How IB Biology Is Marked: Papers 1A, 1B, 2 and the IA

IB Diploma Biology has a new course, first assessed in 2025. Its content is organised into four themes, and its papers lean heavily on data and experimental work. If you are revising from older past papers or notes written for the previous course, it is worth being clear about what the current exam actually looks like — and how its markschemes decide which answers earn credit.

The details below come from the IB’s own subject brief and specimen papers for the course first assessed in 2025. Your teacher and the subject guide are the final word for your session.

The course: four themes, SL and HL

The syllabus is organised into four themes: unity and diversity, form and function, interaction and interdependence, and continuity and change. Each theme is studied at several levels of biological organisation, from molecules up to ecosystems, so be ready for questions that draw on more than one part of the course.

HL students study the same themes as SL in greater depth, plus additional topics that are taught only at HL. The IB recommends 150 teaching hours for SL subjects and 240 for HL, and the difference shows in the papers: they are longer, and worth more marks, at HL.

How the assessment is weighted

The weighting is the same at SL and HL. External exams make up 80% of the final grade and the internal assessment the other 20%:

• Paper 1 — 36%. SL 1 hour 30 minutes; HL 2 hours.
• Paper 2 — 44%. SL 1 hour 30 minutes; HL 2 hours 30 minutes.
• Internal assessment (the scientific investigation) — 20%.

Paper 1: 1A and 1B

Paper 1 is sat as one exam in two parts.

Paper 1A is multiple choice, worth 30 marks at SL and 40 at HL in the specimen papers. It rewards breadth: a question can come from anywhere in the syllabus, and the wrong options are written to catch half-learned ideas. The fix is the same as for any multiple-choice paper — read every option, and test each one as a statement rather than picking the first that sounds familiar.

Paper 1B is made up of four data-based questions that are related to the syllabus and address all of the themes, worth 25 marks at SL and 35 at HL in the specimen papers. Expect unfamiliar experiments, tables and graphs, and questions that ask you to read values, calculate, identify variables, and evaluate a method or a conclusion. Together, 1A and 1B are worth 55 marks at SL and 75 at HL. A calculator is required.

Paper 2: short answers, then extended responses

Paper 2 is in two sections. Section A contains data-based and short-answer questions, and every question must be answered. Section B contains extended-response questions with a choice: in the specimen papers, HL students answer two questions from three and SL students answer one from two. The whole paper is worth 80 marks at HL and 50 at SL.

Section B is where planning pays. Each extended-response question is split into parts — in the specimen papers, one part worth 7 marks alongside smaller 4-mark parts — and the IB awards one additional mark for the construction of your answer. That mark is for a clear, logically organised response — not for length. Spend a minute deciding which question you can answer best across all of its parts, not just the first one.

The internal assessment

The internal assessment is a scientific investigation: an open-ended task in which you gather and analyse data to answer a research question you have formulated yourself. It is assessed through a written report, with a maximum of 3,000 words, and the IB allocates 10 hours to it. At 20% of the final grade, it is the single piece of work you have most control over, so it is worth getting the research question focused early — a narrow, measurable question makes every later part of the report easier to do well.

The course’s experimental programme also includes the collaborative sciences project, in which students work together across science subjects on a shared problem.

How IB markschemes work

IB biology markschemes follow a set of conventions, printed at the front of every specimen markscheme. Knowing them changes how you write:

• Each marking point ends with a semicolon. Short-answer questions are marked as a list of separate points, very like the indicative content on GCSE schemes.
• There are often more points than marks. A question marked “max” will accept any of a longer list, up to the total — so for a four-mark answer, make four clearly distinct points rather than one point four ways.
• Underlined words are essential. If a word is underlined in the markscheme, the mark is not awarded without it. This is where precise vocabulary counts.
• Equivalent wording is accepted. Markschemes mark answers with the same meaning, often noted as OWTTE (“or words to that effect”), and examiners are reminded that many candidates write in a second language, so effective communication matters more than grammar.
• Error carried forward applies. If you use an earlier wrong answer correctly in a later step, the later marks can still be awarded — the same principle as method marks at GCSE.
• Units and significant figures are not penalised unless the markscheme notes say so — but where a note says “units required”, a missing unit costs the mark. Since you cannot see the notes in the exam, always give the unit.

Where marks are lost

Most IB biology marks that slip away do so for the same few reasons.

• Describing data instead of using it. Data-based questions ask for trends, comparisons and calculations. “It goes up” rarely earns a mark; “the rate increases from 2 to 8 units between 20 and 40 °C, then falls” often does. Quote values from the data, with units.
• Missing the command term. IB uses its own command terms, and “outline”, “explain”, “compare and contrast” and “evaluate” ask for different things. A “compare and contrast” answer needs both similarities and differences. The same thinking applies as with GCSE command words.
• Vague vocabulary. With essential words underlined in markschemes, “the enzyme stops working” and “the active site changes shape so the substrate no longer fits” are not equivalent. Learn the exact terms.
• Unstructured extended responses. A Section B answer that lists everything you know about the topic, in no order, risks the construction mark and makes it harder for the examiner to find the marking points. Plan, then write in a sequence.
• Weak evaluation. Paper 1B and Section A both ask you to judge experiments. Name the specific limitation (sample size, an uncontrolled variable, the range of the independent variable) and its effect on the conclusion, rather than writing “more repeats” on its own.

How to practise

The habits above are learned by writing answers and finding out exactly which marking points they earned. On Exaim, every written answer is marked against the markscheme point by point, so you can see that you had the idea but missed the underlined word, or described the graph without quoting a value — and fix it on the next attempt.

Start with the free revision notes, flashcards, mind maps and worksheets for IB Biology HL or IB Biology SL, organised by topic and subtopic, or browse every course from the revision resources page.