International Baccalaureate · IB Diploma

Biology · HL

Papers, samples and curriculum documents for this course. · ⁨Papers, amostras e documentos curriculares deste curso.⁩

Course units and learning goals · ⁨Unidades do curso e objetivos de aprendizagem⁩

These lessons teach selected course objectives. Check the remaining coverage gaps; the material is not a complete preparation programme. · ⁨Estas aulas ensinam objetivos selecionados do curso. Verifique as lacunas de cobertura restantes; o material não é um programa completo de preparação.⁩

Lessons · ⁨Lições⁩ →Vocabulary practice · ⁨Prática de vocabulário⁩ →

A.1.1 · Water

Attraction between molecules of the same substance.
The same intermolecular interactions help explain surface tension and the energy needed to change temperature. Heating changes molecular motion and interactions; it does not normally break the covalent O–H bonds.
Compare equal drops on clean surfaces under fixed temperature. Measure contact angle or spreading consistently. A detergent changes the system, so record concentration rather than treating all liquids as equivalent.

cohesion: Attraction between molecules of the same substance
adhesion: Attraction between different substances

A.1.2 · Nucleic acids

Formation of RNA using a DNA template.
A codon comprises three bases. The genetic code is degenerate: more than one codon can specify the same amino acid. A substitution can therefore be silent, while insertions or deletions can shift the reading frame.
Keep DNA template, coding DNA and mRNA distinct. State the strand used and write sequences in the required direction. Use a codon table for mRNA, not an unexplained DNA triplet.

transcription: Formation of RNA using a DNA template
translation: Formation of a polypeptide using an mRNA sequence

A.2.1 · Origins of cells

One organism living within another in an evolutionary association.
Independent observations strengthen a model when they make a consistent prediction. Similarity is evidence to evaluate alongside ancestry, sequence comparisons and alternative explanations, rather than proof from one feature.
Use attributed micrographs and sequence data. Separate observations from inferences in a table. Check whether each comparison concerns a mitochondrion, chloroplast, bacterial cell or eukaryotic nucleus.

endosymbiosis: One organism living within another in an evolutionary association
inference: An explanation drawn from observations and reasoning

A.2.2 · Cell structure

Image length divided by actual length.
A scale bar provides a known real distance in the same image. Convert the image length and real length to the same unit before dividing. Magnification is a ratio and has no unit.
Focus a prepared slide at low power first. Move to a higher power and use fine focus. Make a clear line drawing, label structures with straight lines, and record the scale rather than shading the image.

magnification: Image length divided by actual length
resolution: Ability to distinguish two close points

A.2.3 · Viruses

An agent that causes disease.
After vaccination, memory cells can support a faster secondary response. Antibiotic resistance arises through heritable variation and selection; an individual bacterium does not choose to become resistant because it needs to survive.
Use published infection data to compare rates per equal population size. Distinguish prevalence at a time from new cases over a period. In school, use safe simulations or approved cultures rather than collecting unknown pathogens.

pathogen: An agent that causes disease
antigen: A structure recognized by a specific immune response

A.3.1 · Diversity of organisms

A named group in a classification.
Read relationships from the branching order. Rotating branches at a node does not change ancestry. A branch length represents time or change only when the diagram gives a scale.
Record character states in a matrix before drawing a tree. Define which state is ancestral using an appropriate comparison. Test whether a new molecular dataset supports the same grouping.

taxon: A named group in a classification
clade: A common ancestor and all its descendants

A.3.2 · Classification and cladistics

A named group in a classification.
Read relationships from the branching order. Rotating branches at a node does not change ancestry. A branch length represents time or change only when the diagram gives a scale.
Record character states in a matrix before drawing a tree. Define which state is ancestral using an appropriate comparison. Test whether a new molecular dataset supports the same grouping.

taxon: A named group in a classification
clade: A common ancestor and all its descendants

A.4.1 · Evolution and speciation

A heritable feature benefiting survival or reproduction in a context.
Explain both a benefit and a cost. Reducing stomatal opening conserves water but can limit carbon dioxide entry. A feature cannot be judged as universally best without its ecological context.
Compare replicated observations across a measured environmental gradient. Account for relatedness, leaf area and age. Use local permitted plant observations rather than removing protected species.

