Explain how substances move across cell membranes by facilitated diffusion and describe how this differs from simple diffusion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Facilitated diffusion moves substances down a concentration gradient, from a higher to a lower concentration.
It uses protein channels or carrier proteins embedded in the membrane.
It is a passive process and does not require ATP.
In contrast, simple diffusion also moves substances down a gradient but does not use protein channels; molecules pass directly through the phospholipid bilayer.
Facilitated diffusion allows larger or charged/polar molecules, such as glucose or ions, to cross the membrane, which cannot pass directly through the bilayer.
It uses protein channels or carrier proteins embedded in the membrane.
It is a passive process and does not require ATP.
In contrast, simple diffusion also moves substances down a gradient but does not use protein channels; molecules pass directly through the phospholipid bilayer.
Facilitated diffusion allows larger or charged/polar molecules, such as glucose or ions, to cross the membrane, which cannot pass directly through the bilayer.
Examiner tips
- Use the phrase "down a concentration gradient" to show direction. Mention both channel and carrier proteins. State that it is passive and no ATP is needed. Contrast with simple diffusion by noting lack of proteins and the type of molecules that can cross.
- common_mistakes
- :
- Confusing facilitated diffusion with active transport. Saying it requires energy. Forgetting to mention that simple diffusion does not use proteins or that facilitated diffusion can move larger/charged molecules.
Mark scheme (5 marks)
- Facilitated diffusion moves substances from a higher concentration to a lower concentration (down a concentration gradient)
- Facilitated diffusion uses protein channels (carrier proteins / channel proteins) in the membrane
- Facilitated diffusion does not require energy (ATP) / is a passive process
- Simple diffusion does not use protein channels — substances pass directly through the phospholipid bilayer
- Facilitated diffusion allows larger or charged/polar molecules (e.g. glucose, ions) to cross the membrane that cannot pass directly through the bilayer
Key terms in this question
facilitated diffusion · simple diffusion
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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