IB Diploma Programme Biology: Syllabus Guide

This guide covers the full syllabus structure of IB Diploma Programme Biology, first assessment 2025, for both Standard Level (SL) and Higher Level (HL) students. It complements the subject overview, which covers the course’s aims and philosophy, and the assessment revision notes, which cover how the course is examined. This guide instead lists what is actually taught – the four syllabus themes and their sub-topics.

Recommended teaching time for syllabus content is 110 hours at SL and 180 hours at HL. On top of this, all students complete an experimental programme of practical work, a collaborative sciences project and an individual scientific investigation, totalling a further 40 hours at SL and 60 hours at HL. Sub-topics marked HL only are additional content that SL students are not examined on.

Theme A – Unity and diversity (19 hours SL / 33 hours HL)

This theme asks what all living things have in common, and where their diversity comes from.

• Water – the properties of water (polarity, hydrogen bonding, cohesion) that make it essential to life, and why no known life functions without it.
• Nucleic acids – the structure of DNA and RNA as information carriers, and how that structure supports their role in storing and transmitting genetic information.
• Origins of cells (HL only) – hypotheses for how the first living cells could have arisen from non-living chemistry.
• Cell structure – the shared and distinguishing features of cells, including what separates prokaryotic from eukaryotic organisation.
• Viruses (HL only) – the structure and diversity of viruses as non-living infectious particles that depend entirely on host cells to replicate.
• Diversity of organisms – classification and the diversity of life, and the practical challenge of grouping millions of species sensibly.
• Classification and cladistics (HL only) – using shared derived characteristics, rather than surface similarity, to build evolutionary trees.
• Evolution and speciation – the mechanisms (variation, selection, isolation) driving change in species over time and the formation of new species.
• Conservation of biodiversity – why biodiversity matters ecologically and economically, and how it is threatened and protected.

Theme B – Form and function (26 hours SL / 39 hours HL)

This theme connects biological structure at every scale to the job that structure performs.

• Carbohydrates and lipids, proteins – the structure and function of these macromolecule groups, and why their shape determines what they can do.
• Membranes and membrane transport – how substances cross cell membranes, by passive and active means.
• Organelles and compartmentalization – the internal structure of cells, and why dividing a cell into compartments is functionally useful.
• Cell specialization – how cells differentiate to perform specific roles despite sharing the same genome.
• Gas exchange, transport – how organisms exchange gases with their environment and move substances internally across scales, from single cells to large multicellular bodies.
• Muscle and motility (HL only) – the structural and biochemical mechanics of movement.
• Adaptation to environment, ecological niches – how an organism’s form suits it to where and how it lives.

Theme C – Interaction and interdependence (31 hours SL / 48 hours HL)

This theme moves from individual organisms to how living systems interact – internally between cells and organs, and externally between organisms.

• Enzymes and metabolism, cell respiration, photosynthesis – the biochemistry that powers living systems and links energy capture to energy use.
• Chemical signalling (HL only) and neural signalling – the two main ways organisms coordinate activity internally.
• Integration of body systems – how organ systems work together rather than in isolation.
• Defence against disease – immune responses to pathogens, from physical barriers to specific immunity.
• Populations and communities, transfer of energy and matter – ecology at the level of interacting organisms and ecosystems, and how energy and nutrients move through food webs.

Theme D – Continuity and change (34 hours SL / 60 hours HL)

This is the largest theme by teaching hours at both SL and HL, covering how information and traits persist and change across generations.

• DNA replication, protein synthesis – how genetic information is copied accurately and expressed as functional proteins.
• Mutations and gene editing – how genetic material changes, naturally and by deliberate human intervention.
• Cell and nuclear division – mitosis and meiosis, and why an organism needs both.
• Gene expression (HL only) – the regulation of which genes are switched on or off in a given cell.
• Water potential, reproduction, inheritance – further mechanisms of continuity between generations, spanning both plant and animal contexts.
• Homeostasis – how organisms maintain a stable internal environment despite external change.
• Natural selection, sustainability and change – evolution and its consequences at population and ecosystem scale, closing the loop back to Theme A’s evolution and speciation content.

How to approach it

Because DP Biology is organised by theme rather than by a strict linear sequence of biological levels, it is worth building your own map of which sub-topics from which themes connect to each other – cell respiration (Theme C) and photosynthesis (Theme C) both depend on concepts from membranes and organelles (Theme B), for instance. Treat the four themes as four different lenses on the same body of living systems rather than four unconnected blocks, since Paper 2’s data-based and extended-response questions are set across the whole syllabus and reward students who can draw connections between themes, not just recall facts within one.

If you are studying at HL, keep a running list of exactly which sub-topics are HL-only (marked above) as you go, since these carry extra weight in Papers 1 and 2 and are the material most often under-revised by HL students who default to SL-level depth out of habit.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Biology, first assessment 2025, published January 2022.