adaptation: A heritable feature benefiting survival or reproduction in a context
acclimatization: An individual adjustment to changed conditions during life

A.4.2 · Conservation of biodiversity

The number of species recorded.
Compare surveys with similar area, effort, season and identification rules. A diversity index is meaningful only with its formula and conventions specified. Habitat fragmentation can affect movement and gene flow even when total area changes little.
Use non-destructive field sampling approved by the school. Identify organisms with a suitable key and record uncertain identifications rather than inventing species. Combine ecological evidence with stakeholder perspectives on land use.

species richness: The number of species recorded
evenness: How evenly individuals are distributed among species

B.1.1 · Carbohydrates and lipids

A covalent linkage between amino-acid residues.
Condensation forms a linkage with a small molecule such as water released in the simplified model; hydrolysis uses water to break such a linkage. Polymer sequence, branching, polarity and three-dimensional shape matter. Saturated fatty acids have no carbon-carbon double bond, while unsaturated fatty acids have at least one; this difference can influence packing under stated conditions.
Use labelled molecular models and identify the actual linkage rather than memorizing shapes alone. Compare an attributed structure with the proposed function, mark polar and non-polar regions where justified, and distinguish a monomer from a residue within a polymer. Food tests provide evidence of chemical groups under specific conditions, not complete molecular structures.

peptide bond: A covalent linkage between amino-acid residues
hydrolysis · ⁨hidrólise⁩: Breaking a linkage using water

B.1.2 · Proteins

A covalent linkage between amino-acid residues.
Condensation forms a linkage with a small molecule such as water released in the simplified model; hydrolysis uses water to break such a linkage. Polymer sequence, branching, polarity and three-dimensional shape matter. Saturated fatty acids have no carbon-carbon double bond, while unsaturated fatty acids have at least one; this difference can influence packing under stated conditions.
Use labelled molecular models and identify the actual linkage rather than memorizing shapes alone. Compare an attributed structure with the proposed function, mark polar and non-polar regions where justified, and distinguish a monomer from a residue within a polymer. Food tests provide evidence of chemical groups under specific conditions, not complete molecular structures.

peptide bond: A covalent linkage between amino-acid residues
hydrolysis · ⁨hidrólise⁩: Breaking a linkage using water

B.2.1 · Membranes and membrane transport

Net water movement through a partially permeable membrane.
Use percentage change to compare samples with different initial masses. A zero percentage change estimates a solution concentration with no net water movement. This is an estimate from a trend, not proof that water molecules stop moving.
Use equal-length cylinders from similar tissue, fixed solution volume, temperature and immersion time. Blot each cylinder in the same way before weighing. Repeat each concentration and plot mean percentage change against concentration.

osmosis: Net water movement through a partially permeable membrane
control variable: A factor kept constant for a fair comparison

B.2.2 · Organelles and compartmentalization

Image length divided by actual length.
A scale bar provides a known real distance in the same image. Convert the image length and real length to the same unit before dividing. Magnification is a ratio and has no unit.
Focus a prepared slide at low power first. Move to a higher power and use fine focus. Make a clear line drawing, label structures with straight lines, and record the scale rather than shading the image.

magnification: Image length divided by actual length
resolution: Ability to distinguish two close points

B.2.3 · Cell specialization

Development of specialized cell structure and function.
Link a named feature to a mechanism and then to the function. Increased area can support exchange, but membrane proteins, gradients and metabolic demand also matter. Stem cells retain different degrees of developmental potential.
Compare scaled drawings of named cells. Label the structure, state the function and explain the causal link. Use tissue-specific examples rather than saying every specialized cell has every adaptation.

differentiation: Development of specialized cell structure and function
gene expression: Use of genetic information to produce a functional product

