Explain how the process of simple diffusion differs from facilitated diffusion in terms of the molecules involved, the membrane structures used, and the direction of net movement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Simple diffusion:
1. Small, non‑polar or lipid‑soluble molecules (e.g. O₂, CO₂, steroids) move directly through the phospholipid bilayer.
2. No membrane proteins are required.
3. Net movement is down the concentration gradient (high to low) and no ATP is needed.
Facilitated diffusion:
1. Polar, charged or large molecules (e.g. glucose, ions) that cannot cross the bilayer unaided.
2. Requires intrinsic membrane proteins – channel proteins forming hydrophilic pores or carrier proteins that change conformation.
3. Net movement is also down the concentration gradient and is ATP‑independent.
Rate difference: simple diffusion rate increases linearly with gradient; facilitated diffusion rate rises to a maximum (Vmax) when all transport proteins are saturated.
1. Small, non‑polar or lipid‑soluble molecules (e.g. O₂, CO₂, steroids) move directly through the phospholipid bilayer.
2. No membrane proteins are required.
3. Net movement is down the concentration gradient (high to low) and no ATP is needed.
Facilitated diffusion:
1. Polar, charged or large molecules (e.g. glucose, ions) that cannot cross the bilayer unaided.
2. Requires intrinsic membrane proteins – channel proteins forming hydrophilic pores or carrier proteins that change conformation.
3. Net movement is also down the concentration gradient and is ATP‑independent.
Rate difference: simple diffusion rate increases linearly with gradient; facilitated diffusion rate rises to a maximum (Vmax) when all transport proteins are saturated.
Examiner tips
- Use the word ‘requires’ to show the need for proteins in facilitated diffusion.
- Mention the gradient direction and ATP independence for both processes.
- Include the rate‑saturation point for facilitated diffusion to hit the extra mark.
- Keep each point short and use exam‑specific terminology.
Mark scheme (4 marks)
- Simple diffusion involves small, non-polar/lipid-soluble molecules (e.g. O₂, CO₂, steroids) moving directly through the phospholipid bilayer, whereas facilitated diffusion involves polar, charged, or large molecules (e.g. glucose, ions) that cannot cross the bilayer unaided.
- Facilitated diffusion requires intrinsic/integral membrane proteins — either channel proteins (forming hydrophilic pores) or carrier proteins (that undergo conformational change) — whereas simple diffusion requires no protein.
- Both simple and facilitated diffusion result in net movement of molecules down their concentration gradient (from high to low concentration), and neither process requires ATP/metabolic energy.
- The rate of simple diffusion increases linearly with concentration gradient, whereas the rate of facilitated diffusion plateaus/reaches a maximum (Vmax) when all carrier/channel proteins are saturated.
Key terms in this question
simple diffusion · facilitated diffusion
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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