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B2.1.3—Simple diffusion

Simple diffusion is passive net movement down a concentration gradient, allowing small non-polar molecules to cross membranes without ATP; This process depends on random motion, permeability, and differences between internal and external concentrations.

Syllabus
First assessment 2025
Objective
B2.1.3
Level
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–5

Common command terms

  • Compare
  • Outline
  • Explain
  • Identify

Scoring notes

Common mistake
Adding channel proteins or ATP to simple diffusion when the row requires direct passive movement.

Recent exam appearances

November 2025Paper2 ["SL"] · TZ33(c)[ 3 ]B2.1.3—Simple diffusion
November 2018Paper1 ["SL"] · TZ05[ 1 ]B2.1.3—Simple diffusion
May 2012Paper2 ["SL"] · TZ12(b)[ 2 ]B2.1.3—Simple diffusion
May 2010Paper2 ["SL"] · TZ27(a)[ 5 ]B2.1.3—Simple diffusion

Coverage 2010–2025 · Updated 15 Jul 2026

Random Motion Produces Net Diffusion

Diffusion is passive net movement from higher to lower concentration, driven by random kinetic motion. Particles continue moving both ways at equilibrium, but net movement becomes zero.

Show gradient-driven membrane crossing so the learner can track Simple Diffusion.

Steeper gradient + larger surface area + higher temperature → faster diffusion. Greater distance or membrane thickness → slower diffusion.

SL Summary: Choose a Membrane Route

Particle property → gradient → route → energy.
Non-polar: bilayer. Water: osmosis/aquaporin. Ion or polar solute downhill: channel/carrier. Uphill: ATP-coupled pump.

The bilayer creates the barrier; proteins make it selective; carbohydrates create an outward interface; component mobility makes the membrane adaptable.

Simple diffusion

[ 3 ]

Explain how the rate of diffusion of oxygen into a cell is affected by the concentration of oxygen outside of the cell.

Concept essentials

  • Simple diffusion is passive movement from higher to lower concentration.
  • Small non-polar molecules can cross the phospholipid bilayer without membrane proteins.
  • Diffusion slows as the concentration gradient is reduced.
  • Facilitated diffusion shares passive movement but requires a transport protein.