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I just started learning about the structure and composition of cell's membrane and there is something that I fail to understand.

The membrane is composed of a phospholipid bilayer. The phospholipid has a polar, hydrophilic head and a nonpolar, hydrophobic tail. My question: Why doesn't the hydrophilic head dissolve in the extracellular fluid outside the cell? I suppose that the extracellular fluid is watery, so how come that the water-loving head doesn't interact with the water from the extracellular fluid?

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We should first understand what happens when a substance dissolves. During dissolution water interacts with the solute molecule; if the strength of interaction between the molecule and water is higher than the strength of interaction among the solute molecules then the solute dissolves.
(Also have a look at this post).

Phospholipid is an amphipathic molecule — it has both hydrophobic and hydrophilic parts (this you understand very well). The hydrophobic part is a long chain of hydrocarbon (hence tail) whereas the hydrophilic part is a small but highly polar region (hence head). You should note that these parts are not disjoint but are covalently bonded.

enter image description here

Even though the hydrophilic part interacts with water (which helps the cell survive in an aqueous environment), the strength of covalent bond (between hydrophilic and hydrophobic regions) is very strong and cannot be broken by physical interaction with water. The lipid bilayer (even a monolayer) stays together because the water cannot interact with hydrophobic parts but they interact with each other via a pseudo-interaction called hydrophobic interaction.

I call it a pseudo-interaction (not a standard usage) because there is not actual molecular interaction that stabilizes the system. Hydrophobic interaction happens via increase in entropy of the system. Read the wikipedia page for details.

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    $\begingroup$ Why do you say that hydrophobic interaction is a pseudo force ? Pseudo force has a very different meaning in physics. $\endgroup$
    – biogirl
    May 30, 2015 at 0:39
  • $\begingroup$ @biogirl it is similar to the pseudo-force in physics. I did not have a better word for it so I called it pseudo force. But like the pseudo-force in physics hydrophobic interaction is not really an interaction and arises because of non-interaction of hydrophobic parts with water. Hydrophobic interaction basically allows an increase in entropy which finally brings down the free energy of the system. $\endgroup$
    – WYSIWYG
    May 30, 2015 at 9:57
  • $\begingroup$ @TomD By 'pseudo-force', I meant that there is no actual interaction between the solute and the solvent or between the solute molecules. There is no molecular interaction that would cause a reduction in enthalpy. The "interaction" is because of increase in entropy (not the loss of entropy). $\endgroup$
    – WYSIWYG
    Jul 27, 2015 at 4:12
  • $\begingroup$ @TomD So that is what happens to the solvent and that is what I said. Water makes a structured cage around the lipid molecule (low entropy; no enthalpic reduction because the lipid and water don't interact). When there is another lipid molecule they both come together and increase the net entropy of the system (lesser water cage compared to cage per molecule- think of it as surface area). $\endgroup$
    – WYSIWYG
    Jul 27, 2015 at 9:47
  • $\begingroup$ @TomD We are saying the same stuff. I said that increase in entropy stabilizes the system and you are saying that loss of entropy destabilizes the system. $\endgroup$
    – WYSIWYG
    Jul 27, 2015 at 9:54
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The answer to your question is basically: It is a bilayer. There are two layers of phospholipids, thus tucking the hydrophilic ends safely away from any extracellular and intracellular fluids. The whole surface of the bilayer is hydrophilic.

phospholipid bilayer

(Picture from here.)

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The hydrophilic region of phospholipids is actually interacting with water or other hydrophilic molecules. At the same time hydrophobic region of phospholipids would be repelled from water because water is forming hydrogen-bond networks, which is a more stable state. You could imagine hydrophobic molecules in water would disturb the network. As a result the hydrophobic parts of phospholipids aggregate together and phospholidis would not come out into water not to expose the hydrophobic parts.

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