The course and the exams

🔄What changed, in four sentences

The options are gone — there is no longer a Neurobiology, Ecology, Biotechnology or Human Physiology option, and no Paper 3 built around one. The old core and AHL topics have been reorganised into a grid of four themes and four levels of organization, with forty topics between them. Practical work is no longer separately assessed through a group 4 project and a practical portfolio: there is one internal assessment, the scientific investigation, plus a collaborative sciences project that is required but not marked. And the papers were rebuilt: Paper 1 is now split into 1A multiple choice and 1B data-based, sat together.

What did not change is most of the biology. Photosynthesis is photosynthesis. If you use an older resource for material that still exists, the science is usually fine; what is unreliable is its account of what is examinable, at which level, and in what format.

📚Themes and levels of organization

Every topic sits at the intersection of a theme and a level of biological organization. The letter is the theme; the first digit is the level; the second digit numbers the topic.

Those hours are a map of where the effort goes. Theme D is the largest at both levels, and at HL it is a third of all the syllabus content. If you are HL and behind, that is where the marks are.

The grid is not meant to be taught in order. Your school may start with any topic, and many courses follow a theme across levels, or a level across themes. Each topic is headed by guiding questions, and the guide also suggests linking questions that connect topics in different parts of the grid. Examiners use exactly these connections: a Paper 2 question can begin with enzymes and end with ecosystems.

Woven through everything is the nature of science (NOS): observation, patterns, hypotheses, experiments, models, evidence, theories, falsification, and the ethics and global impact of science. It is not a separate section of the exam. It turns up inside ordinary questions — “explain why this correlation does not prove causation”, “suggest why this hypothesis is difficult to test”.

🎓SL or HL

SL is 150 teaching hours and HL 240. Of those, syllabus content takes 110 hours at SL and 180 at HL; the rest is the experimental programme — practical work (20 h SL, 40 h HL), the collaborative sciences project (10 h) and the scientific investigation (10 h).

The extra HL content is not a separate syllabus. In most topics it is additional understandings folded into the same topic, labelled Additional higher level in the guide and flagged AHL on these pages. Six topics are AHL in their entirety and do not exist at SL:

Seven topics have no AHL content at all, so SL and HL study exactly the same material: A4.2, B1.1, B4.1, B4.2, C3.2, C4.1 and C4.2. In every topic, all of the SL content is also examined at HL.

If you are choosing: take HL biology if you need it for medicine, veterinary science, biomedical science or biology at university. Take it because a course needs it, not because it sounds impressive — a 6 at SL is worth more than a 4 at HL to almost everyone.

📝How you are assessed

External assessment is 80% of your grade, in two papers. Calculators are permitted in both. Unlike physics and chemistry, biology has no data booklet: every formula you need (magnification, Lincoln index, chi-squared, Simpson’s reciprocal index, Hardy–Weinberg) must be known, or will be given in the question.

Internal assessment is 20%: one scientific investigation, about 10 hours of class time, a report of at most 3,000 words, marked out of 24 by your teacher and moderated by the IB. The requirements are identical at SL and HL.

Two things about the weighting that students consistently misread. First, Paper 1B is a substantial part of Paper 1 and is entirely about handling data and experimental method — you cannot revise for it by memorizing content. Second, both papers test AO3 — analyse, evaluate, synthesize — heavily, alongside knowledge and understanding. In Section B, an extended response that lists facts without organizing them into an argument scores poorly.

🔧Tools and inquiry run through everything

Alongside the content, the guide defines three tools — experimental techniques, technology and mathematics — and a three-stage inquiry process: exploring and designing, collecting and processing data, concluding and evaluating. There is no separate chapter on them. They are assessed in Paper 1B, in Paper 2 Section A, and in the IA.

The most examinable parts, and the ones students are least ready for:

• Calculations you must be able to do: magnification and actual size from a scale bar; percentage change and percentage difference; mean, median and mode; rates of change from graphs and tables; the Lincoln index; Simpson’s reciprocal index; allele frequencies (HL, Hardy–Weinberg); and scientific notation.
• Statistics: range, standard deviation, standard error and interquartile range, used to judge variation and reliability; the chi-squared test and the t-test; the correlation coefficient \( r \) and the coefficient of determination \( R^{2} \). You apply and interpret them — you do not need to memorize the standard deviation formula.
• Uncertainties: record them as a range (±) to an appropriate precision; show standard deviation or standard error as error bars; and discuss whether overlapping error bars mean a difference is real.
• Graphs: sketches with labelled but unscaled axes; plotted graphs with scales, lines or curves of best fit and error bars; box-and-whisker plots; logarithmic scales for population growth.
• Techniques you must have met: chromatography, colorimetry, serial dilutions, molecular modelling, light microscopy with an eyepiece graticule, temporary mounts, identifying organisms and dichotomous keys, random and systematic sampling, karyotyping, and cladogram analysis.
• Correlation and causation: the single idea most often tested through NOS. A correlation, however strong, does not establish cause.

📋Command terms — read the verb

Every question is built on a command term with a defined meaning, and answering the wrong one is the cheapest way to lose marks. The ones that most often go wrong in biology:

• State and List — a name or brief answer, no explanation. Define — the precise meaning of a word or phrase.
• Outline — a brief account. Describe — a detailed account of what. Explain — a detailed account including reasons or causes. If your answer to “explain” contains no “because”, it is a description.
• Distinguish — make clear the differences. Compare — similarities, referring to both throughout. Compare and contrast — similarities and differences, referring to both throughout. Two separate paragraphs, one on each thing, do not compare them.
• Annotate — add brief notes to a diagram. Labels alone are not annotations; biology drawing questions routinely require functions.
• Deduce — reach a conclusion from the information given. Suggest — propose a possible answer; more than one may be creditworthy, so an answer outside the syllabus can still score.
• Evaluate — weigh strengths and limitations and reach a judgement. Discuss — a balanced review of a range of arguments, factors or hypotheses, with a conclusion. Both need a conclusion to reach the top marks.
• Calculate and Determine — working is credited. An unsupported wrong final answer scores zero where the working would have scored most of the marks.

🧭How to use this site

The rail on the left lists every topic, grouped by theme, and the prev/next links at the foot of each page walk through the whole course in syllabus order. If you are revising for a test on one topic, open that page and work down it; if you are revising for a mock, use the hub and take a theme at a time.

Each topic page ends with six practice questions whose answers state the reasoning, not just the result. Attempt each one on paper before revealing the answer — reading a worked solution feels like learning and mostly is not.