Nonpolar and small polar moleculescan pass through the cell membrane, so they diffuse across it in response to concentration gradients. Carbon dioxide and oxygen are two molecules that undergo this simple diffusionthrough the membrane. The simple diffusion of water is known as osmosis.
This arrangement means that only small, uncharged molecules can pass through the membrane. The membrane is called semipermeable, meaning that some things can pass through without assistance, while other things cannot. Water is a charged molecule, so it cannot get through the lipid part of the bilayer. In order to allow water to move in and out, cells have special proteins that act as a doorway. These proteins are called aquaporins (aqua = water, porin = pore).
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