IB DP Biology Theme A: Unity and Diversity — Study Guide

This guide covers Theme A – Unity and diversity (19 hours SL / 33 hours HL) of IB Diploma Programme Biology, first assessment 2025. It complements the full syllabus guide, which lists every theme at a glance, and the Theme A revision notes, which condense this content for late-stage recall. This guide instead explains each sub-topic in full, for first study or where a fuller explanation is needed than a condensed recap can provide.

Where this theme fits

Theme A opens the DP Biology syllabus by asking two connected questions: what do all living things have in common, and where does their diversity come from? Every sub-topic below answers one or both questions in a different way – water and nucleic acids establish shared molecular foundations, cell structure and viruses examine what does and does not count as a shared living design, and classification, evolution and conservation turn to how diversity arises, is organised, and is protected. Theme A’s content underpins later themes directly: nucleic acid structure here is revisited in Theme D for the mechanism of replication and expression, and the variation this theme introduces in evolution and speciation is the same variation Theme D examines at the genetic level.

Water

Water’s biological importance follows entirely from its molecular structure. Oxygen pulls the shared electrons in a water molecule more strongly than hydrogen does, leaving a partial negative charge near the oxygen atom and partial positive charges near the two hydrogen atoms. This uneven charge distribution makes water a polar molecule, and polarity is what allows hydrogen bonds to form between neighbouring water molecules. Hydrogen bonding in turn explains cohesion (water molecules sticking to one another, which underlies phenomena such as surface tension and the rise of water through plant xylem), water’s unusually high specific heat capacity (which helps buffer organisms and environments against rapid temperature change), and its effectiveness as a solvent for polar and ionic substances – the basis for water’s role as the medium in which most biochemical reactions occur. No known form of life functions without water, which is why this sub-topic opens the theme: it is the single shared chemical foundation beneath everything that follows.

Nucleic acids

DNA and RNA are both polymers built from repeating nucleotide units, and each nucleotide is built from three parts: a phosphate group, a pentose sugar, and a nitrogenous base. DNA’s specific double-helix structure arises from complementary base pairing – adenine with thymine, cytosine with guanine – held together by hydrogen bonds between the two strands. This structure explains two of DNA’s defining biological properties: it can be copied accurately (because each strand specifies the sequence of its complementary partner), and it stores genetic information stably over long periods. Theme A introduces this structure; Theme D returns to it for the mechanism of DNA replication and protein synthesis, so a solid grasp of base pairing here pays off directly later in the course.

Origins of cells (HL only)

This sub-topic asks how the first living cells could plausibly have arisen from non-living chemistry, and it is conceptually different from the rest of Theme A because it deals with competing scientific hypotheses rather than an agreed mechanism. Students should be able to outline more than one hypothesis for the origin of cells and state what kind of evidence each draws on, rather than treating any single account as settled fact – the syllabus explicitly frames this as an open scientific question, and exam responses should reflect that framing.

Cell structure

All cells share certain basic features – a plasma membrane, cytoplasm, and genetic material – but divide into two fundamentally different organisational plans: prokaryotic and eukaryotic. Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, and typically carry a single circular chromosome; eukaryotic cells have both a membrane-bound nucleus and membrane-bound organelles, and typically carry multiple linear chromosomes. Eukaryotic cells are also typically much larger than prokaryotic cells. This prokaryotic-versus-eukaryotic contrast is one of the most frequently examined ideas in Theme A precisely because it demonstrates both unity (every cell shares the same basic components) and a fundamental point of divergence (how those components are organised) in a single comparison.

Viruses (HL only)

Viruses are explicitly non-living in this syllabus’s framing: a virus consists of genetic material enclosed in a protein coat (capsid), with no independent metabolism of its own, and depends entirely on a host cell’s machinery to replicate. Because viruses are non-living, describing them with cell-based vocabulary – growth, metabolism, reproduction in the sense a cell reproduces – is a category error the syllabus specifically wants students to avoid. Viruses are diverse in structure and in the hosts they infect, but every virus shares this same basic dependent relationship with a host cell.

Diversity of organisms

With millions of species to account for, classification systems exist to group organisms in a way that is detailed enough to be biologically useful while remaining simple enough to apply consistently. This sub-topic covers the logic behind classification as a practical and scientific challenge, rather than the details of any one classification scheme – the emphasis is on why classification is necessary and what trade-offs any scheme must balance.

Classification and cladistics (HL only)

Traditional classification often relied on surface similarity, but cladistics classifies organisms by their shared derived characteristics – features inherited from a common ancestor – rather than how alike two organisms happen to look. This is represented in a cladogram, a branching diagram in which the point where two lineages diverge shows their most recent common ancestor. The key exam skill is reading and constructing a simple cladogram: identifying which two groups share the most recent common ancestor, and explaining why a cladistic classification can group organisms differently from one based purely on physical resemblance (unrelated organisms can resemble each other through convergent evolution, while closely related organisms can look quite different).

Evolution and speciation

Evolutionary change and the formation of new species follow a three-step logical sequence: variation exists within a population (arising ultimately from mutation and genetic recombination); natural selection acts on that variation because some variants survive and reproduce more successfully than others in a given environment; and reproductive isolation (geographic or otherwise) allows diverging populations to accumulate enough genetic difference that they can no longer interbreed, producing new species. This sequence connects backward to Water and Nucleic acids (the molecular basis of the variation that selection acts on) and forward to Theme D’s treatment of natural selection at the genetic level, so this sub-topic should not be revised as an isolated definition of “evolution” but as a mechanism built from earlier and later syllabus content.

Conservation of biodiversity

Biodiversity matters both ecologically (diverse ecosystems tend to be more resilient and support more complex food webs) and economically (biodiversity underpins resources such as medicine, agriculture and ecosystem services). Threats to biodiversity include habitat loss, invasive species, overexploitation, climate change, and pollution; responses include protected areas, legal frameworks, captive breeding programmes, and habitat restoration. Students should be able to name specific categories on both sides of this balance – specific threats and specific protective responses – rather than making a general claim that biodiversity is “important,” which does not answer the kind of applied question this sub-topic is examined with.

How Theme A is examined

Because Paper 1 and Paper 2 draw on the whole syllabus rather than testing one theme in isolation, Theme A content frequently appears alongside material from other themes – a question on conservation might expect a candidate to draw on classification to explain why the loss of one species affects overall diversity, or on evolution to explain how a shrinking population’s genetic variation is affected. Preparing Theme A sub-topics with attention to how they connect to each other and to later themes, rather than as nine isolated definitions, is the more exam-effective approach the syllabus itself is structured around.

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 and the Theme A revision notes, which first reproduced this theme’s sub-topic names and HL-only markers from it.