Facilitated Diffusion: The Back Door for Polar Molecules
Imagine a crowded room with only one narrow door. Small, nimble people slip through easily — that's simple diffusion for small nonpolar molecules like oxygen or carbon dioxide. But what about a large person carrying a heavy box? They can't fit through that door at all. They need a wider, specially designed entrance.
That wider entrance is facilitated diffusion.
The Core Problem
The cell membrane is a lipid bilayer — think of it as a greasy, oily barrier. Small nonpolar molecules (oxygen, carbon dioxide, steroids) dissolve right through this grease and cross easily. But polar molecules (glucose, amino acids, ions) are like water-loving marbles — they simply cannot dissolve in the oily membrane. They're stuck.
Yet cells need glucose desperately. How does it get in?
The Solution: Protein Helpers
The cell embeds special transmembrane proteins that act as selective gates or tunnels. These proteins provide an alternative path through the membrane — a path that bypasses the lipid barrier entirely.
Facilitated diffusion is the passive movement of molecules across a membrane down their concentration gradient, using specific transmembrane proteins. It requires no cellular energy (ATP) — the driving force is purely the concentration difference.
Two Types of Transport Proteins
1. Channel proteins form water-filled pores. Ions or small polar molecules pass through like water through a pipe. These are often gated — they open or close in response to signals.
2. Carrier proteins bind the molecule on one side, then change shape to release it on the other. This is slower but more selective — like a revolving door that only admits one specific guest at a time.
The Specific Examples You Asked About
| Protein Type | What It Does | Example |
|---|
| Porins | Large channel proteins in outer membranes of bacteria, mitochondria, and chloroplasts. Let through molecules up to about 5000 Da. | Outer membrane of E. coli |
| Uniport | Carries one molecule in one direction. | GLUT1 — transports glucose into red blood cells |
| Symport | Carries two molecules in the same direction. One moves down its gradient, pulling the other along. | Sodium-glucose symporter in kidney tubules |
| Antiport | Carries two molecules in opposite directions. One enters as the other exits. | Sodium-potassium pump? No — that uses ATP. A true antiport example: the chloride-bicarbonate exchanger on red blood cells |
Symports and antiports are NOT always active transport. They are secondary active transport only if the driving ion's gradient was created by ATP. If both molecules move down their own gradients, it's still passive facilitated diffusion. The key question: Is ATP being spent directly? If no, it's passive.
The Key Properties
- Saturation: The proteins have a maximum transport rate. At high concentrations, all binding sites are occupied — the rate plateaus. This is unlike simple diffusion, which keeps increasing linearly. …