The membrane's oily core is what stops ions
- The idea
- Predict
- See it
- Why
- Check
The idea
“The charged heads on the surface repel ions, trap them, or pull them through.”
Phospholipid heads are charged, so it's easy to picture them picking which ions get in. Ions reach the heads fine. The oily middle stops them.
The short version
The mistake: The charged heads on the surface repel ions, trap them, or pull them through.
What's actually true: The barrier is the nonpolar core of the bilayer. Ions of either charge stay with water and can't easily enter it.
- The phospholipid tails point inward and make the middle of the bilayer nonpolar, like a thin layer of oil.
- An ion in water is wrapped in water molecules drawn to its charge. Shedding them to enter the oily core costs too much energy.
- The heads can hold some ions at the surface. Repulsion would block only one sign, though, and binding spots fill up.
- Small nonpolar molecules like O2 and CO2 have no shell to shed. They cross by simple diffusion.
On the exam: Ions of either sign are stopped by the oily core. They cross through channels, carriers or pumps.