B.3.1 · Gas exchange

Movement of air or water over an exchange surface.
Distinguish movement of the whole medium from diffusion across the membrane. Ventilation and perfusion work together, while haemoglobin helps transport oxygen in blood. A change in breathing rate alone does not measure oxygen uptake.
Use an approved model or published respiratory data. Compare exchange surface area, diffusion path and flow. For human demonstrations use voluntary resting measurements; do not induce breathlessness or hyperventilation.

ventilation: Movement of air or water over an exchange surface
perfusion: Blood flow through tissue or an exchange surface

B.3.2 · Transport

Breakdown of large food molecules.
Absorption moves soluble products into blood or lymph. Thin exchange surfaces and a large surface area shorten diffusion paths and increase transfer. Enzyme activity and transport are different processes.
Use Benedict reagent with controlled heating for reducing sugars, iodine for starch, Biuret reagent for protein, and the ethanol emulsion test for lipids. Keep ethanol away from flames. Use positive and negative controls.

digestion: Breakdown of large food molecules
absorption: Movement of soluble products into the body

B.3.3 · Muscle and motility

The repeating contractile unit between two Z lines.
During shortening, overlap increases. The A-band length stays approximately constant while I bands and the H zone become smaller. Movement at a joint also depends on tendon insertion, lever geometry and opposing muscle activity.
Annotate two sarcomere diagrams at different lengths using the same scale. Track band widths separately from filament length. Use prepared models or existing images, not painful or maximal-exertion tasks.

sarcomere: The repeating contractile unit between two Z lines
cross-bridge: A myosin-actin interaction during the contraction cycle

B.4.1 · Adaptation to environment

A heritable feature benefiting survival or reproduction in a context.
Explain both a benefit and a cost. Reducing stomatal opening conserves water but can limit carbon dioxide entry. A feature cannot be judged as universally best without its ecological context.
Compare replicated observations across a measured environmental gradient. Account for relatedness, leaf area and age. Use local permitted plant observations rather than removing protected species.

adaptation: A heritable feature benefiting survival or reproduction in a context
acclimatization: An individual adjustment to changed conditions during life

B.4.2 · Ecological niches

The resource use, conditions and interactions associated with an organism’s role.
Separate a habitat observation from a causal claim about a niche boundary. Absence from a site can reflect dispersal, detection, abiotic conditions or interactions. Resource partitioning can reduce overlap, but an observational difference alone does not prove that competition caused it. Compare alternative explanations and the timescale of evidence.
Use permitted field observations or an attributed dataset. Record species identification confidence, resource category, time, abiotic conditions and sampling effort. Repeat observations at several locations rather than choosing only patches that fit the hypothesis. Non-destructive comparisons or approved models should be used instead of removing native species or introducing competitors.

ecological niche: The resource use, conditions and interactions associated with an organism’s role
resource partitioning: Differences in resource use that can reduce overlap among organisms

C.1.1 · Enzymes and metabolism

A biological catalyst.
Measure rate using product formed per unit time or a fixed endpoint. For an endpoint test, 1/time is a rate proxy if the same amount of product or substrate change defines the endpoint each time.
For starch digestion, equilibrate enzyme and starch in a water bath, control pH with buffer, mix measured volumes, and test samples with iodine at fixed intervals. Use a clean spot for each test. Do not put iodine into the reaction mixture.

enzyme: A biological catalyst
denaturation: A structural change that disrupts function

C.1.2 · Cell respiration

Cell reactions that transfer energy from substrates.
Anaerobic processes allow ATP production when oxygen supply cannot support the required aerobic rate, but give less ATP per glucose. In humans lactate can accumulate; yeast can produce ethanol and carbon dioxide.
A respirometer can measure oxygen uptake when carbon dioxide is absorbed. Control temperature with a water bath and use a comparison containing inert material. Keep absorbent separated from organisms and follow the school risk assessment.

respiration: Cell reactions that transfer energy from substrates
ATP: A molecule that couples energy transfers in cells

C.1.3 · Photosynthesis

A factor whose shortage restricts rate.
Change only one factor when testing a limiting factor. At low light, extra light may increase rate. At a plateau, the changed factor is no longer the main limit in that range; the graph alone does not identify which other factor is limiting.
Measure collected gas volume over a fixed time instead of assuming all bubbles have the same volume. Control temperature, plant size and carbon dioxide supply. Allow the plant to adjust before each reading and repeat.

