[[Cell Structure and Function]]

Type of passive diffusion that allows molecules that can't cross the hydrophobic core on their own using channel or transport proteins. Requires no energy.

# Summary
* Aquaporins are channel proteins that move large quantities of water across the membrane.
* Facilitated diffusion uses transport and channel proteins to move molecules that can't cross the hydrophobic core on their own across the membrane. (More info in Membrane Transport)
* Movement of ions across membrane can polarize it, creating an uneven charge distribution

**How molecular structure decides the route:**
- Small and nonpolar: passes directly through the bilayer.
- Large and polar (like glucose): needs a transport protein.
- Charged ion (like Na⁺ or K⁺): needs a channel protein.

Water can cross the membrane on their own but it's slow, aquaporins transport large quantities.

| Transport Type        | Energy Required? | Direction   | Examples                                |
| --------------------- | ---------------- | ----------- | --------------------------------------- |
| Simple Diffusion      | No               | High to Low | O₂, CO₂                                 |
| Facilitated Diffusion | No               | High to Low | Glucose, ions, water through aquaporins |
| Active Transport      | Yes (ATP)        | Low to High | Na⁺/K⁺ pump, calcium pumps              |
Key difference between simple and facilitated is the protein.