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Biology Extended Essay Guide
A guide to designing, conducting, and writing a rigorous IB Biology Extended Essay with strong experimental methodology.
What is a Biology Extended Essay?
The Biology Extended Essay (EE) is one of the most challenging and rewarding parts of the IB Diploma. Below you will find everything you need to choose a strong topic, design your methodology, and write a high-scoring EE in Biology.
Word Count
3,500 – 4,000 words
Excluding bibliography, abstract, and appendices. Budget every section carefully before you start writing.
Assessment
Graded A–E
Combined with the TOK essay for up to 3 bonus IB Diploma points. A strong EE and TOK = maximum bonus.
Timeline
12–18 months
From initial topic to final submission. Most students underestimate this. Start early and plan backwards from your deadline.
Supervision
3 required meetings
Initial, interim, and final (viva voce). Each requires an RPPF reflection. Your supervisor approves your RQ.
Strong Biology EE Topic Ideas
| Topic Idea | Why It Works | Difficulty |
|---|---|---|
| How does temperature affect the rate of catalase enzyme activity in potato tissue? | Classic, measurable, easy to control | Easy |
| To what extent does caffeine concentration affect the heart rate of Daphnia magna? | Live organism ethics approachable, clear variables | Easy |
| How does antibiotic concentration affect the inhibition zones of E. coli growth in agar plates? | Microbiology, clear hypothesis, quantifiable | Medium |
| To what extent does pH affect the effectiveness of salivary amylase in breaking down starch? | Enzyme biology, measurable, theory-rich | Easy |
| How does UV exposure duration affect the rate of melanin production in exposed vs. unexposed skin cells? | Secondary analysis if primary not feasible | Hard |
| To what extent does the gut microbiome composition correlate with BMI in human subjects? A meta-analysis | Secondary analysis, microbiome biology, data-rich | Hard |
Subject-Specific Advice
Variables
Define and control all variables
State independent variable, dependent variable, and ALL controlled variables explicitly. Any uncontrolled variable is a limitation to discuss.
Replication
Minimum 5 trials per condition
Biological systems are variable. Repeat each condition at least 5 times and report mean ± standard deviation. Single measurements are not scientifically credible.
Statistics
Choose the right test
t-test (two groups), ANOVA (multiple groups), Chi-squared (categorical data), Pearson’s r (correlation). State your null hypothesis and interpret your p-value.
Biology Theory
Explain mechanisms
Do not just report what you observed — explain the biological mechanism behind your results using cellular biology, biochemistry, or physiology theory.
Assessment & Common Mistakes
Assessed on: biological knowledge, experimental design, data analysis, and critical evaluation. Examiners expect: a controlled experiment with replicated trials, appropriate statistics, and a discussion grounded in biological theory — not just a description of results.
⚠ Common Pitfall
Students conduct experiments with too few trials and too little statistical analysis, then discuss their results without explaining the underlying biological mechanism. An EE that just says “the rate increased with temperature” without explaining enzyme kinetics and the Arrhenius equation is missing the core biological analysis.
Biology EE Checklist
- ✓ My RQ is specific, testable, and asks about a causal or correlational biological relationship
- ✓ I have identified and controlled all major variables
- ✓ I conducted minimum 5 trials per condition
- ✓ I have applied appropriate statistical analysis (t-test, ANOVA, etc.) with correct interpretation
- ✓ My discussion explains the biological mechanism behind my results
- ✓ I have discussed biological limitations (enzyme variability, organism age, sample size)
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