IB Biology Extended Essay: Topics, Criteria & Structure
By Michael Thompson · Education Specialist; 10 years teaching the IB at Bromsgrove School · Published 4 September 2026 · Updated 4 October 2026
A biology extended essay is a 4,000-word independent research paper that forms a compulsory part of the IB Diploma Programme core, assessed from May 2027 against five criteria worth up to 30 marks. Choosing the right research question and understanding how examiners apply each criterion to biology work is what separates a focused, high-scoring essay from a descriptive one. The essay can be experimental or based on secondary data, and both routes are equally valid - the difference lies in what each route demands methodologically. This guide covers the criteria in full, how to build a focused research question, what experimental and secondary-data essays each need, and where marks are most often dropped.
Key Takeaways
- Five criteria, 30 marks from May 2027: Framework for the essay (6), Knowledge and understanding (6), Analysis and line of argument (6), Discussion and evaluation (8) and Reflection (4) - Discussion and evaluation (Criterion D) carries the most weight.
- Both experimental and literature-based routes are accepted: Secondary-data essays must make their selection criteria for sources explicit, and the analysis of the data must be your own, not copied from the papers you used.
- The research question must be expressed as a question: Per IB guidance, it must not be a hypothesis or a statement of intent, and it must be focused enough to allow analysis and a sustained line of argument.
- The 4,000-word limit is a hard ceiling: Examiners are instructed not to read or assess anything beyond 4,000 words, so tight scope matters from the start.
- Description without analysis is the most common mark-loser: Essays that report results or summarise sources without interpreting them score poorly under Criterion C (Analysis and line of argument), regardless of how much data they contain.
- Reflection is one 500-word statement: Criterion E assesses only a reflective statement of no more than 500 words on the Reflection and progress form (RPF), written after the viva voce, the last of three mandatory reflection sessions.
In This Article
- What the Biology Extended Essay Is and How It Works
- The Five IB Biology Extended Essay Criteria Explained
- Crafting a Strong Biology Extended Essay Research Question
- Experimental vs Literature-Based Essays: What Each Route Needs
- IB Biology Extended Essay Structure and Reflection
- Where Biology Extended Essays Most Often Lose Marks
- What to Do Next
1. What the Biology Extended Essay Is and How It Works
The biology extended essay is a mandatory, externally assessed 4,000-word independent research paper that every IB Diploma Programme student must complete as part of the DP core. It is not a class assignment marked by your teacher. The IBO assesses it externally, and the grade contributes to your overall DP points alongside Theory of Knowledge.
Biology sits under "Sciences" in the subject-specific guidance of the IB's Extended essay guide (first assessment 2027), alongside Chemistry, Physics, Computer Science, Design Technology, and Sports, Exercise and Health Science. That grouping matters because the guide's advice on applying each criterion is written for the sciences as a group.
This guide follows the new Extended Essay the IBO launched in February 2025, with first assessment in May 2027. It brings a revised set of five criteria worth 30 marks, a single reflective statement in place of the old three-part reflection form, and a formal interdisciplinary pathway. If you sit your exams in May 2027 or later, this is your model; November 2026 candidates should read the note at the end of section 2.
The research process follows a fixed sequence: choose a subject area and topic, formulate a focused research question, then select a method, whether experimental, fieldwork-based, or literature-driven. Getting that order right matters more than most students expect. A weak research question makes every subsequent step harder to execute.
2. The Five IB Biology Extended Essay Criteria Explained
From May 2027 the biology extended essay is marked out of 30 points across five criteria, the same criteria every subject uses. Knowing the mark allocation before you start shapes every decision you make about scope, method, and writing style.
| Criterion | Name | Marks |
|---|---|---|
| A | Framework for the essay | 6 |
| B | Knowledge and understanding | 6 |
| C | Analysis and line of argument | 6 |
| D | Discussion and evaluation | 8 |
| E | Reflection | 4 |
| Total | 30 |
Criterion D carries 8 of the 30 marks, more than any other single criterion. A biology essay that describes results clearly but never discusses their significance, weighs alternative explanations or evaluates its own limitations will lose a disproportionate share of marks, regardless of how well the practical work was conducted.
