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If the hydrophobic hydrocarbon chain of the phospholipid prevents the movement of polar molecules through the membrane. Why does the hydrophilic phosphate head of the phospholipid not prevent the movement of non-polar molecules?

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    $\begingroup$ You might be interested in this. $\endgroup$ Commented Nov 29, 2016 at 15:33
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    $\begingroup$ See if this helps: biology.stackexchange.com/questions/52371/… $\endgroup$
    – bpedit
    Commented Nov 29, 2016 at 17:59
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    $\begingroup$ Possible duplicate of Why lipophilic molecules can pass phospholipid bilayer, in spite of 2 hydrophilic layers? $\endgroup$
    – user24284
    Commented Apr 30, 2017 at 8:43
  • $\begingroup$ Polar molecules needs electrochemical gradient and protein carrier. ...whereas non polar molecules needs kinetic energy and these molecule continuously bouncing to come out from the cell membrane through the channel provide by lipoprotein structure of cell membrane and concentration gradient also effective for movement of molecules all types of molecules. $\endgroup$
    – lovisha
    Commented Jan 22, 2018 at 9:03

3 Answers 3

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The plasma membrane consists of hydrophobic and hydrophillic characteristics. Towards the outsides, they are hydrophillic, so they can create bonds with water. The insides are hydrophobic, allowing no water inside and keeping them tight together due to the polar forces.

An non-polar particle (if small), can pass through this because it does not interfere with the hydrophobic/hydrophillic (polar) nature of the plasma membrane. However, polar particles would not have the opportunity to move in, because the insides (hydrophobic) are literally afraid of water, or charges, don't allow polar substances to pass through.

So only hydrophobic (nonpolar), gases, and small particles (nonpolar) can pass through. There are exceptions of $H_2O$ passing through the membrane in small amounts because their electric charge is very minor.

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First, we should understand what causes molecules to be hydrophilic or hydrophobic.

A molecule is hydrophilic if it contains atoms with different electronegativities, that is atoms that pull electrons with different strengths. The factors determining the strength of the pull are the size of the atom (including the pre-existing electron shells) and the number of protons. Make the atom too big and electrons are too far from the nucleus, make it too small and there is not enough protons to pull the electrons. An atom that has a balance between these two factors is oxygen which has the second highest electronegativity in the periodic table. Oxygen was thought to be the maker of acids, which is literally in the name. Now we know this is not the case because all you need is to be sufficiently more electronegative than hydrogen so that it becomes essentially a proton that bonds ionically with you and is released into the solution, becomes looser. Anyway, the electronegative atoms in a polar molecule form hydrogen bonds with oxygen and hydrogen in water molecules because they have opposite partial charges, partial because the bond is not ionic. This includes water forming hydrogen bonds with other water molecules that gives rise to cohesion, surface tension as well as hydrophobic interactions which we shall explain in the next paragraph.

What happens with hydrophobic interactions (not bonds) is that water molecules want to pair with each other so they push the so-called hydrophobic molecules out of the way and clump them together. They are not hydrophobic in the literal sense, they essentially don't pair with water because they are not polar enough. Examples are hydrocarbons, where hydrogen and carbon have very close electronegativities.


The cell membrane is formed of phospholipids, these are molecules with polar hydrophilic phosphate heads and non-polar hydrophobic fatty acid tails. They align so that the phosphate heads face water and the fatty acid tails hide inside the membrane.

Polar molecules can't pass the membrane because they are being pulled by water outside and not being pulled by the membrane which has the hydrophobic fatty acids inside. Channels essentially form hydrophilic passages for these polar molecules so they can pass according to their concentration gradients.

Non-polar molecules are essentially hated by both bodies of water, extracellular and intacellular fluids. The question then becomes: why don't they stay inside the membrane with the hydrophobic fatty acids? Well, we need to think about this in terms of forces. You essentially have two forces:

  1. Surface tension of the water that lines both sides of the membrane and keeps the hydrophobic molecules inside.
  2. Pressure force (or normal force) that arises as the hydrophobic molecules accumulate and push on the surfaces of the membrane.

It has to happen that eventually the hydrophobic pressure overcomes the surface tension unless you put infinite amounts of salt in the water (increases the number of bonds) or you freeze the water which increases the surface tension. It then becomes clear that the molecules are more likely to have a net movement towards the body of water that initially had less of them. This is known as simple diffusion. It's also good to know that membrane fluidity which is the continuous lateral movement of the phospholipids parallel to the membrane constantly breaks other bonds between the hydrophobic molecules and the fatty acid tails like Van der Waal forces.

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Outside membrane there in water and inside membrane there is lipid.

Hydrophilic molecules are more stable in water as compared to lipid so they prefer to stay in water and not go inside lipid.

Hydrophobic molecules are more stable in lipid as compared to water so they prefer to leave water and move freely into lipid.

You are saying that hydrophilic part of membrane will obstruct the movement of hydrophobic molecules but do not forget that they are already in water, which is polar, so it doesn't matter to them.

Analogy :- 'If you are on roof of your house you can very easily jump to roof of adjacent house' similarly hydrophobic molecules which are already unstable in water can move towards hydrophilic part of membrane because they are equally unstable there.

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