|MadSci Network: Cell Biology|
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 side. 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.
Try the links in the MadSci Library for more information on Cell Biology.