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IB Biology HL Study Guide & Review

Study the current IB Biology HL syllabus by connecting mechanisms across biological scales, interpreting unfamiliar data, practising investigation skills and turning precise knowledge into clear Paper 1 and Paper 2 responses.

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  1. 01Learn concepts
  2. 02Practise questions
  3. 03Review mistakes

Syllabus knowledge tree

Explore the IB Biology HL syllabus

The current IB Biology HL syllabus organises biology through four themes explored across molecules, cells, organisms and ecosystems. Use the Unit and Topic map to connect recurring ideas, locate HL-only depth, and choose the next focused study action.

4
syllabus groups
40
mapped topics

Preview the course map

Explore the course units and topics. Sign in to open the full mastery workspace and see your progress.

A Unity and Diversity
A1.1 Water
A1.2 Nucleic acids
A2.1 Origins of cells
A2.2 Cell structure
A2.3 Viruses [HL only]
A3.1 Diversity of organisms
A3.2 Classification and cladistics [HL only]
A4.1 Evolution and speciation
A4.2 Conservation of biodiversity
B Form and Function
B1.1 Carbohydrates and lipids
B1.2 Proteins
B2.1 Membranes and membrane transport
B2.2 Organelles and compartmentalization
B2.3 Cell specialization
B3.1 Gas exchange
B3.2 Transport
B3.3 Muscle and motility [HL only]
B4.1 Adaptation to environment
B4.2 Ecological niches
C Interaction and Interdependence
C1.1 Enzymes and metabolism
C1.2 Cell respiration
C1.3 Photosynthesis
C2.1 Chemical signalling [HL only]
C2.2 Neural signalling
C3.1 Integration of body systems
C3.2 Defence against disease
C4.1 Populations and communities
C4.2 Transfers of energy and matter
D Continuity and Change
D1.1 DNA replication
D1.2 Protein synthesis
D1.3 Mutation and gene editing
D2.1 Cell and nuclear division
D2.2 Gene expression [HL only]
D2.3 Water potential
D3.1 Reproduction
D3.2 Inheritance
D3.3 Homeostasis
D4.1 Natural selection
D4.2 Stability and change
D4.3 Climate change

How to study IB Biology HL

IB Biology HL becomes manageable when you stop treating its four themes as separate chapters. Use the syllabus map to follow one idea across molecules, cells, organisms and ecosystems. For every process, identify the changing structure or variable, explain the mechanism, and state the biological consequence. This produces answers that are connected rather than lists of memorised terms. At HL, keep standard-level foundations connected to additional depth in molecular biology, physiology, ecology and genetics; do not revise HL-only material as a detached list.

Use Concept to build a mechanism, Mastery to retrieve it without prompts, and the Question Bank to apply it to unfamiliar graphs, experiments and extended responses. After marking, classify the loss as missing knowledge, broken causality, weak evidence use or imprecise terminology, then repeat a matched question instead of simply rereading the topic.

Practise Biology topics in different ways

Match the activity to the thinking demanded by the topic instead of using one revision technique across every part of the course.

Mechanisms at molecular and cellular scale

Water, membranes, enzymes, respiration and photosynthesis

Redraw structures and pathways from memory. Then change one condition or component and predict the effect, explaining each causal step rather than naming the final outcome only under exam conditions.

Open molecular and cell questions

Genetics, inheritance and change

DNA replication, protein synthesis, inheritance and natural selection

Separate the process from the evidence. Work through an example step by step, then use its result to justify a genetic or evolutionary conclusion with the correct level of biological organisation.

Practise continuity and change

Physiology and ecology

Gas exchange, transport, homeostasis, populations and ecosystems

Link adaptation to function, then function to a system-level consequence. For graphs, describe the measured pattern with comparative values before proposing a biological explanation or evaluating competing causes.

Practise interactions

Investigation and unfamiliar data

Variables, uncertainty, graphical evidence, conclusions and evaluation

Identify variables and units before interpreting a trend. For evaluation, name the limitation, explain its likely effect on the evidence and propose a specific improvement that addresses that limitation.

Open data-based practice

Where to start

Choose one starting point, then use the same explain–retrieve–apply loop for every Biology topic.

Choose your starting point

  1. I know the topic

    Open its exact place in the current syllabus and stay inside the scope for your level.

    Browse the syllabus

  2. I do not know what is weak

    Run a short mixed diagnostic and begin with the first error pattern, not the easiest chapter.

    Start diagnostic

Use the three-step learning loop

  1. Explain the mechanism

    Draw or narrate the complete causal sequence, clearly labelling structures, variables and biological consequences.

    Study a Concept

  2. Retrieve across scales

    Close the explanation and reconstruct it from molecule to system, checking terminology only afterwards.

    Use Mastery

  3. Apply and repair

    Answer a small mixed set, diagnose every lost mark and repeat one matched question.

    Practise questions

IB Biology HL assessment format

External assessment contributes 80% and the scientific investigation contributes 20%. Use the component differences to balance rapid retrieval, unfamiliar data, extended biological explanations and investigation skills for HL.

Paper / componentQuestionsTime% of grade
Paper 175 marksPaper 1A multiple-choice questions and Paper 1B compulsory data-based questions addressing the organising themes.How to prepare: Combine fast retrieval with regular interpretation of unfamiliar tables, graphs and experimental contexts; explain why rejected options conflict with biological evidence.2 hours36%
Paper 280 marksData-based and short-answer questions followed by extended-response questions.How to prepare: Practise complete causal chains under time pressure, using precise terminology and selecting evidence that directly answers the command term.2 hours 30 minutes44%
Scientific investigation24 marksOne individual open-ended scientific investigation, internally assessed and externally moderated.How to prepare: Develop a focused research question, controlled method, defensible data analysis, evidence-based conclusion and evaluation tied to specific limitations.10 recommended course hours20%

SourceInternational Baccalaureate Organization · IB Biology subject brief — first assessment 2025IB Biology · First assessment 2025

IB Biology HL questions

Use a short diagnostic if the weakness is unclear; otherwise open the exact syllabus Topic. Explain its mechanism without notes, then attempt a small matched question set and use lost marks to choose the next action.

Memorise required terminology and examples precisely, but prioritise relationships and mechanisms inside the current HL syllabus. Test whether you can use the knowledge to explain a change, interpret evidence and justify a conclusion.

Identify variables and units, describe the pattern with comparative values, then explain it biologically. Keep description and explanation separate, and when evaluating evidence state the limitation, its likely effect and a specific improvement.

Begin short timed sets once individual Topics can be explained without notes. Move to mixed Paper 1 and Paper 2 work after several Topics are secure, preserving unseen current-syllabus papers for full simulations where possible.