limiting factor: A factor whose shortage restricts rate
photosynthesis: Light-driven formation of carbohydrate

C.2.1 · Chemical signalling

A molecule that binds a compatible signal.
Distinguish signal reception, transduction and response. Amplification can allow a small extracellular change to produce a larger intracellular effect. Different receptors or pathways can give different responses to one signal.
Use an attributed dataset comparing signal concentration and response. Include an untreated control and, where supplied, receptor-blocking evidence. Do not infer the whole pathway from a concentration-response graph alone.

receptor: A molecule that binds a compatible signal
signal transduction: Conversion of signal reception into intracellular changes

C.2.2 · Neural signalling

A brief propagating change in membrane potential.
The all-or-none principle applies to an individual action potential. Stimulus information can be encoded by frequency and recruitment. Myelination permits saltatory conduction between nodes rather than making ions move freely through myelin.
Interpret supplied voltage-time traces and compare time intervals. Identify threshold, rising phase and recovery before explaining ions. Use classroom models rather than electrical stimulation of people.

action potential: A brief propagating change in membrane potential
synapse: A junction for communication between cells

C.3.1 · Integration of body systems

Maintenance of suitable internal conditions.
When blood glucose is high, insulin helps increase glucose uptake and storage as glycogen. When it is low, glucagon supports release of glucose from stores. These responses are coordinated, not identical effects of two hormones.
Interpret a time graph by identifying the initial disturbance, the response and the return toward the normal range. Mark the delay before a response. Do not assume a graph shows an instantaneous correction.

homeostasis: Maintenance of suitable internal conditions
negative feedback: A response opposing the original change

C.3.2 · Defence against disease

An agent that causes disease.
After vaccination, memory cells can support a faster secondary response. Antibiotic resistance arises through heritable variation and selection; an individual bacterium does not choose to become resistant because it needs to survive.
Use published infection data to compare rates per equal population size. Distinguish prevalence at a time from new cases over a period. In school, use safe simulations or approved cultures rather than collecting unknown pathogens.

pathogen: An agent that causes disease
antigen: A structure recognized by a specific immune response

C.4.1 · Populations and communities

A defined area used for sampling.
Estimate total abundance by multiplying mean density by area, with consistent units. This assumes sampled areas represent the habitat. Patchiness and too few samples widen uncertainty.
Choose coordinates with random numbers before visiting the patches. Record quadrat area and counting rules. For a transect, use fixed distances and measure a relevant abiotic variable. Do not damage habitats or sample unsafe locations.

quadrat: A defined area used for sampling
population: Organisms of one species in a defined area

C.4.2 · Transfer of energy and matter

Production at one trophic level divided by production at the preceding level over a common interval.
Use the same energy units and period in numerator and denominator. Distinguish ingestion, assimilation and production: consumed energy is not all assimilated, and assimilated energy is not all stored as new biomass. An energy pyramid records a flow per area per time; a standing biomass snapshot is a different quantity.
Analyse attributed or fictional ecosystem budget data. Draw a boundary and arrows for feeding, detritus and heat transfer. Identify which values are measured and which inferred, and check whether the budget includes decomposers. Avoid treating a fixed 10% rule as a universal measurement for every ecosystem.

trophic transfer efficiency: Production at one trophic level divided by production at the preceding level over a common interval
assimilation: Uptake of digested material into an organism’s usable internal pool

D.1.1 · DNA replication

DNA copying in which each daughter double helix retains one parental strand.
New DNA is synthesized in the 5′ to 3′ direction. In a simplified fork model, one new strand can be synthesized continuously while the other is produced in fragments that are joined. Distinguish these molecular mechanisms from the later separation of chromosomes during cell division. Copying errors can occur despite proofreading and repair.
Use labelled strand models or an attributed experimental diagram. Mark old and new material with both labels and colours so the meaning survives monochrome viewing. Track successive rounds using a stated starting population and assumptions. A classroom model illustrates predictions; it is not direct evidence about enzyme activity or a substitute for experimental results.

