Proteins open the door; the gradient picks the direction. A plateau in uptake means every carrier is busy.
Facilitated diffusion is diffusion with a door. A membrane protein gives ions and large polar molecules a way through the oily bilayer, but it adds no energy. The concentration gradient still decides which way they go.
Key terms
- Facilitated diffusion — passive movement of a solute down its concentration gradient through a membrane protein. No ATP is spent.
- Channel protein — a tunnel with a hydrophilic pore for ions or water. Many are gated: they open and close on a signal.
- Carrier protein — binds a specific solute, changes shape, and releases it on the other side of the membrane.
- Aquaporin — a channel for water. It lets water cross far faster than water can slip through the bilayer alone.
- Saturation — every carrier is busy, so adding more solute no longer speeds transport. The rate levels off.
- Specificity — a carrier moves only solutes whose shape fits its binding site, the way a substrate fits an enzyme's active site.
How it works
- The bilayer's hydrophobic core blocks ions and large polar molecules like glucose. On their own, they barely cross.
- A channel opens a hydrophilic path. A carrier grips the solute, changes shape, and lets go on the far side.
- The protein adds no energy. Net movement runs from high concentration to low, into or out of the cell.
- The rate depends on how steep the gradient is and how many transport proteins the membrane holds. More proteins → more flow.
- Carriers can saturate. Once every one is busy, extra solute outside waits its turn and the rate levels off.
- At equal concentrations, molecules still cross both ways, at equal rates. Net movement stops; movement doesn't.
Why it matters
On the exam this topic shows up as data. Uptake that plateaus as you add more solute points to a transport protein. Uptake that keeps climbing in a straight line points to diffusion directly through the bilayer. A blocker that wipes out fast water entry points to aquaporins. Ion channels also set up later units: when sodium or potassium ions flow through them, charge shifts across the membrane. Nerve cells signal by opening and closing these channels.
Exam traps
| Trap | Actually |
|---|
| A carrier always brings its solute into the cell. | Net flow follows the gradient, in or out. Liver cells export glucose through the same transporter family muscle cells use to import it. |
| A protein in the path means the cell is spending ATP. | Facilitated diffusion is passive. The protein lowers the barrier; the gradient supplies the push. |
| Uptake levels off because inside and outside have become equal. | In initial-rate data, a plateau means every carrier is occupied. That's saturation, not equilibrium. |
| Carriers sort molecules by size or mass. | They sort by shape. A sugar with glucose's mass but a different shape barely gets carried. |
Worked example
Cells are placed in solutions of S, a polar sugar. The initial rate of S uptake (units/min) at outside concentrations of 2, 4, 8, 16, and 32 mM is 12, 18, 24, 30, and 33. Given time, S inside the cells rises to match the outside concentration but never exceeds it. How does S cross the membrane?
- Look for a ceiling. Doubling S from 16 to 32 mM raises uptake only from 30 to 33. The rate is leveling off.
- A ceiling means a limited number of proteins, nearly all occupied. Diffusion straight through the bilayer would keep rising in step with S.
- Now check direction. S stops accumulating once inside equals outside. Nothing pushes it uphill, so no pump is at work.
Facilitated diffusion through carrier proteins: passive, down the gradient, and saturable.
The plateau shows a protein is in the path, and stopping at equal concentrations shows the movement is passive. The near-miss is active transport: students see a protein and assume a pump. A pump would push S above the outside level.