Hi Diploma students
These are comprehensive revision notes for IB Diploma Programme Biology, free and open to anyone. They cover the syllabus first taught in 2023 and first examined in 2025 — all forty topics across the four themes, standard level and higher level, plus the internal assessment and the core.
The 2023 course is not the old one reshuffled. The six core topics and the options have gone; the content is now a grid of four themes crossed with four levels of biological organization, and a good deal of material is new. If you have inherited notes or past papers from an older student, read Start here before you trust them.
Please take note that although succinct notes are essential towards your revision, they should not be the only materials you cover — working through problems from your textbook and past papers is just as important, and in biology so are practical work and fieldwork.
Happy revising. — Mr. Suta
🧭Start here
How the themes and levels of organization fit together, what separates SL from HL, how Papers 1A, 1B and 2 are built, the skills that are examined inside every paper, and the command terms. Ten minutes here will save you a term of guessing.
📚The syllabus
Four themes, each studied at four levels of organization — molecules, cells, organisms and ecosystems — giving forty topics. A topic’s code tells you both: C1.2 is Theme C at level 1 (molecules). Material marked AHL on a page is additional higher level, for HL candidates only; six topics are AHL in their entirety.
Theme A — Unity and diversity
Why all life shares so much, and how it came to be so varied: water and nucleic acids, the first cells, cell structure, viruses, species and classification, evolution, and the conservation of what evolution has produced. SL 19 hours, HL 33.
Theme B — Form and function
How structure fits purpose at every scale: carbohydrates, lipids and proteins; membranes and organelles; specialized cells; gas exchange, transport and movement; and how organisms are adapted to their habitats and niches. SL 26 hours, HL 39.
Theme C — Interaction and interdependence
Systems made of interacting parts: enzymes and metabolic pathways, respiration and photosynthesis, chemical and neural signalling, the coordination of body systems, defence against disease, and the populations, communities and energy flows of ecosystems. SL 31 hours, HL 48.
Theme D — Continuity and change
How life copies itself and how it changes: DNA replication, protein synthesis and mutation; cell division and gene expression; reproduction, inheritance and homeostasis; and natural selection, ecosystem stability and climate change. The largest theme by a distance, and at HL a quarter of the course: SL 34 hours, HL 60.
Assessment and the core
The scientific investigation is 20% of your grade and is marked on four criteria worth six marks each — so research design is worth as much as your conclusion, and most lost marks are lost before any data is collected. The Extended Essay and the Collaborative Sciences Project sit outside the syllabus but inside your two years.
🧩How to use these pages
Every topic page has the same shape: what you need to be able to do, then the biology in the order of the syllabus understandings, a worked example, six practice questions with full answers, and a clearly separated list of other people’s resources.
Each worked example ends by naming the specific mistake that costs marks on that topic — writing that enzymes are “killed”, taking \( q^{2} \) for \( q \) in Hardy–Weinberg, saying the sarcomere’s filaments shorten. Those paragraphs are the most useful part of the page and the easiest to skip.
The course is designed around connections, and exam questions routinely begin in one topic and finish in another. Where topics link, the link is in the text. Follow them.
🎓Official IB resources
These are the documents that actually govern the course. Your school has access to all of them through the Programme Resource Centre, so ask your teacher — or me — if you cannot reach one.
- The Biology guide (first assessment 2025) — the definitive statement of what can be asked. Each understanding is numbered, and anything not there is not examinable.
- The syllabus roadmap — the one-page grid of themes and levels of organization; worth pinning above your desk.
- Specimen papers and markschemes — written for this syllabus, unlike most past papers you will find online.
- Subject reports — examiners describing, in plain language, exactly what candidates got wrong last session. Almost nobody reads them.
📜A note on past papers
Past papers are the most valuable revision in this subject, and your school can give you properly licensed ones. Two cautions when you use them.
First, anything from 2024 or earlier is the old syllabus. Much of the biology is still good practice — a Punnett grid does not change — but the options and the old Paper 3 are gone, some topics have moved between SL and HL, and a good deal of current material (viruses, cladistics at HL, climate change, gene expression) was examined differently or not at all. Use old papers for skills, current specimens for format.
Second, mark your own work against the markscheme rather than against a model answer. IB markschemes award specific points, and you cannot see which points you missed by comparing paragraphs of prose.