Unity and Diversity
IB Biology 2023 Guide — Theme A: Unity and Diversity (A1–A6)
Six subtopics, one recurring exam habit: every question here is really asking either what do all living things share? or why do they differ? Water, nucleic acids and the cell are the shared toolkit — unity. Evolution, speciation and classification are the engine that builds diversity on top of that toolkit, and conservation is what's at stake if we lose the product. Learn the vocabulary, but drill the arithmetic just as hard — water potential, Simpson's index, surface area : volume, Chargaff's rule and PCR doubling all turn up as calculations, not just definitions.
Overview
Theme A sits at the base of the whole course: it establishes the molecules (water, DNA), the structure (the cell) and the process (evolution) that every other theme builds on. Diversity of organisms and conservation then close the loop — classification organises the variety evolution produces, and conservation asks how to keep it.
- Unity strand: water's properties, nucleic acid structure, and cell theory — features shared by (almost) all life.
- Diversity strand: evolution and speciation generate variation; classification and cladistics organise it; conservation protects it.
- Numeracy is examined hard here — water potential, Simpson's index, SA:V, Chargaff percentages and PCR doubling are all fair game as calculations, not just recall.
The shape of the chapter
Command terms that decide your mark here
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| State | Give a specific answer, no reasoning needed | AO1 | One correct fact scores full marks — extra explanation wastes time, not marks |
| Describe | Give a detailed account without justifying it | AO1/AO2 | Needs specific detail or data — a vague trend alone loses marks |
| Explain | Give reasons — build a causal chain | AO2/AO3 | Each link in the chain is usually a separate mark point; missing one link loses that mark |
| Compare | Give similarities AND differences, explicitly linked | AO3 | Listing features of each thing separately (no linking word) loses the comparison marks |
| Distinguish | State the difference between items | AO2 | Must contrast both items in the same sentence, not just describe one |
| Calculate | Obtain a numerical answer, working shown | AO2 | No working shown = no method marks, even if the final number is right |
| Deduce | Reach a conclusion from the data/diagram given | AO3 | Must reference the specific data given, not general knowledge |
| Evaluate | Weigh strengths and limitations, reach a judgement | AO3 | A one-sided answer (only benefits or only limitations) caps at half marks |
Key point
Overview