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I’m having trouble grasping why small polar molecules can cross the hydrophobic region of the membrane and not ions. Won’t the polar molecules be attracted to the watery extracellular medium and not be inclined to move through the hydrophobic region? (This is my reasoning for why ions won’t go through.)

Also, why do lipid-soluble particles go through the membrane? I understand they have an affinity for the hydrophobic region, but won’t they want to stay there, sandwiched between the two watery layers? Isn’t that the point of the orientation of the bilayer? Why would the lipid-soluble particles move from the hydrophobic region into the watery interior of the cell? Furthermore, why are lipid molecules not repelled by the hydrophilic heads on the exterior of the membrane?

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    $\begingroup$ Polar does not equal charged. Polar molecules have dipole moments, but they do not carry a charge. Ions on the other hand are charged particles. There is a difference between trying to get something polar through the membrane and something that is charged through the membrane. $\endgroup$ – AMR Nov 6 '15 at 21:03
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Passive diffusion.

This question specifically refers to passive diffusion.

Ill reiterate and clarify what was mentioned in the question.

Passive diffusion involves a molecule dissolving into the membrane, diffusing across it, and then dissolves in the aqueous solution on the other side.

The second part of your question can be answered briefly: this diffusion event occurs from time to time, and often at a high rate.

Importantly only small, relatively hydrophobic molecules are able to diffuse across a phospholipid bilayer at significant rates. Thus, gases (such as O2 and CO2), hydrophobic molecules (such as benzene), and small polar but uncharged molecules (such as H2O and ethanol) are able to diffuse across the plasma membrane. Other biological molecules, however, are unable to dissolve in the hydrophobic interior of the phospholipid bilayer. Consequently, larger uncharged polar molecules such as glucose are unable to cross the plasma membrane by passive diffusion, as are charged molecules of any size (including small ions such as H+, Na+, K+, and Cl-).

-Cooper, 2000

Hydrophobic fatty acids repel charge.

The additional or missing electron gives the ion its charge. The hydrophilic head groups of the bilayer are charged, but the fatty acid chains are not, and these repel charge.

There are also repulsive forces between hydrophobic and polar residues (hence the bilayer exists in water), however polarity is essentially a spectrum. A small relatively non-polar polar molecule will not experience enough repulsion to stop diffusion, but a larger, or more polar molecule might.

Active and passive transport.

Although not mentioned in the question, these ions and other molecules can permeate the membrane via protein channels that allow, often with great specificity, ions and large molecules to traverse the membrane.

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In the structure of the bilayer, there are proteins sticking out through the membrane as either transmembrane (through the whole membrane), extrinsic (on the outside) or intrinsic (on the inside) proteins. These act as gates or channels to allow polar, water soluble substances through. This process is known as facilitated diffusion. Check out page 11 of this article

Some ions are allowed in but this is a mediated process of active transport, where ion channels or gates regulate it. This is seen in neurons during the movement of the electrochemical signal along the axon, however, this does not happen automatically, unlike facilitated diffusion.

As for the second part of your question i feel like you can check out this question here

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