biophysics - Why lipophilic molecules can pass phospholipid bilayer, in spite of 2 hydrophilic layers?

It is commonly told that, hydrophobic/ lipophilic/ nonpolar molecules can quite easily pass phospholipid bilayer, and hydrophilic (polar or ionic) molecules can't pass (when no protein aid that); because hydrophobic nature of the lipid.

But in the same logic, hydrophobic molecules shouldn't pass through the bilayer. Because there are 2 hydrophilic layers in the membrane. i.e.

A and A' .

Then how the hydrophobic molecules can pass through A and A'?

I guess, it is due to thin diameter of A and A' . Is that?

Answer

Good question. This is my take.

It's not just the surface of the membrane that's polar. There is water (polar) on both sides of the membrane. In most animal cells there is also an unequal distribution of charges across the membrane. The environment outisde of the cell is typically positive due to an excess of positive ions, especially sodium. The inside of the cell is typically negative due to an excess of negative ions such as phosphate.

This means the hydrophobic molecules aren't any more at home in the environment outside, or inside, the membrane than they are at the surface. There's no reason to suppose any more repulsion at the surface. So, just due to their random kinetic motion they will find themselves at the membrane's surface, some with the necessary kinetic energy to cross.

There's another way to view this. We shouldn't think of the membrane as allowing hydrophobic substances to enter. We should think of it as NOT allowing hydrophyllic substance to enter without a proper ID check by proteins in the membrane.