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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.

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Polar molecules, ions, and large substances (ex: carbohydrates and proteins) typically have a difficult time making it through the cell membrane without assistance. These particles generally utilize channel and carrier proteins or active transport pumps to cross the semipermeable barrier