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We are taught in the classroom that "likes dissolve likes". Thus, since water is polar, it is thought that it would be soluble in other polar molecules. So the fact that water is able to pass through the nonpolar center of the phospholipid bilayer seems counterintuitive. There are two mechanisms that allow this to occur: the lipid pathway and the water channel pathway.
The lipid pathway refers to water passing through the cell membrane via osmosis, which relies on a concentration gradient. Water moves down the concentration gradient (from high to low concentration).
In some cells, the water flux is very high and cannot be accounted for water diffusion across the lipid bilayers. Thus, the hypothesis, known as the water channel pathways, assumes that there must be a protein that provides a channel through which water passes much like facilitated diffusion.
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