semi-conservative replication: DNA copying in which each daughter double helix retains one parental strand
DNA polymerase: An enzyme that synthesizes DNA using a template

D.1.2 · Protein synthesis

Formation of RNA using a DNA template.
A codon comprises three bases. The genetic code is degenerate: more than one codon can specify the same amino acid. A substitution can therefore be silent, while insertions or deletions can shift the reading frame.
Keep DNA template, coding DNA and mRNA distinct. State the strand used and write sequences in the required direction. Use a codon table for mRNA, not an unexplained DNA triplet.

transcription: Formation of RNA using a DNA template
translation: Formation of a polypeptide using an mRNA sequence

D.1.3 · Mutations and gene editing

Formation of RNA using a DNA template.
A codon comprises three bases. The genetic code is degenerate: more than one codon can specify the same amino acid. A substitution can therefore be silent, while insertions or deletions can shift the reading frame.
Keep DNA template, coding DNA and mRNA distinct. State the strand used and write sequences in the required direction. Use a codon table for mRNA, not an unexplained DNA triplet.

transcription: Formation of RNA using a DNA template
translation: Formation of a polypeptide using an mRNA sequence

D.2.1 · Cell and nuclear division

Fraction of scored cells undergoing mitosis.
Count chromosomes by centromeres under the stated convention. DNA amount and chromosome number are different quantities. Crossing over and independent assortment contribute to meiotic variation.
Use a prepared root-tip slide or a labelled image. Define a counting rule and field selection before counting. Distinguish dividing cells from damaged or ambiguous cells and report exclusions.

mitotic index: Fraction of scored cells undergoing mitosis
chromatid: One copy of a replicated chromosome before separation

D.2.2 · Gene expression

A protein that influences transcription.
Distinguish a measured mRNA change from a protein or functional change. Translation, degradation and protein modification can all intervene. Regulation is specific to a cell context and environmental conditions.
Compare treated and control expression data after checking normalization and replicates. Use evidence from a supplied perturbation to discuss causal models, rather than equating every association with a direct mechanism.

transcription factor: A protein that influences transcription
epigenetic modification: A change affecting expression without changing the DNA sequence

D.2.3 · Water potential

A pressure-equivalent measure of the tendency of water to move relative to a reference.
Compare the complete water potentials of the two sides rather than solute concentrations alone. A turgid cell can have the same total water potential as its surroundings despite unequal solute potential. At equilibrium there is no net transfer, but water molecules can continue moving in both directions.
Use provided potential values and labelled plant-cell models. State the reference and units before adding components. For school tissue investigations, control temperature, tissue dimensions, time and blotting, and estimate a zero-change point from repeated measurements. Do not claim the mass method separately measures cell pressure without further evidence.

water potential: A pressure-equivalent measure of the tendency of water to move relative to a reference
pressure potential: The pressure contribution to water potential in the stated model

D.3.1 · Reproduction

Having one chromosome set in the stated life cycle.
Distinguish homologous chromosomes from sister chromatids, and chromosome number from DNA quantity. Replication before meiosis copies DNA without doubling the number of chromosome sets. Crossing over and independent assortment contribute to genetic variation, while random fertilization further changes combinations. These processes do not make every offspring genetically distinct under every conceivable condition.
Use a labelled model organism with a stated chromosome number. Track chromosome sets through replication, meiosis I, meiosis II and fertilization. Use anonymous model data rather than personal family or reproductive-health information. Compare organism life cycles carefully; flowering-plant and animal reproductive structures require their own additional teaching.

haploid: Having one chromosome set in the stated life cycle
fertilization: Fusion of gamete nuclei to form a zygote

D.3.2 · Inheritance

A variant of a gene.
In a simple monohybrid cross Aa × Aa, gametes carry A or a. Combining independent gametes gives AA, Aa, Aa and aa. The predicted probabilities describe many possible fertilizations, not a fixed order of children.
Write parental genotypes and gametes before making the grid. State the inheritance model and phenotype key. Use a pedigree to check consistency with a model; do not infer certainty from a small family alone.

