IB Diploma Biology SL and HL: papers, the four themes, the IA and exams

IB Biology assessment

The weightings are the same at SL and HL. These details come from the IB's Biology subject brief for first assessment in 2025.

The four themes

SL or HL?

The IB recommends 150 teaching hours for SL and 240 for HL, with 110 and 180 hours of syllabus content. HL students also study six topics that SL leaves out: origins of cells, viruses, classification and cladistics, muscle and motility, chemical signalling, and gene expression, as well as more depth across the rest.

IB Biology exam dates 2027

The IB's May 2026 schedule says the May 2027 exams will run from Friday 23 April to Wednesday 19 May 2027, with no exams on 1 May, subject to confirmation. Your IB coordinator has the subject-by-subject timetable for your zone.

IB grades and points

Each subject is graded from 1 to 7. Six subjects at up to 7 points each, plus up to 3 points from theory of knowledge and the extended essay, make a maximum of 45. The IB says the diploma needs at least 24 points and that other conditions must also be met. Grade boundaries are set for each subject after every session, so a raw mark from one year doesn't map exactly onto the next. Our IB mocks guide explains how points add up and how predicted grades work.

Worked examples

Cell structure: magnification

A cell is 30 mm long in a micrograph and 15 μm long in real life. Convert to the same unit first: 30 mm = 30,000 μm.

Inheritance: multiple alleles

Parents with blood groups A (genotype IᴬI) and B (IᴮI) can have children with genotypes IᴬIᴮ, IᴬI, IᴮI and II, so each of the blood groups AB, A, B and O has a 1 in 4 chance. Iᴬ and Iᴮ are codominant, and both are dominant to I.

Populations: a capture-mark-release estimate

40 snails are marked and released. Later, 50 are caught and 10 of them are marked. The population estimate (the Lincoln index) is 40 × 50 ÷ 10 = 200, assuming the marked snails mixed fully back into the population.

Decide whether a description is mitosis or meiosis in our Genetics & DNA course, then press Mark to check your answer in the course:

Use the daily paper as a short timed mock

Uni-Sine publishes a free daily paper for IB Biology SL and IB Biology HL. Each is a short timed mock: 8 questions worth 25 to 35 marks, with a suggested 35 to 45 minutes, marked when you finish. It is not a full IB paper, and the questions are original, not past papers.

• What it covers well: cell structure and microscopy, biological molecules, enzymes, membranes and transport, nucleic acids, DNA replication and protein synthesis, gas exchange and transport, immunity, photosynthesis and respiration, inheritance, gene expression, homeostasis, nervous coordination, muscle contraction, ecology and evolution.
• What it does not cover: the data-based questions of Paper 1B and the scientific investigation. Climate change, conservation and sustainability, origins of cells and viruses get little or no attention.

For full papers under exam conditions, use IB past papers from your teacher, and read our IB mocks guide for how mocks feed your predicted grades.

Common mistakes in IB Biology

• Mixed units in magnification. Convert image and actual size to the same unit.
• Describing data without numbers. Quote values from the graph or table in data-based answers.
• Confusing codominance and incomplete dominance. In codominance both alleles show fully.
• Forgetting the assumptions. Capture-mark-release assumes no births, deaths or migration between samples.

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