Educerie · IB Diploma · Biology
Theme A — Unity and diversity
Theme A asks one question at four scales: what do all living things share, and how did they come to differ? A1 answers it in molecules, A2 in cells, A3 in whole organisms, A4 across ecosystems and evolutionary time.
A1 Molecules — water and nucleic acids
Water
Water is polar: oxygen pulls the shared electrons harder than hydrogen does, leaving oxygen slightly negative and each hydrogen slightly positive. Every property below follows from that one fact, and exam answers that begin with polarity almost always score.
- Cohesion — hydrogen bonds hold water molecules to each other. This is what lets a continuous column of water be pulled up a xylem vessel without breaking.
- Adhesion — water hydrogen-bonds to other polar surfaces, such as cellulose cell walls.
- Solvent properties — polar and ionic substances dissolve, so they can be transported. Non-polar substances do not, which is why lipids need carrier proteins in blood.
- Thermal properties — a high specific heat capacity means aquatic habitats change temperature slowly; a high latent heat of vaporisation means sweating removes a lot of heat for little water.
- Buoyancy and surface tension — hydrogen bonding at the surface supports small organisms.
The comparison that gets examined is water against methane. Both are small molecules of similar mass, but methane is non-polar, forms no hydrogen bonds, and so is a gas at room temperature with none of water's thermal properties. If a question asks why water is unusual, the methane comparison is the answer.
Nucleic acids
DNA is a double helix of two antiparallel strands, held by hydrogen bonds between complementary bases: A pairs with T by two hydrogen bonds, G pairs with C by three. Each nucleotide is a phosphate, a deoxyribose sugar and a base, joined into a strand by covalent phosphodiester bonds between the sugar of one and the phosphate of the next.
RNA differs in three ways, and all three are needed for full marks: it is single-stranded, its sugar is ribose not deoxyribose, and it uses uracil in place of thymine.
Because G–C pairs have three hydrogen bonds and A–T only two, a DNA region rich in G and C takes more energy to separate. That is a favourite suggest question.
A2 Cells — structure, origins and viruses
The cell theory states that living organisms are composed of cells, that the cell is the smallest unit of life, and that cells arise only from pre-existing cells. Note the exceptions the syllabus expects you to know — striated muscle fibres are multinucleate, fungal hyphae are not divided into discrete cells, and giant algae are single cells of enormous size.
Prokaryotic versus eukaryotic is the distinction most often examined, and it must be written as a comparison:
| Prokaryotic | Eukaryotic | |
|---|---|---|
| Nucleus | none; DNA free in cytoplasm | true nucleus with envelope |
| DNA | circular, naked | linear, associated with histones |
| Organelles | no membrane-bound organelles | mitochondria, ER, Golgi and others |
| Ribosomes | 70S | 80S (70S in mitochondria and chloroplasts) |
| Size | typically 1–5 µm | typically 10–100 µm |
That last ribosome row is the evidence for endosymbiosis: mitochondria and chloroplasts have their own circular DNA, 70S ribosomes and double membranes, and divide independently — because they descend from free-living prokaryotes engulfed by a larger cell.
Viruses are not cells. They have no cytoplasm, no ribosomes and no metabolism, and replicate only inside a host. The lytic cycle destroys the host cell immediately; the lysogenic cycle integrates viral DNA into the host genome, where it is copied silently until triggered.
A3 Organisms — diversity and classification
A species is traditionally a group of organisms that can interbreed and produce fertile offspring. Say fertile — a mule is offspring, but sterile, which is exactly why horses and donkeys are separate species. The definition fails for organisms that reproduce asexually and for extinct species known only from fossils, which is why molecular methods now supplement it.
Classification is hierarchical: domain, kingdom, phylum, class, order, family, genus, species. The binomial name is genus then species, genus capitalised, species not, both italicised or underlined: Homo sapiens.
A natural classification groups organisms by shared ancestry, so every group is a clade — an ancestor and all its descendants. Reptiles as traditionally defined are not a clade, because birds descend from the same ancestor but were excluded.
Cladistics builds these groups from evidence, now chiefly molecular. Base or amino acid sequences are compared: the more differences between two species, the longer since they diverged. Read a cladogram by where branches join, not by how close the tips look on the page — two adjacent tips can be distantly related.
A4 Ecosystems — evolution, speciation and conservation
Evolution is the change in heritable characteristics of a population over time. The evidence the syllabus expects: the fossil record, selective breeding, homologous structures — the pentadactyl limb across mammals with quite different lifestyles — and molecular sequence similarity.
Note the logic of homologous versus analogous. Homologous structures share form but differ in function, indicating common ancestry (divergent evolution). Analogous structures share function but differ in form, indicating similar selection pressures on unrelated lineages (convergent evolution). Getting these the wrong way round is a standard error.
Speciation requires reproductive isolation, then divergence:
- Allopatric — a geographical barrier separates populations, which then experience different selection pressures and diverge until they can no longer interbreed.
- Sympatric — isolation without a physical barrier, by temporal, behavioural or polyploidy mechanisms within the same area.
Conservation of biodiversity covers in situ methods, which protect species in their habitat and preserve the ecological relationships that sustain them, and ex situ methods — zoos, seed banks, captive breeding — which are a last resort and cannot preserve a species' role in its ecosystem.
What actually loses marks in this theme
- Describing water's properties without ever mentioning polarity or hydrogen bonding.
- Writing gene where allele is meant, or amount where concentration is meant.
- Answering distinguish with two separate descriptions instead of a comparison.
- Omitting "and fertile offspring" from the species definition.
- Reading a cladogram by tip proximity rather than by branch points.
- Confusing homologous and analogous structures.
- Forgetting that speciation needs isolation first, then divergence.
- Describing a process when the question said explain.
Educerie · written from the published IB syllabus structure for Biology Theme A, first assessment 2025. Original text. Last reviewed 5 September 2026.