MadSci Network: Cell Biology

Re: Why can water pass over cell membranes (the phospholipid bilayer) so easily

Date: Wed Oct 11 15:07:14 2000
Posted By: Michael Maguire, Faculty,Case Western Reserve Univ.
Area of science: Cell Biology
ID: 971284731.Cb

1.  I answered essentially the same question a few days ago as 
Message ID Number: 970817495.Bc

2.  It is a mistake to think of water as charged.  Polarity is not charge, 
or you can possibly think of it as a weak partial charge.  Nonetheless, 
water is more than sufficiently NON-polar to be able to penetrate a 
membrane.  The depth of penetration of the membrane by a water molecule is 
a function of the water molecule's own motional energy and the motion of 
the lipids in the bilayer.  Since the energy available for penetration is 
a Gaussian function, there will be some molecules with sufficient energy 
simply to diffuse in far enough, so that they have a probability of coming 
out the other side, as the least energetically expensive path.  You can 
look at it as a cartoon in a way.  Here's water molecules moving around 
and here's the bilayer moving around with each lipid moving somewhat 
independently.  At any given moment, a fraction of the water molecules are 
moving towards the plane of the bilayer.  Most will not get past the lipid 
head groups because of a) charge and polarity and b) bulk size (physically 
blocked).  However suppose at the moment a water molecule hit the plane of 
the bilayer that the lipid that was directly underneath it a moment before 
moves a bit leaving an "open" space.  This would allow the water molecule 
to penetrate deeper into the bilayer.  Normally, this would be a 
relatively unfavorable state energetically and virtually all the water 
molecules would come back out the same direction and not penetrate very 
far.  An even smaller fraction however would penetrate rather deeply and 
at some point a few would penetrate far enough to get through to the other 

3.  Also remember that within the membrane are other various lipids 
besides phospholipids and that some of the fatty acids on the 
phospholipids are unsaturated.  These various lipids and the double bonds 
can and will supply "electrons" that will at least partially stabilize 
the "polar" water molecule.  This will add to the probability of the 
scenario in #2 above.

4.  The amount of water that penetrates most membranes is indeed very 
small, but it can go through.

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