IB DP Biology Theme A: Unity and Diversity – Revision Notes

These notes go deeper into Theme A – Unity and diversity (19 hours SL / 33 hours HL) than the full syllabus guide, which lists every theme at a glance. Use this alongside the subject overview and the assessment revision notes, which cover aims and exam structure rather than theme content. Theme A asks what all living things have in common, and where their diversity comes from – the two questions every sub-topic below answers in a different way.

Water

Water’s properties – polarity, hydrogen bonding and cohesion – are why no known life functions without it. Revise this sub-topic by being able to explain each property in terms of the water molecule’s structure, not just name the property: polarity arises because oxygen pulls shared electrons more strongly than hydrogen, giving the molecule a partial negative charge near oxygen and partial positive charges near the hydrogens. This uneven charge distribution is what allows hydrogen bonds to form between neighbouring water molecules, which in turn explains cohesion (water molecules sticking to each other) and several of water’s other biologically important properties, such as its high specific heat capacity and its behaviour as a solvent for polar and ionic substances.

Nucleic acids

DNA and RNA as information carriers. The exam-relevant core is structural: both are polymers of nucleotides, each nucleotide built from a phosphate group, a pentose sugar and a nitrogenous base. DNA’s double-helix structure, with complementary base pairing (adenine-thymine, cytosine-guanine) held together by hydrogen bonds, is what allows it to be copied accurately and to store genetic information stably. Be ready to connect this sub-topic forward to Theme D (DNA replication, protein synthesis) – Theme A introduces the structure, Theme D revisits it for the mechanism of copying and expression.

Origins of cells (HL only)

Hypotheses for how the first living cells could have arisen from non-living chemistry. This is HL-only and conceptually different from the rest of Theme A in that it deals with competing scientific hypotheses rather than settled mechanisms – revision should focus on being able to outline more than one hypothesis and state what evidence each draws on, rather than presenting a single “correct” account of the origin of cells.

Cell structure

The shared and distinguishing features of cells, and specifically what separates prokaryotic from eukaryotic organisation. A reliable way to revise this is a direct comparison table: presence or absence of a membrane-bound nucleus, membrane-bound organelles, chromosome structure (circular vs linear), and typical cell size. Prokaryotic and eukaryotic organisation is one of the most frequently tested unity-vs-diversity contrasts in Theme A, precisely because it shows both a shared feature (all cells have a plasma membrane, cytoplasm and genetic material) and a fundamental point of divergence.

Viruses (HL only)

The structure and diversity of viruses as non-living infectious particles that depend entirely on host cells to replicate. Because viruses are explicitly non-living in this syllabus’s own framing, a common exam trap is describing them with cell-based vocabulary (metabolism, growth) that does not actually apply – revise viruses on their own terms: a protein coat (capsid) enclosing genetic material, with no independent metabolism, entirely reliant on host-cell machinery to reproduce.

Diversity of organisms

Variation and how species are recognised and named (A3.1) – not the building of classification schemes, which is the HL-only sub-topic below. Revise: variation between organisms as a defining feature of life and the basis for naming and classifying organisms; species as groups of organisms with shared traits; the binomial system (genus then species, e.g. Homo sapiens); the biological species concept (organisms that can interbreed to produce fertile offspring); why populations that do not interbreed can be hard to distinguish from species as they diverge; diversity in chromosome numbers of plant and animal species; karyotyping and karyograms; and genome diversity.

Classification and cladistics (HL only)

Using shared derived characteristics, rather than surface similarity, to build evolutionary trees. The key exam skill here is reading and constructing a simple cladogram – being able to identify which two groups share the most recent common ancestor from a branching diagram, and explaining why a cladistic classification (based on shared derived characteristics) can group organisms differently from a classification based on superficial physical similarity.

Evolution and speciation

The mechanisms – variation, selection, isolation – driving change in species over time and the formation of new species. Structure your revision around the sequence: variation exists within a population, natural selection acts on that variation because some variants survive and reproduce more successfully, and reproductive isolation (geographic or otherwise) allows diverging populations to become genetically distinct enough to form new species. This sub-topic connects directly back to Water and Nucleic acids (the molecular basis of variation) and forward to Theme D’s natural selection content, so revising it in isolation from those connections leaves an incomplete picture.

Conservation of biodiversity

Why biodiversity matters ecologically and economically, and how it is threatened and protected. Be able to give named categories of threat (habitat loss, invasive species, overexploitation, climate change, pollution) and named categories of protection response (protected areas, legal frameworks, captive breeding, restoration), rather than a general statement that biodiversity is “important.”

How this theme is tested

Because Paper 1 and Paper 2 draw on the whole syllabus rather than one theme at a time, Theme A content rarely appears in isolation – a question on conservation of biodiversity might expect you to draw on classification (to explain why losing one species matters to overall diversity) or on evolution (to explain how a shrinking population affects genetic variation). Revise Theme A sub-topics with an eye to how they connect to each other, not just to their own definitions.

Self-test

1. Explain, in terms of molecular structure, why water is described as a polar molecule.
2. What are the three components common to a nucleotide?
3. Name two features that distinguish a eukaryotic cell from a prokaryotic cell.
4. Why are viruses described as non-living, and what do they depend on host cells for?
5. (HL) What does a cladogram show that a classification based on surface similarity might miss?
6. Outline the three-step logic connecting variation, selection and isolation to speciation.
7. Name two named threats to biodiversity and two named conservation responses.

Answers: 1. The oxygen atom pulls the shared electrons more strongly than the hydrogen atoms, giving the molecule an uneven (partial negative near oxygen, partial positive near the hydrogens) charge distribution. 2. A phosphate group, a pentose sugar and a nitrogenous base. 3. Any two of: presence of a membrane-bound nucleus, presence of membrane-bound organelles, circular vs linear chromosome structure, typical cell size. 4. They have no independent metabolism and cannot reproduce without hijacking a host cell’s machinery. 5. It shows evolutionary relationships based on shared derived characteristics (most recent common ancestor), which can group organisms differently from a classification based only on how similar they look. 6. Variation exists within a population; natural selection favours variants better suited to the environment; reproductive isolation lets diverging populations become genetically distinct enough to become separate species. 7. Any two threats (habitat loss, invasive species, overexploitation, climate change, pollution) and any two responses (protected areas, legal frameworks, captive breeding, restoration).

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Biology, first assessment 2025, published January 2022 – the same source already cited by the full syllabus guide, which first reproduced this theme’s sub-topic names and HL-only markers from it.