allele: A variant of a gene
genotype: The alleles an organism carries

D.3.3 · Homeostasis

Maintenance of suitable internal conditions.
When blood glucose is high, insulin helps increase glucose uptake and storage as glycogen. When it is low, glucagon supports release of glucose from stores. These responses are coordinated, not identical effects of two hormones.
Interpret a time graph by identifying the initial disturbance, the response and the return toward the normal range. Mark the delay before a response. Do not assume a graph shows an instantaneous correction.

homeostasis: Maintenance of suitable internal conditions
negative feedback: A response opposing the original change

D.4.1 · Natural selection

A heritable feature benefiting survival or reproduction in a context.
Explain both a benefit and a cost. Reducing stomatal opening conserves water but can limit carbon dioxide entry. A feature cannot be judged as universally best without its ecological context.
Compare replicated observations across a measured environmental gradient. Account for relatedness, leaf area and age. Use local permitted plant observations rather than removing protected species.

adaptation: A heritable feature benefiting survival or reproduction in a context
acclimatization: An individual adjustment to changed conditions during life

D.4.2 · Sustainability and change

A defined area used for sampling.
Estimate total abundance by multiplying mean density by area, with consistent units. This assumes sampled areas represent the habitat. Patchiness and too few samples widen uncertainty.
Choose coordinates with random numbers before visiting the patches. Record quadrat area and counting rules. For a transect, use fixed distances and measure a relevant abiotic variable. Do not damage habitats or sample unsafe locations.

quadrat: A defined area used for sampling
population: Organisms of one species in a defined area

D.4.3 · Climate change

Actions addressing causes of environmental change.
Distinguish mitigation, which addresses drivers, from adaptation, which reduces harm from impacts. A policy assessment needs evidence about effectiveness, cost, equity and uncertainty; one criterion is not the entire decision.
Compare multi-year data using consistent baselines. State the region, timescale and uncertainty. At HL, connect a management decision to law, economics and ethics rather than treating these lenses as extra definitions only.

mitigation: Actions addressing causes of environmental change
adaptation: Actions reducing harm from environmental impacts

Practical · Experimental programme

A quantified limitation on a measured result.
For a product or quotient, adding fractional uncertainties is a common maximum-uncertainty approximation. For a difference, add absolute uncertainties. A nonzero intercept can reveal an offset or an incomplete model.
Show units on axes and choose a sensible scale. Plot uncertainty bars where justified, draw a best-fit line rather than joining every point, and estimate steepest and shallowest plausible gradients when the course method calls for them.

uncertainty: A quantified limitation on a measured result
systematic error: A consistent measurement bias

Preparing for this qualification · ⁨Preparação para esta qualificação⁩

Unity/diversity, form/function, interaction/interdependence and continuity/change organize the course; there are no old option topics.
SL excludes origins of cells, viruses, classification/cladistics, muscle/motility, chemical signalling and gene expression as HL-only topics; mixed topics also have AHL details needing the guide.
Paper 1: 1A MCQ and 1B data; Paper 2: short/extended responses. SL durations 1.5/1.5 h; HL 2/2.5 h; weights 36%/44%. Scientific investigation 20%, maximum 3,000 words.
Practical work 40 h, collaborative sciences project 10 h and individual scientific investigation 10 h.

Teaching coverage still needed · ⁨Cobertura do ensino ainda necessária⁩

Full statement-level SL and AHL content for this topic awaits the full 2025 Biology guide; listed cases teach only their stated focus.
Individual investigation and collaborative sciences project are school-supervised; full practical-skills guide remains needed.

Specifications and sample documents · ⁨Especificações e documentos de exemplo⁩

Biology — Subject brief · 2025 ↗
First assessment: 2025

Course materials · ⁨Materiais do curso⁩

Course preparation · ⁨Preparação do curso⁩

Documents are available. Board-specific notes, assessments and interactive past-paper practice are not yet available for every course. · ⁨Os documentos estão disponíveis. Notas específicas da banca, avaliações e prática interativa de provas anteriores ainda não estão disponíveis para todos os cursos.⁩

Lessons · ⁨Lições⁩ →