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Forgive me if this is a silly question. I can't understand the basics. Why doesn't the cell membrane just break apart? What's keeping the layers in the phospholipid bilayer together? I know that the membrane is embedded with proteins and lipids, but I still can't wrap my head around the "why". Are the hydrophobic interactions in the middle "stronger" than the hydrophilic interactions on the outside? What's keeping the individual phosphate heads together instead of, say, one of them just drifting away due to a nearby water molecule?

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The membrane bilayer is held together by hydrophobic forces. This is an entropy driven process. When a greasy or hydrophobic molecule is suspended in water, the water molecules form an organized "cage" around the hydrophobic molecule. When two hydrophobic molecules come into contact, they force the water between them out. This increases the entropy because the freed waters don't need to be organized into the cage. Lipid bilayers have many many many hydrophobic lipids that squeeze out a lot of water and greatly increase entropy. The polar phosphates allow the water to interact with the surface of the membrane, without a polar head group the lipids would form a spherical blob instead of a membrane.

Read this section on wikipedia for more.

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    $\begingroup$ Perhaps worth adding that the formation of the bilayer increases the entropy of the water, but the entropy associated with the phospholipids will decrease. The net change in entropy is favourable. $\endgroup$ – Alan Boyd Sep 17 '14 at 18:06
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    $\begingroup$ Also the bilayer maximises van der Waals interactions between the closely-packed hydrocarbon chains, again contributing to the stability of the bilayer. $\endgroup$ – Alan Boyd Sep 17 '14 at 18:36
  • $\begingroup$ also surface tension.. which is basically because of the interactive forces.. $\endgroup$ – WYSIWYG Sep 18 '14 at 4:22
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    $\begingroup$ Artificial vesicles can be produced using the same principle. They are quite stable, but the larger ones (giant unilamellar vesicles) are fragile to any strain: cell-sized vesicles would not resist in a tissue or blood flow, and the cell actomyosin cortex or RBC spectrin network are necessary for this. $\endgroup$ – Joce Sep 19 '14 at 7:32
  • $\begingroup$ without a polar head group the lipids would form a spherical blob instead of a membrane I am not so sure about this sentence. Even with the polar head-group, spherical blobs do form as micelles. $\endgroup$ – James Jun 10 '16 at 9:17

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