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What I Think I Know: Hydrophilic and hydrophobic things repel each other. Since the cell membrane contains hydrophobic tails, it is difficult for hydrophilic molecules to pass through the cell membrane.

Question: Why don't the heads of phospholipid bilayers repel hydrophobic molecules? In other words, if the phospholipid membrane has both hydrophilic and hydrophobic portions, why do only the hydrophobic portions act as repellents?

Stipulation: I need a quotation from a textbook, university, or other credible source.

Evidence of Prior Research:

Why lipophilic molecules can pass phospholipid bilayer, in spite of 2 hydrophilic layers?: This question has a comment that links a graph from a textbook. I can't understand the graph by itself. There are no other references.

Understanding what passes through and doesn't pass through the plasma membrane: All references discuss the topic, but none answer the question.

I understand that this topic is addressed quite frequently, but the question I'm asking is never answered. Texts tend to say something like, "Ions can't get through the core because of their charge" and completely gloss over the concept of lipids getting through the heads. Please don't dismiss this as a homework question that I didn't take the time to research. This is my second try asking this question (on this site); I have spent hours trying to find an answer.

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    $\begingroup$ Taylor, you've done an excellent job updating your question and providing evidence of additional research effort. Thanks very much for making the extra effort. I don't foresee this question getting marked/closed as homework now, and I hope you get a good answer! Good luck! :). $\endgroup$ – theforestecologist Dec 3 '18 at 1:05
  • $\begingroup$ @Taylor Do you mean chemicals in the intra/extracellular fluid or chemicals embedded in the plasma membrane? $\endgroup$ – Jam Dec 4 '18 at 23:49
  • $\begingroup$ @Jam I'm talking about molecules/chemicals that pass from the intracellular fluid to the extracellular fluid, or vice versa. $\endgroup$ – Taylor Dec 5 '18 at 21:36
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Your question is rooted in a misundertsanding of the hydrophobic effect. Hydrophillic and hydrophobic molecules do not repel but, rather, attract one another through van der Waals interactions. The tendency of hydrophobic molecules to aggregate in aqueous solution (ie the hydrophobic effect) is, instead of some repulsive force, actually driven entropically. I don’t think I will go into this in detail since it has been explained well in many places. That said, it is also explained very poorly in many places (which I suspect you have encountered). I recommend this website to learn about it and other intermolecular interactions.

Once you have a firm grasp on that, consider that in order for a hydrophobic molecule to reach a plasma membrane, it must already be solvated by water. The transfer of a hydrophobe from one hydrophillic environment (water) to another (head groups of the phospholipids in the plasma membrane) should be energetically negligible. The limiting step for passive diffusion across a membrane is transfer from the hydrophillic environment of the phospholipid head groups to the hydrophobic environment of their tails. In fact, the rate of diffusion across a plasma membrane increases with hydrophobicity.

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