pyruvate is reduced to lactate
- the liver is converting glucose into lactate for the muscles
Show the answer and why
Answer: C
Both pathways run at once during exercise; lactate accumulates when the anaerobic contribution grows, not when aerobic respiration stops.
D3.2 · Inheritance
A pedigree shows an affected father, and all his daughters affected but none of his sons. The best interpretation is
- X-linked recessive inheritance
- Y-linked inheritance
- autosomal recessive inheritance
- X-linked dominant inheritance
Show the answer and why
Answer: D
A father gives his X to every daughter and to no son — a dominant allele on that X paints exactly this pattern.
