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B2.1.8—Selectivity in permeability

Selective permeability depends on molecule properties and membrane proteins, separating direct bilayer diffusion from protein-mediated transport; This selectivity explains why different substances need different crossing mechanisms.

Syllabus
First assessment 2025
Objective
B2.1.8
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1A
Typical marks1

Common command terms

  • Identify

Scoring notes

Common mistake
Saying facilitated diffusion requires ATP because it uses a protein.

Recent exam appearances

November 2025Paper1A ["SL"] · TZ13[ 1 ]B2.1.8—Selectivity in permeability
Practice this objective

Coverage 2025–2025 · Updated 15 Jul 2026

Concept essentials

  • Selective permeability depends on both solute properties and membrane components.
  • Facilitated diffusion and active transport both use proteins, but only active transport uses ATP.
  • Small non-polar molecules can cross the bilayer more readily than large polar molecules.
ConceptIB Biology SL