Either a sodium ion, glucose molecule, or an oxygen molcule
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There are few molecules that can enter and exit the cell without mediation. Some small non-polar molecules, particularly oxygen - O2 and carbon dioxide - CO2, tend to dissolve in lipid bilayers.  This makes them capable of passing through the cell membrane through simple diffusion. In addition, some small uncharged but polar molecules, such as water or urea can also diffuse across the membrane but do so at a slow rate.

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