Hello!
To answer your question:
The cell membrane's main trait is its selective permeability, which means that it allows some substances to cross it easily, but not others. Small molecules that are nonpolar (have no charge) can cross the membrane easily through diffusion, but ions (charged molecules) and larger molecules typically cannot.
Two examples are:
Charged Ions
An ion is a molecule that is charged because it has lost or gained an electron.
Large Polar Molecules
Large uncharged molecules, such as glucose, also cannot easily permeate the cell membrane.
Hope this helps!
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