2026-08-17 13:28:28: Tonicity and Osmoregulation

biology/Cell Structure and Function/Facilitated Diffusion.md

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+[[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.