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

C2.2.1—Neurons

C2.2.2—Resting potential generation

C2.2.3—Nerve impulses as action potentials

C2.2.4—Variation in impulse speed

C2.2.5—Synapses as junctions

C2.2.6—Neurotransmitter release

C2.2.7—Excitatory postsynaptic potential

C2.2.8 (HL)—Depolarization and repolarization

C2.2.9 (HL)—Action potential propagation

C2.2.10 (HL)—Oscilloscope traces

C2.2.11 (HL)—Saltatory conduction

C2.2.12 (HL)—Exogenous chemicals

C2.2.13 (HL)—Inhibitory neurotransmitters

C2.2.14 (HL)—Summation

C2.2.15 (HL)—Pain perception

C2.2.16 (HL)—Consciousness

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

IB Biology HL C2.2.9 (HL)—Action potential propagation Question Bank

• Local currents from Na⁺ diffusion depolarize the next axon region

• Refractory regions behind the impulse help ensure one-way propagation

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