The current IB Biology syllabus, first assessed in 2025, organizes biology through four conceptual themes studied at the molecular, cellular, organism and ecosystem levels. SL and HL follow the same framework and assessment weightings, but HL contains more content, greater conceptual depth and longer examinations.
This reference breakdown explains the syllabus structure, SL versus HL differences, teaching-hour allocations, examination format and the most effective way to organize revision.
IB Biology SL vs HL comparison
The IB recommends 150 teaching hours for SL and 240 teaching hours for HL. HL students study the SL foundations plus additional higher level content, commonly described as AHL content, including some entirely HL-only topics and extensions within shared topics.
Feature
Biology SL
Biology HL
Recommended teaching time
150 hours
240 hours
Syllabus content
110 hours
180 hours
Experimental programme
40 hours
60 hours
Paper 1
1 hour 30 minutes, 55 marks
2 hours, 75 marks
Paper 2
1 hour 30 minutes, 50 marks
2 hours 30 minutes, 80 marks
External assessment
80%
80%
Scientific investigation
20%
20%
HL is therefore not a separate course with an unrelated syllabus. It uses the same biological framework but requires broader knowledge, more detailed mechanisms and stronger connections between topics. The IB Biology SL versus HL revision comparison provides a further breakdown of these practical differences.
How the IB Biology syllabus is organized
The syllabus uses a roadmap formed by four themes and four levels of biological organization. Each topic code identifies its position within this structure. For example, C1.2 means Theme C, level 1 and topic 2.
Theme
Central idea
Representative content
A: Unity and diversity
Shared features and biological variation
Water, nucleic acids, cells, classification, evolution and biodiversity
B: Form and function
Relationships between biological structures and their functions
Biomolecules, membranes, specialization, gas exchange and transport
C: Interaction and interdependence
Interactions within and between biological systems
Enzymes, respiration, photosynthesis, signalling, immunity and ecology
D: Continuity and change
Inheritance, regulation and biological change
DNA, protein synthesis, reproduction, inheritance, homeostasis and selection
The four organizational levels are molecules, cells, organisms and ecosystems. This arrangement encourages students to connect ideas across scales rather than treat every chapter as an isolated body of facts. The IB's official Biology curriculum page displays the syllabus roadmap and explains how the themes and organizational levels intersect.
The IB provides recommended teaching hours for each theme, but these are not fixed examination weightings. A theme receiving more teaching time may represent more content, yet students should not assume that the same percentage will appear on every examination.
Syllabus area
SL hours
HL hours
A: Unity and diversity
19
33
B: Form and function
26
39
C: Interaction and interdependence
31
48
D: Continuity and change
34
60
Total syllabus content
110
180
Experimental programme
40
60
Total course
150
240
Theme D has the largest recommended allocation, especially at HL, but selective revision remains risky. Paper 1B includes syllabus-related data questions addressing the themes, while Paper 2 can require connections between several areas. Use the hours to estimate content volume, not to predict an exact mark distribution.
How assessment works
Both levels have the same component weightings:
Component
Format
Weighting
Paper 1A
Multiple-choice questions
Part of Paper 1
Paper 1B
Syllabus-related data-based questions
Part of Paper 1
Paper 1 total
Paper 1A and 1B completed in one examination period
36%
Paper 2
Data-based, short-answer and extended-response questions
44%
Scientific investigation
Individual written investigation
20%
The examinations account for 80% of the final result. HL papers are longer and assess both shared and AHL material, but HL students do not take a separate Paper 3. The former option topics have also been removed as separate units, although some of their content has been incorporated elsewhere in the course.
The experimental programme and scientific investigation
Practical work is part of the syllabus rather than an optional addition. The recommended experimental programme totals 40 hours at SL and 60 hours at HL, including practical work, the collaborative sciences project and 10 hours for the scientific investigation.
The scientific investigation is an open-ended task in which each student gathers and analyses data to answer a self-formulated research question. It produces an individual report of no more than 3,000 words, is marked by the teacher and externally moderated by the IB. It is worth 20% at both levels and is marked out of 24 using research design, data analysis, conclusion and evaluation criteria.
The RevisionDojo Biology IA guide explains these criteria and how to structure evidence without turning the report into a descriptive laboratory narrative.
How to revise from the syllabus
Treat the syllabus as a checklist of assessable understandings, not merely a list of chapter names. For each topic:
Learn the mechanism and required terminology from syllabus-aligned notes.
Explain how the topic connects to another level of organization.
Complete multiple-choice, data-based and written questions.
Compare the answer with the mark scheme and record the precise error.
Reattempt the weak area after a delay.
HL students should label AHL material clearly, but they should not revise it in isolation from the SL foundations supporting it. Jojo AI can help clarify unfamiliar mechanisms and provide feedback on practice responses, while the topic-by-topic Biology revision guide shows how to turn the syllabus codes into a practical study sequence.
Conclusion
The current IB Biology syllabus gives SL and HL students the same conceptual framework, assessment weightings and scientific investigation format. HL adds substantially more content and depth, supported by 90 additional recommended teaching hours and longer examinations. RevisionDojo Study Notes, the Biology Questionbank and Jojo AI are most useful when combined in a cycle of learning, targeted practice, feedback and retesting.
Sarah holds a PhD in Cell Biology and taught IB Biology across Europe and Asia for 18 years, latterly as a science department lead. Outside of the papers, her focus lies with the Biology EE, especially with its new format, closing the gap between understanding and application.
