Mechanisms across scales
Connect molecular and cellular processes to organism function, population outcomes, ecosystem interactions, and evolutionary change.
IB Sciences • SL
IB Biology SL
Use this IB Biology SL study guide to browse the syllabus topic structure, study materials, and practice resources.
Structured practice
IB Biology SL Question Bank
Jump straight into grouped unit practice without losing your place in the syllabus overview.
A - Unity and Diversity
A1.1 Water
A1.1.1 Water as the medium for life
A1.1.2 Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
A1.1.3 Cohesion of water molecules due to hydrogen bonding and consequences for organisms
A1.1.4 Adhesion of water to materials that are polar or charged and impacts for organisms
A1.1.5 Solvent properties of water as a medium for metabolism and transport
A1.1.6 Physical properties of water and the consequences for animals in aquatic habitats
A1.2 Nucleic acids
A1.2.1 DNA as the genetic material of all living organisms
A1.2.2 Components of a nucleotide
A1.2.3 Sugar–phosphate bonding and the sugar–phosphate “backbone” of DNA and RNA
A1.2.4 Bases in each nucleic acid that form the basis of a code
A1.2.5 RNA as a polymer formed by condensation of nucleotide monomers
A1.2.6 DNA as a double helix linked by hydrogen bonding between complementary base pairs
A1.2.7 Differences between DNA and RNA
A1.2.8 Complementary base pairing in allowing genetic information to be replicated and expressed
A1.2.9 Diversity of possible DNA base sequences and its capacity for storing information
A1.2.10 Conservation of the genetic code as evidence of universal common ancestry
A2.1 Origins of cells (HL)
A2.1.3 Challenge of explaining the spontaneous origin of cells
A2.1.4 Evidence for the origin of carbon compounds
A2.1.5 Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers
A2.1.6 RNA as a presumed first genetic material
A2.1.7 Evidence for a last universal common ancestor
A2.1.8 Approaches used to estimate dates of the first living cells
A2.1.9 Evidence for the evolution of the last universal common ancestor
A2.2 Cell structure
A2.2.1 Cells as the basic structural unit of all living organisms
A2.2.2 Microscopy skills
A2.2.3 Developments in microscopy
A2.2.4 Structures common to cells in all living organisms
A2.2.5 Prokaryote cell structure
A2.2.6 Eukaryote cell structure
A2.2.7 Processes of life in unicellular organisms
A2.2.8 Differences in eukaryotic cell structure between animals, fungi, and plants
A2.2.9 Atypical cell structure in eukaryotes
A2.2.10 Cell types and cell structures viewed in light and electron micrographs
A2.2.11 Drawing and annotation based on electron micrographs
A3.1 Diversity of organisms
A3.1.1 Variation between organisms as a defining feature of life
A3.1.2 Species as groups of organisms with shared traits
A3.1.3 Binomial system for naming organisms
A3.1.4 Biological species concept
A3.1.5 Difficulties distinguishing between populations and species
A3.1.6 Diversity in chromosome numbers of plant and animal species
A3.1.7 Karyotyping and karyograms
A3.1.8 Unity and diversity of genomes within species
A3.1.9 Diversity of eukaryote genomes
A3.1.10 Comparison of genome sizes
A3.1.11 Current and potential future uses of whole genome sequencing
A4.1 Evolution and speciation
A4.1.1 Evolution as change in the heritable characteristics of a population
A4.1.2 Evidence for evolution from base sequences in DNA or RNA and amino acid sequences
A4.1.3 Evidence for evolution from selective breeding of domesticated animals and crop plants
A4.1.4 Evidence for evolution from homologous structures
A4.1.5 Convergent evolution as the origin of analogous structures
A4.1.6 Speciation by splitting of pre-existing species
A4.1.7 Roles of reproductive isolation and differential selection in speciation
A4.2 Conservation of biodiversity
A4.2.1 Biodiversity as the variety of life in all its forms, levels and combinations
A4.2.2 Comparisons between current number of species on Earth and past levels of biodiversity
A4.2.3 Causes of anthropogenic species extinction
A4.2.4 Causes of ecosystem loss
A4.2.5 Evidence for a biodiversity crisis
A4.2.6 Causes of the current biodiversity crisis
A4.2.7 Need for several approaches to conservation of biodiversity
A4.2.8 Evolutionarily distinct and globally endangered species in conservation prioritization
Overview
Course overview
How to study IB Biology SL
IB Biology SL connects biological understanding across four themes—unity and diversity, form and function, interaction and interdependence, and continuity and change—and four levels of organisation: molecules, cells, organisms, and ecosystems. The current course began teaching in August or September 2023 and was first assessed in 2025. SL prioritises secure core mechanisms, accurate data interpretation, and concise evidence-based explanations.
Evidence and inquiry
Interpret experiments, graphs, images, sampling, uncertainty, and primary or secondary data before drawing a biological conclusion.
Precise explanations
Use correct terminology, labelled diagrams, and explicit cause-and-effect sequences instead of replacing a mechanism with a memorised list.
Assessment at a glance
For the current IB Biology SL course first assessed in 2025, external assessment contributes 80% and the individual scientific investigation contributes 20%. Confirm the examination session and practical requirements with your school.
| Component | Format | Weighting |
|---|---|---|
| Paper 1 | 1 hour 30 minutes; Paper 1A multiple choice and Paper 1B data-based questions | 36% |
| Paper 2 | 1 hour 30 minutes; data-based, short-answer, and extended-response questions | 44% |
| Internal assessment | 10 hours; individual scientific investigation, maximum 3,000 words | 20% |
Sources: official IB Biology subject brief (first assessment 2025) and official IB Biology course overview.
Revision plan
- 1.Map each topic to its theme and level of organisation so connections across molecules, cells, organisms, and ecosystems remain visible.
- 2.Reconstruct biological mechanisms from memory with labelled diagrams and ordered cause-and-effect steps.
- 3.Practise Paper 1B and Paper 2 data questions by separating observation, interpretation, conclusion, and limitation.
- 4.Use mixed-topic sets to connect form with function, interactions with consequences, and continuity with change.
Exam strategy
Read the command term and identify the biological scale being tested. Build mechanisms in a logical order, support claims with the supplied evidence, label diagrams precisely, and distinguish a description of the data from an explanation of why the pattern occurs.
Practise IB Biology SL questionsReview BioNinja current-course IB Biology notesFrequently asked questions
What does IB Biology SL cover?
IB Biology SL studies life through four themes: unity and diversity, form and function, interaction and interdependence, and continuity and change. Topics are connected across molecules, cells, organisms, and ecosystems.
Which syllabus applies to the current IB Biology course?
The current Biology course began teaching in August or September 2023 and was first assessed in 2025. Students should confirm their examination session and any guide updates with their school.
How is IB Biology SL assessed?
External assessment is worth 80%: Paper 1 lasts one hour and 30 minutes and contributes 36%, while Paper 2 lasts one hour and 30 minutes and contributes 44%. The scientific investigation internal assessment contributes 20%.
What is in IB Biology SL Paper 1?
Paper 1 combines Paper 1A multiple-choice questions with Paper 1B data-based questions. Paper 1B uses syllabus-related data across the course themes, so students need both biological understanding and careful evidence interpretation.
What is the current IB Biology scientific investigation?
The scientific investigation is an open-ended individual task in which a student gathers and analyses data to answer a self-formulated research question. The written report has a maximum overall word count of 3,000 words and contributes 20% at both SL and HL.