How do plasma membranes prevent ions from diffusing?

[Darkmisc]

TL;DR Summary

Why exactly can't ions diffuse through plasma membranes (without help from proteins)?

Hi everyone

Explanations of diffusion that I've seen mention that the polarity of phospholipid layer stops ions from diffusing without specifying how. I'm assuming that ions with the same polarity as the membrane will be repelled. Would it be correct to say that ions with the opposite polarity as the layer are unable to diffuse because they get stuck to the polar heads of the phospholipids? If so, would a cell be covered in a layer of ions? And would that layer be covered by other ions of opposite charge?

Thanks

 

[BillTre]

The lipid molecules in cell membranes have two parts. They have charge at one end, which is attracted to the water (which is polar in nature, hydrophilic). The rest of the lipid molecules have no charge and are non-polar (lipophilic). Water finds them repulsive and wants to keep away from them (to anthropomorphize the thermodynamic forces involved). The non-polar, non-charged part of the lipid molecules get grouped together (to minimize free energy) and form micelles or membranes, if their concentrations are high enough. This makes a situation where the charged part of the phospholipid contacts the water on either side of the membrane, while the non-charged part is in contact with the non-charged part of the other phospholipid molecules in the middle of the membrane.

Ions, having charge, like being in the watery areas (they are hydrophilic; -phillic is liking or loving). As a result they are also lipophobic (-phobic is fearing) and do not like being in the non-charged, non-polar part of the membrane (the middle part).

When it comes to charged molecules, like ions, diffusing around. They will move fine in an aqueous environment (either inside or outside of the cell), but they won't diffuse easily through the membrane (even though it is very thin). An occasional charged molecule can get through the membrane just because it has a moment near the membrane with enough kinetic force to blast through the membrane, but those are rare. The membrane acts as a effective barrier to their movement.
Channel proteins can let ions through because they provide a aqueous (or hydrophilic) pathway through the membrane due to the structure of the protein.

These are thermodynamic controls on the diffusion of ions. It happens without expending energy.

Specific charges on the ions don't matter concerning their ability to get through the membrane, unless protein channels are involved. They can be ion specific.
The charges are important in other effects.