IB Biology IA Topics — 2025 Syllabus

Curated research-question templates grouped by the four 2025 themes. Each includes variables, controls, recommended statistical test, and difficulty — so you can pick a topic you can actually deliver.

Unity and Diversity

Enzyme kinetics, cell-level processes, ecological diversity — topics here let you showcase mechanism-level biology on a predictable timeline.

Effect of temperature on catalase activity in potato extract

Accessible

RQ template: How does substrate temperature (°C) affect the initial rate of O₂ evolution by catalase extracted from Solanum tuberosum?

Independent variable: Temperature of H₂O₂ substrate (e.g. 10, 20, 30, 40, 50, 60 °C)
Dependent variable: Initial rate of O₂ gas production (cm³ s⁻¹) measured via gas syringe or displacement
Control variables: Mass of potato extract (g) · Concentration of H₂O₂ (% w/v) · pH of substrate · Volume of substrate (cm³) · Stirring rate
Suggested statistical test: One-way ANOVA followed by Tukey HSD; report r² from regression

Why it works: Classic, reliable, and shows clear enzyme kinetics — denaturation above ~50 °C gives a strong discussion point.

Effect of salt concentration on Allium cepa (onion) epidermal cell plasmolysis

Accessible

RQ template: How does the concentration of NaCl solution (mol dm⁻³) affect the percentage of plasmolysed Allium cepa epidermal cells after 15 minutes of immersion?

Independent variable: NaCl concentration (0.0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³)
Dependent variable: % of plasmolysed cells from a random field of view of ≥50 cells
Control variables: Immersion time (min) · Epidermal layer position on the bulb · Microscope magnification · Temperature · Cell counting protocol
Suggested statistical test: Chi-squared test for goodness of fit; logistic regression for incipient plasmolysis point

Why it works: Allows a strong link to water-potential theory and identification of incipient plasmolysis concentration.

Comparing amylase activity across germinating and non-germinating seeds

Moderate

RQ template: Does the specific activity of α-amylase differ significantly between germinating and non-germinating Triticum aestivum seeds measured by the iodine starch disappearance assay?

Independent variable: Seed condition (germinating vs dry)
Dependent variable: Time (s) for starch-iodine complex to fully disappear at 25 °C
Control variables: Seed mass (g) · Starch concentration · Temperature (°C) · pH · Extraction protocol
Suggested statistical test: Two-sample t-test (unequal variances) or Mann-Whitney U

Why it works: Good for linking to gibberellin signalling and germination metabolism.

Biodiversity survey: Simpson index in two contrasting microhabitats

Moderate

RQ template: How does invertebrate Simpson diversity (1−D) differ between a mown lawn and an adjacent untouched grass strip on the school campus?

Independent variable: Microhabitat (mown vs unmown)
Dependent variable: Simpson diversity index 1−D
Control variables: Sampling method (pitfall traps/quadrat) · Sample duration (h) · Time of day · Weather conditions
Suggested statistical test: Bootstrap confidence intervals on 1−D; Shannon as secondary index

Why it works: Strong for ecosystem-scale reasoning; ethical collection protocol required.

Form and Function

Physiology, anatomy, leaf and respiratory adaptations. Best when you can measure a rate or count a density cleanly.

Effect of caffeine concentration on Daphnia heart rate

Accessible

RQ template: How does caffeine concentration (mg dm⁻³) affect the heart rate (beats min⁻¹) of Daphnia magna at 20 °C?

Independent variable: Caffeine concentration (0, 50, 100, 200, 400 mg dm⁻³)
Dependent variable: Heart rate (beats min⁻¹) counted from video at ×100
Control variables: Daphnia body length (mm) · Exposure time (min) · Water temperature (°C) · Light intensity during observation
Suggested statistical test: ANOVA with post-hoc; Pearson correlation for concentration vs rate

Why it works: Ethics: return organisms to stock tank; use minimum effective doses. Age-match Daphnia to reduce variance.

Stomatal density vs light exposure in leaves of the same species

Accessible

RQ template: How does sun-versus-shade leaf position affect stomatal density (stomata mm⁻²) on the abaxial surface of Hedera helix leaves?

