Compare simple and facilitated diffusion
Both are passive net movement down a gradient, but facilitated diffusion uses selective membrane proteins.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which feature distinguishes facilitated diffusion from simple diffusion?
What you will learn
- Explain simple diffusion down a concentration gradient
- Compare simple diffusion with channel- and carrier-mediated facilitated diffusion
Simple diffusion crosses the bilayer
Particles move randomly, but a concentration difference produces net movement from higher to lower concentration. Small non-polar molecules can diffuse through the hydrophobic bilayer more readily than charged or polar particles.
No metabolic energy is required for simple diffusion. Net movement continues while a gradient exists; particles do not stop moving individually at equilibrium, but there is no net change in distribution.
Facilitated diffusion uses proteins
Facilitated diffusion also moves particles down their concentration or electrochemical gradient without direct ATP use, but the route is a membrane protein. Channel proteins form selective hydrophilic pathways; carrier proteins bind particles and change shape to move them.
Transport is selective because protein structure and properties limit which particles pass. Do not call facilitated diffusion active transport just because a protein is involved.
Pause and explain
At dynamic equilibrium, what is true of diffusion?
Worked example
An ion moves from higher to lower concentration through a selective membrane channel. Classify the process and explain why.
Show the worked solution
- The ion moves down its concentration gradient.
- It uses a channel protein and requires no direct metabolic energy.
Answer This is facilitated diffusion: the ion moves down its concentration gradient through a selective channel protein without direct ATP use.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A small non-polar molecule moves across a membrane from high to low concentration without a transporter. Name the process.
Give me a hint
Use the gradient and the lack of a protein route.
Compare my reasoning
- Net movement is down its concentration gradient.
- No membrane transport protein is involved.
This is simple diffusion through the phospholipid bilayer down the concentration gradient.
Look for these in your work
- Process named
- Direction stated
A glucose molecule moves down its gradient by binding to a carrier that changes shape. Classify the transport.
Give me a hint
Consider whether it uses a protein and whether it is down the gradient.
Compare my reasoning
- The carrier makes the route protein-mediated.
- Down-gradient movement without energy is facilitated diffusion.
This is facilitated diffusion because glucose uses a carrier protein and moves down its concentration gradient without direct metabolic energy.
Look for these in your work
- Carrier route identified
- Passive direction classified
Two solutions have equal concentration of a permeable solute. Explain what happens after equilibrium is reached.
Give me a hint
Distinguish particle movement from net movement.
Compare my reasoning
- Particles continue random movement in both directions.
- Opposing rates are equal, so there is no net diffusion.
Particles continue to move randomly across the membrane, but movement in both directions balances on average. There is no net diffusion at equilibrium.
Look for these in your work
- Random movement retained
- No-net-movement explained
Common mistakes
- Calling every protein-assisted movement active transport
- Saying diffusion is the movement of all particles in one direction only
- Claiming there is no particle movement at equilibrium
What can you explain now?
Which feature distinguishes facilitated diffusion from simple diffusion?
Compare with the explanation
It uses a selective membrane protein
A channel or carrier provides the facilitated route.
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Make it stick
How do simple diffusion and facilitated diffusion differ, and what does equilibrium mean for net movement?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.