The Complete IB Biology Guide: What to Know Before Choosing

Whether you're heading towards medicine, environmental science or a microbiology career, IB Biology can be where that path starts taking shape. But doing well in it begins with knowing exactly what the syllabus includes.

‍

IB Biology is a Group 4 science offered at SL and HL. Its current syllabus, first examined in 2025, groups content into four themes, with two exam papers and a scientific investigation making up your final grade.

‍

In this guide, we'll discuss what each syllabus theme covers, how SL and HL differ, how assessment works and where the subject leads.

‍

Let's start by defining the course.

Key takeaways

• IB Biology is a Group 4 experimental science in the Diploma Programme, studied over two years at SL or HL. SL has 150 recommended teaching hours, while HL has 240.
• The current syllabus, first examined in 2025, spreads 40 topics across four themes. These are Unity and diversity, Form and function, Interaction and interdependence, and Continuity and change.
• Both levels share 34 topics, and HL adds six more, such as Viruses and Gene expression. HL also goes deeper into shared topics and doubles practical work to 40 hours. Its exams run four and a half hours, against three at SL.
• Two exam papers make up 80% of the grade, with Paper 1 worth 36% and Paper 2 worth 44%. The remaining 20% comes from the IA, a scientific investigation of up to 3,000 words that's identical at SL and HL.
• In May 2026, the mean grade for IB Biology was 4.5 at HL and 4.3 at SL. It isn't a direct equivalent of AP or A-level Biology.
• HL Biology suits medicine, biomedical science and veterinary medicine, where many courses ask for it, often alongside HL Chemistry. Topics like conservation and gene expression also lead towards environmental science, biotechnology and genetics.

What is IB Biology?

IB Biology is a Group 4 experimental science in the Diploma Programme, taken over two years at SL or HL. SL has 150 recommended teaching hours, while HL has 240.

‍

The syllabus organises content around four themes and four levels of organisation: molecules, cells, organisms and ecosystems. Each topic code combines both. For example, C2.2 Neural signalling sits in Theme C, Interaction and interdependence, at the cell level.

‍

Those hours also include dedicated lab time. SL students spend 20 hours on practical work, HL students 40. Everyone completes a 10-hour collaborative sciences project, an interdisciplinary task on a real-world problem. 

‍

This hands-on work can also lead to a biology extended essay, which our IB Extended Essay Guide helps you plan.

IB Biology HL vs SL: what's the difference?

HL keeps SL's four themes but adds six HL-only topics, deeper content in shared topics and 90 more teaching hours.

‍

Here are the key differences between the two levels at a glance.

The IB Biology syllabus, topic by topic

The IB Biology syllabus spreads 40 topics across four themes, from Unity and diversity to Continuity and change.

‍

Here are the topics everyone studies, followed by the six extra HL topics.

Core topics for SL and HL

Both levels share 34 topics, covering everything from water and cell structure to inheritance and climate change.

‍

Here's what each theme covers.

• Unity and diversity: What do a bacterium, an oak tree and you have in common? The answer starts with water, nucleic acids and cell structure, before the theme moves on to evolution and conservation.
• Form and function: In biology, the way something is built shapes what it can do. You'll see this in proteins and membranes, specialised cells, gas exchange surfaces and organisms adapted to their environments.
• Interaction and interdependence: Vaccines, antibiotic resistance and COVID-19 data all feature here. Behind them sits the story of how living systems depend on each other, from enzymes to ecosystems. Along the way, you'll cover photosynthesis, nerve signals and how body systems coordinate.
• Continuity and change: Life has to stay stable and keep changing at the same time. This theme covers both sides, from DNA replication, inheritance and homeostasis to mutation, natural selection and climate change.

IB Biology HL-only topics

Beyond those 34 shared topics, HL students take on six more, plus extra depth across many existing ones. The six new topics don't form a fifth theme, since each slots into one of the four you've just seen.

‍

Here are the six HL-only topics and what each one adds.

• Unity and diversity: HL takes this theme back to the very beginning of life. You'll explore how the first cells formed on early Earth, then move on to viruses and how they evolve rapidly. Classification gets an upgrade too, as cladistics uses gene and protein sequences to map how species are related.
• Form and function: Every step you take relies on muscles contracting, and HL explains exactly how through the sliding filament model. You'll also see how muscles work with joints and skeletons, and how marine mammals are built for swimming.
• Interaction and interdependence: Adrenaline changes how your cells behave without ever entering them. HL shows how this works, as signalling chemicals bind to receptors that trigger a response within the cell. Other hormones, such as oestradiol, take a different route and act inside the cell directly.
• Continuity and change: Identical twins share the same DNA, so why can they still differ? The answer lies in how genes are switched on and off, which HL explores through gene expression. You'll also see how epigenetic tags can pass from one generation to the next.

How IB Biology is assessed

Now that you know what the syllabus covers, it's worth knowing how the IB will assess you on it. SL and HL follow the same structure, with the same weighting for each component.