Independent variable: Leaf position (full sun vs shaded canopy)
Dependent variable: Stomatal density per mm²
Control variables: Leaf maturity (only fully expanded leaves) · Plant species · Location of leaf imprint (mid-lamina) · Microscope calibration
Suggested statistical test: Two-sample t-test; if non-normal, Mann-Whitney U

Why it works: Use nail-varnish peel technique for high-quality imprints; count blind to reduce bias.

Effect of exercise intensity on lung ventilation rate

Moderate

RQ template: How does exercise intensity (W on a cycle ergometer) affect tidal-volume-corrected minute ventilation (dm³ min⁻¹) in healthy 16–18 year-olds?

Independent variable: Power output (50, 100, 150, 200 W)
Dependent variable: Minute ventilation (dm³ min⁻¹)
Control variables: Participant sex/age bracket · Warm-up protocol · Ambient temperature · Cadence (rpm)
Suggested statistical test: Repeated-measures ANOVA; regression of MV on workload

Why it works: Requires ethics/PAR-Q; uses the physiology ↔ form-and-function link directly.

Effect of salinity on xylem transport rate in Tradescantia stems

Moderate

RQ template: How does the NaCl concentration of the root medium (mol dm⁻³) affect the dye-ascent rate (mm min⁻¹) in excised Tradescantia stems?

Independent variable: NaCl concentration (0.00, 0.10, 0.25, 0.50, 0.75 mol dm⁻³)
Dependent variable: Rate of eosin dye ascent (mm min⁻¹)
Control variables: Stem diameter (mm) · Stem length (cm) · Humidity · Temperature · Light intensity
Suggested statistical test: Regression; ANOVA across concentrations

Why it works: Links transpiration, water potential and abiotic stress.

Interaction and Interdependence

Ecological interactions, photosynthesis and symbiosis — stronger topics when you can isolate one variable from the wider ecosystem.

Allelopathy: effect of Eucalyptus leaf leachate on radish germination

Accessible

RQ template: How does the concentration of Eucalyptus globulus leaf leachate affect the percentage germination of Raphanus sativus seeds over 5 days?

Independent variable: Leachate concentration (0, 25, 50, 75, 100 %)
Dependent variable: % germination at day 5
Control variables: Seed batch · Incubation temperature · Light regime · Moisture volume (cm³) · Petri dish size
Suggested statistical test: Chi-squared; logistic dose-response fit

Why it works: Strong interspecies-interaction framing; clear biochemical mechanism to discuss.

Effect of light wavelength on algal (Chlorella) photosynthetic O₂ production

Moderate

RQ template: How does the wavelength of illumination affect the rate of O₂ evolution (measured as % saturation change) in Chlorella vulgaris suspensions of fixed optical density?

Independent variable: LED wavelength (approx. 450, 525, 590, 625, 660 nm)
Dependent variable: O₂ % saturation change over 10 minutes
Control variables: Optical density (600 nm) · Irradiance (μmol m⁻² s⁻¹) · Temperature · CO₂ availability (pH buffer)
Suggested statistical test: ANOVA; map to absorption spectrum shape

Why it works: Links absorption spectra to action spectra; data-loggers make this realistic at school labs.

Effect of pH on Lactobacillus growth (milk pH drop method)

Moderate

RQ template: How does the starting pH of milk affect the time (h) taken for Lactobacillus bulgaricus cultures to reduce milk pH to 4.5 at 37 °C?

Independent variable: Initial pH (5.5, 6.0, 6.5, 7.0, 7.5)
Dependent variable: Time to reach pH 4.5 (h)
Control variables: Inoculum size · Temperature (°C) · Milk fat content · pH meter calibration
Suggested statistical test: Regression; ANOVA across pH levels

Why it works: Good ecosystem/symbiosis angle — can discuss fermentation and gut microbiota.

Continuity and Change

Genetics, mutation, selection, evolution, climate data. Database IAs often sit here and can score highly if data processing is rigorous.

Effect of UV exposure on yeast (S. cerevisiae) survival

Moderate

RQ template: How does UV-C exposure time (s) affect the viability (% CFU survival) of Saccharomyces cerevisiae on nutrient agar plates?

