Certain compounds, such as water, can pass across the cell membrane by traveling in the lipid bilayer.

Water passes the cell membrane by travelling through protein channels in the lipid bilayer. 
The lipid bilayer has a non-polar interior, meaning polar molecules are not able to pass through them. To facilitate the movement of such molecules in and out of the cell, there are protein channels present in the membrane. The channels that assist the movement of water molecules are called aquaporins.

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