‍

In the new syllabus, your IB Biology grade comes from two papers and a scientific investigation, which we’ll break down below. 

External assessment: Papers 1 and 2

The first component is two written papers, which together make up 80% of your final IB Biology grade.

‍

Here's how each paper compares at SL and HL.

Internal assessment: the scientific investigation

The second component is the internal assessment (IA), a scientific investigation worth 20% of your final grade. Unlike the exams, it's exactly the same for SL and HL students.

‍

You answer your own research question in a report of up to 3,000 words. Data can come from experiments, fieldwork, databases or simulations.

‍

Teachers mark the report out of 24 across four criteria: research design, data analysis, conclusion and evaluation. Each carries six marks, so evaluation matters as much as design.

Is IB Biology hard?

IB Biology is demanding, but it's very manageable. According to the IB Diploma Programme Statistical Bulletin (May 2026), Biology's mean grade was 4.5 at HL and 4.3 at SL. Both sit slightly below the overall Diploma mean grade of 4.9.

‍

At HL, six extra topics and longer exams add weight, all on top of the extended essay, TOK and CAS. Data skills matter too, since Paper 1B and the IA both centre on analysing data. Learning the IB's command terms, like 'evaluate' and 'discuss', also sharpens every answer you write.

IB Biology vs AP Biology vs A-level Biology

None of IB, AP or A-level Biology is simply harder than the others, and they aren't direct equivalents either. Each challenges you differently, and the biggest difference is how each one assesses you.

‍

Here's what makes each one demanding.

• IB Biology: The challenge is consistency, as assessment is spread across two papers and the IA over two years. HL adds six extra topics and four and a half hours of exams.
• AP Biology: Everything rides on one three-hour exam, split evenly between 60 multiple-choice and six free-response questions. Many questions centre on interpreting graphs and experimental data.
• A-level Biology: In England, the pressure lands at the end. Your grade comes entirely from exams after two years, usually three written papers. Practical skills are assessed separately through the practical endorsement, which doesn't count towards your grade.

Where IB Biology can take you

IB Biology is a natural fit for degrees like medicine, biomedical science and veterinary medicine. Many of these courses ask for HL Biology, often alongside HL Chemistry.

‍

Individual topics point towards other paths too, beyond the classic medical route. Conservation and climate change lead towards environmental science, while gene expression suits biotechnology and genetics.

‍

Defence against disease leads into microbiology and public health, while populations and communities suit marine biology and zoology. Muscle and motility also links to sports science.

‍

If you want to look beyond degrees and into careers, explore our 20 Careers in Biology guide.

Explore biology with Immerse Education

The IA asks you to design your own investigation, often before you've ever framed a research question. That's a skill textbooks can't fully teach. It's best built through real research.

‍

Our Biology Summer School gives you that chance in Oxford or Cambridge. Over two weeks, participants aged 16 to 18 explore biological molecules, human physiology, genetics, evolution and ecology.

‍

Each participant also completes a personal research project with mentor support, ideal practice for your own IA. You'll learn from tutors who hold, or are working towards, a doctorate. You'll approach IB Biology with real research experience and a clearer idea of where the subject could take you.

FAQs

What is IB Biology?

IB Biology is a Group 4 experimental science in the IB Diploma Programme, taken at SL or HL. The two-year course covers four themes and includes 20 to 40 hours of practical work.

Is IB Biology HL or SL harder?

HL is the more demanding level, because it covers more content in greater depth than SL. It adds six topics, 90 teaching hours and an extra 90 minutes of exam time.

What's on the IB Biology syllabus?

The IB Biology syllabus has 40 topics across four themes. These are Unity and diversity, Form and function, Interaction and interdependence, and Continuity and change. SL covers 34 topics, while HL adds six more.

What is the IB Biology IA?

The IB Biology IA is a scientific investigation built around your own research question. It's a report of up to 3,000 words, worth 20% of your grade at both SL and HL.

What are typical IB Biology grade boundaries? 

Grade boundaries for IB Biology change every exam session, because the IB sets them after marking. In May 2026, a grade 7 at HL needed between 71% and 78% of total marks, depending on time zone. 

Is IB Biology equivalent to AP Biology?

No, IB Biology and AP Biology aren't direct equivalents. AP Biology ends in a single three-hour exam. IB Biology runs over two years and combines two exam papers with an internal assessment worth 20%.

Conclusion: making the most of IB Biology

Success in IB Biology starts with understanding how the course works, from the four themes to the IA. With that picture in place, every choice that follows becomes much easier, starting with your level.

‍

Choose HL if you're aiming for medicine or another biology-heavy degree, and SL if your focus lies in other subjects.

‍

Whichever level you take, start early on the key skills: analysing data and designing your investigations.

‍

Want a head start? Spend two weeks researching biology in Oxford or Cambridge on our Biology Summer School, with 2027 places now open.