Independent variable: UV-C exposure time (0, 10, 30, 60, 120 s)
Dependent variable: CFU survival (%) normalised to control plate
Control variables: Cell density at plating · Agar composition · UV-C lamp-to-plate distance · Incubation time/temperature
Suggested statistical test: ANOVA; log-linear survival curve fit

Why it works: Clear link to DNA-damage and repair mechanisms — strong for the mutation section.

Effect of pesticide dose on Drosophila development time

Challenging

RQ template: How does the concentration of a household-grade permethrin solution in the larval medium affect egg-to-adult development time (days) in Drosophila melanogaster at 25 °C?

Independent variable: Permethrin concentration (0, 0.1, 0.5, 1.0, 2.0 ppm)
Dependent variable: Mean egg-to-adult development time (days)
Control variables: Rearing temperature (°C) · Medium composition and volume · Number of eggs per vial · Light/dark cycle
Suggested statistical test: ANOVA with post-hoc; Kaplan–Meier survival curves for viability

Why it works: Strong selection-pressure discussion — links directly to resistance evolution.

Hardy–Weinberg analysis of PTC tasting in a consented volunteer cohort

Accessible

RQ template: Do observed PTC-tasting phenotype frequencies in a sample of ≥80 age-matched volunteers differ significantly from Hardy–Weinberg expectations?

Independent variable: Phenotype category (taster vs non-taster)
Dependent variable: Observed vs expected allele and genotype frequencies
Control variables: Age bracket · Standardised paper-strip protocol · Repeat testing for borderline responders
Suggested statistical test: Chi-squared goodness of fit against H-W expected

Why it works: Zero-apparatus, high-engagement IA; ethics review for taste-testing must be completed.

Database-driven: latitude vs CO₂ concentration anomaly trend

Accessible

RQ template: Does the rate of monthly CO₂ concentration rise (ppm yr⁻¹) in NOAA Mauna Loa data correlate with the latitude-adjusted warming trend between 2000 and 2024?

Independent variable: Latitude band (equatorial, mid-latitude, high-latitude) — from HadCRUT
Dependent variable: CO₂ concentration rise rate (ppm yr⁻¹)
Control variables: Dataset selection · Time window · Quality-flag filtering
Suggested statistical test: Pearson correlation; regression with residual analysis

Why it works: Database IAs are fully accepted under the 2025 guidance. Strong option for students without lab access.

IA Topics — Frequently Asked Questions

How do I choose a good IB Biology IA topic?▾

Good IA topics are narrow enough that you can collect 5 levels × 5 replicates of data, grounded in IB-level biology (you can explain a mechanism), and measurable with the equipment you actually have. Avoid topics like "effect of pollution on plants" — prefer something like "effect of NaCl concentration on Allium cepa plasmolysis".

Can an IA topic be a database study or simulation?▾

Yes. The 2025 IB Biology IA accepts database investigations (e.g. NCBI GenBank, HHMI BioInteractive, WHO Global Health Observatory) and simulations (e.g. PhET) as long as you process data yourself and draw biological conclusions. This is often the best route if your school has limited lab facilities.

How many levels of the independent variable do I need?▾

Examiners typically expect at least 5 different levels of a quantitative independent variable, each repeated at least 5 times (the "5 × 5 minimum"). For categorical independent variables, have at least 2 groups with ≥10 replicates each. Fewer levels make it very hard to score full marks for Data Analysis and Conclusion.

Does the topic need to match one of the four IB themes?▾

Your IA does not need to be explicitly labelled under a theme, but the topic should be anchored in IB Biology content. Grouping by theme (Unity & Diversity; Form & Function; Interaction & Interdependence; Continuity & Change) helps you map your chosen mechanism to the syllabus and write a stronger Conclusion.

Is it safe to pick an IA topic my friend used last year?▾

Pre-2025 topics often need to be adapted for the 2025 rubric. You can reuse a successful methodology but you should still make the research question your own, collect your own data, and structure your report around the 4-criterion 2025 rubric. IB has upgraded plagiarism detection, so never copy text or data.

Next Steps

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