obsidianvault/biology/Cell Structure and Function/Facilitated Diffusion.md

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.