The Most-Tested Topics in IB Biology HL
Every IB Biology HL past-paper question, tagged to its syllabus topic — ranked by how often each is tested (2016–2025).
All topics (HL) ()
41 papers analysed · top 15 of 33 topics
| # | Topic | % of all HL questions | Questions | Avg marks |
|---|---|---|---|---|
| 1 | Circulatory system; heart; blood vessels; blood; plant transport (xylem/phloem) | 10.5% | 91 | 3.8 |
| 2 | Cell organelles; prokaryotic vs eukaryotic; microscopy | 9.6% | 83 | 2.3 |
| 3 | DNA and RNA structure; replication; transcription; translation | 9.1% | 79 | 2.3 |
| 4 | Ecosystems; food chains/webs; trophic levels; energy flow; nutrient cycling | 9.1% | 79 | 4 |
| 5 | Hormones; homeostasis; feedback mechanisms; blood glucose regulation | 8.9% | 77 | 2.5 |
| 6 | Natural selection; adaptation; speciation; evidence for evolution | 8.3% | 72 | 3.8 |
| 7 | Mitosis; meiosis; cell cycle; chromosome behaviour | 8.3% | 72 | 2.9 |
| 8 | Amino acids; peptide bonds; protein structure (1-4); enzymes | 7.1% | 62 | 2.5 |
| 9 | Population ecology; succession; climate change impacts | 6.9% | 60 | 5.2 |
| 10 | Transcription; translation; gene expression; epigenetics (HL) | 6.7% | 58 | 2.5 |
| 11 | Alleles; genotype; phenotype; dominance; codominance; sex-linkage | 6.3% | 55 | 2.5 |
| 12 | Gene mutations; chromosomal mutations; CRISPR (HL); GMOs | 6.1% | 53 | 3.7 |
| 13 | Neurons; action potential; synaptic transmission; reflex arc | 6% | 52 | 4.1 |
| 14 | Fluid mosaic model; osmosis; diffusion; active transport; endocytosis | 5.9% | 51 | 2.5 |
| 15 | Enzymes; active site; substrate specificity; factors affecting rate; inhibition | 5.3% | 46 | 3.3 |
How this was measured
Based on OwlsPrep's database of IB Biology HL past papers (2016–2025), 869 questions each tagged to its syllabus topic. “% of all HL questions” = the share of all tagged IB Biology HL questions that test the topic. Because one question can test several topics, shares do not sum to 100%. HL papers include both core and AHL (HL-only) content. Figures are aggregate statistics from question metadata — no exam questions or mark schemes are reproduced.
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