Why do large polar molecules have difficulty crossing the phospholipid bilayer?
There are really no polar molecules that are able to diffuse through the phospholipid bilayer of a cell. The reason for this is due to the fact that polar molecules can not dissolve in nonpolar materials, i.e. phospholipid bilayers. Polar materials are charged, and non-polar materials are not charged, and therefore they don’t mix. It’s like putting oil in a glass of water, all the oil will come together in one group with all the water surrounding it. In short, non-polar and polar materials are not miscible with each other. Always remember: “Like Dissolves Like”. Polar molecules require a cell membrane receptor for gaining access to a cell, and most of the time the molecule never enters the cell at all, but stimulates a second messenger system to carry out the function the polar molecule is used to produce. Of course, there are execptions. There are open channels on some cells that allow very small polar molecules to travel through them, but as for large polar molecules, well, I think I