Per the IB's Extended essay guide (first assessment 2027), each criterion breaks down into named strands, and the sciences guidance explains how they apply:
- Criterion A (Framework for the essay): Research question, Research methods, Structure. For experimental work, the method must be detailed enough for the investigation to be replicated, and data tables, diagrams and graphs must be clearly labelled, with uncertainties shown in example calculations. If you carry out the research in a university or other external lab, the methods must be your own, not just those the organisation already uses.
- Criterion B (Knowledge and understanding): Knowledge, Terminology, Concepts. Biology examiners expect you to explore the underlying processes and theories, not just define terms, and this matters most in a secondary-data essay. The guide's own example: an essay on the turnover rate of an enzyme must discuss enzyme kinetics.
- Criterion C (Analysis and line of argument): Analysis, Line of argument. Analysis should follow standard qualitative, quantitative and statistical methods where appropriate and address the research question directly. Restating what the data shows is not analysis; explaining why a trend deviates from the expected pattern, and what that implies, is. Every element of the essay should link back to the question.
- Criterion D (Discussion and evaluation): Discussion, Evaluation. The discussion should be critical, set in a scientific context and tied to the research question, with sources used to support conclusions. The conclusion should synthesise the results rather than repeat them, and the evaluation should account for unresolved issues and suggest improvements.
- Criterion E (Reflection): Assessed only from your reflective statement of no more than 500 words on the Reflection and progress form (RPF). Examiners look for evaluative reflection with specific examples and evidence of growth and transfer of learning, not a diary of meetings.
The non-obvious gotcha: Criterion A's research methods strand is assessed separately from Criterion C's analysis strand. A student can lose marks for an underspecified method even if their analysis of the resulting data is excellent.
> November 2026 candidates only: your session is the last on the old 34-mark scheme: Focus and method (6), Knowledge and understanding (6), Critical thinking (12), Presentation (4) and Engagement (6), with Engagement assessed from the Reflections on planning and progress form (RPPF), three reflections totalling up to 500 words.
3. Crafting a Strong Biology Extended Essay Research Question
The IB requires the research question to be framed as a question, not a hypothesis or a statement of intent. It is not a title either. That distinction matters for Criterion A, which has a dedicated strand for the research question alone.
The four-step formulation process
The 2027 guide sets out a four-step process:
- Confirm your focused topic - a biology topic you are genuinely interested in, realistic for your school's facilities and narrow enough for 4,000 words.
- Do a preliminary investigation - check what has been published, whether sources are accessible, and whether you can collect the data you need in the time available, safely and ethically.
- Draft and evaluate questions - brainstorm open questions framed with "how", "why" or "to what extent", then keep only those that are clear, focused and arguable. If the answer is simply yes or no, revise.
- Choose and refine - make one your provisional research question, define and justify its variables, and stay ready to amend it as the research develops.
Strong vs weak: worked examples
| Research question | Why it works or fails | |
|---|---|---|
| Strong | "To what extent does the concentration of caffeine affect the heart rate of Daphnia magna?" | Named organism, measurable independent variable, command term signals evaluation |
| Weak | "How does pollution affect ecosystems?" | No organism, no measurable variable, no route to a reasoned argument |
The IB's own example in the 2027 guide asks how a stated range of potassium nitrate concentrations affects the growth rate of onion roots. It illustrates the required specificity: named substance, defined range of values, named organism and a measurable outcome. The guide also recommends a pilot experiment to fine-tune the independent variable.
The consistency gotcha
The research question must remain identical throughout the biology extended essay. One IB sample essay, marked under the pre-2027 criteria, was criticised partly because the conclusion quoted a slightly different version of the question from the one stated at the start. Copy the exact wording into your conclusion rather than paraphrasing from memory.
