The big idea: Some particles cannot slip straight through the phospholipid bilayer.
Ions (like sodium and potassium) and large polar molecules (like glucose) are charged or water-loving, so the oily, water-hating core of the bilayer blocks them.
Facilitated diffusion is how they get across: they pass through a transport protein embedded in the membrane — a channel protein or a carrier protein — while still moving down the concentration gradient.
It is passive: no ATP (cell energy) is used. The protein only provides a route.
The membrane contains both a channel protein (an open hydrophilic pore) and a carrier protein (which changes shape to ferry a molecule across) — these are the proteins that make facilitated diffusion possible.
Interactive diagram
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- Facilitated diffusion
- The passive movement of ions and large polar molecules across a membrane, through a channel or carrier protein, down the concentration gradient (no ATP used).
- Channel protein
- A membrane protein with a water-filled pore (tunnel). It lets specific ions or polar molecules pass straight through.
- Carrier protein
- A membrane protein that binds a specific molecule and changes shape to move it from one side of the membrane to the other.
- Concentration gradient
- The difference in concentration of a substance between two areas. Particles diffuse down the gradient — from high to low concentration.
- Passive transport
- Movement that does NOT use the cell's energy (ATP), because particles move down their concentration gradient.
Why a protein is needed at all: The middle of the membrane is made of hydrophobic (water-hating) fatty-acid tails.
Charged ions and water-loving molecules are repelled by this oily core, so they cannot diffuse straight through like oxygen can.
A transport protein gives them a hydrophilic (water-loving) path across — that is the whole point of facilitated diffusion.
There are two kinds of transport protein used in facilitated diffusion, and they work in different ways.
A channel protein is like an open doorway: it forms a water-filled pore that lets the particle slip straight through.
A carrier protein is more like a revolving door: the molecule binds to it, and the protein changes shape to carry the molecule from one side to the other.
Channel proteins — an open pore: A channel protein has a hydrophilic pore running right through the membrane.
Specific ions (such as sodium, Na⁺, or potassium, K⁺) and small polar molecules pass straight through the pore, down the gradient.
Because the route is open, channels are fast. (Aquaporins, the water channels from 2.3.2, are a special type of channel protein.)
Carrier proteins — bind and change shape: A carrier protein binds the specific molecule it transports (for example glucose or fructose) on one side of the membrane.
It then changes shape to move the molecule across and release it on the other side.
Because it must change shape each time, a carrier is slower than a channel — but it can handle larger molecules.
| Feature | Channel protein | Carrier protein |
|---|---|---|
| What it is | A protein with a water-filled pore through the membrane | A protein that binds the molecule on one side |
| How it works | Opens a hydrophilic tunnel; the molecule passes straight through | Changes shape to move the molecule from one side to the other |
| Speed | Faster (an open route) | Slower (must change shape each time) |
| Typical passengers | Ions (e.g. sodium, potassium) and water (aquaporins) | Larger molecules such as glucose and fructose |
| Direction | Down the concentration gradient | Down the concentration gradient |
| ATP used? | No — passive | No — passive |
Still passive — no ATP: This is the point examiners check most. Facilitated diffusion moves particles down the concentration gradient (from high to low).
Because the particles move the way they would naturally go anyway, no ATP is used — it is passive, exactly like simple diffusion.
The only difference from simple diffusion is the route: facilitated diffusion goes through a protein, while simple diffusion goes straight through the bilayer.
Facilitated diffusion (second panel): ions and large polar molecules pass through a channel or carrier protein, still moving DOWN the concentration gradient and using no ATP — it is passive, just like simple diffusion.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Channel protein
- An open water-filled pore through the membrane
- The molecule passes straight through
- Fast — the route is open
- Carries ions (Na⁺, K⁺) and water (aquaporins)
Carrier protein
- Binds the molecule, then changes shape
- Ferries the molecule from one side to the other
- Slower — must change shape each time
- Carries larger molecules (glucose, fructose)
A memory hook: Channel = an open channel/tunnel the particle slips through.
Carrier = it carries the molecule by changing shape.
And for both: down the gradient, no ATP — facilitated diffusion is passive.
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How this is tested: A 4-mark Describe question on Paper 2 asks how substances move across the membrane by facilitated diffusion — score it with separate points: the particle is polar / an ion, it passes through a channel or carrier protein, it moves down the concentration gradient, and no ATP is used.
A 1-mark question can ask you to explain why a polar molecule like glucose still crosses down its gradient even though it needs a protein — the protein only gives a route; the molecule still moves high → low.
A short Outline (2 marks) may name a sugar such as fructose moving by facilitated diffusion — say it binds a carrier protein and moves down the gradient, passively.
IB-style question — describe facilitated diffusion
Describe how a polar molecule such as glucose crosses the cell membrane by facilitated diffusion. [4]
How to score all four marks
- Say why a protein is needed. Glucose is large and polar (water-loving), so it cannot pass straight through the hydrophobic core of the phospholipid bilayer.
- Name the route. It crosses through a transport protein in the membrane — a carrier protein (or channel protein) specific to that molecule.
- State the direction. It moves down the concentration gradient, from where glucose is at higher concentration to where it is lower.
- State that it is passive. No ATP is used, so it is passive transport. (The carrier binds glucose and changes shape to move it across.) Award 1 mark each for: needs a protein because polar/large; through a channel/carrier protein; down the gradient; passive / no ATP.
Final answer
Glucose is too large and polar to cross the bilayer, so it passes through a channel/carrier protein, moving down its concentration gradient without using ATP — facilitated diffusion is passive.
✓ Why this scores full marks: Each sentence is a separate, distinct point — why a protein is needed, the protein route, the direction, and 'no ATP'.
The mark most students lose is forgetting to say down the gradient and passive / no ATP — without those, an examiner cannot tell facilitated diffusion from active transport.
| Feature | Simple diffusion | Facilitated diffusion |
|---|---|---|
| Route across the membrane | Straight through the phospholipid bilayer | Through a channel or carrier protein |
| What can cross | Small, non-polar molecules (e.g. O₂, CO₂) | Ions and large polar molecules (e.g. glucose, Na⁺) |
| Direction | High → low (down the gradient) | High → low (down the gradient) |
| Energy (ATP) | None — passive | None — passive |
| At high solute concentration | Rate keeps rising (straight line) | Rate levels off — proteins become saturated |