IB Biology Extended Essay
The criteria on this page are the ones for exams from May 2027, when the Extended Essay is marked out of 30. See what changed. If you sit your exams in November 2026, the older 34-mark criteria are summarised after them.
The IB Biology Extended Essay (EE) is a 4,000-word independent research project. Its grade, combined with your TOK grade, decides up to three bonus points on your Diploma score. Biology is a popular EE subject, but students often underestimate how different a Biology EE is from a Biology IA. This guide covers what examiners reward, which research questions work, and what an A-grade essay does.
How the Biology EE differs from the IA
The Biology IA is a scientific investigation written up in a report of up to 3,000 words and marked on four science criteria. The Biology EE is a 4,000-word essay marked on the Extended Essay criteria, and it asks for deeper engagement with published literature and a sustained argument. The key distinction: the EE must pursue a question whose answer is not already in the textbook, and argue its way to an answer.
Choosing a research question for Biology EE
The research question is the most important decision in your Extended Essay. A strong Biology EE research question has four characteristics:
- Measurable: You can collect quantitative data, not just observe qualitatively.
- Narrow: It specifies the organism, the variable, and the condition being tested.
- Not trivially answered by existing literature: If the answer is in your textbook, it is not an EE question.
- Feasible: You can run the experiment or access the data with available equipment and time.
Examples of strong Biology EE research questions
- How does soil pH affect the germination rate and root elongation of Phaseolus vulgaris seeds over a 14-day period?
- How does light intensity affect the rate of oxygen production by Elodea canadensis, measured over 20-minute intervals?
- How does wavelength of light during the vegetative phase affect anthocyanin concentration in Ocimum basilicum leaves?
- What is the relationship between dissolved oxygen levels and macroinvertebrate species diversity in three sections of a local river?
Common research question mistakes
The most frequent error is a question that is too broad. "How does temperature affect enzyme activity?" is not an EE question; it is a review article topic. "How does temperature (10-50 degrees C, in 10 degree increments) affect the rate of starch hydrolysis by a purchased fungal amylase, measured by iodine absorbance at 580 nm?" is an EE question. The IB's experimentation guidelines also set limits: no body fluids such as saliva, and no experiments that give drugs to animals or push them beyond their natural tolerance.
IB Biology EE assessment criteria
From May 2027, the Biology EE is marked out of 30 on five criteria:
- Criterion A: Framework for the essay (6 marks). Is the research question clearly stated and focused? Is the method appropriate, detailed and reproducible, and does the structure of the essay serve the question?
- Criterion B: Knowledge and understanding (6 marks). Does the essay show knowledge and understanding of the biology behind the question, with correct terminology and relevant, reliable sources?
- Criterion C: Analysis and line of argument (6 marks). Does your analysis go beyond describing data, with findings that stay relevant to the question and build a coherent argument?
- Criterion D: Discussion and evaluation (8 marks). The highest-weighted criterion. Do you discuss what your findings mean against published research, with evidence, and evaluate the investigation itself: its anomalies, its limitations and what they do to your conclusion?
- Criterion E: Reflection (4 marks). Marked on the reflective statement in your RPF. It rewards evaluative reflection, with specific examples, on how the investigation changed you as a learner.
Grade boundaries for the new 30-mark scale are set only after the May 2027 session is marked; on the older scale, 27 or more out of 34 has been a grade A in recent sessions. Either way, essays most often fall short in discussion and evaluation: results are summarised without being connected to published research, or without evaluating why they differ from what the literature predicts.
If you sit your exams in November 2026, the essay is marked out of 34 instead: Focus and method (6), Knowledge and understanding (6), Critical thinking (12), Presentation (4) and Engagement (6, marked on the three RPPF reflections). The advice above carries over, because research, analysis, and discussion and evaluation together make up Critical thinking.
How to structure a Biology EE
Within the 4,000-word limit, Biology EEs that score well share this structure:
- Introduction (400-500 words): State the research question, provide biological context, cite at least two peer-reviewed sources. End with a clear hypothesis if experimental.
- Background Theory (500-700 words): Explain the biology your investigation depends on, going beyond the syllabus where the question needs it. This is where Criterion B marks are won.
- Methodology (400-600 words): Write a complete, reproducible method. State variables explicitly: independent, dependent, controlled, and uncontrolled. Justify sample size and replication numbers.
- Results and Analysis (700-900 words): Present data in labelled tables and graphs. Apply appropriate statistical tests with error bars. Identify trends and anomalies.
- Discussion (800-1,000 words): Compare your findings to published literature explicitly. Do not save evaluation for this section: the guide wants it integrated throughout the essay, so weigh the strengths and weaknesses of your method, data and sources where they arise, then draw their effect on the conclusion together here and suggest specific improvements.
- Conclusion (200-300 words): Answer the research question directly. State your confidence in the answer given the limitations identified.
Biology EE topics that score well
- Microbiology and antimicrobial activity: Testing the effect of plant extracts on bacterial growth using disc diffusion assays, with known non-pathogenic cultures from a supplier, incubation at or below 25°C and no antibiotic-resistance testing, as the IB's experimentation guidelines require. Yields measurable quantitative data.
- Ecology and population studies: Comparing species richness across disturbed and undisturbed habitats using quadrat sampling. Strong for discussion and evaluation (Criterion D) because you can compare findings to published biodiversity indices.
- Plant physiology: Effect of different light spectra on chlorophyll production, stomatal density, or growth rate. Feasible with school equipment and produces clear quantitative results.
- Biochemistry and enzymes: The classic choice, but only when you go beyond basic kinetics, for example by estimating the Michaelis constant, investigating inhibitors, or comparing enzyme activity across pH and ionic strength.
The reflection: what Criterion E rewards
From May 2027, Criterion E is marked on a single reflective statement of up to 500 words in the RPF, written after your final reflection session; through November 2026 it is marked on the three RPPF reflections. Weak reflection describes what happened: "I collected my data this week and it went well." Strong reflection evaluates what the experience taught you, with specific examples: "My first results contradicted the literature. Checking the calibration showed a zero-point error of about 12%, and since then I test an instrument before I trust a surprising result, in every practical, not just this one." Specific, evaluative reflection that shows growth is what the upper levels of Criterion E describe.
Analyse your Biology EE with IBLens
IBLens reads Extended Essays against the assessment criteria for your session. Paste a draft of your Biology EE to receive criterion-by-criterion feedback identifying where you are losing marks, especially on analysis and discussion (Criteria C and D) and the focus of your research question in Criterion A.
An AI check is outside help on your EE, so ask your supervisor before you use IBLens or any other tool, and tell them if you do.
Analyse your Biology EE with IBLens