4. Experimental vs Literature-Based Essays: What Each Route Needs
Biology is one of the few IB subjects where both primary experimental work and secondary literature analysis are fully accepted routes. Choosing the wrong one for your topic is a common mistake worth avoiding early.
Experimental essays rest on student-collected data. The 2027 guide states that the methodology must be detailed enough for the investigation to be replicated, that dependent variables must be measured under standardised conditions, and that SI units must be used. The IB sample antimicrobial metals essay, marked under the pre-2027 scheme, illustrates the pitfall: the examiner commentary flags that running multiple t-tests on the same dataset produces a high risk of compounded error, and that the methodology lacked sufficient detail for replication.
There is also a practical and ethical boundary on scope. The guide warns against topics that need sophisticated equipment not usually found in school labs, or where collecting enough data would be unethical or impractical. Experiments involving animals, human participants or microorganisms must follow the IB's ethical guidelines for extended essay research and its sciences experimentation guidelines, which cover areas such as human physiology experiments, animal behaviour, microbiological studies and risk assessment. Agree any such plan with your supervisor first. If your topic drifts into territory you cannot study safely and ethically in school, a literature-based approach is usually the more practical route.
Literature-based essays are not a soft option. The guide requires the selection criteria for the studies and data you include to be made explicit, and the sources must use similar standardised approaches and controls so that comparisons are valid. You may reproduce graphs or tables from your sources, but the (statistical) analysis must be your own. The IB sample APP/Alzheimer's essay uses secondary data exclusively and earned its marks by analysing and evaluating evidence, not by summarising it. An essay that chains together findings from peer-reviewed papers without interrogating their limitations will not reach the upper descriptors, regardless of how many sources it cites.
| Route | What Criterion A looks for | Main risk |
|---|---|---|
| Experimental | Replicable method, controlled variables, safe and ethical scope | Insufficient procedural detail; statistical errors |
| Literature/secondary | Explicit selection criteria, comparable sources, your own analysis | Summarising rather than critically analysing evidence |
Pick the route that your research question genuinely suits, not the one that sounds more impressive.
5. IB Biology Extended Essay Structure and Reflection
From May 2027 there is no separate presentation criterion: structure is a strand of Criterion A (Framework for the essay), and the sciences guidance is specific about it. Results must be presented in a standardised format with clear headings, units and numbering, data must be clearly processed, and example calculations should show the uncertainties on your measurements. A well-labelled graph that clarifies a trend helps the examiner; a decorative one does not.
Standard section order for a biology extended essay:
- Title page
- Contents page
- Introduction (stating the research question and the biology behind it)
- Methodology or approach
- Results and analysis
- Discussion and evaluation
- Conclusion
- References
- Appendices (if needed)
The 4,000-word ceiling is absolute. Examiners are instructed not to read or assess anything beyond 4,000 words, and an essay that runs over is compromised across all the criteria. Tables, graphs, diagrams, equations and calculations do not count towards the total, and nor do the contents page, citations or the bibliography. Anything essential to your argument must sit in the body, because examiners are not required to read appendices or footnotes. The counter-intuitive gotcha: a 3,800-word essay can outscore a 4,100-word one, because the examiner never reads the conclusion of the longer submission.
All sources must be referenced consistently throughout. The guide also warns against over-extensive quoting: your knowledge needs to be evident in your own writing. Pick one referencing style and apply it to every entry.
Reflection is assessed separately from the essay. You attend three mandatory reflection sessions with your supervisor: an initial session, an interim session and the final viva voce, a short interview held after the essay is finished. Attendance is recorded on the Reflection and progress form (RPF), and at the end of the process you write one reflective statement of no more than 500 words on the RPF. That statement is the only thing Criterion E assesses.
6. Where Biology Extended Essays Most Often Lose Marks
Most marks lost on a biology extended essay come from the same handful of errors, and several are counterintuitive enough that students repeat them even after feedback.
Description instead of analysis (Criterion C) is the single most common weakness. Reporting what your results show, or summarising what a source argues, is not the same as interpreting it. Criterion C - Analysis and line of argument - rewards students whose analysis consistently produces relevant findings and whose argument links the research question, findings and conclusions.
Thin discussion and evaluation (Criterion D) is the most expensive weakness, because D is worth 8 marks. A conclusion that simply repeats the results, or an evaluation that lists generic errors without linking them to the research question and method, will not reach the top bands. Negative results are fine: the guide says they are as valid as positive ones, provided you evaluate them critically.
Weak methodology (Criterion A) catches experimental essays that omit enough detail for replication. The antimicrobial metals sample essay was marked down partly because its methodology lacked replication detail. That same essay also applied multiple t-tests to the same dataset, which the examiner flagged as raising the Type I error rate, a statistical trap that reads as rigorous but actually undermines the analysis.
Scope problems cut both ways. A research question that is too broad produces a descriptive survey with no analytical depth. One that is too narrow may not generate enough material to sustain 4,000 words. Neither version meets the IB's requirement for a clear, focused research question.
Inconsistent research question wording is a smaller but costly error. In the same sample essay, the conclusion quotes a slightly different version of the research question from the one stated at the start. A mismatch between introduction and conclusion weakens both the research question strand of Criterion A and the line of argument under Criterion C, not just the presentation.
Poor source integration (Criterion B) affects literature-based essays in particular. Citing a source without explaining its relevance, or failing to situate your argument within the underlying biology, leaves the Knowledge and understanding strands unsatisfied. The guide is explicit: avoid over-extensive quoting, because your knowledge needs to be evident in your own writing.
7. What to Do Next
The most common early mistake is leaving reflection until the final term. The IB requires three mandatory reflection sessions with your supervisor, and your attendance at each is recorded on the Reflection and progress form (RPF).
This week, email your supervisor and book your first reflection session. Bring a possible topic, some initial reading and an idea of the research methods you might use. Write up what was discussed the same day: those notes, together with your record of the viva voce, are what you will draw on when you write the 500-word reflective statement at the end.
Also check your session. If you sit your exams in May 2027 or later, you are on the 30-mark model described here; only November 2026 candidates remain on the old scheme. Grade boundaries for the 30-mark scale have not been published yet and will only be set after the first session, so ignore any site that claims to know them.
FAQ
Is the biology extended essay hard?
The biology EE is demanding primarily because it requires sustained independent research and genuine analysis rather than description - the research question and methodology decisions made early on determine how manageable the rest of the essay is.
How do you write a biology extended essay?
Start by choosing a specific, answerable research question using a biology command term, decide on an experimental or literature-based approach, draft a replicable methodology, then structure the essay so every section addresses the research question rather than surveying the topic generally.
How is a biology extended essay marked?
From May 2027 it is marked on the five criteria every subject shares - Framework for the essay (6), Knowledge and understanding (6), Analysis and line of argument (6), Discussion and evaluation (8) and Reflection (4) - a total of 30 marks that converts to a grade from A to E.
What are the best topics for a biology extended essay?
The strongest biology EE topics are those with a clear, measurable variable or a well-defined evaluative question - areas like antimicrobial resistance, enzyme kinetics, neurological disease therapeutics, or population ecology tend to generate focused, analysable research questions.
Can a biology extended essay use secondary data?
Yes - a biology EE based on published studies or databases is fully accepted, provided the essay makes its selection criteria for sources explicit and the analysis of the data is your own; the IB's own sample essays include a secondary-data essay on Alzheimer's disease therapeutics.
What is the word limit for a biology extended essay?
The biology extended essay has a 4,000-word limit; examiners are instructed not to read or assess anything beyond 4,000 words, although tables, graphs, equations and calculations do not count towards the total.
References
- Extended essay - International Baccalaureate® - https://ibo.org/programmes/diploma-programme/curriculum/dp-core/extended-essay
- DP extended essay updates - International Baccalaureate® - https://ibo.org/university-admission/latest-curriculum-updates/dp-extended-essay-updates